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Physics · Electricity

Circuit symbols, charge and current

Draw and interpret circuit diagrams with standard symbols, and use \(Q = It\) to link charge, current and time.

  • 6 key terms
  • All boards
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Warm-up

Answer each one, then check.

  1. 1

    What is the unit of current?

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    The ampere (A)

  2. 2

    What is the unit of time in physics calculations?

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    The second (s)

  3. 3

    Which particles carry current in a metal wire?

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    Electrons

  4. 4

    What does a battery provide?

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    A potential difference (a push for charge)

  5. 5

    Convert 2 minutes to seconds.

    Show answerHide answer

    120 s

Learning Objectives

  1. 1Draw and interpret circuit diagrams using standard symbols.
  2. 2State that a source of potential difference is needed to make charge flow in a closed circuit.
  3. 3Recall and apply \(Q = It\).
  4. 4State that the current has the same value at any point in a single closed loop.

CHARGE AND CURRENT

charge flow \(=\) current \(\times\) time \(Q = It\)

Electric current is a flow of electrical charge. The size of the current is the rate of flow of charge. Charge is in coulombs (C), current in amperes (A) and time in seconds (s).

Drawing a Circuit Diagram

Use a ruler and standard symbols.

  1. 1 Start with the power supply

    Draw a cell or battery.
  2. 2 Add components in series

    Draw straight lines with square corners; no gaps.
  3. 3 Add the ammeter in series

    It goes in the main loop where you want to measure current.
  4. 4 Add the voltmeter in parallel

    It goes across the component you are measuring.
  5. 5 Check

    Every component is joined by lines and the loop is closed.

Charge Flow

A current of 3.0 A flows through a lamp for 20 s. Calculate the charge that flows.

Show the solutionHide the solution
  1. 1 Write the equation \(Q = It\)
  2. 2 Substitute \(Q = 3.0 \times 20\)
  3. 3 Answer \(Q = 60\) C

Answer60 coulombs

Finding the Current

240 C of charge flows through a heater in 2 minutes. Calculate the current.

Show the solutionHide the solution
  1. 1 Convert the time \(2\) min \(= 120\) s
  2. 2 Rearrange \(I = Q \div t\)
  3. 3 Substitute \(I = 240 \div 120\)
  4. 4 Answer \(I = 2.0\) A

Answer2.0 A

Key Ideas

Learn these.

  • A source of pd is needed

    For charge to flow round a closed circuit there must be a source of potential difference, such as a cell.

  • Current in a series loop

    The current has the same value at every point in a single closed loop.

  • Current is not used up

    Charge keeps flowing round the loop; the components transfer energy from the charge.

  • Ammeter and voltmeter

    Ammeter in series measures current. Voltmeter in parallel measures pd.

Circuit Doodles

Draw circuit diagrams for: (a) a cell, a switch and a lamp in series; (b) the same circuit with an ammeter to measure the current in the lamp and a voltmeter to measure the pd across it.

1. Draw the loop with a ruler.

2. Add the meters in the right way.

A good answer shows: (a) A loop containing a cell, closed switch and lamp. (b) The ammeter is in the loop (series); the voltmeter is drawn in parallel with the lamp only.

Can I...?

  1. 1Recognise standard circuit symbols.
  2. 2Draw a series circuit.
  3. 3Place an ammeter and voltmeter correctly.
  4. 4Use \(Q = It\).
  5. 5Rearrange for current or time.
  6. 6Convert minutes to seconds.
  7. 7Explain why current is the same round a series loop.
  8. 8State what is needed for charge to flow.

Summary & Exam Focus

  • \(Q = It\).
  • Symbols: cell, lamp, ammeter (A), voltmeter (V), and so on.
  • Ammeters in series, voltmeters in parallel.
  • Current is the same at every point in a series loop.

Exam focus

A current of 0.40 A flows for 30 s. Calculate the charge that flows. (2 marks) (2 marks)

Write the equation, substitute, and include the unit C.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Electric current
The rate of flow of electrical charge.
Charge
A property of matter that can be positive or negative; measured in coulombs.
Ampere
The unit of current; one coulomb per second.
Coulomb
The unit of charge.
Ammeter
Measures current; connected in series.
Potential difference
The energy transferred per unit of charge; measured in volts.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Calculate 2 marks

    A current of 0.40 A flows through a lamp for 30 s. Calculate the charge that flows through the lamp. Use the equation: charge flow = current × time

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    Model answer

    \(Q = 0.40 \times 30 = 12\) C

    Mark scheme

    • Correct substitution — 1 mark
    • 12 C — 1 mark
  2. Question 2 Draw 4 marks

    Draw a circuit diagram to show a cell, a switch and a lamp connected in series, with an ammeter to measure the current through the lamp and a voltmeter to measure the potential difference across the lamp. Use the correct circuit symbols.

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    Model answer

    A single closed loop containing a cell, a switch, a lamp and an ammeter, with a voltmeter connected in parallel across the lamp only.

    Mark scheme

    • Cell, switch and lamp in a closed series loop — 1 mark
    • Ammeter in series — 1 mark
    • Voltmeter in parallel with the lamp — 1 mark
    • All symbols correct — 1 mark
  3. Question 3 Calculate 3 marks

    180 C of charge flows through a heater in 1.0 minute. Calculate the current in the heater.

    Show answerHide answer

    Model answer

    \(t = 60\) s; \(I = Q \div t = 180 \div 60 = 3.0\) A

    Mark scheme

    • Converts 1.0 minute to 60 s — 1 mark
    • Correct substitution — 1 mark
    • 3.0 A — 1 mark
  4. Question 4 Explain 2 marks

    In a series circuit, ammeter A1 reads 0.50 A. A second ammeter, A2, is on the other side of a lamp. What does A2 read? Explain your answer.

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    Model answer

    0.50 A. The current has the same value at every point in a single closed loop; the lamp does not use up the current.

    Mark scheme

    • 0.50 A — 1 mark
    • Current is the same at every point in a series loop — 1 mark
  5. Question 5 Calculate 3 marks

    A charge of 12 C flows through a resistor when the current is 0.50 A. Calculate the time for which the current flows.

    Show answerHide answer

    Model answer

    \(t = Q \div I = 12 \div 0.50 = 24\) s

    Mark scheme

    • Rearranges to t = Q ÷ I — 1 mark
    • Correct substitution — 1 mark
    • 24 s — 1 mark

Quick check

  1. An ammeter must be connected...

    1. Ain parallel
    2. Bin series
    3. Cacross a cell only
    4. Dwith no wires
    Show answerHide answer

    B: in series

    An ammeter measures the current so it goes in the loop.

  2. A voltmeter is connected...

    1. Ain series with the component
    2. Bacross the switch only
    3. Cin parallel with the component
    4. Dafter the cell
    Show answerHide answer

    C: in parallel with the component

    It measures the pd across a component.

  3. Charge flow in coulombs is calculated by...

    1. Acurrent ÷ time
    2. Bcurrent + time
    3. Cvoltage × time
    4. Dcurrent × time
    Show answerHide answer

    D: current × time

    \(Q = It\).

  4. A current of 2 A flows for 10 s. The charge is...

    1. A20 C
    2. B5 C
    3. C12 C
    4. D0.2 C
    Show answerHide answer

    A: 20 C

    \(2 \times 10 = 20\) C.

  5. In a series circuit, the current is...

    1. Abiggest at the cell
    2. Bthe same everywhere in the loop
    3. Csmallest at the lamp
    4. Dzero at the switch
    Show answerHide answer

    B: the same everywhere in the loop

    The same charge flows round the whole loop.

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