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3.3.4Electricity

Ohm’s Law Experiment — SQA National 5 Physics Revision Notes & Topical Questions

5 min read 5 parts Lesson 8 of 16

📚 SQA National 5 Physics Hub / Topical Revision
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Curriculum Equation Key Ohm & Kirchhoff Circuit LawsFast Exam Solution
V = I × R | P = V × I = I²R = V²/R | ΣI_in = ΣI_out | ΣV = 0
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Part 1 of 4Planning the Circuit

3.3.4 Ohm's Law Experiment

The set-up must measure the current through, and the voltage across, the resistor.

Components
  • Use a power supply, a fixed resistor, a variable resistor or adjustable supply, an ammeter, a voltmeter and connecting leads.
Ammeter
  • Connect the ammeter in series with the resistor so the same current flows through both.
Voltmeter
  • Connect the voltmeter in parallel across the resistor only.
Variable control
  • The variable resistor changes the current, which lets many different voltage readings be taken.

Part 2 of 4Analysing the Results

The shape of the graph shows whether the resistor obeys Ohm's law.

Analysing the Results
Straight line
  • A straight line through the origin shows that current is proportional to voltage, so Ohm's law is obeyed.
Gradient
  • The gradient of the V against I graph is the resistance of the resistor.
Controlled variable
  • Keep the temperature steady by using low currents and switching off between readings.
Errors
  • Zero errors on meters and small scale divisions limit accuracy, which repeating readings helps to reduce.
Safety
  • Use low voltages and avoid touching hot resistors, which can become warm if the current is too large.
V (V)I (A)V ÷ I (Ω)
1.00.1010
2.00.2010
3.00.3010
4.00.4010

Part 3 of 4Worked Example

Question

  • From the graph of a resistor, the voltage rises from 1.0 V to 5.0 V while the current rises from 0.10 A to 0.50 A.
  • Calculate its resistance from the gradient.

  1. Change in voltage ΔV = 5.0 − 1.0 = 4.0 V.
  2. Change in current ΔI = 0.50 − 0.10 = 0.40 A.
  3. Gradient = ΔV ÷ ΔI = 4.0 ÷ 0.40 = 10 Ω.

Answer The resistance is 10 Ω.

Part 4 of 4Exam Checklist & Tips

3.3.4 – Key Points for the Exam

  • Variable resistor A resistor whose value can be changed to alter the current.
  • Best-fit line A line drawn through the points to show the overall trend.
  • Reliability How consistent results are when the experiment is repeated.
  • Gradient
    • The slope of the graph;
      • here it equals the resistance.

Common Mistakes

  • Putting the voltmeter in series with the resistor.
  • Letting the resistor heat up, which changes its resistance and bends the graph.
  • Joining the plotted points dot-to-dot rather than drawing a best-fit line.
  • Taking only one reading and drawing a conclusion from it.

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