Senior 1 practice book · Unit 8 of 9

Electric Current and Ohm's Law

50 questions that complete the Senior 1 quiz for this unit: 29 core and 21 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

A battery is a store of current, and the current gets used up as it goes round the circuit.A battery is a store of energy. The current is the same all the way round a series circuit; it is energy, not charge, that is transferred to the components.
Adding more resistors always increases the total resistance.Resistors added in series increase the total, but each resistor added in parallel gives the current another path, so the total resistance goes DOWN.
Electrons flow out of the positive terminal of a cell.Electrons leave the negative terminal and flow round to the positive terminal; conventional current is drawn the other way, from + to −.
The kilowatt-hour is a unit of power.The kilowatt-hour is a unit of ENERGY: the energy used by a 1 kW appliance in 1 hour, equal to 3.6 × 10⁶ J.
A bigger fuse is always safer because it does not blow so often.A fuse must be rated just above the normal current of the appliance; one that is too large will not melt when a fault makes the current dangerously high.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Electric current and charge: I = Q/t
Potential difference, e.m.f. and energy: V = W/Q
Circuit components, symbols and circuit diagrams
Using ammeters and voltmeters: connection, scale reading and zero error
Conventional current and electron flow
Ohm's law and V = IR
I–V graphs of ohmic and non-ohmic conductors (lamp, diode, thermistor)
Factors affecting resistance and resistivity: R = ρL/A
Resistors in series and in parallel
Current and potential difference in series and parallel circuits
Effects of electric current: heating, magnetic and chemical
Electrical safety: fuses, earthing, insulation
Electrical power and energy: P = VI, the kilowatt-hour and the cost of electricity

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 21 harder ones are marked ★ Challenge.

1True or false
The kilowatt-hour is a unit of power.
Show answer
Answer: False
The kilowatt-hour is a unit of energy (power × time); the kilowatt is the unit of power.
Common mistakeMixing 'kW' and 'kWh' is common; the extra 'h' (hours) makes it energy.
2Fill in the blank · ★ Challenge
Three 12 Ω resistors connected in parallel have a combined resistance of ______ Ω.
Show answer
Answer: 4
1/R = 3 × 1/12 = 1/4, so R = 4 Ω (equal resistors: R ÷ n = 12 ÷ 3).
Common mistakeWriting 36 Ω adds them as in series.
3True or false
In a metal wire, an electric current is a flow of protons.
Show answer
Answer: False
In metals the moving charges are free electrons; protons stay fixed in the nuclei.
Common mistakeLearners link 'current' with 'positive' because of conventional current, but in metals only electrons move.
4True or false · ★ Challenge
For a component that does not obey Ohm's law, R = V/I can still be used to find its resistance at a particular p.d.
Show answer
Answer: True
Resistance is always defined as V/I; for a non-ohmic component its value is different at different p.d.s.
Common mistakeLearners think R = V/I applies only to ohmic conductors; it always defines resistance, but only for ohmic ones is it constant.
5True or false
A fuse should be connected in the live wire of an appliance.
Show answer
Answer: True
When the fuse in the live wire melts, the appliance is disconnected from the high p.d. and is safe.
Common mistakePutting the fuse in the neutral wire would cut the current but leave the appliance connected to the live p.d.
6True or false
In a parallel circuit, if one lamp breaks, all the other lamps go out.
Show answer
Answer: False
Each lamp is on its own branch; a break in one branch does not stop current in the others.
Common mistakeThis is true for SERIES circuits, where there is only one path; learners mix up the two.
7Multiple choice · ★ Challenge
Two lamps are in parallel across a 12 V battery. The currents in the lamps are 0.5 A and 1.5 A. What is the current from the battery, and the p.d. across each lamp?
1. A2.0 A; 6 V across each lamp
2. B1.0 A; 12 V across each lamp
3. C0.75 A; 6 V across each lamp
4. D2.0 A; 12 V across each lamp
Show answer
Answer: D. 2.0 A; 12 V across each lamp
In parallel the branch currents add: 0.5 + 1.5 = 2.0 A, and each branch has the full 12 V across it.
Common mistakeChoosing 6 V across each lamp shares the p.d. as in a series circuit; parallel branches each get the whole supply p.d.
8True or false
Conventional current and the flow of electrons in a wire are in the same direction.
Show answer
Answer: False
Conventional current is drawn from + to − outside the cell; electrons actually flow from − to +.
Common mistakeLearners assume the arrows on a circuit show the electrons; they show conventional current, the opposite way.
9Short answer · ★ Challenge
When a car battery is recharged, a current is passed through it from a charger. Name the effect of current used and explain what happens.
Show answer
Model answer: The chemical effect. The current drives chemical reactions in the cell backwards, changing the chemicals at the plates back into their charged state, so that electrical energy is stored as chemical energy.
Common mistakeLearners say the charger 'fills the battery with current'; current is not stored, the energy is stored chemically.
10True or false
Electroplating a spoon and recharging a phone battery both use the chemical effect of current.
Show answer
Answer: True
In both, the current drives chemical changes: depositing metal on the spoon, or reversing the reactions in the cell.
Common mistakeLearners often say charging uses the heating effect because chargers get warm; the useful effect is chemical.
11True or false
A voltmeter should have a very high resistance, so that it takes almost no current from the circuit.
Show answer
Answer: True
A voltmeter is connected in parallel; a high resistance means it draws almost no current and does not disturb the circuit.
Common mistakeLearners think every meter should have low resistance; a voltmeter needs high resistance.
12Short answer · ★ Challenge
You have three 6 Ω resistors. Show how to connect all three to get a total resistance of 9 Ω.
Show answer
Model answer: Connect two of them in parallel: 6 × 6 ÷ (6 + 6) = 3 Ω. Connect this pair in series with the third resistor: 3 + 6 = 9 Ω.
Common mistakeLearners try all three in series (18 Ω) or all in parallel (2 Ω); a mixed combination is needed.
13True or false
Connecting another resistor in parallel with an existing resistor decreases the total resistance of the circuit.
Show answer
Answer: True
The extra branch gives the current another path, so more current flows for the same p.d.; the total resistance falls.
Common mistakeLearners think 'more resistors = more resistance'; that is true in series but not in parallel.
14Short answer · ★ Challenge
A pupil wants to dim and brighten a torch lamp powered by a battery. Name the component she should add, describe how it is connected, and explain how it works.
Show answer
Model answer: A rheostat (variable resistor) connected in series with the lamp. Sliding the contact changes the length of resistance wire in the circuit: more wire means more resistance, a smaller current and a dimmer lamp; less wire gives a brighter lamp.
Common mistakeConnecting the rheostat in parallel with the lamp is a common error; then it does not control the lamp's current properly.
15True or false
For a current to flow there must be a complete conducting path from one terminal of the cell, through the components, back to the other terminal.
Show answer
Answer: True
A current only flows in a closed (complete) circuit; any gap, such as an open switch, stops it.
Common mistakeSome learners think current flows out of a battery even if the circuit is broken; without a complete loop no current flows.
16Short answer · ★ Challenge
Describe an experiment to verify Ohm's law for a resistance wire. Name the apparatus and say how the results are used.
Show answer
Model answer: Apparatus: battery, rheostat, ammeter in series with the wire, voltmeter across the wire, switch. Vary the rheostat to get at least five different currents; record V and I each time, keeping the wire cool (switch off between readings). Plot I against V: a straight line through the origin shows I ∝ V, so Ohm's law holds; R = V/I is constant (1 ÷ gradient).
Common mistakeForgetting to keep the temperature constant is the usual weakness; a hot wire changes its resistance and the graph curves.
17Fill in the blank
Conventional current was defined before electrons were discovered; it is the direction in which ______ charge would flow.
Show answer
Answer: positive
Conventional current flows from the positive terminal to the negative terminal outside the cell, as positive charge would.
Common mistakeWriting 'negative' describes electron flow, which is opposite to conventional current.
18Multiple choice · ★ Challenge
A 2400 W electric kettle is used on a 240 V supply. Fuses of 3 A, 5 A, 13 A and 30 A are available. Which should be fitted?
1. A3 A
2. B5 A
3. C13 A
4. D30 A
Show answer
Answer: C. 13 A
Normal current I = P/V = 2400 ÷ 240 = 10 A. Choose the fuse just above 10 A, which is 13 A.
Common mistakeChoosing 30 A seems 'safe' but would not blow in a fault until the current was dangerously large.
19Fill in the blank
One kilowatt-hour is equal to ______ J.
Show answer
Answer: 3 600 000 (3.6 × 10⁶)
1 kWh = 1000 W × 3600 s = 3 600 000 J.
Common mistakeWriting 1000 J forgets to multiply by the 3600 seconds in an hour.
20Multiple choice
In a copper wire connected to a battery, the electrons move:
1. AFrom the positive terminal, through the wire, to the negative terminal
2. BFrom the negative terminal, through the wire, to the positive terminal
3. CBackwards and forwards with no overall direction around the circuit
4. DFrom both terminals towards the middle of the wire, where they meet
Show answer
Answer: B. From the negative terminal, through the wire, to the positive terminal
Electrons are negative, so they are repelled by the negative terminal and attracted to the positive terminal.
Common mistakeChoosing '+ to −' gives the direction of conventional current, not the direction in which electrons actually move.
21Short answer · ★ Challenge
A student connects a voltmeter IN SERIES with a lamp and a 6 V battery. Describe what she observes and explain why.
Show answer
Model answer: The lamp does not light (or is extremely dim), and the voltmeter reads almost 6 V. The voltmeter's very high resistance allows almost no current to flow, so almost all the battery's p.d. is across the voltmeter.
Common mistakeExpecting the voltmeter to read 0 V is a common error; it reads nearly the whole supply p.d. because it has nearly all the resistance.
22Multiple choice
Which component's resistance falls when more light shines on it?
1. AThermistor (temperature-dependent resistor)
2. BDiode
3. CLight-dependent resistor (LDR)
4. DFuse
Show answer
Answer: C. Light-dependent resistor (LDR)
An LDR's resistance decreases in bright light; it is used in automatic street lights.
Common mistakeChoosing thermistor confuses light with temperature; a thermistor responds to heat.
23Multiple choice
A component that allows current to flow in one direction only is a:
1. ARheostat
2. BDiode
3. CFuse
4. DSwitch
Show answer
Answer: B. Diode
A diode has a very low resistance one way and a very high resistance the other way.
Common mistakeChoosing a switch is tempting, but a switch stops current in both directions when open.
24Multiple choice · ★ Challenge
In a lighting circuit, a 20 Ω lamp and a 5 Ω heater are placed side by side, connected between the same two points (in parallel). What is their combined resistance?
1. A25 Ω
2. B100 Ω
3. C4 Ω
4. D12.5 Ω
Show answer
Answer: C. 4 Ω
1/R = 1/20 + 1/5 = 1/20 + 4/20 = 5/20, so R = 4 Ω (or 20 × 5 ÷ (20 + 5) = 4 Ω).
Common mistakeChoosing 25 Ω adds them as if in series; in parallel the total is less than the smallest resistor.
25Multiple choice
Which device works mainly because of the heating effect of a current?
1. AAn electric bell
2. BA loudspeaker
3. CA lifting electromagnet
4. DAn electric iron
Show answer
Answer: D. An electric iron
An electric iron has a high-resistance element that converts electrical energy into heat.
Common mistakeChoosing the electric bell confuses effects; a bell uses the magnetic effect of current.
26Short answer · ★ Challenge
A circuit diagram shows arrows for the current going from the + terminal of the battery round to the − terminal. A pupil says the arrows are wrong because electrons go the other way. Who is right? Explain.
Show answer
Model answer: Both statements are true. The arrows show conventional current, which by agreement flows from + to −. The electrons really move from − to +. For effects such as heating, a flow of negative charge one way is equivalent to a flow of positive charge the other way, so the convention gives correct results.
Common mistakeSaying 'the diagram is wrong' treats the convention as an error; it is a definition, and electron flow is simply opposite to it.
27Multiple choice
In a series circuit containing a cell and two different lamps, the current is:
1. AThe same at every point in the circuit
2. BLargest just after it leaves the cell
3. CSmaller after each lamp it passes through
4. DLargest in the lamp with the most resistance
Show answer
Answer: A. The same at every point in the circuit
In series there is only one path, so the same current flows through every component.
Common mistakeChoosing 'smaller after each lamp' is the 'current is used up' idea; energy is transferred, but the current is the same.
28Multiple choice · ★ Challenge
A resistor that obeys Ohm's law has 6 V across it and carries 0.2 A. What p.d. is needed to make 0.5 A flow through it?
1. A2.4 V
2. B9 V
3. C15 V
4. D3 V
Show answer
Answer: C. 15 V
R = V/I = 6 ÷ 0.2 = 30 Ω. V = IR = 0.5 × 30 = 15 V.
Common mistakeChoosing 2.4 V uses V = 6 × 0.2 × 2; first find R from the first reading, then use it with the new current.
29Multiple choice
For a fixed resistor at constant temperature, which change doubles the current through it?
1. ADoubling the p.d. across it
2. BHalving the p.d. across it
3. CDoubling its length of wire
4. DDoubling the time it is on
Show answer
Answer: A. Doubling the p.d. across it
I = V/R; with R fixed, the current is proportional to the p.d., so doubling V doubles I.
Common mistakeChoosing 'doubling the time' confuses current (a rate) with charge; leaving it on longer moves more charge but the current stays the same.
30Fill in the blank
A current of 1 ampere is a flow of 1 ______ of charge past a point every second.
Show answer
Answer: coulomb
I = Q/t, so 1 A = 1 C/s.
Common mistakeWriting 'volt' or 'joule' mixes up units; the ampere is coulombs per second.
31Multiple choice · ★ Challenge
A voltmeter has a 0–15 V scale divided into 30 equal small divisions. The pointer is on the 17th division. What is the reading?
1. A17 V
2. B8.5 V
3. C5.7 V
4. D7.5 V
Show answer
Answer: B. 8.5 V
Each division = 15 ÷ 30 = 0.5 V. Reading = 17 × 0.5 = 8.5 V.
Common mistakeChoosing 17 V reads the division number as volts; first find the value of one division.
32Multiple choice
Why should an ammeter have a very low resistance?
1. ASo that almost no current flows through it at all
2. BSo that it can be safely connected across the supply
3. CSo that it measures the voltage at the same time
4. DSo that it hardly reduces the current it measures
Show answer
Answer: D. So that it hardly reduces the current it measures
An ammeter is placed in series, so all the current goes through it; a low resistance means it hardly changes that current.
Common mistakeChoosing 'so that almost no current flows' describes a voltmeter, which has a very HIGH resistance.
33Multiple choice · ★ Challenge
Which description fits the I–V graph of a silicon diode?
1. AA straight line through the origin, the same for both forward and reverse directions
2. BA curve that bends towards the voltage axis, the same in both directions
3. CA large current in reverse, with almost no current in the forward direction
4. DAlmost no current in reverse; forward current only above about 0.6 V
Show answer
Answer: D. Almost no current in reverse; forward current only above about 0.6 V
A diode conducts in one direction only, and only after the forward p.d. reaches about 0.6 V; in reverse its resistance is very high.
Common mistakeChoosing the straight line through the origin describes an ohmic resistor, not a diode.
34Fill in the blank
A potential difference of 1 volt means that 1 ______ of energy is transferred for each coulomb of charge.
Show answer
Answer: joule
V = W/Q, so 1 V = 1 J/C.
Common mistakeWriting 'ampere' confuses p.d. with current.
35Multiple choice
Why does the resistance of a filament lamp increase as the current through it increases?
1. AThe filament gets hotter and its vibrating atoms obstruct the electrons more
2. BThe filament gets longer as it heats up, so it adds more metal to the circuit
3. CMore electrons flow, and electrons repel each other so they slow one another
4. DThe glass bulb heats up and starts to conduct, taking current from the filament
Show answer
Answer: A. The filament gets hotter and its vibrating atoms obstruct the electrons more
A larger current heats the filament; its metal ions vibrate more and collide more often with the electrons, so resistance rises.
Common mistakeChoosing 'it gets longer' points to a tiny effect; the main cause is the faster vibration of the atoms at higher temperature.
36Multiple choice · ★ Challenge
A family uses a 1.5 kW electric cooker for 2 hours a day for 30 days. Electricity costs 250 Frw per kWh. What does the cooking cost?
1. A750 Frw
2. B22 500 Frw
3. C11 250 Frw
4. D22 500 000 Frw
Show answer
Answer: B. 22 500 Frw
Energy = 1.5 kW × 2 h × 30 = 90 kWh. Cost = 90 × 250 = 22 500 Frw.
Common mistakeChoosing 22 500 000 Frw uses 1500 W instead of 1.5 kW; the price is per kilowatt-hour, so power must be in kW.
37Short answer · ★ Challenge
A cell has an e.m.f. of 1.5 V, but when it lights a lamp the p.d. across the lamp is only 1.3 V. Explain the difference between e.m.f. and p.d., and suggest why the two values differ.
Show answer
Model answer: The e.m.f. is the energy the cell gives to each coulomb of charge (1.5 J/C); the p.d. across the lamp is the energy each coulomb transfers in the lamp (1.3 J/C). The rest (0.2 J/C) is used up inside the cell because the cell has internal resistance.
Common mistakeLearners treat e.m.f. and p.d. as identical; the cell's own resistance wastes some energy, so the terminal p.d. is less than the e.m.f.
38Multiple choice
Why is copper used for house wiring, while nichrome is used for the heating element of a kettle?
1. ACopper has a high resistivity; nichrome has a low resistivity and does not get hot
2. BCopper is cheaper than nichrome, and both have exactly the same resistivity
3. CCopper has low resistivity; nichrome has high resistivity and melting point
4. DCopper is magnetic, while nichrome is not, so nichrome warms the water better
Show answer
Answer: C. Copper has low resistivity; nichrome has high resistivity and melting point
Wiring should waste little energy (low resistivity); a heating element should convert electrical energy to heat and survive high temperatures.
Common mistakeChoosing 'both have the same resistivity' ignores that resistivity is a property of the material, and nichrome's is about 60 times copper's.
39Multiple choice
Two lamps X and Y are in series with a 12 V battery. The p.d. across X is 7.5 V. What is the p.d. across Y?
1. A4.5 V
2. B7.5 V
3. C12 V
4. D19.5 V
Show answer
Answer: A. 4.5 V
In series the p.d.s add up to the supply: V(Y) = 12 − 7.5 = 4.5 V.
Common mistakeChoosing 7.5 V assumes both lamps share equally; the lamps are different, so subtract from the supply.
40Multiple choice · ★ Challenge
A phone battery is labelled 3000 mAh. For how long can it supply a steady current of 0.25 A?
1. A750 h
2. B12.0 h
3. C0.75 h
4. D1.2 h
Show answer
Answer: B. 12.0 h
3000 mAh = 3.0 Ah. Time = 3.0 Ah ÷ 0.25 A = 12 h.
Common mistakeChoosing 750 h multiplies 3000 by 0.25; the charge (in Ah) must be DIVIDED by the current, after changing mAh to Ah.
41Multiple choice
A 5 Ω resistor and a 7 Ω resistor are joined end to end in one line. What single resistor could replace them?
1. A2.9 Ω
2. B12 Ω
3. C35 Ω
4. D2 Ω
Show answer
Answer: B. 12 Ω
Joined end to end means in series, so the resistances add: R = 5 + 7 = 12 Ω.
Common mistakeChoosing 2.9 Ω uses the parallel rule (product ÷ sum); in series, simply add.
42Multiple choice
A 60 W lamp works on a 240 V supply. What current flows through it?
1. A4 A
2. B14 400 A
3. C300 A
4. D0.25 A
Show answer
Answer: D. 0.25 A
P = VI, so I = P/V = 60 ÷ 240 = 0.25 A.
Common mistakeChoosing 4 A divides the wrong way round (240 ÷ 60); I = P ÷ V.
43Multiple choice · ★ Challenge
The flash tube of a camera discharges 0.5 C in just 25 ms. Find the average current during the flash.
1. A0.02 A
2. B50 A
3. C20 A
4. D0.0125 A
Show answer
Answer: C. 20 A
t = 25 ms = 0.025 s. I = Q/t = 0.5 ÷ 0.025 = 20 A.
Common mistakeChoosing 0.02 A comes from dividing 0.5 by 25 without changing ms to s; 25 ms = 0.025 s.
44Short answer
Give two reasons why the lamps in a house are connected in parallel rather than in series.
Show answer
Model answer: Each lamp gets the full mains p.d., so it works at its proper brightness. Each lamp can be switched on and off on its own, and if one lamp breaks the others stay lit.
Common mistakeSome learners say parallel 'uses less electricity'; the main reasons are full p.d. for each lamp and independent operation.
45Multiple choice · ★ Challenge
Wires A and B are made of the same metal and have the same length. A has a diameter of 1 mm and B a diameter of 3 mm. How does the resistance of A compare with that of B?
1. AR of A = 9 × R of B
2. BR of A = 3 × R of B
3. CR of A = R of B ÷ 9
4. DR of A = R of B ÷ 3
Show answer
Answer: A. R of A = 9 × R of B
Area ∝ diameter², so B has 3² = 9 times the area of A. R ∝ 1/A, so A has 9 times the resistance of B.
Common mistakeChoosing 3 times forgets that cross-sectional area depends on the diameter SQUARED.
46Short answer
Explain how the earth wire protects a person using an appliance with a metal case, such as a fridge.
Show answer
Model answer: The earth wire connects the metal case to the ground. If a live wire touches the case, a large current flows through the earth wire instead of through a person; this large current blows the fuse and cuts off the supply.
Common mistakeSaying the earth wire 'stores extra electricity' is wrong; it gives a low-resistance path to ground so the fuse blows.
47Short answer · ★ Challenge
A solar home system has a 12 V battery that runs four 3 W LED lamps for 5 hours each evening. Calculate the energy used each evening in watt-hours and the current taken from the battery.
Show answer
Model answer: Total power = 4 × 3 = 12 W. Energy = 12 W × 5 h = 60 Wh. Current I = P/V = 12 ÷ 12 = 1 A.
Common mistakeA common error is to use one lamp's power (3 W) for the whole system; add the powers of all lamps first.
48Short answer
Explain, in terms of electrons, why a thick copper cable has a smaller resistance than a thin copper wire of the same length.
Show answer
Model answer: The thick cable has a larger cross-sectional area, so there are more free electrons available and more 'paths' side by side. For the same p.d., more charge flows per second, so the current is larger and the resistance smaller.
Common mistakeSaying thick wires 'have less metal in the way' gets it backwards; they have more metal, giving more room for electrons to flow.
49Short answer · ★ Challenge
A thermistor is tested at a constant p.d. of 2.0 V. At 20 °C the current is 0.010 A; at 60 °C it is 0.040 A. Calculate its resistance at each temperature and describe how its resistance depends on temperature.
Show answer
Model answer: At 20 °C: R = 2.0 ÷ 0.010 = 200 Ω. At 60 °C: R = 2.0 ÷ 0.040 = 50 Ω. The resistance decreases as the temperature rises (it is an NTC thermistor).
Common mistakeSome learners assume all resistances rise with temperature as in a metal lamp; a thermistor's resistance falls.
50Multiple choice
A 12 V car battery moves 5 C of charge round a circuit. How much energy does it transfer?
1. A60 J
2. B2.4 J
3. C17 J
4. D0.42 J
Show answer
Answer: A. 60 J
W = QV = 5 × 12 = 60 J.
Common mistakeChoosing 2.4 J divides 12 by 5; from V = W/Q, energy W = Q × V.