What is electric current and how is it measured?

Electric current is the flow of electric charge through a conductor, typically measured in amperes (A). In the context of Cambridge International A Levels Physics, it is important to understand that current is the rate at which charge flows past a point in a circuit. One ampere is defined as one coulomb of charge passing through a point in one second.

How does electric current relate to voltage and resistance?

Electric current, voltage, and resistance are related by Ohm's Law, which is a fundamental concept in Cambridge International A Levels Physics. Ohm's Law states that the current (I) through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) of the conductor. This relationship is expressed by the formula I = V/R.

What are the differences between direct current (DC) and alternating current (AC)?

Direct current (DC) is the flow of electric charge in one constant direction, while alternating current (AC) changes direction periodically. In Cambridge International A Levels Physics, students learn that DC is typically used in batteries and electronic devices, whereas AC is used for power distribution in homes and industries due to its ability to be easily transformed to different voltages.

Why is understanding electric current important in the study of electricity?

Understanding electric current is crucial because it is a fundamental aspect of electricity that affects how electrical circuits function. In Cambridge International A Levels Physics, students explore how current interacts with other electrical quantities like voltage and resistance, which is essential for analyzing and designing electrical systems and devices.

What role do electrons play in electric current?

Electrons are the primary charge carriers in most conductors, and their movement constitutes electric current. In the context of Cambridge International A Levels Physics, it is important to recognize that when a voltage is applied across a conductor, it creates an electric field that causes electrons to drift from the negative to the positive terminal, resulting in a flow of current.

Why is "Saying conventional current is direction of electron flow" a common mistake in Electric current?

Why it happens: Historical convention. How to avoid it: Conventional current = direction POSITIVE charge moves = OPPOSITE to electron motion. Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Electric current

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Short Notes - Electric current

Detailed Study Notes

Detailed notes on Electricity for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Electric Current Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Charge, current, drift velocity. The microscopic picture behind electric circuits.

At a glance

Current $I = Q/t$ . Units A.
Charge quantised: $e = 1.6 \times 10^{-19}$ C.
Conventional current opposite to electron flow.
$I = nAve$ microscopic.
Drift velocity is SLOW (~mm/s).

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

9.1.1 — Define current and charge.
9.1.2 — Use $I = nAve$ .
9.1.3 — Distinguish conventional and electron flow.

Current and charge

$I = Q/t$ .

Current. Rate of flow of charge. $I = Q/t$ .

Units. A = C/s.

Charge. Quantised: $e = 1.6 \times 10^{-19}$ C. Total charge $= ne$ for $n$ electrons.

Direction. Conventional current flows from + to − externally. Electron flow opposite.

Cambridge tip. State direction convention.

Figure 1 — Current is the rate of flow of charge

Current is measured at a point, not along a length: count the charge passing one cross-section each second.

Figure 2 — Conventional current and electron flow

The convention was fixed before the electron was discovered. All circuit rules use conventional current, so state which one you mean when a question asks about direction.

$I = Q/t$ .
Quantised in $e$ .
Conventional vs electron flow.

See the full worked example for electric current →

Drift velocity

$I = nAve$ .

Microscopic model. Current = number of charge carriers per unit volume × cross-sectional area × drift velocity × charge per carrier. $I = nAve$

Drift velocity. Average net velocity of carriers along wire. Usually mm/s — very slow compared to RANDOM thermal motion (~ $10^5$ m/s).

Signal propagation at near $c$ doesn't require fast individual electrons — it's a chain-reaction effect.

Example. Copper $n = 8.5 \times 10^{28}$ /m³, 3 A in $10^{-6}$ m²: $v \approx 2.2 \times 10^{-4}$ m/s.

Cambridge tip. Drift velocity is much less than thermal velocity — KEY exam misconception.

Figure 3 — Microscopic model of current: I = nAvq

The current equals the number density n × cross-section A × drift velocity v × charge per carrier q. For electrons in a metal q = e.

Figure 4 — Random thermal motion compared with drift velocity

The arrows in both boxes are the same length because the random speed barely changes. What a current adds is a tiny, steady drift on top of it.

Figure 5 — Why a lamp lights instantly despite a slow drift velocity

The signal travels at close to the speed of light because it is the field that propagates, not the electrons — a favourite exam misconception.

$I = nAve$ .
Drift ~mm/s.
Signal speed unrelated.

See the full worked example for electric current →

Quick recap

$I = Q/t$ and $I = nAve$ .
Conventional vs electron flow.
Drift velocity slow; signal fast.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

$I = Q/t$ .
$I = nAve$ .
Conventional current opposite to electrons.

How it’s examined

Electric current on every Paper 1. Most-tested: $Q = It$ (4-5 marks), $I = nAve$ (6 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.

Take this whole topic with you

Step-by-step worked examples — Electric current

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

1Charge from current (4 marks)
Extended• Adapted from 9702/22 May/Jun 2024• charge
Question
A current of 2.5 A flows for 30 s. Find the charge transferred. (4 marks)
Step-by-step solution
1. Step 1
  $Q = It$ .
  $Q = 2.5 \times 30 = 75 \text{ C}$
Answer
$Q = 75$ C.
2Number of electrons (5 marks)
Extended• electrons
Question
How many electrons pass a point per second when current is 1.6 A? $e = 1.6 \times 10^{-19}$ C. (5 marks)
Step-by-step solution
1. Step 1
  Charge per second = 1.6 C.
2. Step 2
  Number = total charge / $e$ .
  $n = \dfrac{1.6}{1.6 \times 10^{-19}} = 1.0 \times 10^{19}$
Answer
$1.0 \times 10^{19}$ electrons per second.
3Drift velocity (6 marks)
Extended• drift velocity
Question
A copper wire ( $n = 8.5 \times 10^{28}$ electrons/m³) carries 3.0 A with cross-section $1.0 \times 10^{-6}$ m². Find drift velocity. (6 marks)
Step-by-step solution
1. Step 1
  $I = nAve$ . Rearrange.
  $v = \dfrac{I}{nAe} = \dfrac{3.0}{(8.5 \times 10^{28})(1.0 \times 10^{-6})(1.6 \times 10^{-19})}$
2. Step 2
  Evaluate.
  $v \approx 2.2 \times 10^{-4} \text{ m/s}$
Answer
$v \approx 2.2 \times 10^{-4}$ m/s (very slow!).

Key Formulae — Electric current

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

Current
$I = \dfrac{\Delta Q}{\Delta t}$
$I$
current (A)
$Q$
charge (C)
When to use
Rate of flow of charge. Units: ampere (A) = C/s.
Drift velocity
$I = nAve$
$n$
number density of charge carriers
$A$
cross-sectional area
$v$
drift velocity
$e$
charge per carrier
When to use
Current in terms of charge-carrier flow.

Key Definitions and Keywords — Electric current

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

Electric current
Examiner keyword
Rate of flow of electric charge. $I = Q/t$ . Units: A.
Electric charge
Examiner keyword
Property of matter that causes electric force. Quantised in units of $e$ . Units: C.
Conventional current
Examiner keyword
Direction in which POSITIVE charge would flow. Opposite to electron flow in a wire.
Drift velocity
Examiner keyword
Average velocity of charge carriers due to applied field. Slow (mm/s) despite signal propagation at near-light speed.

Common Mistakes and Misconceptions — Electric current

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

✕Saying conventional current is direction of electron flow
9702 Examiner Reports 2022-2024
Why it happens
Historical convention.
How to avoid it
Conventional current = direction POSITIVE charge moves = OPPOSITE to electron motion.

Electric current — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Cambridge International A Levels Physics (9702)

Electric current

0% Complete

3 tasks remaining

Short Notes - Electric current

Detailed Study Notes

Detailed notes on Electricity for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Electric Current Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Charge, current, drift velocity. The microscopic picture behind electric circuits.

At a glance

Current $I = Q/t$ . Units A.
Charge quantised: $e = 1.6 \times 10^{-19}$ C.
Conventional current opposite to electron flow.
$I = nAve$ microscopic.
Drift velocity is SLOW (~mm/s).

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

9.1.1 — Define current and charge.
9.1.2 — Use $I = nAve$ .
9.1.3 — Distinguish conventional and electron flow.

Current and charge

$I = Q/t$ .

Current. Rate of flow of charge. $I = Q/t$ .

Units. A = C/s.

Charge. Quantised: $e = 1.6 \times 10^{-19}$ C. Total charge $= ne$ for $n$ electrons.

Direction. Conventional current flows from + to − externally. Electron flow opposite.

Cambridge tip. State direction convention.

Figure 1 — Current is the rate of flow of charge

Current is measured at a point, not along a length: count the charge passing one cross-section each second.

Figure 2 — Conventional current and electron flow

The convention was fixed before the electron was discovered. All circuit rules use conventional current, so state which one you mean when a question asks about direction.

$I = Q/t$ .
Quantised in $e$ .
Conventional vs electron flow.

See the full worked example for electric current →

Drift velocity

$I = nAve$ .

Microscopic model. Current = number of charge carriers per unit volume × cross-sectional area × drift velocity × charge per carrier. $I = nAve$

Drift velocity. Average net velocity of carriers along wire. Usually mm/s — very slow compared to RANDOM thermal motion (~ $10^5$ m/s).

Signal propagation at near $c$ doesn't require fast individual electrons — it's a chain-reaction effect.

Example. Copper $n = 8.5 \times 10^{28}$ /m³, 3 A in $10^{-6}$ m²: $v \approx 2.2 \times 10^{-4}$ m/s.

Cambridge tip. Drift velocity is much less than thermal velocity — KEY exam misconception.

Figure 3 — Microscopic model of current: I = nAvq

The current equals the number density n × cross-section A × drift velocity v × charge per carrier q. For electrons in a metal q = e.

Figure 4 — Random thermal motion compared with drift velocity

The arrows in both boxes are the same length because the random speed barely changes. What a current adds is a tiny, steady drift on top of it.

Figure 5 — Why a lamp lights instantly despite a slow drift velocity

The signal travels at close to the speed of light because it is the field that propagates, not the electrons — a favourite exam misconception.

$I = nAve$ .
Drift ~mm/s.
Signal speed unrelated.

See the full worked example for electric current →

Quick recap

$I = Q/t$ and $I = nAve$ .
Conventional vs electron flow.
Drift velocity slow; signal fast.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

$I = Q/t$ .
$I = nAve$ .
Conventional current opposite to electrons.

How it’s examined

Electric current on every Paper 1. Most-tested: $Q = It$ (4-5 marks), $I = nAve$ (6 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.

Take this whole topic with you

Step-by-step worked examples — Electric current

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

1Charge from current (4 marks)
Extended• Adapted from 9702/22 May/Jun 2024• charge
Question
A current of 2.5 A flows for 30 s. Find the charge transferred. (4 marks)
Step-by-step solution
1. Step 1
  $Q = It$ .
  $Q = 2.5 \times 30 = 75 \text{ C}$
Answer
$Q = 75$ C.
2Number of electrons (5 marks)
Extended• electrons
Question
How many electrons pass a point per second when current is 1.6 A? $e = 1.6 \times 10^{-19}$ C. (5 marks)
Step-by-step solution
1. Step 1
  Charge per second = 1.6 C.
2. Step 2
  Number = total charge / $e$ .
  $n = \dfrac{1.6}{1.6 \times 10^{-19}} = 1.0 \times 10^{19}$
Answer
$1.0 \times 10^{19}$ electrons per second.
3Drift velocity (6 marks)
Extended• drift velocity
Question
A copper wire ( $n = 8.5 \times 10^{28}$ electrons/m³) carries 3.0 A with cross-section $1.0 \times 10^{-6}$ m². Find drift velocity. (6 marks)
Step-by-step solution
1. Step 1
  $I = nAve$ . Rearrange.
  $v = \dfrac{I}{nAe} = \dfrac{3.0}{(8.5 \times 10^{28})(1.0 \times 10^{-6})(1.6 \times 10^{-19})}$
2. Step 2
  Evaluate.
  $v \approx 2.2 \times 10^{-4} \text{ m/s}$
Answer
$v \approx 2.2 \times 10^{-4}$ m/s (very slow!).

Key Formulae — Electric current

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

Current
$I = \dfrac{\Delta Q}{\Delta t}$
$I$
current (A)
$Q$
charge (C)
When to use
Rate of flow of charge. Units: ampere (A) = C/s.
Drift velocity
$I = nAve$
$n$
number density of charge carriers
$A$
cross-sectional area
$v$
drift velocity
$e$
charge per carrier
When to use
Current in terms of charge-carrier flow.

Key Definitions and Keywords — Electric current

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

Electric current
Examiner keyword
Rate of flow of electric charge. $I = Q/t$ . Units: A.
Electric charge
Examiner keyword
Property of matter that causes electric force. Quantised in units of $e$ . Units: C.
Conventional current
Examiner keyword
Direction in which POSITIVE charge would flow. Opposite to electron flow in a wire.
Drift velocity
Examiner keyword
Average velocity of charge carriers due to applied field. Slow (mm/s) despite signal propagation at near-light speed.

Common Mistakes and Misconceptions — Electric current

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

✕Saying conventional current is direction of electron flow
9702 Examiner Reports 2022-2024
Why it happens
Historical convention.
How to avoid it
Conventional current = direction POSITIVE charge moves = OPPOSITE to electron motion.

Electric current — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Electric current

Short Notes - Electric current

Detailed Study Notes

At a glance

What you’ll learn

Current and charge

Drift velocity

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Charge from current (4 marks)

2Number of electrons (5 marks)

3Drift velocity (6 marks)

Current

Drift velocity

Electric current

Electric charge

Conventional current

Drift velocity

✕Saying conventional current is direction of electron flow

Electric current — frequently asked questions

Electric current

Short Notes - Electric current

Detailed Study Notes

At a glance

What you’ll learn

Current and charge

Drift velocity

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Charge from current (4 marks)

2Number of electrons (5 marks)

3Drift velocity (6 marks)

Current

Drift velocity

Electric current

Electric charge

Conventional current

Drift velocity

✕Saying conventional current is direction of electron flow

Electric current — frequently asked questions