What are the key components of a practical D.C. circuit?

A practical D.C. circuit typically includes a power source such as a battery, conductive paths like wires, and components such as resistors, capacitors, and switches. These elements work together to control the flow of electric current, allowing the circuit to perform specific functions. Understanding these components is crucial for Cambridge International A Levels Physics students studying D.C. circuits.

How do you calculate the total resistance in a series circuit?

In a series circuit, the total resistance is the sum of the individual resistances. This is because the current flows through each resistor sequentially, adding up the resistance values. For example, if you have three resistors with resistances of 2 ohms, 3 ohms, and 5 ohms, the total resistance would be 2 + 3 + 5 = 10 ohms. This concept is fundamental in the study of practical circuits in the Cambridge International A Levels Physics curriculum.

What is the role of a capacitor in a D.C. circuit?

In a D.C. circuit, a capacitor stores electrical energy in an electric field and can release it when needed. Capacitors are used to smooth out voltage fluctuations, filter signals, and store energy for later use. Understanding the function of capacitors is essential for students learning about practical circuits in the Cambridge International A Levels Physics course.

Why is it important to understand the concept of voltage drop in practical circuits?

Voltage drop refers to the reduction in voltage as electric current flows through a component or conductor in a circuit. It is important to understand because it affects the performance and efficiency of the circuit. Excessive voltage drop can lead to insufficient power being delivered to components, which is a critical consideration in designing and analyzing practical circuits for Cambridge International A Levels Physics students.

How can you measure current in a practical D.C. circuit?

To measure current in a practical D.C. circuit, you use an ammeter, which must be connected in series with the circuit components. This allows the ammeter to measure the flow of electrons through the circuit. Accurate measurement of current is vital for analyzing circuit behavior and is a key skill for students studying D.C. circuits in the Cambridge International A Levels Physics syllabus.

Why is "Saying ideal ammeter has high resistance" a common mistake in Practical circuits?

Why it happens: Confusing with voltmeter. How to avoid it: Ammeter SERIES, LOW R (to not block current). Voltmeter PARALLEL, HIGH R (to not draw current). Source: 9702 Examiner Reports 2022-2024

D.C. CircuitsCambridge International A Levels Physics (9702)

Practical circuits

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Practical Circuits Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

EMF, internal resistance, terminal PD. Ammeters and voltmeters.

What you’ll learn

Mapped to the Cambridge IGCSE 9702 syllabus (2025-2027).

10.1.1 — Use EMF + internal resistance.
10.1.2 — Compute terminal PD.
10.1.3 — Describe meter properties.

EMF and internal resistance

Real cells aren't ideal.

EMF. Total energy per unit charge supplied by cell.

Internal resistance $r$ . Real cells have small internal resistance — energy lost as heat inside.

Kirchhoff's voltage law around circuit: $E = I(R + r) = V_t + Ir$ where $V_t = IR$ is terminal PD (across external load).

Terminal PD < EMF when current flows. Lost PD = $Ir$ .

No load. When $I = 0$ , $V_t = E$ (e.g. voltmeter reading with circuit open).

Example. $E = 6$ V drives 0.5 A through $R = 10$ Ω: $6 = 0.5(10 + r) \Rightarrow r = 2$ Ω. Terminal PD = $6 - 0.5 \times 2 = 5$ V.

Cambridge tip. Real-world batteries have $r$ from milliohms (car battery) to ohms (small cell).

$E = I(R + r)$ .
$V_t = E - Ir$ .
Open circuit: $V_t = E$ .

See the full worked example for practical circuits →

Ammeter and voltmeter

Opposite requirements.

Ammeter.

Measures current.
Connected in SERIES with component.
Ideal: ZERO resistance (so doesn't reduce circuit current).

Voltmeter.

Measures PD across component.
Connected in PARALLEL with component.
Ideal: INFINITE resistance (so doesn't divert current).

Cambridge tip. Mark scheme often asks 'why does voltmeter need high resistance?' — answer: to draw minimal current and not disturb the PD being measured.

Ammeter: series, low R.
Voltmeter: parallel, high R.
Properties opposite.

See the full worked example for practical circuits →

How it’s examined

Practical circuits common on Paper 2. Most-tested: internal resistance (7-8 marks), meter placement (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

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Step-by-step worked examples — Practical circuits

Step-by-step solutions to past-paper-style questions on practical circuits, written exactly the way a tutor would explain them at the board.

1Internal resistance (7 marks)
Extended• Adapted from 9702/22 May/Jun 2024• internal resistance
Question
A cell EMF 6.0 V drives 0.50 A through a 10 Ω external resistor. Find internal resistance $r$ . (7 marks)
Step-by-step solution
1. Step 1
  $E = I(R + r)$ (EMF equals total PD around circuit).
2. Step 2
  Substitute.
  $6.0 = 0.50(10 + r)$
3. Step 3
  Solve.
  $12 = 10 + r \Rightarrow r = 2.0 \text{ }\Omega$
Answer
$r = 2.0$ Ω.
2Terminal PD (5 marks)
Extended• terminal PD
Question
For the same cell, find terminal PD when current is 0.50 A. (5 marks)
Step-by-step solution
1. Step 1
  Terminal PD = EMF − $Ir$ .
  $V_t = 6.0 - 0.50 \times 2.0 = 5.0 \text{ V}$
Answer
$V_t = 5.0$ V.
3Meter placement (5 marks)
Extended• meters
Question
Describe ideal placement and properties of (a) ammeter, (b) voltmeter. (5 marks)
Step-by-step solution
1. Step 1
  Ammeter in SERIES. Must have very LOW resistance to avoid disturbing circuit. Ideal: zero resistance.
2. Step 2
  Voltmeter in PARALLEL. Must have very HIGH resistance to draw minimal current. Ideal: infinite resistance.
Answer
Ammeter: series, low R. Voltmeter: parallel, high R.

Key Formulae — Practical circuits

The formulae you need to memorise for practical circuits on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

EMF with internal resistance
$E = I(R + r) = V_t + Ir$
$E$
EMF
$r$
internal resistance
$V_t$
terminal PD
When to use
Real cell with internal resistance. EMF splits across internal $r$ and external $R$ .

Key Definitions and Keywords — Practical circuits

Definitions to memorise and the exact keywords mark schemes credit for practical circuits answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Internal resistance
Examiner keyword
Resistance within an EMF source itself. Causes terminal PD to be less than EMF when current flows.
Terminal PD
Examiner keyword
PD across the external terminals of an EMF source. $V_t = E - Ir$ .

Common Mistakes and Misconceptions — Practical circuits

The traps other students keep falling into on practical circuits questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Saying ideal ammeter has high resistance
9702 Examiner Reports 2022-2024
Why it happens
Confusing with voltmeter.
How to avoid it
Ammeter SERIES, LOW R (to not block current). Voltmeter PARALLEL, HIGH R (to not draw current).

Practice questions

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Practical circuits — frequently asked questions

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Practical circuits

Short Study Notes in the form of Slides

Detailed Notes

What you’ll learn

How it’s examined

1Internal resistance (7 marks)

2Terminal PD (5 marks)

3Meter placement (5 marks)

EMF with internal resistance

Internal resistance

Terminal PD

✕Saying ideal ammeter has high resistance

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Practical circuits — frequently asked questions