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3.3.2Electricity

Calculating Potential Difference, Current and Resistance — SQA National 5 Physics Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 16

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Part 1 of 5Ohm's Law

3.3.2 Calculating Potential Difference, Current and Resistance

For many conductors at constant temperature the current is proportional to the voltage.

Equation
  • V = IR, where V is potential difference in volts, I is current in amperes and R is resistance in ohms.
Meaning
  • For a fixed resistance, doubling the voltage doubles the current through the component.
Resistance
  • Resistance measures how strongly a component opposes current; a larger resistance allows less current.
Limits
  • The law applies to components at steady temperature, so it does not describe a hot filament lamp.
Worked ratio
  • If the voltage across a fixed resistor changes from 4 V to 8 V, the current also doubles because R is constant.
To findRearranged formUnit
VoltageV = IRvolt (V)
CurrentI = V ÷ Rampere (A)
ResistanceR = V ÷ Iohm (Ω)

Part 2 of 5Using Prefixes

Electronics often uses small and large units that must be converted first.

Using Prefixes
Milliamp
  • 1 mA = 0.001 A, so 3.0 mA is 0.0030 A.
Kilohm
  • 1 kΩ = 1000 Ω, so 2.2 kΩ is 2200 Ω.
Megohm
  • 1 MΩ = 1 000 000 Ω, which is common for very high resistances.
Check
  • Always convert to base units before substituting into V = IR, then state the unit of the answer.
Standard form
  • Large values such as 4.7 × 10⁶ Ω are often written in standard form, and the power of ten must be included.

Part 3 of 5Solving a Calculation

A repeatable layout earns marks and prevents slips.

1
List the quantities given in the question and the one you need to find.
2
Convert any prefixes into volts, amperes and ohms.
3
Write the correct form of the relationship before substituting values.
4
Substitute the numbers, calculate and match the number of significant figures to the data.
5
State the answer with its unit and check that it is sensible.

Part 4 of 5Worked Example

Question

  • A 2.2 kΩ resistor carries a current of 3.0 mA.
  • Calculate the potential difference across it.

  1. ConvertR = 2.2 kΩ = 2200 Ω and I = 3.0 mA = 0.0030 A.
  2. Use V = IR.
  3. V = 0.0030 × 2200 = 6.6 V.

Answer

  • The potential difference is 6.6 V.
  • (Check: 9.0 V across 45 Ω gives I = 9.0 ÷ 45 = 0.20 A.)

Part 5 of 5Exam Checklist & Tips

3.3.2 – Key Points for the Exam

  • Ohm's law The statement that V = IR for a conductor at constant temperature.
  • Resistance A measure of how much a component opposes current, in ohms.
  • Ohm The unit of resistance; 1 Ω = 1 V/A.
  • Milliampere One thousandth of an ampere.

Common Mistakes

  • Using resistance in kilohms without converting to ohms.
  • Rearranging the equation the wrong way, for example R = IV.
  • Writing the unit as amps for voltage or volts for current.
  • Using the voltage of the whole supply when the question asks about one component.

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