Electric current is a flow of electrons in a conductor. The SI unit of electric current is ampere.

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1 C h a p t e r at G l a n c e 4. Electric Current : Electric current is a flow of electrons in a conductor. The SI unit of electric current is ampere. Current = Charge time i.e, I = Q t The SI unit of charge is coulomb. While drawing circuit diagrams, electrical symbols are used. Electric Potential and Potential difference : The potential difference between two points in an electric field is defined as the amount of work done in moving a unit positive charge from one point to another point. Work done Potential difference = Quatity of charge transferred The SI unit of potential difference is volt. esistance and Ohm's Law : Ohm's law states that electric current flowing in a metallic conductor is directly proportional to potential difference across it terminals, I or I = constant This constant is called resistance () of the conductor. esistance : Electric current is the flow of electrons through a conductor. When the electrons flow from one part to the other part of conductor, they collide with other electrons and also with the atoms and ions present in the conductor. Due to these collisions there is some obstruction or oppositon & this property of conductor to oppose the flow of current is resistance. Unit of resistance is ohm (). esistivity (specific resistance) : Specific resistance of material of a conductor is defined as resistance of a conductor of unit length and unit area of cross section. esistance of a system of resistors : There are two methods of joining the resistors together. They are :. esistors in series and. esistors in parallel. esistors in series : For 'n' number of resistors connected in series s = n. This combination is used to increase resistance in a circuit.

2 SCIENCE & TECHNOLOGY - S.S.C. 8 esistors in parallel : For 'n' number of resistors connected in parallel = p 3 n This combination is used to decrease resistance in the circuit. Heating effect of electric current : Heating effect of electric current has many useful applications. The most common examples are electric iron, electric toaster, electric oven, electric kettle and electric heater. Joule's Law : Joule's law can be stated as : The quantity of heat (H) generated in a conductor of resistance (), when a current (I) flows through it for a time (t), is directly proportional to : (i) the square of the current(i), (ii) the resistance() of the conductor and (iii) the time(t) for which the current flows. H = I t 4.8 cal or H = Electric Power : I t cal 4.8 Electric power is the electrical work done per unit time. The S.I. unit of electric power is watt. kwh = joule. Electric power = voltage current P = I or P = Electrical energy = power time Commercial unit of electrical energy is kilowatt-hour. MULTIPLE CHOICE QUESTIONS (MCQ's) (Each question carries one mark) MCQ's BASED ON PACTICALS Experiment : I. To study the dependence of current (I) on the potential difference () across a resistor and determine its resistance. II. To determine the equivalent resistance of two resistors when connected in series. III. To determine the equivalent resistance of two resistors when connected in parallel. ;;;

3 8 Unique MCQ's. When the resistance of the conductor increases then the current will : a increase b decrease c remain same d none of these. What will happens to current passing through a resistance, if potential difference across it is doubled and the resistance is halved. a remains unchanged b becomes double c becomes half d becomes four times 3. When current is plotted against potential difference, a straight line plot is obtained, which of the following law is verified? a Faraday's law b Maxwell's law c Ohm's law d Fleming's left hand rule 4. In a voltmeter there are 0 divisions between the 0 mark and 0.5 v. The least count of voltmeters is: a 0.00 volts b 0.05 volts c volts d 0.50 volts 5. Keeping the potential difference constant the resistance of circuit is made four times. How much dose the current change? a remains unchanged b becomes four times c becomes one fourth d becomes half 6. If the resistance is to be increased then the number of resistances should be connected in : a series b parallel c mixed arrangement d None of the above 7. What is the equivalent resistance of three resistances of ohms, 3 ohms and 4 ohms are connected in series? a 6 b c 9 d 9 8. In series combination which remains constant? a current b voltage c both current and voltage d both are variable 9. You are given three equal resistances, in how many combinations can they be arranged? a three b four c five d two 0. A resistor of is connected to a cell when magnitude of current is A, if another resistor of connected in series with the first one, then magnitude of current will be : a IA b more than IA c less then IA d zero. What device is used for measuring potential difference? a galvanometer b variable resistor c ammeter d voltmeter. Which instrument is used to measure electric current? a potentiometer b variable resistor c ammeter d voltmeter 3. How is ammeter connected with the electric circuit? a series or parallel b series c parallel d anywhere in the circuit 4. Which symbol is used to represent electric cell in a circuit diagram? a b c d

4 SCIENCE & TECHNOLOGY - S.S.C For what purpose the resistor is used in an electric circuit? a to change direction of current b to control current c to stop the current d to decrease the current 6. Which physical quantity has the unit ohm? a resistance b resistivity c current d potential difference 7. Which combination is used to decrease resistance in the circuit? a series b parallel c both series and parallel d none of these 8. How would the three resistance of 3 each be connected so as to produce an equivalent resistance of. a all in parallel b all in series c two in parallel and other in series d two in series and the other in parallel 9. Which is the better way to connect lights and other electrical appliances in domestic wiring? a series c parallel b both series and parallel d none of these Hint : In parallel circuits, each electrical appliances has its own switch due to which it can be turned on or turned off independently, without effecting other appliances. 0. If we want to connect a large number of electric bulbs for decorating buildings which circuit is better? a series c both series and parallel b parallel d none of these Hint : All the bulbs connected in series can be controlled with just one switch and also current in series circuit is smaller.. Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change? a double b 4 th c half d remains same Hint : I = as the current is inversely proportional to resistance, if the resistance is doubled the current will get halved. MCQ's BASED ON CHAPTE Moderate Level. Which particles constitute the electric current in a metallic conductor? a electrons b protons c neutrons d atoms. What is the SI unit of electric charge? a ampere b volt c coulomb d ohm 3. What is the SI unit of electric current? a coulomb b ampere c ohm d volt

5 84 Unique MCQ's 4. What coulomb of electric charge flows through the circuit if a radio set draws a current of 0.5A for 30 minutes. a 0.9 b 5 c.5 d What does the symbol represents in electric circuit diagram? a electric cell b tapkey c wire crossing d electric bulb 6. Who invented battery? a Alessandro olta b G. S. Ohm c Andre Ampere d Joule 7. Which metal has lowest resistivity? a copper b aluminium c iron d silver 8. Which of the following is a very good insulator? a silicon b carbon c rubber d aluminium 9. What is the commercial unit of electrical energy? a watt b kilowatt - hour c kilowatt d horse power Elementary Level. By what other name is the unit Joules/Coulomb called? a volt b ampere c ohm d ohm - metre. Which of the following is an incorrect statement? a one milli ampere = 0 3 A b one kilo volt = 0 3 c one mega volt = 0 6 d one microampere = 0 6 A 3. What amount of work is done if 4C of charge is moved through a potential difference of 8? a 30 J b J c J d 3 J 4. Which is the correct statement? a resistivity of a substance depends on its length. b resistivity of a substance depends on its thickness. c resistivity of a substance does not depend on the temperature. d resistivity of a substance depends on the nature of the substance. 5. What is the SI unit of resistivity? a ohm b ohm-metre c ampere d ohm-centimeter 6. Which of the following is a best conductor of electricity? a silver b copper c aluminium d gold 7. Which of the following is an alloy of Cu and Ni? a nichrome b manganin c constantan d brass 8. Which material is used to make filament of an electric bulb? a magnesium b tungsten c nichrome d aluminium 9. Which device work on the principle of heating effect of electric current. a telephone b ammeter c voltmeter d fuse 0. Which of the following substances have infinitely high resistance? a rubber b aluminium c silver d copper. What is the resistivity of silver? a m b m c m d m

6 SCIENCE & TECHNOLOGY - S.S.C. 8 5 Complex Level. Which of the following indicate Ohm's law? a H = I t b = W c Q. What constituents make the alloy manganin? a copper, magnesium and nickel I = Q t d = I b aluminium, manganese and nickel c copper, manganese and nickel d iron, manganese and nickel 3. What is the potential difference between the ends of a conductor of 0 resistance, when a current of.5 A flows through it? a 50 b 0. 5 c 5 A d 5 Hint : = I 4. What is the resistance of an electric oven, if the potential difference between the terminals is 0 and the current is 5 A? a 40 b 0 c 4 d 44 Hint : = I 5. An electric circuit has a 4 battery and a resistor of 30 ohms. What will be the current in the circuit? a 0. 4 A b 0. 8 A c 80 A d A Hint : = I 6. Two resistance X and Y are connected turn by turn: (i) In parallel and (ii) In series In which case the resultant resistance will be less than either of the individual resistances? a in parallel c both in parallel and in series 7. How is electric power calculated? a Electric power = current time b in series d none of these b Electric power = Potential difference current c Electric power = Work Current d Electric power = time voltage 8. How much work is done in moving a charge of coulombs from a point at 8 volts to a point at 38 volts? a 40 joules b 0 joules c 4.4 joules d 0 joules Hint : P. D = W Q 9. Which of the following does not represent electrical power in a circuit? a I b I c I d

7 86 Unique MCQ's 0. Which of the following is a correct statement for joules law? a heat produced = I t b I c H I d H Mysterious Level. What material is used in electric iron as a heating element? a copper b nichrome c aluminium d silver Hint : It has very high resistivity, hence, produces a lot of heat. Nichrome does not undergo oxidation easily even at high temperature.. A resistor of 6 is connected in parallel with another resistor X. The resultant resistance of the combination is. What is the value of the resistor X? a 8 b 6 c 4 d 3 Hint : = + p 3. You are given three resistances of, and 3 ohms. To get.5 ohms as resultant how will you connect the resistances? a All in series b All in parallel c and resistances are connected in series and this combination is put in parallel with 3 resistances. d and 3 resistance in parallel and this combination is put in series with resistance. Hint : s = +, = + p 4. What is the highest resistance which can be obtained by combining four resistors having resistances 0, 8, and 5? a 35 b c 5 d 40 Hint : Series combination is used to increase resistance in a circuit. s = = = How much electrical energy is transferred by a 5 A current flowing through resistor of for half an hour? a 5 kwh b kwh c 0. 5 kwh d. 5 kwh Hint : Electrical energy = power time Power = I 6. A bulb is rated at 50-00w. What is its resistance? a 0 b 5 c 50 d 5 Hint : P = 7. In the circuit diagram given alongside, find: Total resistance of the circuit a 3 b 6 c 0 d 4 Hint : p = + 3

8 SCIENCE & TECHNOLOGY - S.S.C In the circuit diagram shown alongside find : Total resistance a 0 Moderate Level Elementary Level Complex Level b 0. 4 c 0. 7 d. 5 Hint : s = + = + p 3 s 9. An electric oven draws a current of 0A from a 0 supply. What is the cost of using the oven for hrs everyday for 30 days if the cost of unit is s..50? a s. 330 b s. 0 c s. 3 d s. 440 Hint : P = I MCQ's BASED ON PACTICAL SKILLS Answers - b - d 3 - c 4 - b 5 - c 6 - a 7 - c 8 - a 9 - b 0 - c - d - c 3 - b 4 - a 5 - b 6 - a 7 - b 8 - a 9 - c 0 - a - c MCQ's BASED ON CHAPTE - a - c 3 - b 4 - d 5 - c 6 - a 7 - d 8 - c 9 - b - a - c 3 - d 4 - d 5 - b 6 - a 7 - c 8 - b 9 - d 0 - a - c - d - c 3 - d 4 - d 5 - b 6 - a 7 - b 8 - a 9 - b 0 - a Mysterious Level - b - d 3 - c 4 - a 5 - b 6 - b 7 - c 8 - d 9 - a