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Electric Current

Key idea: Learn what electric current is, how to use I = Q/t and Q = It, and how to distinguish conventional current from electron flow (O Level).

  • SEC G3 Physics 2027
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Learning objectives

  • State current as rate of charge flow measured in amperes
  • Distinguish conventional current from electron flow
  • Apply charge equals current multiplied by time
  • Define source e.m.f. as work done per unit charge around a circuit
  • Calculate total e.m.f. for sources in series
  • Define component potential difference as work done per unit charge
  • State resistance as potential difference divided by current
  • Apply resistance equals potential difference divided by current
  • Apply wire-resistance proportionalities for length and cross-sectional area
  • Describe the effect of temperature on metallic resistance
  • Sketch and interpret required current–voltage characteristics

1. Definition

Electric current, I (A), is the rate of flow of electric charge, Q (C), past a point:

I = Q/t so 1 A = 1 C s⁻¹

What you need for this course

You should be able to:

  • state that current is the rate of flow of charge (A)
  • distinguish between conventional current and electron flow
  • use Q = It

2. Key Ideas

  • Current I is measured in ampere (A).
  • Charge Q is measured in coulomb (C).
  • Time t is measured in second (s).
  • Relationship: Q = It.
  • Conventional current direction is from + to −.
  • In metals, electrons flow from − to + (opposite to conventional current).
  • An ammeter measures current and must be connected in series.

3. Detailed Explanations

A. Electric current: overview

Electric current is the movement of electric charge through a conductor (e.g. electrons through a metal wire). In static electricity, charges stay on surfaces and do not flow continuously around a circuit.

B. Measuring current (ammeter)

To measure current, connect the ammeter in series so the same current flows through the ammeter and the component.

Ammeter connection in seriesSimple circuit with cell, resistor and ammeter in series. Includes warning against connecting ammeter in parallel.AResistorCellResistor and ammeter share one pathWrong: ammeter in parallelA
Connect the ammeter in series to measure current in the circuit.
Safety

Never connect an ammeter directly across a cell/power supply (that would act like a short circuit).

C. Conventional current vs electron flow

Charge-flow directions in a simple circuitA closed circuit containing a cell and lamp. Conventional current arrows run clockwise from the positive terminal; electron-flow arrows run anticlockwise. The source electromotive force and component potential difference are labelled as quantities measured in volts.+−lampconventional currentelectron drift in metalsource: e.m.f.measured in voltscomponent: p.d.measured in voltsCharge circulates; it is not used up.In a single series loop, current has the same value at every point.
Conventional current follows the direction positive charge would move; electrons in the metal drift the opposite way. Charge circulates around the closed loop and is not used up.

In a metal wire, the charge carriers are electrons (negative). So electron flow is:

  • from negative terminal → positive terminal

However, by convention, we define conventional current as the direction positive charge would flow:

  • from positive terminal → negative terminal

Current is not “used up” by a lamp. In a single series loop, the rate of charge flow is the same before and after the lamp; the lamp transfers energy, not charge, to the surroundings.

Link

Direct current (d.c.) vs alternating current (a.c.) is covered here: Direct Current & Alternating Current.

4. Common Mistakes

  • Connecting the ammeter in parallel (it must be in series).
  • Saying conventional current flows from negative → positive (that is electron flow in metals).
  • Saying a component “uses up current”; charge continues around the circuit while the component transfers energy.
  • Using Q = It but forgetting to convert time to seconds (e.g. minutes → seconds).

5. Exam Tips

  1. Write the relationship first: Q = It.
  2. Substitute with units (C, A, s), then rearrange if needed.
  3. If asked for direction, state: “conventional current is from + to −”.

6. Worked Examples

Modelled example 1

Charge carriers in a metal

Core

Problem

Identify the mobile charge carriers in a metal wire and compare their flow direction with conventional current.
Study the worked solution

  1. Identify the carriers

    Method

    Select electrons as the mobile charges in a metal.

    Reason

    Metal ions remain in the lattice while delocalised electrons drift.

    Working

    Electron flow: negative terminal → positive terminal.

  2. Apply the direction convention

    Method

    Draw conventional current opposite to electron flow.

    Reason

    Conventional current is defined as the direction positive charge would move.

    Working

    Conventional current: positive terminal → negative terminal.

Guided practice 2

Finding current

About 4 min

Problem

A charge of 60 C passes a point in a circuit in 20 s. Find the current.

Use charge flow per second

Unit: A

Hints

Hint 1: start from the definition
Current is charge divided by time.
Hint 2: substitute SI values
Both coulombs and seconds are already in the required units.
View solution step by step

  1. Calculate current

    Method

    Divide the charge passing the point by the elapsed time.

    Reason

    Current is the rate of charge flow.

    Working

    I = Q/t = 60/20 = 3.0 A

Common misconception 3

Finding charge (time in minutes)

Find and correct the mistake

Learner response

A current of 0.50 A flows for 3 minutes. A student substitutes t = 3 into Q = It and obtains 1.5 C. Diagnose the error.

Make the time unit consistent with amperes

Unit: C

View solution step by step

  1. Convert the time

    Method

    Express 3 minutes as 180 s.

    Reason

    One ampere is one coulomb per second.

    Working

    t = 3(60) = 180 s

  2. Calculate charge

    Method

    Multiply current by the consistent time.

    Reason

    Q = It accumulates the charge passing over the full interval.

    Working

    Q = (0.50)(180) = 90 C

Examiner practice 4

Finding time

3 marks

Examination question

How long does it take for 24 C of charge to pass a point if the current is 4.0 A? [3 marks]

Show relationship, rearrangement and result

View solution step by step

  1. Calculate the time

    3 marks

    Method

    Start from Q = It, rearrange and substitute.

    Reason

    Time equals total charge divided by charge passing each second.

    Working

    t = Q/I = 24/4.0 = 6.0 s

Challenge 5

Time in minutes

Minimal support

Output-unit transfer

A current of 0.30 A flows. How long, in minutes, does it take for 90 C to pass a point?

Calculate in SI, then report the requested unit

Hints

Hint 1: find seconds first
Using coulombs and amperes gives time in seconds.
Hint 2: convert the result
Divide the seconds by 60.
View solution step by step

  1. Calculate time in seconds

    Method

    Divide charge by current.

    Reason

    Amperes are coulombs per second.

    Working

    t = 90/0.30 = 300 s

  2. Convert to the requested unit

    Method

    Divide by 60 s min⁻¹.

    Reason

    The question requests minutes, not the SI calculation unit.

    Working

    t = 300/60 = 5.0 min

7. Mind Stretchers

Mind stretcher 1: Comparing currentsExtension

Circuit A has 2.0 C of charge passing a point every second. Circuit B has 120 C passing a point in 1 minute. Which circuit has the larger current?

Show Answer

Circuit A: I = 2.0/1 = 2.0 A.

Circuit B: t = 60 s, so I = 120/60 = 2.0 A.

They are the same.

Mind stretcher 2: Ammeter connectionExtension

Why must an ammeter be connected in series to measure the current in a component?

Show Answer

In a series connection, the same current that flows through the component also flows through the ammeter, so the ammeter measures that current. If connected in parallel, it would not measure the component’s current correctly and could act like a short circuit.

8. Practice and next step

Complete the current-and-charge questions in the Current Electricity Quiz, then use Structured Current Electricity for multi-step Q = It problems. Continue to E.m.f. and potential difference to connect each coulomb of moving charge to energy transfer.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027