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3.2.2Electricity

Potential Difference — SQA National 5 Physics Revision Notes & Topical Questions

5 min read 5 parts Lesson 4 of 16

📚 SQA National 5 Physics Hub / Topical Revision
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Part 1 of 5What Is Potential Difference?

3.2.2 Potential Difference

A cell does work on charges, and circuit components then transform that energy.

Energy per coulomb
  • The potential difference between two points is the energy transferred to or from each coulomb of charge.
The volt
  • A potential difference of 1 volt means that 1 joule of energy is transferred per coulomb, so 1 V = 1 J/C.
Supply
  • A cell or battery gives energy to charges as they pass through it.
Components
  • Resistors and lamps transform the electrical energy into heat and light.
Chemical energy
  • Inside a cell, chemical energy is transferred to the charges, which then carry energy around the circuit.

Part 2 of 5The Equation E = QV

Energy, charge and potential difference are linked in a simple relationship.

The Equation E = QV
Equation
  • E = QV, where E is energy in joules, Q is charge in coulombs and V is potential difference in volts.
Finding V
  • Rearranging gives V = E ÷ Q, which allows the voltage to be found from the energy and the charge.
Finding Q
  • Rearranging gives Q = E ÷ V, which finds the amount of charge moved for a given energy.
Combining
  • This can be combined with Q = It when the current and time are given rather than the charge.
QuantitySymbolUnit
EnergyEjoule (J)
ChargeQcoulomb (C)
Potential differenceVvolt (V)

Part 3 of 5Measuring Voltage

A voltmeter compares the energy per charge between two points in a circuit.

In parallel
  • A voltmeter is connected in parallel across the component whose potential difference is being measured.
Supply voltage
  • Connected across a cell, a voltmeter shows the supply potential difference.
Series circuits
  • The potential differences across components in a series circuit add up to the supply voltage.
Zero reading
  • A voltmeter across a wire of negligible resistance shows almost no potential difference.

Part 4 of 5Worked Example

Question

  • A 6.0 V battery moves 25 C of charge around a circuit.
  • Calculate the energy transferred.

  1. Write the relationship E = QV.
  2. SubstituteE = 25 × 6.0.
  3. E = 150 J.

Answer

  • The battery transfers 150 J.
  • (Rearranging, 900 J at 12 V would move Q = 900 ÷ 12 = 75 C.)

Part 5 of 5Exam Checklist & Tips

3.2.2 – Key Points for the Exam

  • Potential difference The energy transferred per unit charge between two points, in volts.
  • Volt
    • The unit of potential difference;
      • 1 V = 1 J/C.
  • Voltmeter A meter connected in parallel that measures potential difference.
  • Joule The unit of energy.

Common Mistakes

  • Placing a voltmeter in series with a component rather than across it.
  • Mixing up charge in coulombs with current in amperes.
  • Saying voltage flows through a circuit; it is current that flows.
  • Leaving out units, or giving energy in watts instead of joules.

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