p I = Q (charge) t (time)

Transcription

1 ClXB-B-ElfT I M C T R I C I T Y 3-08 *Demonstrate and explain how static and current electricity are alike and different Vocabulary: discharge, circuit, circuit diagram, switch, dry cell, battery, load, positive terminal, negative terminal 3-09 *know what a coulomb is *Define electric current as charge per unit time p I = Q (charge) t (time) Understand the concept of current (what it really is) *Know what instrument current is measured with and where to attach it to the circuit *Solve current problems Vocabulary: coulomb, current, charge, ampere (amp), ammeter 3-10 *Define voltage (electric potential difference) as the energy per unit charge between two points along a conductor p V= E (energy) Q (unit of charge) Understand the concept of voltage (what it really is) Understand what potential difference is *Know what instrument current is measured with and where to attach it to the circuit *Solve voltage problems Vocabulary: joule, volt (joules per coulomb), potential difference, voltmeter 3-11 identify 5 sources of electrical energy and technologies Vocabulary: chemical, thermoelectric, photoelectric, piezoelectric 3-12 *Define resistance using the particle model "Understand the concept of resistance (what it really is) "Understand 4 factors that affect resistance R= V (voltage) I (unit of charge) p notes p Vocabulary: resistance, resistor, ohm, load, Ohm's Law Summary Concept Map Three-Point Approach

2 VOCABULARY CURRENT ELECTB1C1TY (AND HEADINGS TO HELP YOU MAKE A CURRENT ELECTRICITY CONCEPT MAP) Parts of a Circuit discharge, circuit, circuit diagram, switch dry cell, battery, load, positive terminal, negative terminal Current coulomb, current, charge, ampere (amp), ammeter Voltage joule, volt, potential difference, voltmeter Resistance resistance, resistor, ohm, load, Ohm's Law 5 sources of electrical energy chemical, thermoelectric, photoelectric, piezoelectric Other

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5 Making Light Bulbs Glow If negative charges can flow on a path from negative terminal to a positive terminal, we say that we have an electric current. We will have a closed circuit. We will see the light bulb glow. That is the electrons giving off light energy. We feel the bulb heat up. That is the electrons giving off heat energy. The electrical energy ofthe electrons is changed to heat and light energy. As electrons flow through thefilamentto the bulb, heat energy and light energy are released. The bulb is an example of a load because it resists the movement of electrons through the circuit. r

6 DAT*. NAME) CLASS: SHcFSff^~Making Light Bulbs Glow Goal Apply whac you learned in the Starting Point Activit)': Making Light Bulbs Glow. What to Do In each of the eight diagrams below, the electric light will not work. Use the space provided to explain what is wrong with each arrangement, then adjust each diagram so that the light will work. Diagrams

7 Introduction to Current Electricity Activity How does a socket make sure the electricity travels to the right ptacm on the bulb? One wire coming to the socket attaches to a piece of metal that connects to the threads of the socket The other wire connects to a piece of metal in the socket that connects to the bottom of the bub. The bottom of the socket is made of an insulating material so that the two connections do not make a SHORT CIRCUIT. Whydoes a lightbulb light up when an electric current flows through it? A current is the flow of electrons. The filamant of a light bulb offers a lot of friction to electrons. (Friction in electricity is called RESISTANCE. So we say the fltament has a high resistance, compared to the wires in the circuit) The friction between the filament and the electrons causes the fltament to heat up. Th* ftomenl gels so hot that it glows.

8 EI.cfrkClm.te ( p. ^ - t t During your Intipdu^^ M ^ t > You alto discovered that die wires needed to make contact with the battery and bulb in certain places: the battery - the bum - These contact points were important to create a closed path through which the electricity could flow. Such a conductive, closed path is called a, You can describe a circuit in words, but it is easier to use a. These diagrams use to represent each of the components, or dementi, in a circuit. The basic symbols are shown below. Conducting Wire Lamp Cell Switch Battery Resistor Notice the battery symbol is similar to that of a cell. A battery is a combination of cells. A resistor is used to represent any device that converts electrical energy into another form of energy. Such devices are often called. A resistor symbol could stand for a toaster, a radio, a lamp, a microwave, a kettle, a television - anything that resists theflowof electric charge.

9 Measuring Electricity Electricity can be thought of as electrons moving. Electrons are too small to "count" so we use coulomb of electrons which is 6 billion billion electrons or the symbol Q We will study 3 measurements of these coulombs of electrons. How fast? called Current or the symbol I using the equation How much energy? called Voltage or the symbol v using the equation How easily? called Resistance or the symbol R using the equation l = Q t V = E Q R= V I u ; 7t- # "tec' (i;jto i

10 How do we measure a "bunch" of electrons? 6 Billion billltn electroni a 1 Negatlfe Caufomi

11 nvehhmmtra Circuit Symbols CIRCUIT DMGfUtf SYMBOLS conducting wire cei wires Joined *>#>**y (with 3 ce»s) I wires not joined lamp ' 1 ground connector switch ammeter fuse voltmeter *3#J Copnigtee Copyright O McGrw.Hil fcw. i P«w <nwd m fhr pwrw, 4- fc

12 * Main parts of a Basic Circuit 1. Source of Potential (Voltage Source) 2. Load Symbol «1 The source of potential in a circuit can be a battery, a thermocouple, a photocell, an electric generator, or any other device that is capable of causing electrons to flow. Symbol / V\A/ S (resistor) The load is the device in the drcuit that uses the electric energy. It may be a light bulb, an electrical appliance, a machine, a motor, etc. Whatever it is, the load offers resistance to the flow of electricity, and thus the symbol for a resistor is often used to represent any load. 3. Conductors Symbol * * The conductors are what carry the flow of electrons from the source to the load and back again. In almost all cases, the conductors are wires. 4. Switch Closed Switch (on) Symbol The switch controls when the load is working or not. Electricity canflowthrough an "open circuit.* The switch is a convenient way to open and close the drcuit. We will just connect and disconnect wires to act as a switch in our circuits.

13 M e a s u r i n g C h a r g e Charg* depends on the number of protons or electrons you are discussing. For example, if you fltv. an object 3 ( billion extra electrons, you are giving It a nsgatlvo chargo* i In practice though. It Is not usually helpful to count Individual electrons. It Is to tsdlous and precise (sort of { like measuring tho distance between Canada and Australia In millimetres). Instead, sclontlsts count largo j groups of sloctrons called Coulombs. j * i One Coulomb electrons or protons (about 0 billion billion) j We write this number using scientific notation.!! 1 C 0.24 x 10" electrons or protons If tho amount of chargo Is negative, then wo aro counting electrons. If tho amount of chargo Is positive, then we aro counting protons. Wo also use a smaller unit of chargo called a micro coulomb fac) 1 nc * 1 x 10* Coulombs

14 Current The movement of charge along a conductive path is called MM 111' Wh«we metjura currei^ Using the symbol I for,qr* and t for time, the nttthcmifical model tor current is:. Q charge moving past a point current * - time Charge, Q, ia measured in Time, t, is measured ia Current is measured in or more commonly. Acurreotof 1 Amp means that 1 Coulomb of charge isflowingpast a point in the circuit every second We use an to measure currents in circuits. The ammeter must be connected into the drcuit in. connect to the The currentflowingfrom the negative end ofthe battery must terminal of the ammeter. The positive end of the battery must connect to the _ terminal of the ammeter. A must always be present in the circuit to limit the currentflowthrough im ammeter.

15 What is a coulomb and why do we use it? How much current exists if 245coulombs pass a point every 5 seconds? 0 If the amperage is 2.3 A. how many coulombs pass by a point in 8 seconds? Complete the tables below Name and akbrtvki-fcm Measured m Uiaf anrrsp Abbreviation. kr-hum. Current Time j I 2A 12 Q* t = '

16 r Science, Gr. 9 Characteristics of Electricity - Measuring Electricity Current & Charge Calculations Worksheet current = charge moving past a point time 1. Find the unknown quantity: t Units: I is A (amperes) Q is C (coulombs) t is s (seconds) a) I = 0.4A Q = t = 20s b) I =? Q = 240 C t = 300 s c) I = 2 A Q = 400 C t =? 2. Find the unknown quantity (CONVERT FIRST to seconds) a) l = Q = 140 C t = 4 min = s b) I = 0.3 A Q = t = 1.5 hours = s c) I = 0.9 A Q = t = 3 min = s WORD PROBLEMS 1. If there is a current of 10 amperes in a circuit for 10 minutes, what quantity of electric charge flows in through the circuit? 2. How much current must there be in a circuit if 100 coulombs flow past a point in the circuit in 4 seconds? 3. How much time is required for 10 coulombs of charge to flow past a point if the rate of flow (current) is 2 amperes? )

17 General Science 10F Name: Date: Problems Involving Electric Current Complete the following problems on loose-leaf. Show all of your working including what you know, what you need to know, the equation that you used, the steps in solving the equation, the units, and the final answer. You should use the following equations to solve the problems. I = q/t q = Ixt 1 coulomb of charge is 6.25 x electrons 1. A small portable stove is plugged into a wall unit It is determined that 360 C of negative charge run past a point on the coil every minute. How many amperes of current flow pas this point in that minute. 2. In the above question, how many electrons pass that point every minute? 3. An ammeter is connected to an electric kettle. It reads 12 A. How many coulombs of charge flow past a given point in the wire in 10 seconds? 4. A voltaic cell is producing 2.6 coulombs of electricity every 2.0 seconds. What is the current flowing through the ammeter? 5. A 100 watt lamp passes a current of 0.8 amperes. How many coulombs of charge would pass a given point in its electrical plug in half an hour? How many electrons would this represent? 6. If there were 24 coulombs passing a section of a conductor in 1.6 seconds, what is the electric current in the conductor? 7. A microwave oven has a current of 11.7 amperes. How many coulombs of charge would pass a given point in its electrical plug in three minutes, the time it takes to warm up supper. 8. What are the units for current, charge, and time when dealing with electric current? 9. Draw a diagram that demonstrates the movement of charge as it flows through a wire from one end of a battery to another. The Nature of Electricity Problems Involving Electric Current

18 ( ( Electric Potential In a circuit, what causes the current to flow? The answer is that the cells provide a 'push 1 which makes the current flow round the circuit. The push or energy given to each electron in a circuit is called the electric potential or Volts. When you measure volts, you compare one location to another. This is why you must connect the volt meter at two points on the circuit. Volts is calculated with the formula V=E/Q V is the electric potential E is the energy required in Joules (J) Q is the amount of electrons in coulombs

19 Science, Gr. 9 Characteristics of Electricity - Measuring Electricity Potential Difference Calculations Worksheet potential difference = energy charge 1. Find the unknown quantity: V = E Q Units: V is V (volts) Q is C (coulombs) E is J (joules) a) V =? E = 45 J Q = 15C b) V = 9 V E =? Q = 150 C c) V = 1.5V E = 225 J Q =? 2. Find the unknown quantity (CONVERT FIRST to volts, joules, or coulombs) a) V = 1000 mv= V E =? Q = 20 C b) V =? E = 1.25 kj = Q = 1500C J c) V = 1.21 GV= V E =? Q = C WORD PROBLEMS 1. The potential difference between the two terminals on a battery is 9 volts. How much work (energy) is required to transfer 10 coulombs of charge across the terminals? 2. Ten joules of work (energy) are required to transfer 2 coulombs of charge from X to Y. What is the difference in potential between these two points? 3. It requires 600 joules of energy to transfer a quantity of charge between points C and D of a circuit, which have a potential difference of 30 volts. How much charge is transferred?

20 General Science 10F Name: Date: Voltage Problems Complete the following problems on loose-leaf. Show all of your working including what you know, what you need to know, the equation that you used, the steps in solving the equation, the units, and the final answer. You should use the following equations to solve the problems. V = E/q E = Vxq 1. Calculate the potential difference of an electric device that produces 30 joules of energy and has a charge of 0.2 coulombs. 2. Calculate the potential difference of an electric device that produces the same amount of energy as in question 1, but has a charge of 0.4 coulombs. 3. Calculate the potential difference of an electric device that produces the same amount of energy as in question 1, but has a charge of 0.15 coulombs. 4. Based on questions 1, 2, and 3 what can you conclude about the energy produced by an electric device, and the voltage of that device? 5. How much energy is in 5 coulombs of charge in a 12 volt battery? 6. How much energy is in 20 coulombs of charge in a 120 volt outlet? 7. How much energy is in 20 coulombs of charge in a 220 volt outlet? 8. Most outlets in your home are 120 volt outlets, however, some large appliances or heaters may use outlets that are 220 volts. Use your answers to numbers 6 and 7 to explain why some large appliances may be plugged in to outlets that have high voltages. The Nature of Electricity

21 An electron traveling through the wires and loads of a circuit encounters ) resistance. Resistance is the HINDRANCE to the flow of charge. For an electron, the journeyfromnegative terminal of the battery to positive terminal ofthe battery is not a direct route. Rather, it is a zigzag path which results from countless COLLISIONS with fixed atoms within the wire or the load. The electrons encounter resistance - a hindrance to their movement. The resistance affects how fast the electrons can move. Variables that Affects Resistance. Resistance to the flow of coulombs of charge in an electric circuit * length of the wire (longer wire - more resistance because there are more collisions) *how wide the wire is (narrow wire - more resistance) *what conducting material the wire is made of (silver is one of the best conductors - silver would allow the coulombs of electrons to move with the least resistance. Because silver is expensive, we use copper and aluminum instead for household wiring.) I

22 Science, Gr. 9 Characteristics of Electricity - Measuring Electricity Resistance Calculations Worksheet v RI resistance = potential difference current 1. Find the unknown quantity: R = V I Units: R is 0 (ohms) V is V (volts) I is A (amperes) a) I = 10 A R = V =? b) I =? R = 200 Q V = 240 V c) I = 15 A R =? V = 110V 2. Find the unknown quantity (CONVERT FIRST to the unit with no prefix) a) I =? R = 2000 O V = 20 mv = V b) I = 25 ma = A R =? V = 110 V c) I = 1 ka = A R =? V = 50 mv = V WORD PROBLEMS 1. How much resistance does a light bulb create if it has a current of 25 ma around it in a 9 V circuit? 2. A heating coil offers a resistance of 2.5 ko. What voltage is required so that 1.5 A of current pass through it? 3. The human body offers a very small amount of resistance (let's say 1 mfl for argument). If a lightning bolt (said to have 1.21 GV of potential according to a famous movie called Back to the Future released in 1984), hits you how much current is flowing through your body.