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KS3 Science · Year 9Chapter 10 · Circuits, current, voltage and resistance
Year 9 · Chapter 10

Circuits, current, voltage and resistance

By the end of this chapter you will be able to

  • Name the circuit symbols and draw a simple series or parallel circuit
  • Say what current and potential difference are, and where to place an ammeter and a voltmeter
  • Use the rules for current and voltage in series and parallel circuits
  • Calculate resistance from R = V / I, and describe an I-V graph
  • Describe how an LDR and a thermistor change resistance, and why mains electricity is dangerous

You should already know

  • Electricity in Year 7 and Year 8 Science (cells, bulbs and simple circuits)
  • Speed, distance and time, and reading graphs (Year 7 and Year 8 Science)

Where this leads

  • Magnetism and electromagnets in Chapter 11, which uses the current in a circuit

National Curriculum for England, Key Stage 3 science, Year 9. Content: 9P2 electric circuits: cells, batteries and the symbols for cells, lamps, switches, resistors, ammeters and voltmeters, 9P2 current as a flow of charge, in amperes (A); potential difference, in volts (V), 9P2 series circuits: same current, voltages add; parallel circuits: voltage the same across each branch, currents add, 9P2 mains electricity and its dangers (awareness only; no mains practicals), 9P3 resistance R = V / I (ohms); I-V graphs for a resistor, a filament lamp and a diode (basic), 9P3 resistors in series and in parallel (basic); LDR and thermistor as variable resistors (basic).

Exercise P  Do you remember?

1
FWhat is the name of the device that stores and supplies energy in a circuit? Write the word.
2
FWhat is 6 divided by 2? Write the number.
3
FWhich way do electrons flow round a circuit: from negative to positive, or from positive to negative? Write the word.
4
FWhat is the unit of mass in the SI system? Write the word.
5
FWhat is 0.5 + 0.2 + 0.3? Write the number.
10.1

Circuit symbols and components

Year 9
9P2: electric circuits: cells, batteries and the symbols for cells, lamps, switches, resistors, ammeters and voltmeters
Video lesson: Circuit symbols and componentsComing soon
Story. A pupil builds a lamp circuit from a cell, a switch and a bulb. The teacher asks her to draw it using the correct symbols, so that another pupil could build the same circuit from her diagram.

A circuit diagram uses symbols, not pictures. A cell supplies energy; a battery is two or more cells together. A lamp transfers electrical energy to light and thermal energy. A switch opens or closes the circuit. A resistor resists the current. An ammeter measures current and is placed in series. A voltmeter measures potential difference and is placed across a component.

cellAlampVseries: one loop; ammeter in line with the lamp
A series circuit with a cell, an ammeter in line, a lamp, and a voltmeter across the lamp
ComponentWhat it does in a circuit
cell or batterysupplies energy; sets the potential difference
lamptransfers electrical energy to light and thermal energy
ammeter (A in a circle)measures current, placed in series
voltmeter (V in a circle)measures potential difference, placed across a component
resistorresists the current; fixed resistance
switchopens or closes the circuit
LDR and thermistorvariable resistors; resistance changes with light or temperature
Common circuit components and what they do

Key ideas

  • A circuit diagram uses symbols for each component.
  • An ammeter is placed in series (in line with the component).
  • A voltmeter is placed across (in parallel with) a component.
  • A battery is two or more cells together.
Worked example 10.1.1
Which component measures current, and how is it connected? Write the words.

Method. Recall the ammeter.

  1. Recall. An ammeter measures current.
    measures current
  2. Connect. It is placed in series, in line with the component.
    in series
  3. Write the answer.
    ammeter, in series

Check. The ammeter is always in series. ✓

Worked example 10.1.2
Which component measures potential difference, and how is it connected? Write the words.

Method. Recall the voltmeter.

  1. Recall. A voltmeter measures potential difference.
    potential difference
  2. Connect. It is placed across the component, in parallel with it.
    across
  3. Write the answer.
    voltmeter, across the component

Check. The voltmeter is across the component. ✓

Worked example 10.1.3
What does a switch do in a circuit? Write the words.

Method. Think about what a switch controls.

  1. Think. A switch controls whether current can flow.
    controls current
  2. Name it. It opens the circuit to stop current, or closes it to let current flow.
    opens or closes
  3. Write the answer.
    opens or closes the circuit

Check. An open switch breaks the circuit. ✓

Watch out

Watch out: an ammeter is never placed across a component. It is in series. A voltmeter is always placed across a component.

Exam tip

Exam tip. Draw components with their symbols, and make sure all wires join in a complete loop.

Try it (with your teacher, or a grown-up at home). With your teacher, use a low-voltage cell, a switch, a bulb holder and a bulb. Build the circuit and draw it with symbols. Check that the switch opens and closes the circuit. Use low voltage only. Never connect a circuit to a mains socket.

Exercise 10A  Circuit symbols and components

1
FWhich component supplies energy to a circuit? Write the word.
2
FWhat is a battery made of? Write the words.
3
FWhich component is used to measure current? Write the word.
4
FWhich component is used to measure potential difference? Write the word.
5
FWhat does a switch do? Write the words.
6
FWhich component transfers electrical energy to light and thermal energy? Write the word.
7
FWhat is the name of the component that resists the current? Write the word.
8
FShould an ammeter be placed in series or across a component? Write the word.
9
FShould a voltmeter be placed in series or across a component? Write the word.
10
PSIn a circuit, how many cells make up a battery of 3 cells? Write the number.
11
PSA circuit has a cell, a switch, a lamp and a resistor in one loop. How many of these components are symbols that are not cells? Write the number.
12
PSIn a diagram, the switch is open. Is current flowing? Write yes or no.
13
RExplain why a voltmeter must not be placed in series. Write one sentence. Accept any sentence saying a voltmeter has a very high resistance, so placing it in series would stop the current.
14
RChallenge: draw a complete circuit diagram with one cell, one switch and one lamp in a single loop, using the correct symbols. Write yes once it is correct, and name the components in order. Accept: cell, switch, lamp in one loop.
10.2

Current in series and parallel circuits

Year 9
9P2: current as a flow of charge, in amperes (A); series circuits: the same current; parallel circuits: the branch currents add
Video lesson: Current in series and parallel circuitsComing soon
Story. A pupil measures the current at four points in a loop with one cell and two lamps. The reading is the same at every point. Then she measures two branches of a parallel circuit, and the branch currents add up to the main current.

Current is the flow of charge round a circuit. It is measured in amperes, A. In a series circuit there is only one path, so the current is the same at every point.

In a parallel circuit the current splits at a junction. The currents in the branches add up to the main current, so the main current is bigger than the current in either branch.

cellAlamp Alamp Bparallel: each branch has the full supply voltage
A parallel circuit with two branches, each with a lamp, and an ammeter on the main wire
QuantityIn a series circuit
currentthe same at every point
potential differenceshared; the parts add up to the supply
resistancethe resistances add up
Current and voltage in a series circuit
QuantityIn a parallel circuit
currentthe branch currents add to the main current
potential differencethe same across each branch
resistancethe total is less than the smallest branch resistance
Current and voltage in a parallel circuit

Key ideas

  • Current is the flow of charge, in amperes (A).
  • Series: one path, so the current is the same everywhere.
  • Parallel: the branch currents add to the main current.
  • The current splits at a junction, and rejoins at the next.
Worked example 10.2.1
In a series loop, the current at one lamp is 0.4 A. What is the current at the other lamp? Write the number, in A.

Method. Use the rule for series circuits.

  1. Think. In series the current is the same at every point.
    the same at every point
  2. Conclude. So the other lamp also has 0.4 A.
    0.4
  3. Write the answer.
    0.4

Check. One path, one current. ✓

Worked example 10.2.2
In a parallel circuit, branch A has 0.2 A and branch B has 0.3 A. What is the main current? Write the number, in A.

Method. Add the branch currents.

  1. Add. The branch currents add to the main current.
    0.2 + 0.3
  2. Calculate. So the main current is 0.5 A.
    0.2 + 0.3 = 0.5
  3. Write the answer.
    0.5

Check. The main current is bigger than either branch. ✓

Worked example 10.2.3
In a parallel circuit, the main current is 0.9 A and one branch carries 0.4 A. What is the other branch current? Write the number, in A.

Method. Subtract the known branch from the main current.

  1. Subtract. The other branch is 0.9 - 0.4.
    0.9 - 0.4
  2. Calculate. So the other branch carries 0.5 A.
    0.9 - 0.4 = 0.5
  3. Write the answer.
    0.5

Check. 0.4 + 0.5 = 0.9 A. ✓

Watch out

Watch out: in a parallel circuit, the main current is not the same as the current in a branch. It is the sum of the branch currents.

Exam tip

Exam tip. In series, say “the same everywhere”; in parallel, say “the branch currents add up”.

Try it (with your teacher, or a grown-up at home). With your teacher, build a series circuit and a parallel circuit with a low-voltage cell, two bulbs and an ammeter. Measure the current at each point. Write down whether the readings are the same or add up, and say which type of circuit it was.

Exercise 10B  Current in series and parallel circuits

1
FWhat is the unit of current? Write the symbol.
2
FWhat is current? Write the words.
3
FIn a series circuit, is the current the same at every point? Write yes or no.
4
FIn a parallel circuit, do the branch currents add to the main current? Write yes or no.
5
FA series loop has a current of 0.4 A at one point. What is the current at another point? Write the number, in A.
6
PSIn a parallel circuit, the branch currents are 0.2 A and 0.3 A. What is the main current? Write the number, in A.
7
PSIn a parallel circuit, the main current is 0.9 A and one branch has 0.4 A. What is the other branch current? Write the number, in A.
8
PSThree branches in a parallel circuit carry 0.1 A, 0.2 A and 0.3 A. What is the main current? Write the number, in A.
9
PSIn a series circuit, the current is 0.25 A at one lamp. What is the current through the cell? Write the number, in A.
10
PSWhich circuit has a bigger main current for the same cell: a series or a parallel with two branches? Write the word.
11
RExplain why the current in a parallel branch is smaller than the main current. Write one sentence. Accept any sentence saying the main current is shared between the branches, so each branch has a part of it.
12
RChallenge: a pupil says a lamp uses up the current, so the current is smaller after the lamp. Write one sentence to correct her. Accept any sentence saying the current is the same, because charge is not used up in a single loop.
10.3

Potential difference in series and parallel circuits

Year 9
9P2: potential difference, in volts (V); series: voltages add; parallel: the voltage is the same across each branch
Video lesson: Potential difference in series and parallel circuitsComing soon
Story. Two cells are placed in series, and a pupil measures the potential difference across each lamp. The readings add up to the total voltage of the cells. In a parallel circuit, the same cells give the same voltage across each branch.

Potential difference (p.d.) is the energy transferred per unit charge between two points. It is measured in volts, V. A voltmeter is placed across a component to measure it.

In a series circuit, the potential difference of the cell is shared between the components, so the parts add up to the supply. In a parallel circuit, the potential difference is the same across each branch.

CircuitRule for potential difference
seriesthe parts add to the supply voltage
parallelthe same across each branch
Potential difference rules for series and parallel circuits

Key ideas

  • Potential difference is measured in volts (V) with a voltmeter.
  • Series: potential differences across the parts add to the supply.
  • Parallel: the potential difference is the same across each branch.
  • Two 1.5 V cells in series give 3 V.
Worked example 10.3.1
Two lamps in a series circuit have 2 V and 4 V across them. What is the supply voltage? Write the number, in V.

Method. Add the two potential differences.

  1. Add. In series the voltages add to the supply.
    2 + 4
  2. Calculate. So the supply is 6 V.
    2 + 4 = 6
  3. Write the answer.
    6

Check. 6 V is shared as 2 V and 4 V. ✓

Worked example 10.3.2
A parallel circuit has a supply of 6 V. What is the potential difference across each branch? Write the number, in V.

Method. Use the parallel rule.

  1. Think. In parallel the potential difference is the same across each branch.
    same across each
  2. Conclude. So each branch has 6 V.
    6
  3. Write the answer.
    6

Check. Each branch is connected to the full supply. ✓

Worked example 10.3.3
Two 1.5 V cells are placed in series. What is the total voltage? Write the number, in V.

Method. Add the cell voltages.

  1. Add. In series the cell voltages add.
    1.5 + 1.5
  2. Calculate. So the total is 3 V.
    1.5 + 1.5 = 3
  3. Write the answer.
    3

Check. 3 V is twice 1.5 V. ✓

Watch out

Watch out: a voltmeter is connected across a component, not in series. The voltage is measured between two points.

Exam tip

Exam tip. In series, add the voltages; in parallel, read the same voltage on each branch.

Try it (with your teacher, or a grown-up at home). With your teacher, use two low-voltage cells and two bulbs. First connect the bulbs in series, then in parallel. Measure the voltage across each bulb with a voltmeter, and say whether the voltages add up or stay the same.

Exercise 10C  Potential difference in series and parallel circuits

1
FWhat is the unit of potential difference? Write the symbol.
2
FIn a series circuit, do the potential differences across the parts add to the supply? Write yes or no.
3
FIn a parallel circuit, is the potential difference the same across each branch? Write yes or no.
4
FWhich instrument measures potential difference? Write the word.
5
PSTwo lamps in series have 2 V and 4 V across them. What is the supply voltage? Write the number, in V.
6
PSA supply of 9 V is connected in series to a resistor. The resistor has 5 V across it. What is the voltage across the rest of the circuit? Write the number, in V.
7
PSA parallel circuit has a supply of 12 V. What is the potential difference across each branch? Write the number, in V.
8
PSTwo 1.5 V cells are placed in series. What is the total voltage? Write the number, in V.
9
PSThree 1.5 V cells are placed in series. What is the total voltage? Write the number, in V.
10
PSA lamp is placed in parallel with a resistor across a 6 V supply. What is the voltage across the lamp? Write the number, in V.
11
PSA supply of 4.5 V is shared between two resistors in series, with 1.5 V across one. What is the voltage across the other? Write the number, in V.
12
RExplain why the voltage across each branch of a parallel circuit is the same. Write one sentence. Accept any sentence saying each branch is connected directly across the supply.
13
RChallenge: a pupil says the voltage is used up as the current goes round a series loop. Write one sentence to correct her. Accept any sentence saying the energy is transferred to the components, and the voltages add up to the supply.
10.4

Resistance, R = V / I, and I-V graphs

Year 9
9P3: resistance R = V / I (ohms); I-V graphs for a resistor, a filament lamp and a diode (basic)
Video lesson: Resistance, R = V / I, and I-V graphsComing soon
Story. A pupil measures the voltage and current for two components. Each one gives a different value for R. She plots the readings on an I-V graph. The resistor gives a straight line; the lamp curves.

Resistance is the opposition to the current. It is calculated using R = V / I, where R is in ohms (Ω), V in volts and I in amperes. The greater the resistance, the smaller the current for a given voltage.

For a resistor at a constant temperature, the current is proportional to the voltage, so the I-V graph is a straight line through the origin. For a filament lamp, the filament gets hotter and its resistance rises, so the graph curves. A diode lets current flow in one direction only.

I-V graphspotential difference, V (volts)current, I (A)012340.00.10.20.30.4resistor (straight)lamp (curves)diode: current one way
I-V graphs of a resistor (straight line), a filament lamp (curve) and a diode
ComponentPotential difference (V)Current (A)Resistance (ohm)
resistor A6.00.512.0
resistor B9.00.330.0
lamp at low brightness4.50.1530.0
Potential difference, current and resistance for three components

Key ideas

  • R = V / I, with R in ohms, V in volts and I in amperes.
  • Resistor: straight line through the origin (constant R).
  • Filament lamp: curves, because its resistance increases as it heats.
  • Diode: current flows in one direction only.
Worked example 10.4.1
A resistor has 6 V across it and a current of 0.5 A. What is its resistance, in ohms? Write the number.

Method. Use R = V / I.

  1. Write. R = 6 / 0.5.
    6 / 0.5
  2. Calculate. So R = 12 ohms.
    6 / 0.5 = 12
  3. Write the answer.
    12

Check. 12 x 0.5 = 6 V. ✓

Worked example 10.4.2
A component has 9 V across it and a current of 0.3 A. What is its resistance, in ohms? Write the number.

Method. Use R = V / I.

  1. Write. R = 9 / 0.3.
    9 / 0.3
  2. Calculate. So R = 30 ohms.
    9 / 0.3 = 30
  3. Write the answer.
    30

Check. 30 x 0.3 = 9 V. ✓

Worked example 10.4.3
On an I-V graph, a straight line through the origin shows which component? Write the word.

Method. Recall the shape of the graph.

  1. Recall. A straight line through the origin means constant resistance.
    constant resistance
  2. Name it. That is a resistor at a constant temperature.
    resistor
  3. Write the answer.
    resistor

Check. A curved line shows a filament lamp. ✓

Watch out

Watch out: R = V / I, not V = R / I. Multiply V by R only when finding V, not when finding R.

Exam tip

Exam tip. Write R = V / I, substitute the numbers with units, then divide.

Try it (with your teacher, or a grown-up at home). With your teacher, use a low-voltage supply, a resistor, an ammeter and a voltmeter. Change the voltage in steps and read the current each time. Plot the points, and say whether the graph is a straight line through the origin.

Exercise 10D  Resistance, R = V / I, and I-V graphs

1
FWhat is the unit of resistance? Write the word.
2
FWrite the equation for resistance. Write the words.
3
FWhat shape is the I-V graph of a resistor at constant temperature? Write the word.
4
FIn a filament lamp, does the resistance increase as the filament heats up? Write yes or no.
5
FDoes a diode allow current to flow in both directions? Write yes or no.
6
PSA resistor has 6 V across it and a current of 0.5 A. What is its resistance, in ohms? Write the number.
7
PSA component has 9 V across it and a current of 0.3 A. What is its resistance, in ohms? Write the number.
8
PSA resistor has a resistance of 20 ohms and a voltage of 4 V across it. What is the current, in amperes? Write the number.
9
PSA resistor of 10 ohms has a current of 0.6 A. What is the potential difference across it, in volts? Write the number.
10
PSA resistor passes 0.1 A at 2 V. What is its resistance? Write the number, in ohms.
11
PSTwo components both have 6 V across them. One has 0.5 A and the other 0.25 A. Which has the larger resistance? Write the word.
12
RExplain why the I-V graph of a filament lamp is curved. Write one sentence. Accept any sentence saying the resistance of the filament increases as it heats up, so the current rises more slowly.
13
RChallenge: a pupil says a resistor’s I-V graph is a curve because its resistance changes. Write one sentence to correct her. Accept any sentence saying a resistor at constant temperature has a constant resistance, so its graph is straight.
10.5

Resistors in series and parallel, variable resistors and mains safety

Year 9
9P3: resistors in series and in parallel (basic); LDR and thermistor as variable resistors (basic); mains electricity and its dangers (awareness only)
Video lesson: Resistors in series and parallel, variable resistors and mains safetyComing soon
Story. A school alarm uses a light sensor that changes its resistance when a door opens. A teacher explains that the sensor is an LDR, and that pupils must never connect any test to a mains socket, which is 230 V and dangerous.

Two resistors in series add together: 10 ohms and 20 ohms make 30 ohms. Two equal resistors in parallel have a total resistance less than either one: two 10 ohm resistors in parallel make 5 ohms.

An LDR is a light-dependent resistor: its resistance falls in bright light. A thermistor is a temperature-dependent resistor: its resistance falls when it is hotter. Both are used as sensors in circuits.

Mains electricity in the UK is about 230 V. It can cause severe electric shock and burns. Pupils must never test mains equipment, and must never connect a circuit to a mains socket. This is awareness only.

ComponentWhat happens to its resistance
LDRfalls in bright light
thermistorfalls when it gets hotter
fixed resistorstays the same at constant temperature
LDR, thermistor and fixed resistor: how their resistance changes

Key ideas

  • Series resistances add.
  • Two equal resistors in parallel: total = one resistance divided by two.
  • An LDR has lower resistance in bright light.
  • A thermistor has lower resistance when hotter.
  • Mains is about 230 V: never used by pupils; awareness only.
Worked example 10.5.1
Two resistors of 10 ohms and 20 ohms are in series. What is their total resistance, in ohms? Write the number.

Method. Add the resistances.

  1. Add. In series the resistances add.
    10 + 20
  2. Calculate. So the total is 30 ohms.
    10 + 20 = 30
  3. Write the answer.
    30

Check. Series total is more than either resistor. ✓

Worked example 10.5.2
Two equal 10 ohm resistors are in parallel. What is their total resistance, in ohms? Write the number.

Method. Share the resistance by the number of branches.

  1. Think. Two equal branches share the current, so the total is less.
    less
  2. Calculate. 10 / 2 = 5 ohms.
    10 / 2 = 5
  3. Write the answer.
    5

Check. The total is less than 10 ohms. ✓

Worked example 10.5.3
In bright light, an LDR has a lower resistance. Does the current in its circuit increase or decrease? Write the word.

Method. Use the rule that current is higher at lower resistance.

  1. Think. With the same voltage, lower resistance means a bigger current.
    bigger current
  2. Decide. So the current increases in bright light.
    increases
  3. Write the answer.
    increases

Check. Current = voltage / resistance. ✓

Watch out

Watch out: adding a resistor in parallel does not increase the total resistance. The total always falls, below the smallest branch.

Exam tip

Exam tip. In series add the resistances; in parallel the total is always less than the smallest.

Try it (with your teacher, or a grown-up at home). With your teacher, use a low-voltage supply and a light-dependent resistor (LDR), a meter and a torch. Measure the current with the torch off and on. Say how the LDR changes resistance, and never test any mains socket or mains equipment.

Exercise 10E  Resistors in series and parallel, variable resistors and mains safety

1
FWhat happens to the resistance of an LDR in bright light? Write the word: rises or falls.
2
FWhat happens to the resistance of a thermistor when it gets hotter? Write the word: rises or falls.
3
FWhat is the mains supply voltage in the UK, approximately? Write the number, in volts.
4
FShould a pupil test a mains socket in a school practical? Write yes or no.
5
PSTwo resistors of 10 ohms and 30 ohms are in series. What is the total resistance, in ohms? Write the number.
6
PSTwo equal 20 ohm resistors are in parallel. What is their total resistance, in ohms? Write the number.
7
PSThree equal 30 ohm resistors are in series. What is the total resistance, in ohms? Write the number.
8
PSIn a parallel pair, one resistor is 10 ohms and the other is 10 ohms. Is the total less than 10 ohms? Write yes or no.
9
FWhich electrical supply is the one that pupils are allowed to use: a mains socket, or a low-voltage cell? Write the words.
10
RExplain why a thermistor is a useful sensor in a heater. Write one sentence. Accept any sentence saying its resistance changes with temperature, so the circuit can detect a change in heat.
11
RChallenge: explain why adding a resistor in parallel always makes the total resistance less than the smallest branch. Accept any sentence saying the extra branch gives the current another path, so the total resistance falls.

Chapter summary

SkillMethod
Circuit symbolsCell, lamp, switch, resistor, ammeter (in series), voltmeter (across).
CurrentFlow of charge in amperes; the same in series; the branch currents add in parallel.
Potential differenceIn volts; series parts add to the supply; the same across each parallel branch.
ResistanceR = V / I, in ohms; a resistor gives a straight I-V line; a filament lamp curves; a diode is one-way.
ResistorsSeries resistances add; parallel total is less than the smallest branch.
Sensors and mainsLDR falls in bright light; thermistor falls when hotter; mains (230 V) is never used by pupils.

Can you…?

  • I can name the circuit symbols and place an ammeter and a voltmeter correctly
  • I can state the current and potential difference rules for series and parallel circuits
  • I can calculate resistance from R = V / I, and describe the shape of an I-V graph
  • I can work out series and parallel resistances at basic level
  • I can describe an LDR and a thermistor, and explain why mains is dangerous

Key terms

current
the flow of charge round a circuit, measured in amperes
potential difference
the energy transferred per unit charge between two points, measured in volts
series circuit
a circuit with one path for the current
parallel circuit
a circuit with more than one path for the current
resistance
the opposition to the current, measured in ohms, equal to V / I
ohm
the unit of resistance
ammeter
a meter placed in series to measure current
voltmeter
a meter placed across a component to measure potential difference
LDR
a light-dependent resistor whose resistance falls in bright light
thermistor
a temperature-dependent resistor whose resistance falls when it is hotter
diode
a component that lets current flow in one direction only
mains electricity
the supply from the mains socket, about 230 V in the UK, which pupils never test

Exercise 10Z  Check-up

1
What is the unit of current? Write the symbol.
2
In a series circuit, is the current the same at every point? Write yes or no.
3
In a parallel circuit, the branch currents are 0.2 A and 0.3 A. What is the main current? Write the number, in A.
4
Two lamps in series have 2 V and 4 V across them. What is the supply voltage? Write the number, in V.
5
In a parallel circuit with a supply of 12 V, what is the voltage across each branch? Write the number, in V.
6
Calculate the resistance of a component with 6 V across it and 0.5 A through it. Write the number, in ohms.
7
What is the unit of resistance? Write the word.
8
Two resistors of 10 ohms and 20 ohms are in series. What is the total resistance, in ohms? Write the number.
9
What happens to the resistance of an LDR in bright light? Write the word: rises or falls.
10
Explain why the I-V graph of a resistor at constant temperature is a straight line. Write one sentence. Accept any sentence saying the current is proportional to the voltage, because the resistance is constant.

Total 19 marks. Allow about 19 minutes.

Exercise 10X  Practice test questions

1
FWhat is the symbol for the unit of potential difference? Write the symbol.[1 mark]
2
FIn a series circuit, which quantity is the same at every point? Write the word.[1 mark]
3
PSCalculate the resistance of a resistor with a voltage of 9 V and a current of 0.3 A. Write the number, in ohms.[2 marks]
4
PSTwo branches in a parallel circuit carry 0.1 A and 0.4 A. What is the main current, in amperes? Write the number.[2 marks]
5
PSA supply of 6 V is connected in parallel with a lamp. What is the voltage across the lamp? Write the number, in volts.[1 mark]
6
RExplain why the resistance of a filament lamp increases as the current increases. Write two sentences. Accept any answer saying the filament heats up, and its resistance rises as the atoms vibrate more.[3 marks]
7
PSTwo equal 10 ohm resistors are placed in parallel. What is their total resistance, in ohms? Explain in one sentence why it is less than 10 ohms. Write the number, then one sentence. Accept 5 ohms, with a reason that the current has two paths.[3 marks]
8
RDescribe the change in an LDR when the light gets brighter, and say what happens to the current in its circuit. Write two sentences. Accept any answer saying the resistance falls, and the current increases.[2 marks]
9
FState the mains voltage in the UK, approximately, and say whether pupils may test it in a school practical. Write the number and yes or no.[2 marks]
10
RExplain why a voltmeter is connected across a component, not in series. Write one sentence. Accept any sentence saying it measures the voltage across the component without stopping the current.[2 marks]
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