RP06 - Internal Resistance and EMF

• What is a source's internal resistance?

  • A source's internal resistance is the resistance is the resistance of the materials inside the source
  • It is equal to the lost volts per unit current in the source
• What is the emf of a power source?

  • A power supply's emf is the work done by the source per unit charge
  • It is equal to the potential difference across the source when no current flows
• What is meant by the phrase 'lost volts'?

  The lost volts of a battery is the difference between the battery's emf and its terminal potential difference
• Why should the battery be disconnected between readings?

  • The temperature of the circuit should remain constant throughout the experiment so the resistance in the circuit in the circuit remains constant
  • Disconnecting the batteru when not needed mitigates heating
• Why should you avoid using a rechargeable battery when carrying out this experiment?

  Rechargeable power sources have a very low internal resistance which would be hard to measure in this experiment
• Why should a new cell/battery be used when carrying out this experiment?

  • Run down cells and batteries have internal resistances that may fluctuate throughout the experiment
  • Using a new source will result in a more consistent value, that may therefore be more accurately calculated
• What safety precautions should be taken when carrying out this experiment?

  • If connected for long periods of time, the battery and circuitry can become hot
  • Avoid touching bare metal contacts and disconnect the battery when readings are not being taken
• Gradient derivation:

  E = I(R+r)

  E = IR + Ir

  E = V + Ir

  V = -rl + E

  Where:

  V = y

  -r = m

  I = x

  E = c