Current Electricity

14 Grade 10 Physics questions on Current Electricity, with worked marking schemes. 14 questions are available on this topic in full.

1. Question 1

   4 marks

   A cell of e.m.f. 1.5 V drives a current of 0.25 A through a resistor for 4.0 minutes. Determine (i) the charge that flows and (ii) the electrical energy converted in the resistor.

   (4 marks)

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2. Question 2

   1 mark

   A learner connects a third resistor in parallel with two resistors that are already in parallel across a cell. The total resistance of the combination

   A. decreases, because the current now has more paths to flow through

   B. becomes equal to the sum of the three separate resistances

   C. remains the same, because the cell has not been changed

   D. increases, because there are now more resistors in the circuit

   (1 mark)

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3. Question 3

   4 marks

   A household in Machakos uses an electric shower rated 3 000 W for 20 minutes each day. If Kenya Power charges KSh 25 per kilowatt-hour, determine the cost of running the shower for 30 days.

   (4 marks)

   Marking scheme

   1 mark: Power in kilowatts = $3\frac{000}{1}000=3$ kW. 1 mark: Daily time in hours = $\frac{20}{60}=\frac{1}{3}h$, so daily energy = $3\times \frac{1}{3}=1kWh$. 1 mark: Energy in 30 days = $1\times 30=30kWh$. 1 mark: Cost = $30\times 25=KSh$ 750. (Allow error carried forward between stages.)

4. Question 4

   3 marks

   A 6 Ω resistor and a 3 Ω resistor are connected in series with a 9 V battery of negligible internal resistance. Determine the current in the circuit and the potential difference across the 6 Ω resistor.

   (3 marks)

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5. Question 5

   5 marks

   Describe an experiment you would carry out in the school laboratory to show that the resistance of a metal wire increases as its temperature rises.

   (5 marks)

   Marking scheme

   1 mark: Connect a coil of the metal wire in series with a cell, an ammeter and a switch, with a voltmeter connected across the coil. 1 mark: Immerse the coil in water in a beaker together with a thermometer, and record the initial temperature. 1 mark: Close the switch briefly, record the ammeter reading I and the voltmeter reading V, and calculate R = V/I. 1 mark: Heat the water in steps, stirring, and at each recorded temperature repeat the readings of V and I and calculate R. 1 mark: R is found to increase steadily with temperature; a graph of R against temperature gives a line of positive gradient, showing that resistance rises with temperature. (Accept a stated precaution, for example switching on only while taking readings to avoid self-heating, in place of the stirring detail.)

6. Question 6

   3 marks

   Explain why the connecting wires in a lighting circuit remain cool while the filament of the bulb becomes white hot, even though the same current passes through both.

   (3 marks)

   Marking scheme

   1 mark: The connecting (copper) wires are thick and of a good conducting material, so they have a very low resistance. 1 mark: The filament is very thin and made of a high-resistivity metal (tungsten), so it has a high resistance. 1 mark: For the same current, the heat produced depends on resistance ($P=I^{2}R$), so nearly all the energy is converted to heat and light in the filament and very little in the wires. (Accept mention that the filament is long and coiled, adding to its resistance.)

7. Question 7

   3 marks

   State Ohm's law and state two conditions under which it holds.

   (3 marks)

   Marking scheme

   1 mark: The current through a conductor is directly proportional to the potential difference across its ends, provided physical conditions remain constant (V = IR). 1 mark: The temperature of the conductor must remain constant. 1 mark: Other physical conditions, such as length, cross-sectional area and the nature of the material (and freedom from strain), must remain constant. (Accept "the conductor must be ohmic/metallic" for the third mark.)

8. Question 8

   1 mark

   An electric iron rated 1 500 W is used on the 240 V supply in a home in Kisumu. The current it draws is approximately

   A. 16 A

   B. 360 A

   C. 0.16 A

   D. 6.3 A

   (1 mark)

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9. Question 9

   1 mark

   A nichrome wire of uniform thickness has resistance R at room temperature. A second nichrome wire of the same thickness but twice the length is used instead, at the same temperature. Its resistance is

   A. 2R

   B. R/2

   C. R

   D. 4R

   (1 mark)

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10. Question 10

    1 mark

    Three identical bulbs are connected in series to a battery. One of the bulbs blows. What happens to the other two?

    A. They stay exactly as bright as before

    B. They both go out because the circuit is broken

    C. They both become brighter because resistance has fallen

    D. One stays lit and the other goes out

    (1 mark)

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11. Question 11

    1 mark

    In a school laboratory circuit, an ammeter and a voltmeter are used to take readings for a resistance wire. The correct arrangement is

    A. ammeter in series with the wire and voltmeter in parallel with it

    B. both the ammeter and the voltmeter in series with the wire

    C. both the ammeter and the voltmeter in parallel with the wire

    D. ammeter in parallel with the wire and voltmeter in series with it

    (1 mark)

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12. Question 12

    1 mark

    Two resistors of 4 Ω and 12 Ω are connected in parallel across a cell. Their combined resistance is

    A. 48 Ω

    B. 8 Ω

    C. 3 Ω

    D. 16 Ω

    (1 mark)

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13. Question 13

    1 mark

    The potential difference across a lamp in a working circuit is 12 V. This means that

    A. 12 joules of electrical energy are converted in the lamp for every coulomb of charge that passes through it

    B. 12 joules of energy are converted in the lamp every second

    C. the lamp must have a resistance of 12 ohms

    D. 12 coulombs of charge pass through the lamp every second

    (1 mark)

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14. Question 14

    2 marks

    Define electric current and state its SI unit.

    (2 marks)

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