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P4.1 Electric Charge and Current

GCSE Combined Science · AQA · Chapter P4: Electric Circuits

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: charges and the forces between them, circuit symbols, electric current 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

  • Recall that electrons carry negative charge and that like charges repel and unlike charges attract
  • 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 free 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

  • Charge: A property of particles such as electrons (negative) and protons (positive). Unit: coulomb (C).
  • 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).
  • Ammeter: A meter connected in series to measure current.
  • Diode: A component that lets current pass in one direction only.

Charges and the forces between them

Atoms contain positive protons in the nucleus and negative electrons around it. An uncharged object has equal amounts of positive and negative charge. Only electrons can move from place to place: an object that gains electrons becomes negative, and one that loses electrons becomes positive.

In a circuit, the cell provides the push: free electrons in the wire are repelled by the negative terminal and attracted towards the positive terminal, so they drift round the circuit.

Exam tip: Positive objects have lost electrons. They never “gain protons” – protons stay locked in the nucleus.

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 P4.3)
  • 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. Charge only flows if there is a complete closed loop and a source of potential difference, such as a cell.

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

84 C of charge passes through a toaster element in 2.0 minutes.
t = 2.0 × 60 = 120 s
I = Q ÷ t = 84 ÷ 120

Answer: I = 0.70 A

Worked example: finding a charge with milliamps

A smoke alarm draws a current of 12 mA for 25 minutes.
I = 12 ÷ 1000 = 0.012 A; t = 25 × 60 = 1500 s
Q = I t = 0.012 × 1500

Answer: Q = 18 C

Worked example: finding a time

How long does it take 720 C to pass through a lamp carrying 0.30 A?
t = Q ÷ I = 720 ÷ 0.30

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

  • Adding more cells (a bigger potential difference) makes the current bigger; adding more resistance in series makes it smaller.
  • 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.
  • 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.
  • Saying an object becomes positive by gaining protons. It has lost electrons.
  • 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. The diagram shows four circuit symbols, labelled A to D. Which symbol is a thermistor? (Choose from: A, B, C, D.)

Answer: C

Working: C is the thermistor (rectangle crossed by a line with a bent end). A is an LDR (arrows pointing in, circle), B is a diode and D is a variable resistor (arrow through the rectangle).

Question 2. True or false? A balloon becomes positively charged when it gains protons from a jumper.

Answer: False

Working: False. Protons stay in the nucleus. The balloon becomes positive only by losing electrons.

Question 3. A charge of 96 C flows through a toaster in 2.0 minutes. Calculate the current in the toaster.

Answer: 0.8 A

Working: t = 2.0 × 60 = 120 s. I = Q ÷ t = 96 ÷ 120 = 0.80 A.

Related lessons

Continue in the full course

This lesson is section P4.1 (AGCSC3301) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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