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4.2 Current and Charge

GCSE Physics · AQA · Chapter 4: Electric Circuits

A free GCSE Physics lesson for AQA (8463) from OBA School: circuit symbols, what an electric current is, and charge flow Q = I t. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Recognise and draw the standard circuit symbols and state what each component does
  • Explain that a current is a rate of flow of charge, carried by electrons in metals
  • State that the current is the same at every point in a single closed loop
  • Use Q = I t, including rearranging it and converting minutes to seconds and mA to A

Key words

  • Circuit diagram: A drawing that uses agreed symbols to show how components are connected.
  • Cell / battery: A cell pushes charge round a circuit. A battery is two or more cells joined together.
  • Electric current: The rate of flow of electric charge. Unit: ampere (A).
  • Charge: A property of particles such as electrons. Unit: coulomb (C).
  • Ammeter: A meter connected in series to measure current.
  • Diode: A component that lets current pass in one direction only.

Circuit symbols

Circuit diagrams use standard symbols so that anyone can build the circuit. Draw wires as straight lines with a ruler and leave no gaps.

  • switch; What it does opens or closes the circuit (turns the current off or on)
  • fixed / variable resistor; What it does limits the current; a variable resistor lets you change the current
  • lamp / LED; What it does gives out light when there is a current; an LED only works one way round
  • diode; What it does lets current through in one direction only
  • fuse; What it does melts and breaks the circuit if the current is too big
  • heater; What it does transfers energy from the current to heat the surroundings
  • LDR / thermistor; What it does resistance changes with light level / temperature (see 4.4)
  • ammeter / voltmeter; What it does measures current (in series) / potential difference (in parallel)

What is an electric current?

An electric current is a flow of electric charge. In metal wires the charge is carried by free electrons that can drift between the positive metal ions. A cell pushes these electrons round the circuit, but only if there is a complete closed loop.

The size of the current is the rate of flow of charge – how much charge passes a point each second. The more electrons that pass each second, the bigger the current.

Key fact: In a single closed loop the current is the same at every point. Charge is not used up by components: the same number of electrons passes every point each second.

Charge flow: Q = I t

Charge is measured in coulombs (C) and current in amperes (A). A current of 1 A means 1 C of charge passes each second.

charge flow = current × time Q = I t

Q = charge flow in coulombs (C) I = current in amperes (A) t = time in seconds (s)

Rearranged: I = Q ÷ t t = Q ÷ I

Unit checks: 1 mA = 0.001 A; 1 minute = 60 s; 1 hour = 3600 s

Worked example: finding a current

54 C of charge passes through a kettle element in 1.5 minutes.
t = 1.5 × 60 = 90 s
I = Q ÷ t = 54 ÷ 90

Answer: I = 0.60 A

Worked example: finding a charge with milliamps

A digital clock draws a current of 15 mA for 20 minutes.
I = 15 ÷ 1000 = 0.015 A; t = 20 × 60 = 1200 s
Q = I t = 0.015 × 1200

Answer: Q = 18 C

Worked example: finding a time

How long does it take 960 C to pass through a lamp carrying 0.40 A?
t = Q ÷ I = 960 ÷ 0.40

Answer: t = 2400 s (= 40 minutes)

Exam tip: Always change minutes or hours into seconds and mA into A before you substitute.

Controlling the current

  • A variable resistor in series with a lamp changes the current: more resistance gives a smaller current and a dimmer lamp.
  • A diode in series protects equipment: if the batteries are put in the wrong way round, the diode blocks the current, so no damage is done.
  • A fuse melts if the current becomes dangerously large, breaking the circuit.

Common mistakes

  • Thinking current is “used up” by a lamp, so it is smaller after the lamp. In a single loop the current is the same everywhere.
  • Using minutes (or hours) in Q = I t instead of seconds.
  • Forgetting to change mA to A: 250 mA = 0.25 A, not 250 A.
  • Drawing gaps in the wires or a cell with both lines the same length.
  • Mixing up the ammeter (in series, measures current) and the voltmeter (in parallel, measures p.d.).

Revision questions

Try these, then check your answers below each one.

Question 1. A charge of 180 C flows through a lamp in 2.0 minutes. Calculate the current in the lamp.

Answer: 1.5 A

Working: t = 2.0 × 60 = 120 s. I = Q ÷ t = 180 ÷ 120 = 1.5 A.

Question 2. A torch bulb carries a current of 0.30 A for 15 minutes. Calculate the charge that flows through it.

Answer: 270 C

Working: t = 15 × 60 = 900 s. Q = I t = 0.30 × 900 = 270 C.

Question 3. Write a current of 45 mA in amperes.

Answer: 0.045 A

Working: 45 ÷ 1000 = 0.045 A.

Related lessons

Continue in the full course

This lesson is section 4.2 (AGCPH0402) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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