St Roch s Secondary School Physics Department. National 5 Physics Electricity and Energy Class Questions

Transcription

1 St Roch s Secondary School Physics Department National 5 Physics Electricity and Energy Class Questions

2

3 Electricity Alternating and Direct Current Current, Charge and Time In this section you can use the equation: charge = current x time also written as Q = It where Q = charge in coulombs (C) I = current in amperes (A) t = time in seconds (s). 1. Find the missing values in the following table. Charge (C) Current (A) Time (s) (a) 5 30 (b) (c) (d) (e) (f) A bulb draws a current of 1 A. How much charge flows through it in 60 seconds? 12V C of charge flow though an electric heater in 180 seconds. What is the current in the heater?

4 4. A hairdryer operates with a current of 5 A. How much time would it take for C to pass through the hairdryer? 5. A current of 2 1 A flows through an electric shaver for 5 minutes. How much charge flows in this time? 6. A 60 W bulb is switched on for 30 minutes. If 450 C pass through it in this time, what is the current flowing in the bulb? 7. A surround sound speaker system on a TV draws a current of 0 6 A. In an average day C of charge flows through the speaker system. For how long is the TV switched on each day? 8. An electric kettle has a label on it as shown below. MODEL No capacity 1 7 litres 9 2 A / V 2 2 kw After the kettle is switched on it automatically switches off when the water in it has boiled. On one occasion C passed through the kettle before it switched off. Use the information given to work out how long the water took to boil? 9. An electric fire is rated at kw, 230 V, 12 5 A. How much charge will flow through this fire in a period of 2 hours 20 minutes? 10. One day an electric iron was switched on from 1 45 p.m. until 3 15 p.m. What current was drawn by the iron if x 10 4 C passed through it in this time?

5 Resistance Voltage, Current and Resistance In this section you can use the equation: also written as: voltage = current x resistance V = IR where V = voltage in volts (V) I = current in amps (A) R = resistance in ohms ( ). Helpful Hint. Many appliances run from mains voltage which is 230 V ac. Useful units for electricity are: 1 A = A = 1 x 10-6 A 1 ma = A = 1 x 10-3 A 1. Find the missing values in the following table. Voltage (V) Current (A) Resistance ( ) (a) (b) (c) (d) (e) (f) x Look at the following circuits and calculate the supply voltage in each case: (a) Vs (b) Vs (c) Vs 10 A 2 56 A 50 ma

6 3. Look at the following circuits and calculate the current in each case: (a) 24 V (b) 12 V (c) 48 V I I I Look at the following circuits and calculate the unknown resistance in each case: (a) 24 V (b) 12 V (c) 48 V 25 ma 30 A 660 A R R R 5. Calculate the resistance of a lamp if the current through it is 10 ma when operated by a 24 V supply. 6. A power drill is operated at mains voltage and has a resistance of 1 5 k. Calculate the current through the drill. 7. The maximum current an electric motor can safely handle is 10 ma and it has a resistance of 360. Calculate its safe operating voltage. 8. A cooker draws a maximum current of A and has a resistance of 8. At what voltage should it operate? 9. Hairdryers work from the mains voltage and can have currents of up to 15 ma flowing through them. Calculate the resistance of the hairdryer. 10. Overhead cables have resistance of 25 k. If the voltage across the cables is V calculate the current through them.

7 Power, Energy and Time In this section you can use the equation: also written as power = energy time P = E t where P = power in watts (W) E = energy in joules (J) t = time in seconds (s). 1. Find the missing values in the following table. Power (W) Energy (J) Time (s) (a) (b) (c) (d) (e) (f) How long will it take for a 60 W bulb to use 720 J of electrical energy? 3. A bulb uses J of energy in 300 seconds. What is its power rating? 4. A 50 W immersion heater is switched on for 80 seconds. How much electrical energy passes through it in this time? 5. Calculate the power rating of an electric sewing machine which uses J of energy in 8 minutes.

8 6. A W hairdryer is switched on for 20 minutes. How much electrical energy does it use? 7. For how many minutes must a 600 W shaver be switched on in order to use J of electrical energy? 8. An electric fire uses 5 22 MJ of electrical energy in half an hour. Calculate the power rating of the fire.(1 MJ = 1 x 10 6 J = J ) 9. How long will it take a 1 4 kw paint stripper to use 1 68 MJ of electrical energy? 10. A microwave oven is on for twenty minutes each day. If it uses 7 98 MJ of electrical energy in one week, what is its power rating?

9 Power, Current and Voltage In this section you can use the equation: power = current x voltage also written as P = IV where: P = power in watts (W) I = current in amps (A) V = voltage in volts (V). Helpful Hint The voltage of the mains is 230 V ac. 1. Find the missing values in the following table. Power (W) Current (A) Voltage (V) (a) (b) (c) 1 5 x (d) (e) x 10-3 (f) A car battery supplies a voltage of 12 V. One headlamp bulb draws a current of 3 A. What is the power rating of this bulb? 3. An electric shower has a power rating of 12 W and draws a current of 0 11 A. What voltage is required to operate the shower?

10 4. What current flows through a 230 V, 60 W household lamp when it is operating at the correct voltage? 5. Calculate the power rating of an electric drill which draws a current of 3 A when connected to the mains. 6. What would be the reading on the ammeter in the circuit shown? 12 V A 24 W 7. The following information was found on the rating plate of a food processor: BL W 230 V 50/60 Hz made in the U.K Calculate the current flowing in the food processor. 8. A fridge has a power rating of 160 W. When it is plugged into the mains what current will it draw? 9. A radio has a power rating of 6 W and draws a current of 0 5 A when operating normally. (a) What voltage does this radio need? (b) How many 1 5 V batteries would be needed to operate the radio? 10. The circuit shows a heating element (resistor) operating correctly. I = 4 2 A 1 kw What is the voltage of the supply? 11. A torch bulb draws a current of 500 ma. It has a power rating of 1 75 W. What voltage is required to light the bulb to its correct brightness?

11 12. An electric locomotive on the East Coast line gets its electricity from the overhead cables which supply 25 kv a.c. The locomotive has a top speed of 140 mph and it operates at 4 7 MW. Calculate the current flowing to the locomotive. 13. A helium - neon laser emits red light and has many uses in medicine. The laser uses the 230 V mains supply and has a power rating of 5 mw. What current flows in the laser? 14. The current flowing in an electric keyboard is measured as 800 ma. What voltage is required to operate the keyboard if it has a power rating of 7 2 W? 15. An electric fire has three heating elements which can be switched on and off independently. Each element has a power of 1 3 kw and is controlled by a switch on the side of the fire. (a) Calculate the current drawn from the socket when one heating element is switched on. (b) What is the maximum current that the fire could draw?

12 Power and Resistance Helpful Hint By combining the formulae: P = I V and V = I R we have alternative forms of the formula for power. i.e. P = I (IR) and P = (V/R) V P = I 2 R P = V 2 /R Use these alternative forms of power formulae in the following questions. 1. Calculate the power rating of a lawn mower which has a resistance of 70 5 and draws a current of 2 5 amps. 2. What power is dissipated in the element of a toaster if it has a resistance of 64 and draws a current of 3 75 amps? 3. Find the power rating of a television given that it operates at mains voltage and has a resistance of What would be the power rating of an iron if it operates at mains voltage and has a resistance of 45? 5. Calculate the power rating of a hedge trimmer given that it draws a current of 1 67 A and has a resistance of Calculate the power rating of a Karaoke machine if it has a resistance of and operates at mains voltage. 7. How much power is dissipated in curling tongs if they draw a current of 1 05 A and have a resistance of 230 while operating from the mains?

13 8. A microwave oven has a resistance of 68 and works properly from a mains supply. What is the power rating for such an oven? 9. Find the power rating of a Hoover designed with a resistance of 110 when operating at mains voltage. 10. What current is drawn from the supply by a transistor radio if it has a power rating of 3 W and a resistance of 12? 11. Find the resistance of a table lamp which has a power rating of 115 W and draws a current of 0 5 A. 12. An alarm clock is driven by a domestic supply voltage of 230 V. What is its resistance if it has a power rating of 2 W? 13. Calculate the total resistance of an electric fire if it operates safely at 230 V and uses electrical energy at a rate of 750 W. 14. Calculate the voltage of a car alarm which has a power rating of 3 W and resistance of Find the power rating of an electric food mixer if it operates at mains voltage and has a resistance of 400.

14 Section 3 Miscellaneous Questions 1. The following circuit is set up in a laboratory. 3 A 12 V The bulb is switched on for 3 minutes and the voltmeter and ammeter readings remain as shown in the diagram. (a) What is the power rating of the bulb? (b) Calculate the resistance of the bulb. (c) How much electrical energy is supplied to the bulb in the 3 minutes? (d) How much charge passes through the bulb in the 3 minutes? 2. The resistor in the following circuit has a power rating of 0 09 W. 10 k A V (a) What reading would you expect to find on the ammeter? (b) What reading would you expect to find on the voltmeter? (c) How long would it take for 5 4 joules of electrical energy to be supplied to the resistor? (d) How much charge passes through the resistor in 1 minute?

15 3. Some students decide to measure the resistance of a torch bulb and each set up a circuit to do this. The circuits are shown below. V A A Circuit 1 Circuit 2 (a) Which circuit has been set up correctly? Explain your answer. (b) Use the following results to plot a graph of voltage against current and hence or otherwise find the resistance of the torch bulb. V Current (A) Voltage (V) The rating plate on an electric kettle is shown below. Type KE 180 Serie V Hz 2 2 kw (a) Calculate the current flowing in the kettle. (b) What is the resistance of the heating element in the kettle?

16 5. A vacuum cleaner connected to the mains draws a current of 3 1 A. (a) What is the power rating of the vacuum cleaner? (b) What is the resistance of this appliance? 6. An iron rated at 1 1 kw draws a current of 4 6 A. One day C of charge passed through the iron. (a) How long was the iron in use? (b) How much electrical energy was supplied in this time? 7. A bulb in a laboratory operates with a current of 2 A. During an experiment C of charge flow through the bulb, supplying it with J of electrical energy. (a) How long did this experiment last? (b) What was the power rating of the bulb? 8. The power rating of an electric fire is 3 kw and it draws a current of 12 5 A. One afternoon the fire used up 8 1 MJ of electrical energy. (a) For how long was the fire switched on? (b) How much charge passed through the fire in this time? 9. An electrical appliance has a power rating of 2 4 kw. If J of electrical energy are supplied to the appliance by C of charge, calculate the current drawn by the appliance. 10. The following label is attached to a hairdryer. Type kw 230 V / 6 A Calculate the amount of electrical energy supplied to the hairdryer by C of charge.

17 Useful Circuits Series Circuits Helpful Hint The rules for series circuits are: 1. the current is the same at all points in the circuit 2. the voltage of the source is shared amongst the components in the circuit. 1. Two identical 2 5 V bulbs are connected to a supply as shown. What is the voltage of the supply? 2 5 V 2 5 V 2. Four identical resistors are connected across a 12 V supply as shown in the diagram. What is the voltage across each of the resistors? 12 V A B C D 3. A simple circuit with a bulb and resistor in series is shown below. 36 V 12 V, 36 W R (a) If the bulb is operating at its correct voltage and power rating what is the voltage across the resistor R? (b) The current in the bulb is 3 A. What current flows in the resistor?

18 4. Two resistors are connected in series to a supply as shown in the diagram. 15 V (a) The current in the 200 resistor is 0 05 A. What is the current in the other resistor? (b) The voltage across the 100 resistor is 5 V. What is the voltage across the 200 resistor? 5. A rheostat is used as a dimmer switch in a simple series circuit as shown. 14 V The rheostat is adjusted until the bulb is shining brightly. The voltage across the bulb is 13 8 V and the current through the rheostat at this setting is 1 7 A. (a) Calculate the voltage across the rheostat. (b) What is the current flowing in the bulb?

19 Parallel Circuits Helpful Hint The rules for parallel circuits are: 1. the voltage is the same across all the components in parallel. 2. the current from the supply is shared amongst the different branches of the circuit. 1. Two resistors are connected in parallel to a 12 V battery. 0 2 A 0 3 A 12 V R 1 R 2 (a) What is the voltage across R 1? (b) What is the voltage across R 2? (c) What size of current is drawn from the battery? 4 A 6 V 2. Two identical bulbs and a resistor are connected in parallel to a 6 V supply. (a) What is the voltage across L 2? L 1 L 2 (b) A current of 1 8 A flows through each of the bulbs. What is the current flowing through the resistor? R 1 3. An electric fire has three elements which can be switched on and off independently. The elements are connected in parallel to the mains supply. Each element draws a current of 0 3 A when switched on. 230 V (a) (b) (c) What is the voltage across the middle element? What is the total current flowing from the supply when two of the elements are switched on? What is the maximum current drawn from the mains by the fire?

20 4. The headlamps and side lights in a car are connected in parallel. The diagram below shows how they are connected. The side lights (L 1 & L 2 ) may be switched on by themselves using switch S 1. The headlights (H 1 & H 2 ) are switched on by switch S 2 and only come on if the sidelights are already on. S 1 S 2 12 V L 1 L 2 H 1 H 2 (a) What is the voltage across the sidelight L 1? (b) What is the voltage across the headlight H 2? (c) Each sidelight draws a current of 3 A from the car battery. What is the total current drawn from the battery when S 1 only is closed? (d) Each headlight draws a current of 5 A from the car battery. What is the total current drawn from the battery when S 1 and S 2 are closed? 5. A hairdryer contains a motor and heating elements (resistors). The hairdryer shown below has three heat settings- cold, warm and hot. The circuit diagram shows how these settings are achieved using switches A, B and C. A B C 230 V M H 1 H 2 The motor draws a current of 3 A from the mains and the heating elements draw a current of 2 A each from the mains. (a) Which switches must be closed to make the hairdryer blow warm air? (b) What current is drawn from the mains when the hairdryer blows warm air? (c) Which switches must be closed to make the hairdryer blow hot air? (d) What current is drawn from the mains when the hairdryer blows hot air? (e) What is the minimum current drawn from the mains when the hairdryer is on? (f) What is the voltage across the motor?

21 Resistance in Series In this section you can use the equation: R s = R 1 + R 2 + R where R s = total resistance of a series circuit or series section of a circuit ( ). 1. Three resistors R 1, R 2 and R 3 are arranged in series as shown in the diagram below. R 1 R 2 R 3 Find the missing values in the table. R 1 ( ) R 2 ( ) R 3 ( ) R s ( ) (a) (b) (c) (d) (e) (f) Calculate the total resistance of the following circuit k The resistance of the following circuit is 8 8 k. Calculate the resistance of R. 950 R 6 3

22 Resistance in Parallel In this section you can use the equation R = + p R 1 R R 3 where R p = total resistance of a parallel circuit or parallel section of a circuit ( ). 1. Calculate the total resistance of each of the following circuits: (a) (b) (c) (d) (e) (f) (g) (h) (i)

23 2. Calculate the total resistance of each of the following circuits: (a) (b) (c) k 3 k 1 2 k k 600 (d) (e) (f) 280 k k 560 k k The total resistance of the circuit below is 80. Calculate the resistance of R. R The total resistance of the following circuit is Calculate the resistance of resistor A. 300 A The total resistance of the following circuit is 240. Calculate the resistance of resistor X

24 1 200 X Combination Circuits Helpful Hint R s = R 1 + R 2 + R ( for a series section of a circuit) 1 = (for a parallel section of a circuit) R p R 1 R 2 R 3 1. Calculate the total resistance in each of the following networks: (a) 20 (b) (c) (d) (e) (f)

25 2. The following circuit shows part of a car lighting system X 250 Y 560 Z Calculate the resistance between points: (a) X and Y (b) Y and Z (c) X and Z. 3. Calculate the resistance of the network of resistors shown below Which network of resistors has the lowest total resistance? Network A Network B A school technician has different resistors to use in building house wiring models He has two 10 resistors, two 50 resistors and two 100 resistors. How can these resistors be combined to produce a total resistance of: (a) 260 (b) 300 (c) 30 (d) 35?

26 Section 4 Miscellaneous Questions 1. Bulbs B 1, B 2 and B 3 are all identical. What is the voltage across bulb B 2 and bulb B 3? 12 V 8 V B 1 B 2 B 3 2. Look at the circuit shown and find: 24 V (a) the voltage across R 2 (b) the current through R 1. I = 2 A R 1 R 2 16 V R 3 3. In the following circuit the bulbs are identical. Find: (a) the voltage across each bulb (b) the current through each bulb. 36 V I = 0 9 A 12 V

27 4. In the following circuit all the resistors are identical. Find: (a) the voltage across each resistor (b) the current through each resistor. 24 V 3 A A B C D 5. Daral has a selection of resistors and has to combine them in some way to obtain a total resistance of 60. He has two 5, two 20 and two 90 resistors. He sets up three different networks as shown below. Which one is correct? Network Network 2 5 Network 3

28 Section 5 - Behind the Wall In this section you can use the equation: energy = power x time also written as E = Pt where E = energy in kilowatt hours (kwh) P = power in kilowatts (kw) t = time in hours (h). Helpful Hint Since the joule is such a small unit of energy electricity boards use the unit kilowatt hour to measure how much electricity has been used. 1 kwh = W x s 1 kwh = J (1 kwh is often called a unit of electricity ) 1. How many kilowatt hours of energy does a 3 kw electric fire use when it is switched on for 4 hours? 2. A 4 kw immersion heater is left on for 2 hours to heat water for a bath. (a) How many kilowatt hours of energy does it use? (b) How many joules of energy does it use? 3. How many units of energy will a 100 W security light use if it is left on overnight for 8 hours? 4. A 3 5 kw electric fire is left on full power for 4 hours. (a) How many units of electricity did the fire use? (b) If each unit of electricity costs 7 4 p calculate the total cost of running the fire for 4 hours.

29 5. How long would it take a 1 6 kw fridge to use 8 kwh of energy? 6. A 360 W food processor used kwh of energy. For how many minutes was it working? 7. An electric iron uses 0 3 kwh of electricity when left on for 15 minutes. What is the power rating of the iron? 8. A 2 kw hot water urn is left on full power for 30 minutes. (a) How many kilowatt hours of energy does it use in this time? (b) What did this electricity cost if each unit costs 7 4 p? 9. How much more expensive would it be to run a 4 5 kw heater for 15 minutes than a 60 W bulb for 10 hours if each unit of electricity costs 7 4 p? 10. The electricity board issue a bill every three months. The bill shows the electricity meter reading at the beginning and end of this period and the total number of units used. The cost per unit of electricity is given along with the standing charge for use of the electricity board s facilities. An exert from an electricity bill is given below. Meter Reading Detail of charges Present Previous domestic cost per unit 7 4 p standing charge (a) How many units of electricity were used? (b) What was the total cost of this bill? (c) How many joules of electricity were used? (d) Why do you think that electricity companies measure the amount of electricity used in kilowatt hours rather than joules?

30 Using Electricity Revision Questions 1. Clare has set up a circuit, as shown below, in order to switch on a bulb. She wants to measure the voltage across the bulb and the current through it. (a) What should Clare use in order to measure the voltage across the bulb? (b) What should be used to measure the current in the bulb? (c) Redraw the circuit to show the correct positions of these meters. 2. Look at the following circuits: 2 V 6 V 0 3 A R A (a) Calculate the voltage across the 12 resistor. (b) Calculate the current through the 100 resistor. (c) What is the value of resistor R? 3. An electric kettle operates at mains voltage drawing a current of 9 A. (a) What is the value of mains voltage? (b) What is the power rating of the kettle? (c) How much electrical energy is supplied to this kettle each second? (d) How much electrical energy would the kettle use if it was switched on for 3 minutes?

31 4. For each of the following circuits what should be the reading on the ammeter and voltmeter? (a) 2 5 V 3 6 V (b) 0 5 A A A 0 5 A V 0 5 A 1 2 V V (c) 1 5 V A 0 2 A V 5. Calculate the power rating of each of the following electrical appliances. (a) An electric shaver which draws a current of 2 A from the mains supply. (b) An electric fire which uses J of energy in 30 minutes. (c) A lamp which operates on the mains voltage drawing a current of 0 26 A. (d) A hairdryer which uses J of electrical energy in 15 minutes.

32 7. Explain how each of the following situations could result in an accident. (a) Eight appliances are connected to one socket using adapters. (b) An electricity socket is fitted in the bathroom. (c) A 13 A fuse is fitted to an alarm clock/radio.

33 1. A lamp is connected in parallel with a resistor as shown below. The bulb is a 12 V, 36 W bulb. (a) Calculate the current through the bulb when it is operating at its correct voltage. (b) What is the resistance of the bulb? 12 V 6 (c) Calculate the current through the resistor. (d) Calculate the total current drawn from the supply. 2. The rating plate from a hand blender is shown below. Electronic model no V 50 Hz W (a) Calculate the current drawn by the blender. (b) Calculate the resistance of the blender. (c) Explain the significance of the symbol on the rating plate. The blender has to be charged up before use. It is charged for 5 hours and the average charging current is 10 ma. (d) Calculate the charge stored.

34 3. Paul is always being accused of wasting electricity because he has nearly every electrical appliance switched on at once in his room. His stereo, CD player, television, lamp and computer are all connected to the mains via an adapter as shown below. mains voltage hairdryer R = 500 TV R = 480 lamp R = 400 computer R = 2 k stereo R = 2 2 (a) What kind of circuit is this? (b) What simple test could you carry out to prove this? (c) Calculate the total resistance of the circuit when all appliances are on. (d) Calculate the current drawn from the mains when all the appliances are on. (e) What is the power rating of the computer? (f) How many units of energy are used when the computer is on for 6 hours? (g) If a unit of electricity is 7 4p how much does it cost to run the computer for this length of time? (h) The lamp is usually on for 5 hours a day. How much money is saved: (i) per day? (ii) per week? if the lamp is switched off.

35 4. Junior Spark tries his hand at some DIY electrics in and around his new home. He designs two circuits. Circuit 1 is a model for a room light and Circuit 2 is a model for toy car lights. His designs are shown below. Circuit 1 24 V R = 900 Circuit 2 S 2 S 3 S 1 L 1 L 2 H 1 H 2 Sidelights 12 V, 24 W Headlights 12 V, 36 W Look at the circuits and answer the following questions. (a) Describe the function of the variable resistor in circuit 1. (b) The variable resistor can be set at any resistance between 10 and 10 k. Use this range to calculate the minimum and maximum current that could flow through the bulb. (c) In circuit 2, which switches must be closed to switch on the head lights? (d) What size of current is drawn from the battery when all the lights in circuit 2 are on? (e) What is the advantage of having the sidelights and the headlights connected in parallel instead of in series with one another?

36 5. A typical toaster design consists of two heating elements arranged in parallel as shown below. 230 V Calculate the power rating of the toaster. 6. Look at the circuit below and answer the following questions: A 10 S 6 V 20 V 60 (a) Calculate the resistance of the circuit when switch S is open. (b) What is the reading on the ammeter when switch S is open? (c) What is the reading on the voltmeter when switch S is open? (d) Calculate the resistance of the circuit when switch S is closed. 7. Consider the following circuit V 20 A 90 (a) What is the total resistance of this circuit? (b) Calculate the reading on the ammeter. (c) What is the voltage across the 90 resistor? 8. The following circuit was set up in a classroom.

37 Vs 0 04A A 300 V P 0 03A 100 Q 0 25 W (a) What is the current through the bulb? (b) Calculate the voltmeter reading. (c) What should the reading on the ammeter be? (d) Calculate the voltage across the 100 resistor. (e) What is the voltage across the 300 resistor? (f) What is the voltage across PQ? (g) Calculate the supply voltage, Vs. 9. A basic design for an electric bell is shown below. Explain how the bell operates.

38 10. A d.c. electric motor was built using the following design: (a) Name the parts of the motor labelled X, Y and Z. (b) Explain how the d.c. motor operates. (c) Why is carbon used in commercial motors as the material for component X? (d) Explain how the magnetic field is produced in larger commercial motors. (e) How could component Y be improved, from this simple design, for use in large scale motors.

39 Electronics Output Devices Identifying Output Devices 1. Which of the components in the list below are output devices? bulb thermistor relay thermocouple solenoid loudspeaker dynamo LED LDR buzzer motor microphone 2. Which output device could be used in a central locking system of a car? 3. What would be an appropriate output device for a public address system? 4. Select an output device which could be used to raise and lower blinds automatically in a luxury flat. 5. Which output device is useful for digital displays on hi-fi systems? The Light Emitting Diode (LED) 1. Which of the following LED s will light? A B C D E F

40 Helpful Hint When working with LED circuits you can use the equation: When applying this equation remember that the supply voltage is shared between the LED and the resistor. Example A certain LED takes a current of 10 ma and the voltage across it is 2 V. What should be the value of the series resistor when a supply voltage of 5 V is used? 1st. Sketch the circuit 2nd. Calculate the voltage across resistor R. 5 V V R = V supply - V led = 5-2 I = 10 ma = 3 V = A R 3rd. Apply V = IR to find the value of R 2V V = IR V R = 3 V V R = I R I = 0 01 A 3 = 0 01 x R R =? R = 3 / 0 01 R = Use the stages outlined above to find the missing values in the following table. V supply I V led R V R V supply (V) Voltage across LED (V) Current (A) Voltage across R (V) Resistance of R ( ) (a) (b) (c) (d) (e) (f)

41 3. For each of the following circuits calculate the value of the series resistor which will enable the LED to operate at its ideal voltage and current. (a) 4 V (b) 9 V R R (c) 2 1 V, 10 ma 12 V (d) 1 8 V, 12 ma 6 V R R 2 2 V, 10 ma 1 6 V, 11 ma (e) 14 V (f) 9 8 V 12mA 15mA R R 2 V 2 3 V (g) 9 5 V (h) 5 V 10mA 20mA R R 3 V 4 V

42 4. Consider the following circuit. 5 1 V 8mA 400 (a) Calculate the voltage across the 400 resistor. (b) Calculate the voltage across the LED. 5. For the circuit shown below work out the value of the supply voltage which will enable the LED to operate at it s stated rating V, 11 ma 6. The voltage and current specifications for a certain LED are 1 75 V and 10 ma respectively. What should be the value of the series resistor if the LED is powered by a 6 V supply? 7. Calculate the ammeter reading in the following circuit. 12 V A 1 8 V Calculate the voltmeter reading in the circuit shown below. 6 V 8mA 475 V

43 9. What is the supply voltage for the following circuit? 15mA V 10. Calculate the value of resistor R in the circuit below. 9 V 20mA R 4V

44 Input Devices In this section you can use the equation: V = IR where V = voltage in volts (V) I = current in amps (A) R = resistance in ohms ( ). Helpful Hint When choosing an input device for an electronic system, think about what type of energy the device has to detect. Capacitor microphone thermistor solar cell light dependent resistor (LDR) thermocouple 1. Select from the list above a suitable input device for each of the following: (a) Public address system in a railway station (b) Digital thermometer (c) Photographers light meter (d) Time delay circuit for courtesy lights in a car (e) Pilot light flame detector in a gas central heating system (f) Sunlight hours recorder at a weather station. 2. The circuits below show two identical LDR s each connected to a 6 V supply. One LDR is placed in a cupboard and the other is placed beside a window A A 6 V 6 V Circuit (i) Circuit (ii) (a) Calculate the resistance of each LDR.

45 (b) Which circuit shows the LDR in the cupboard? 3. The following circuit shows a thermistor connected to a 5 V supply and placed in a school laboratory. 5 V A In the morning the ammeter gave a reading of 1 25 ma. Later in the same day the reading had risen to 2 5 ma. (a) Calculate the resistance of the thermistor in the morning. (b) What happened to the temperature in the room during the day? Explain your answer. 4. The following information for an LDR was found in a components catalogue. Light Source Illumination (lux) Resistance (k ) moonlight W bulb at 1m fluorescent light bright sunlight This LDR is connected to a 12 V supply with an ammeter in series with it as shown in the diagram. 12 V A (a) What is the resistance in ohms of the LDR when exposed to fluorescent light? (b) What would the ammeter read when a lamp with a 60 W bulb in it is placed 1 m away from the LDR? (c) When the ammeter gives a reading of 0 6 A which light source is being used?

46 5. A pupil uses a thermistor as a simple electronic thermometer. She connects the thermistor to an ammeter and places the thermistor into a beaker of hot water. The ammeter gives a reading of 8 ma. Temperature ( o C) Resistance ( ) 6 V A (a) What is the temperature of the water in the beaker? (b) The pupil adds some more water to the beaker and the ammeter gives a new reading of 1 6 ma. Did the pupil add hot or cold water to the beaker? (c) What is the new temperature of the water? (d) What will the ammeter read when the water is boiling?

47 Voltage Dividers In a series circuit the voltage divides up between the components in the circuit i.e. V s V s = V 1 + V 2 R 1 R 2 where V s = supply voltage V 1 = voltage across R 1 V 2 = voltage across R 2 V 1 V 2 From Ohm s law we know that since current is constant in a series circuit, the higher the resistance of a component the greater the voltage across it. This idea is used in the following example to calculate the voltage across components in a voltage divider i.e. series circuit. Example 24V 4 8 Use the fact that the voltage split across each component is in the same ratio as the resistance of each component. V 1 = R 1 x V s V 2 = R 2 x V s where R t = total resistance R t = 4 x 24 = 8 x = 8V = 16 V ( Remember to check your answer e.g. does V 1 + V 2 = V s ) R t Lastly! Circuit problems in electronics are usually drawn slightly differently than you are used to seeing. e.g. 24 V + 24 V 4 8 would be drawn as 0 V 8 4

48 Find the voltage across each resistor in the following: + 12 V + 24 V (1) (2) (3) + 36 V V 0 V 0 V (4) + 12 V (5) + 10 V (6) +36 V V 0 V 0 V (7) + 24 V (8) + 24 V (9) + 24 V V 0 V 0 V (10) (11) (12) + 36 V + 36 V +36 V V 0 V 0 V

49 + 12 V + 10 V (13) (14) (15) V V 0 V 0 V (16) + 5 V (17) + 36 V (18) + 24 V 2 k k V 0 V 0 V (19) V (20) + 20 V V 0 V

50 Helpful Hint LDR s and thermistors often make up part of a voltage divider circuit in electronic systems. It is important to remember that the resistance of these components varies with external conditions. The following tables indicate how the resistance of an LDR and thermistor vary with external conditions. LDR Light condition Resistance ( ) dark light bright 20 Thermistor Temperature ( o C) Resistance ( ) Use the information above to solve questions The following circuit is part of the input to an electronic frost alarm V 100 If the circuit operates from mains voltage calculate the voltage drop across the thermistor when it is (a) (b) 10 o C 40 o C 0 V 22. The following circuit could be part of a light meter for a camera V Use the information above to find the voltage drop across the LDR when it is: 1 k (a) (b) dark light 0 V

51 23. Calculate the voltage across the resistor in the following circuit when the temperature is: (a) 100 o C (b) 40 o C + 24 V V 24. A young engineer designs part of an electronic system to trigger an alarm when it gets too bright. What will the trigger voltage across the resistor be in the following system when it is bright? + 36 V 1 5k 0 V 25. At what temperature would the following circuit show equal readings on each voltmeter? V V V 2 0 V

52 Helpful Hint In a circuit like the following: + 5 V R 1 X R 2 0 V You are often required to calculate the voltage( or potential) at X This is the same as asking for the voltage (or potential) across resistor R 2 Calculate the voltage at X in questions V + 5 V + 36 V k X X X 0 V 0 V 0 V Switch S open Capacitor C uncharged Switch S closed V +12 V 3 5 k X X 10 k 0 V Capacitor C fully charged 0 V Switch S open.

53 Digital Processes Transistors in Circuits Helpful Hint The symbol for an NPN transistor is shown below. The transistor operates as a switch in a circuit. It switches ON when the voltage across XY, in the diagram below, is above approximately 0 7 V. P X 0 7 V Q Y In the last section you studied voltage dividers. A voltage divider is usually placed between P and Q. Various components can be used to make up the voltage divider but it is always the voltage across XY which affects the transistor! Some examples are shown on the next page.

54 Examples (a) The voltage divider can be two resistors in series +6 V X 3 V Voltage across XY is 3 V so transistor conducts (i.e. switches ON ) and therefore bulb lights. 0 V Y (b) The voltage divider can be a Potentiometer + 6 V X 6 V Voltage across XY is 6 V so transistor conducts and bulb lights. 0 V Y + 6 V X 0 V Voltage across XY is 0 V so transistor does not conduct therefore bulb is OFF. 0 V Y (c) +6 V thermistor The voltage divider may contain a variable resistor X V In warm conditions the resistance of the thermistor is low and so it takes only a small share of the supply voltage. Hence the voltage across XY is high i.e. transistor conducts and bulb is ON. In cold conditions the resistance of thermistor is high and so it takes a large share of the supply voltage. This leaves only a small voltage across XY therefore the transistor does not conduct and the bulb remains OFF. 0 V Y

55 1. For each of the following circuits calculate the voltage across XY and then state whether the output device is ON or OFF. (a) +12 V 400 X V Y (b) +6 V 800 X V Y (c) +5 V 450 X 50 0 V Y

56 (d) +1 5 V 100 X V Y (e) +2 V 100 X 3 9 k 0 V Y (f) (Hint! Think here!) +4 V 10 X 80 0 V Y

57 2. Consider the following circuit. + 6 V B C X 0 V A Y (a) What is the voltage across XY when the sliding connection is at point A? (b) Does the LED light when the sliding connection is at point A? (c) Does the LED light when the sliding connection is at point B? Explain your answer. (d) Does the LED light when the sliding connection is at point C? Explain your answer.

58 3. Study the four circuits shown below. +5 V +5 V 0 V 0 V Circuit A Circuit B +5 V +5 V 0 V Circuit C 0 V Circuit D (a) Which circuit could be used to remind drivers at night to put on their headlamps? Explain your answer. (b) Which circuit would be useful as a warning indicator of low temperature in an elderly person s house? Explain your answer. (c) Which circuit could be used to waken campers when daylight arrives? Explain your answer. (d) Which circuit would be most suitable as a fire alarm?

59 4. The following circuit is used to switch on an electric heater automatically when the temperature in a room falls below a certain value. + 5 V X mains 230 V heater 0 V (a) Explain how the circuit operates. (b) What would be the effect of decreasing the resistance of the variable resistor? (c) Why would it be unsuitable to put the heater at point X instead of the relay? 5. For each of the following circuits state whether the output device is ON or OFF and explain your answer. (a) +5 V (b) +5 V (c) +5 V capacitor uncharged 0 V (d) capacitor charged 0 V +5 V 0 V 0 V

60 (e) (f) +5 V +5 V capacitor charged 0 V switch closed 0 V

61 Logic Gates 1. Identify the logic gate represented by each of the following truth tables. (a) Input Output (b) (c) Input 1 Input 2 Output Input 1 Input 2 Output For each of the following combinations of logic gates complete a truth table of the form: Input A Input B Output (a) A B (b) A B (c) (d) A B A B (e) A B

62 (f ) A B 3. Each of the following combinations of logic gates has three inputs. For each combination complete a truth table of the form: Input A Input B Input C Output (a) A B C (b) A B C (c) A B (d) A B C C (e) A B (f) A B C (g) A C B

63 4. Each of the following combinations of logic gates involve linked inputs, that is two inputs being fed into different gates. For each combination complete a truth table of the form: Input A Input B Output (a) (b) A B A B (c) (d) A B A B 5. Show the pattern of pulses that would appear at the output of each of the following gates as a result of the input pulses shown. (a) (b) (c) (d)

64 Analogue Processes Devices containing amplifiers (1) In which of the following systems do amplifiers play an important part? radio capacitor TV loudspeaker LED solenoid stereo remote control car public address system. Voltage Gain In this section you can use the equation: voltage gain = output voltage input voltage also written as: voltage gain = V out V in where (Note: V out = Output voltage in volts (V) V in = Input voltage in volts (V). Voltage gain has no unit since it is a ratio of input to output voltages.) 1. Find the missing values in the following table. Input voltage (V) Output voltage (V) Voltage Gain (a) 2 10 (b) (c) 3 10 (d) 15 5 (e) (f) Calculate the gain of a guitar amplifier if the input signal is 2 3 V and the output signal is 46 V.

65 3. A student studies the input and output traces from an amplifier and displays the results on an oscilloscope. Use the trace to find the gain of the amplifier. 6 V 0 V 12 V 0 V Input signal Output signal 4. A hearing aid amplifies an input signal by a factor of 20. What would be the voltage of an output signal if the input signal was 0 12 V? 5. Televisions use amplifiers to boost the signal received after transmission. The output voltage of an amplifier is 2 3 mv and its gain is 15. Calculate the size of the input voltage collected at the aerial. 6. A pick up cartridge can produce a 0 2 mv signal which is then amplified. What is the gain of an amplifier which increases this signal to 15 V? 7. If the voltage gain of a pre-amp is and the input signal is 3 mv, calculate the output voltage. 8. A signal generator connected to a loudspeaker is used to investigate the range of human hearing. The output from the loudspeaker is 20 V and the gain is Calculate the size of the input signal. 9. A CD sends signals of approximately 0 25 mv to a pre-amplifier which produces an output voltage of 15 V. Use this data to find the gain of the pre-amplifier. 10. Calculate the output voltage of a personal stereo amplifier if it has a gain of and the input voltage is 3 5 mv.

66 Power Gain In this section you can use the equation: power gain = output power input power also written as: power gain = P out P in where (Note: P out = Output power in Watts (W) P in = Input power in Watts (W). Power gain has no unit since it is a ratio of input to output powers.) 1. Find the missing values in the following table: Input Power (W) Output Power (W) Power Gain (a) (b) (c) 4 x (d) (e) (f) 5 x An amplifier delivers an output of 60 W when the input power is 1 8 W. What is the power gain of the amplifier? 3. What would be the input power to an amplifier with a power gain of 400, if it delivers an output of 40 W? 4. The signal from a compact disc player has a power of 6 mw. The attached amplifier has a gain of What is the power delivered to the speaker?

67 Helpful Hint In this section you can also use the equation where P = V2 R P= power in watts (W) V= voltage in volts (V) R= resistance in ohms ( ). 5. Find the missing values in the following table: Power (W) Voltage (V) Resistance ( ) (a) (b) (c) (d) (e) 3 48 (f) An amplifier is fed with an input voltage of 12 mv. Calculate the input power of the amplifier given that the input resistance is 15 k. 7. An experiment is set up in a laboratory to investigate the input and output voltages of an amplifier. The apparatus is shown below. The input voltage is 8 mv and the input resistance of the amplifier is 220 k. An output signal of 0 8 V, W is fed to a loudspeaker. (a) What is the input power to the amplifier? (b) What is the resistance of the loudspeaker?

68 8. The pick-up cartridge in a record deck delivers a 10 mv signal to the amplifier which has an input resistance of 20 k. (a) What is the input power to the amplifier? (b) The maximum power output from the amplifier is 40 W. Calculate the power gain of the amplifier. (c) The output from the amplifier is delivered to a loudspeaker with an input resistance of 6. What is the voltage across this speaker? 9. 3 mv is fed into an amplifier whose input resistance is 50 k. (a) Calculate the power delivered to the amplifier. (b) If the amplifier has a power gain of 2 5 x 10 9, what is the output power produced by the amplifier? (c) This output signal is sent to a loudspeaker which has a resistance of 20. What is the voltage across the loudspeaker? 10. The tape deck of a hi-fi system produces a signal of 3 mv, 1 6 x W. This signal is passed through an amplifier to a loudspeaker. The loudspeaker has a resistance of 6 and the voltage across it is 6 V. (a) What is the power output of the loudspeaker? (b) Calculate the power gain of this amplifier. (c) What is the input resistance of the amplifier?

69 Electronics Revision Questions General Level 1. The circuit below is set up in a laboratory. A D C B (a) Name components A, B, C and D. (b) Explain why component C is necessary in this circuit. (c) Why is it important that component B is connected the right way round? 2. Look at the following circuits: X Y (a) Name components X and Y. (b) What will happen to the resistance of component X as the temperature increases? (c) What will happen to the current flowing in X as the temperature increases? (d) What will happen to the resistance of component Y as it gets dark? (e) What will happen to the current flowing in Y as it gets dark?

70 3. The resistance of a certain thermistor was recorded at various temperatures. The results are shown in the table below. Temperature ( o C) Resistance ( ) The thermistor was then connected to a 4 V battery and ammeter and exposed to the same range of temperatures 4V A (a) What was the ammeter reading when the temperature was 10 0 C? (b) What temperature is indicated by an ammeter reading of 0 02 A? 4. Look at the following electronic symbols. A B C D (a) Name the device represented by each symbol. (b) Which device can be used as a switch? (c) Which device is represented by the following truth table? A B output Amplifiers play an important part in many electronic systems. (a) Calculate the voltage gain of a guitar amplifier which has an input voltage of 0 8 V and an output voltage of 20 V. (b) A personal stereo amplifier has a voltage gain of to produce an output voltage of 9 V. What is the input voltage for the amplifier? (c) An input voltage of 2 mv is sent to an amplifier which has a voltage gain of Calculate the output voltage produced.

71 Credit Level 1. Some students are given the electronic components shown below to use in their school design projects. A B C D E F (a) Name each of the components. (b) Which of the components would be most suitable to use as the input device for a gardeners frost alarm? (c) Copy and complete the following circuit showing how the input device would be connected. Explain fully how the system operates. 12V M 10 k A X 0V B (d) Use the information in the table below to calculate the voltage across AB at each temperature in the table below. Temperature ( o C) Resistance ( ) (e) The output device will switch on when the voltage across AB is 0 7 V or more. At what temperature does this happen?

72 2. The following circuit is used to indicate when the light level in a room is too low. +5 V X 0 V (a) Why has a variable resistor been used instead of a fixed value resistor at the input? (b) What is the purpose of resistor X in the circuit? (c) Calculate the size of resistor X if the LED is designed to operate at 3 V, 10 ma. (d) Explain how the circuit works. It is suggested that the circuit be adapted slightly and used as part of an automatic street lighting system.two of the possible adapted circuits are shown below. +5 V +5 V street lights mains 230 V street lights 0V Circuit 1 0V Circuit 2 (e) Circuit 1 was described as unsuitable for the purpose. By looking at the circuits, can you suggest what was wrong with circuit 1 and how Circuit 2 is better. (f) Explain the function of the relay in circuit 2.

73 3. Logic gates are used widely in electronic systems. (a) Name each of the following logic gates: (I) (Ii) (iii) (b) For each logic gate draw the corresponding truth table. For each of the following applications name the logic gate that would be most suitable. A B (c) The courtesy lights in a car should come when either of the front doors of the car are opened. (d) A frost alarm is designed to operate only at night. (e) An alarm system in a bank must be able to be operated from two different positions behind the counter. (f) Look at the combination of logic gates shown below. Copy and complete the truth table for this system. C Input A Input B Input C Output

74 4. The diagram below shows a clock pulse generator circuit with its output. R C X Y output (a) Name the logic gate used in this circuit. (b) When the capacitor is uncharged what are the logic levels at X and Y? (c) When the capacitor charges up what happens to the logic levels at X and Y? (d) Use your answers to part (a) and (b) to explain how this circuit produces a series of pulses as shown at the output. (e) The output from this circuit can be altered by adjusting R and / or C. Look at the output shown below and explain how R and / or C have been altered to produce it. The clock pulse generator is connected to a binary counter a decoder and a seven segment display as shown below. clock pulse binary counter decoder seven segment generator display (f) The output from the binary counter is displayed as a series of LED s as shown: off on What number would appear on the seven segment display when the following output was produced by the binary counter?