Required Practicals/Edexcel/Practical 14
14A2CP14

Investigating AC generator output with an oscilloscope (CP14)

Use an oscilloscope to investigate how the peak voltage and frequency of a generator depend on rotation speed.

Apparatus

  • Simple AC generator (rotating coil in a magnetic field, or bicycle dynamo)
  • Oscilloscope
  • Protractor or revolution counter / tachometer
  • Connecting leads

Safety

  • Ensure the generator shaft is securely supported and cannot slip during rotation.
  • Do not connect the generator to mains-voltage equipment.

Method

  1. Connect the generator output to the oscilloscope input. Set the timebase and voltage sensitivity.
  2. Rotate the generator at a steady rate. Observe the sinusoidal trace on the oscilloscope.
  3. Measure the peak voltage $V_0$ (vertical distance from zero to crest, multiplied by V/div setting) and period T (horizontal distance per cycle multiplied by s/div setting).
  4. Vary the rotation rate (use a tachometer to measure rotations per second = frequency f). Record $V_0$ and f for each rate.
  5. Plot $V_0$ against f: straight line through origin.

Key Variables

Independent
Rotation frequency f
Dependent
Peak EMF $V_0$
Controlled
Same generator coil and magnet; Oscilloscope settings constant during a reading

Analysis and Results

  • Peak EMF: $V_0 = BAN\omega = BAN \times 2\pi f$. So $V_0 \propto f$.
  • Plot $V_0$ vs f: straight line through origin, gradient $= 2\pi BAN$.
  • Doubling the rotation speed doubles both the frequency and the peak voltage.
  • Period from oscilloscope: $T = 1/f$; check against tachometer reading.

Common Errors

  • Reading peak-to-peak voltage rather than peak voltage from the oscilloscope (divide by 2).
  • Misreading the timebase: multiply the number of divisions per cycle by the s/div setting.
  • Not maintaining a constant rotation rate during a measurement.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q13 marks
A generator coil has 200 turns, area $30$ cm$^2$, and rotates at 10 Hz in a field of 0.040 T. Calculate the peak EMF.
Q23 marks
Describe what happens to the oscilloscope trace when the rotation speed of the generator is doubled.