Grade 11 Physical Sciences · Electricity and Magnetism

Explain changes in a circuit

Predict and explain how meter readings change when a switch is closed or a resistor is added (adding a parallel resistor lowers total resistance).

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Electric circuits Internal resistance Intro: PHYSICS grade 11 and 12Miss Martins Maths and Science

Electric Circuits - Open and Closed Switch (Grade 12 Physics)Goon School

Physics | Electric circuits | Circuits with switchesMlungisi Nkosi Maths & Science

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Question 8.3 · May/June 2025 · Paper 1 · National (DBE) · 2 marks

In the circuit diagram below, the battery has an emf of

24

V and an internal resistance of

0, 3 Ω

. Light bulb L has a resistance of

9, 5 Ω

and R is a resistor of unknown resistance. Ignore the resistance of the ammeters and the connecting wires. Diagram for this question

Switch

S_{2}

is now OPENED, while switch

S_{1}

remains CLOSED.

8.3 How will this affect the reading on ammeter

A_{2}

? Choose from INCREASES, DECREASES or REMAINS THE SAME. Give a reason for the answer.(2)

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: explain changes in a circuit

Work each one out first, then open the memo and check every tick.

3. Question 8.5 · November 2025 · Paper 1 · National (DBE) · 5 marks
Two identical headlights, $L_{1}$ and $L_{2}$ , and a starter motor, M, of a car are connected to a battery, as shown in the circuit diagram below. The resistance of each headlight is $12 Ω$ , while the resistance of the starter motor is $0, 1 Ω$ . The emf ( $ε$ ) and internal resistance (r) of the battery are unknown.

The ammeters and the connecting wires have negligible resistance, while the voltmeter has a high resistance. Switches $S_{1}$ and $S_{2}$ are initially open.
8.5 Both switches are now closed.

How will the reading on ammeter $A_{1}$ be affected? Choose from INCREASES, DECREASES or REMAINS THE SAME.

Explain the answer WITHOUT the use of a calculation.(5)
Show the memo (5 ticks)Memo
1. ✓
  ChoiceDecrease
  1 mark · Explain changes in a circuit
2. ✓
  Total resistance decreasesTotal/External resistance decreases.
  1 mark · Explain changes in a circuit
3. ✓
  Total current increasesTotal current increases.
  1 mark · Explain changes in a circuit
4. ✓
  Lost volts increase $V_{in t er na l}$ increases.
  1 mark · Explain changes in a circuit
5. ✓
  Voltage across the headlights decreasesVoltage across the light bulbs decreases / $V_{e x t er na l}$ decreases.
  1 mark · Explain changes in a circuit
4. Question 8.4 · May/June 2023 · Paper 1 · National (DBE) · 4 marks
A battery with unknown emf ( $ε$ ) and unknown internal resistance (r) is connected to three resistors, a high-resistance voltmeter, two switches and two ammeters of negligible resistance, as shown below.
Switch S $_{2}$ is now OPENED, while switch S $_{1}$ is CLOSED.
8.4 How does the voltmeter reading change? Choose from INCREASES, DECREASES or REMAINS THE SAME.

Explain the answer.(4)
Show the memo (4 ticks)Memo
1. ✓
  ChoiceIncreases
  1 mark · Explain changes in a circuit
2. ✓
  Resistance increases(Total) resistance increases.
  1 mark · Explain changes in a circuit
3. ✓
  Current decreasesCurrent decreases.
  1 mark · Explain changes in a circuit
4. ✓
  Internal volts decrease $V_{in t er na l}$ /Internal volts decreases.
  1 mark · Explain changes in a circuit
5. Question 8.3.2 · November 2023 · Paper 1 · National (DBE) · 3 marks
The circuit diagram below shows a battery with an emf of $12$ V and an internal resistance of $0, 5 Ω$ connected to three resistors, a light bulb, a switch, an ammeter and connecting wires. The ammeter and connecting wires have negligible resistance.
8.3 Switch S is now OPENED.

Answer the questions that follow.
8.3.2 Explain the answer to QUESTION 8.3.1.(3)
Show the memo (3 ticks)Memo
1. ✓
  Resistance up, current downTotal resistance of the circuit increases and total current in circuit decreases.
  1 mark · Explain changes in a circuit
2. ✓
  Lost volts down, external volts up $V_{in t er na l}$ / internal volts / $V_{l os t}$ decreases and $V_{e x t er na l}$ / external volts / $V_{R L}$ increases.
  1 mark · Explain changes in a circuit
3. ✓
  Power increasesPower output increases, therefore brightness increases.
  1 mark · Explain changes in a circuit
6. Question 8.5 · November 2022 · Paper 1 · National (DBE) · 4 marks
The circuit diagram below shows four resistors connected to a battery of emf $ε$ and internal resistance $r$ . The resistances of the ammeter and the connecting wires are negligible, while the voltmeters have very high resistances.
8.5 Explain the answer to QUESTION 8.4.2.(4)
Show the memo (4 ticks)Memo
1. ✓
  Effect on the $1 Ω$ branchWhen the voltmeter is connected: no/very little current through the $1 Ω$ branch OR the branch with the $1 Ω$ resistor is disabled/bypassed OR a voltmeter has a very high resistance OR the resistance of the parallel branch increases.
  1 mark · Explain changes in a circuit
2. ✓
  Total resistance increases(Total) resistance of the circuit increases.
  1 mark · Explain changes in a circuit
3. ✓
  Current decreasesCurrent in circuit decreases.
  1 mark · Explain changes in a circuit
4. ✓
  Lost volts decrease $V_{in t er na l}$ / internal volts / $V_{l os t}$ decreases.
  Therefore, external volts increase for a constant emf.
  1 mark · Explain changes in a circuit

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