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Series and parallel circuits: GCSE Combined Science Physics practice questions

Foundation and Higher · Grade 5

A second set of questions on series and parallel circuits. They are different from the ones on the topic page. Try each, then open the answer and compare your working.

More questions

Question 1 Hard

A 6.0 V battery is connected to a 12 Ω resistor and a 20 Ω resistor in parallel. Calculate the current from the battery.

Show answer

Answer: 0.80 A

  1. Each branch has the full 6.0 V across it.
  2. I1 = 6.0 ÷ 12 = 0.50 A and I2 = 6.0 ÷ 20 = 0.30 A.
  3. Current from the battery = 0.50 + 0.30 = 0.80 A.
  • Method (1): One branch current: 0.50 A or 0.30 A
  • Method (1): Adds the two branch currents
  • Accuracy (1): 0.80 A

Question 2 Hard

A 12 V supply is connected in series with a lamp and a 16 Ω resistor. The current is 0.50 A. Calculate the resistance of the lamp.

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Answer: 8.0 Ω

  1. R total = V ÷ I = 12 ÷ 0.50 = 24 Ω.
  2. In series R total = R lamp + 16, so R lamp = 24 − 16 = 8.0 Ω.
  • Method (1): R total = 12 ÷ 0.50 = 24 Ω
  • Method (1): 24 − 16
  • Accuracy (1): 8.0 Ω

Question 3 Hard

Two resistors are connected in series with a 9.0 V battery. The current is 250 mA. One resistor is 20 Ω. Calculate the potential difference across the other resistor.

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Answer: 4.0 V

  1. 250 mA = 0.25 A.
  2. p.d. across the 20 Ω resistor = 0.25 × 20 = 5.0 V.
  3. p.d. across the other resistor = 9.0 − 5.0 = 4.0 V.
  • Mark (1): 250 mA = 0.25 A
  • Method (1): 0.25 × 20 = 5.0 V
  • Accuracy (1): 4.0 V

Question 4 Hard

A 15 Ω resistor and a lamp are connected in parallel with a 6.0 V battery. The current from the battery is 0.65 A. Calculate the resistance of the lamp.

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Answer: 24 Ω

  1. Current in the resistor = 6.0 ÷ 15 = 0.40 A.
  2. Current in the lamp = 0.65 − 0.40 = 0.25 A.
  3. R lamp = 6.0 ÷ 0.25 = 24 Ω.
  • Method (1): 6.0 ÷ 15 = 0.40 A
  • Method (1): 0.65 − 0.40 = 0.25 A
  • Accuracy (1): 24 Ω

Question 5 Hard

Which safety device switches off a circuit automatically and can then be reset?

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Answer: circuit breaker

  1. A fuse melts and has to be replaced.
  2. A circuit breaker can be reset.

Quick checks

Short questions with options. Choose one before you open the answer.

Check 1

What is the same at every point in a series circuit?

  1. The current
  2. The potential difference
  3. The resistance
Show answer

The current There is only one path, so the same current passes every point.

Check 2

How do you find the total resistance of resistors in series?

  1. Use the smallest one
  2. Add them: R total = R1 + R2
  3. Divide them
Show answer

Add them: R total = R1 + R2 Each extra resistor in the loop adds to the total.

Check 3

What happens to the supply p.d. in a series circuit?

  1. Each component gets all of it
  2. It is used up before the first component
  3. It is shared between the components
Show answer

It is shared between the components The p.d. values across the components add up to the supply p.d.

Check 4

What is true about the p.d. across each branch of a parallel circuit?

  1. It equals the supply p.d.
  2. It is shared between the branches
  3. It is zero
Show answer

It equals the supply p.d. Each branch is connected across the same two points as the supply.

Check 5

What happens to the currents in the branches of a parallel circuit?

  1. They are all equal
  2. They add up to the current from the supply
  3. They multiply together
Show answer

They add up to the current from the supply The current splits at a junction and the branch currents add back to the supply current.

Check 6

How does the total resistance of two parallel resistors compare with the smaller one?

  1. It is the sum of the two
  2. It is halfway between them
  3. It is less than the smaller one
Show answer

It is less than the smaller one Each new branch gives charge another path, so the total goes down.

Check 7

Write the equation linking potential difference, current and resistance.

  1. V = I × R
  2. V = I ÷ R
  3. V = R ÷ I
Show answer

V = I × R Potential difference = current × resistance.

Check 8

What is the unit of resistance?

  1. The ohm, Ω
  2. The volt, V
  3. The amp, A
Show answer

The ohm, Ω Resistance is measured in ohms.

Check 9

What is the unit of potential difference?

  1. The ohm, Ω
  2. The volt, V
  3. The watt, W
Show answer

The volt, V Potential difference is measured in volts.

Check 10

Which meter is connected in series with a component?

  1. An ammeter
  2. A voltmeter
  3. A thermometer
Show answer

An ammeter An ammeter measures current, so the current must pass through it.

Check 11

How is a voltmeter connected to measure the p.d. across a resistor?

  1. In series, in the loop
  2. Between the cell and the switch only
  3. In parallel, across the resistor
Show answer

In parallel, across the resistor A voltmeter measures the p.d. between two points, so it is connected across them.

Check 12

Which line of the cell symbol is the positive end?

  1. The long line
  2. The short line
  3. The line in the middle
Show answer

The long line The long line is positive and the short line is negative.

Check 13

What is a resistor?

  1. A component that makes it harder for charge to flow
  2. A component that pushes the charge round
  3. A switch that stops the current
Show answer

A component that makes it harder for charge to flow Resistance is measured in ohms.

Check 14

How do you change 250 mA into amps?

  1. Multiply by 1000, to get 250 000 A
  2. Divide by 1000, to get 0.25 A
  3. Leave it as 250 A
Show answer

Divide by 1000, to get 0.25 A 1 A = 1000 mA, so divide by 1000.

Check 15

How do you find the current from the p.d. and resistance?

  1. I = V × R
  2. I = R ÷ V
  3. I = V ÷ R
Show answer

I = V ÷ R Rearranging V = I × R gives I = V ÷ R.

Check 16

Why does adding a resistor in series reduce the current?

  1. The total resistance goes up
  2. The p.d. of the cell goes down
  3. The charge is used up
Show answer

The total resistance goes up A bigger total resistance means a smaller current for the same p.d.

Check 17

Why does adding a branch in parallel lower the total resistance?

  1. It adds more resistance to the loop
  2. It gives the charge another path to flow through
  3. It reduces the p.d. of the cell
Show answer

It gives the charge another path to flow through More paths mean more current for the same p.d., so the total resistance is smaller.

Check 18

Why are the lights in a house wired in parallel?

  1. It uses less electricity
  2. So that they all go out together
  3. Each gets the full p.d. and works on its own
Show answer

Each gets the full p.d. and works on its own If one lamp breaks, the other branches are still complete circuits.

Check 19

One lamp breaks in a series circuit with three lamps. What happens to the others?

  1. They all go out
  2. They get brighter
  3. They stay lit
Show answer

They all go out A break anywhere in a series loop stops the current everywhere.

Check 20

A student says: "Add the resistors to get the total for a parallel circuit." What is wrong?

  1. A parallel total is less than the smallest resistor, so adding is wrong
  2. Nothing is wrong
  3. You should multiply them
Show answer

A parallel total is less than the smallest resistor, so adding is wrong Only add resistances in series.

Check 21

A student says: "Each resistor in a series circuit gets the full p.d. of the battery." What is wrong?

  1. The battery has no p.d.
  2. The p.d. is shared between them
  3. Only the first resistor gets any
Show answer

The p.d. is shared between them Only parallel branches each get the full supply p.d.

Check 22

A 12 V battery is connected in series with a 4.0 Ω and an 8.0 Ω resistor. What is the current?

  1. 3.0 A
  2. 144 A
  3. 1.0 A
Show answer

1.0 A R total = 12 Ω and I = 12 ÷ 12 = 1.0 A.

Check 23

The branches of a parallel circuit carry 0.50 A and 0.25 A. What current comes from the battery?

  1. 0.75 A
  2. 0.25 A
  3. 0.125 A
Show answer

0.75 A The branch currents add up to the supply current: 0.50 + 0.25 = 0.75 A.

Check 24

A current of 0.50 A passes through a 12 Ω resistor. What is the p.d. across it?

  1. 24 V
  2. 6.0 V
  3. 0.04 V
Show answer

6.0 V V = I × R = 0.50 × 12 = 6.0 V.

Check 25

What is the equation linking charge flow, current and time?

  1. Q = I ÷ t
  2. Q = I × t
  3. Q = V × I
Show answer

Q = I × t charge flow = current × time, or Q = I × t.

Check 26

What is the unit of charge?

  1. The ampere, A
  2. The volt, V
  3. The coulomb, C
Show answer

The coulomb, C The coulomb, C.

Check 27

What must a circuit include for charge to flow round it?

  1. A source of potential difference
  2. A resistor
  3. An ammeter
Show answer

A source of potential difference A source of potential difference, such as a cell or battery, in a closed loop.

Check 28

What does an ohmic conductor's I-V graph look like?

  1. A curve that flattens
  2. A straight line through the origin
  3. A flat line, then a steep rise
Show answer

A straight line through the origin A straight line through the origin, because the resistance is constant.

Check 29

Why does the resistance of a filament lamp increase as it gets hotter?

  1. The wire gets longer.
  2. The p.d. gets smaller.
  3. The electrons collide with the vibrating ions more often.
Show answer

The electrons collide with the vibrating ions more often. The ions vibrate more, so the electrons collide with them more often.

Check 30

What does a diode do?

  1. It lets current flow one way only.
  2. It lets current flow both ways equally.
  3. It stores charge.
Show answer

It lets current flow one way only. It lets current flow in one direction only and has a very high resistance in the other.

Check 31

How does the resistance of a thermistor change as the temperature increases?

  1. It increases.
  2. It decreases.
  3. It stays the same.
Show answer

It decreases. It decreases.

Check 32

How does the resistance of an LDR change as the light gets brighter?

  1. It increases.
  2. It stays the same.
  3. It decreases.
Show answer

It decreases. It decreases.

Check 33

Give a use of an LDR in a circuit.

  1. Switching a lamp on when it gets dark
  2. Switching the heating on when it gets cold
  3. Stopping current flowing backwards
Show answer

Switching a lamp on when it gets dark Switching a light on when it gets dark.

Check 34

What are the voltage and frequency of the UK mains supply?

  1. About 12 V and 50 Hz
  2. About 230 V and 50 Hz
  3. About 230 V and 5 Hz
Show answer

About 230 V and 50 Hz About 230 V and 50 Hz.

Check 35

What is the difference between a.c. and d.c.?

  1. A.c. is always bigger than d.c.
  2. D.c. keeps changing direction. A.c. does not.
  3. A.c. keeps changing direction. D.c. does not.
Show answer

A.c. keeps changing direction. D.c. does not. A.c. keeps reversing direction. D.c. flows in one direction only.

Check 36

What colours are the live, neutral and earth wires?

  1. Brown, blue and green and yellow
  2. Red, black and green
  3. Blue, brown and green and yellow
Show answer

Brown, blue and green and yellow Live is brown, neutral is blue and earth is green and yellow stripes.

Check 37

What is the job of the earth wire?

  1. It carries the current to the appliance.
  2. It is a safety wire.
  3. It completes the circuit.
Show answer

It is a safety wire. It is a safety wire that stops the appliance becoming live.

Check 38

Why can a live wire be dangerous even when the switch is open?

  1. An open switch always keeps the wire live.
  2. The earth wire is at 230 V.
  3. The switch may be in the neutral wire.
Show answer

The switch may be in the neutral wire. If the switch is in the neutral wire, the appliance is still connected to the live wire.

Check 39

What is the equation linking power, potential difference and current?

  1. P = V × I
  2. P = V ÷ I
  3. P = I ÷ V
Show answer

P = V × I power = potential difference × current, or P = V × I.

Check 40

What is the second equation for electrical power?

  1. P = I × R
  2. P = I² × R
  3. P = I² ÷ R
Show answer

P = I² × R power = current² × resistance, or P = I² × R.

Check 41

What is the equation linking energy transferred, power and time?

  1. E = P ÷ t
  2. E = t ÷ P
  3. E = P × t
Show answer

E = P × t energy transferred = power × time, or E = P × t.

Check 42

What is the equation linking energy transferred, charge flow and potential difference?

  1. E = Q × V
  2. E = Q ÷ V
  3. E = Q + V
Show answer

E = Q × V energy transferred = charge flow × potential difference, or E = Q × V.

Check 43

A 60 W lamp is on for 10 s. How much energy does it transfer?

  1. 6 J
  2. 600 J
  3. 70 J
Show answer

600 J 600 J, because 60 × 10 = 600.

Check 44

What does a step-up transformer do in the National Grid?

  1. It lowers the p.d. for homes.
  2. It makes electricity.
  3. It raises the p.d. for the transmission cables.
Show answer

It raises the p.d. for the transmission cables. It raises the p.d. from the power station for the transmission cables.

Check 45

Why does the National Grid use a very high potential difference?

  1. The current is smaller, so less energy is wasted as heat.
  2. The current is bigger, so the power is bigger.
  3. The cables have no resistance.
Show answer

The current is smaller, so less energy is wasted as heat. For the same power the current is smaller, so less energy is wasted as heat in the cables.

Check 46

Higher tier. A transformer has Vp = 100 V and Ip = 2 A. The secondary p.d. is 400 V. What is the secondary current?

  1. 8 A
  2. 0.5 A
  3. 2 A
Show answer

0.5 A 0.5 A, because 100 × 2 = 400 × Is.

Check 47

What is the difference between a fuse and a circuit breaker?

Show answer

A circuit breaker can be reset. A fuse has to be replaced.

Check 48

Give one advantage and one disadvantage of the heating effect of a current.

Show answer

It is used in kettles and heaters, but it wastes energy in wires.

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