Electromotive Force and Internal Resistance

• What happens when charge passes through a power supply like a battery?

  It gains electrical energy
• How is electromotive force (e.m.f) defined?

  Energy converted per coulomb of charge

  
• What symbol represents electromotive force?

  ε (epsilon)

  
• In what unit is e.m.f measured?

  Volts (V)
• When is e.m.f equal to the terminal potential difference?

  When no current is flowing
• How can e.m.f be measured?

  Using a high-resistance voltmeter
• What is the terminal potential difference (p.d)?


  Potential difference across the terminals of a cell
• What is the relationship between terminal p.d and e.m.f without internal resistance?

  They are equal
• What is the formula for terminal p.d?

  V = IR
• What does internal resistance cause in a cell?

  Terminal p.d to be lower than e.m.f
• What is 'lost volts' in a circuit?

  Voltage lost due to internal resistance
• How is lost volts represented?

  By the symbol little v
• What is the definition of lost volts?

  Work done per unit charge to overcome internal resistance
• What is the conservation of energy equation involving lost volts?

  v = e.m.f − terminal p.d
• What is the formula for lost volts?

  v = Ir
• What does the e.m.f represent in a circuit?

  Total voltage available to the circuit
• What is internal resistance?

  Resistance of materials within the battery
• What effect does internal resistance have on a power supply?

  Causes loss of voltage over time

  
• How can a cell be modeled in a circuit?

  As a source of e.m.f with internal resistance
• What is the circuit diagram representation of a power supply with internal resistance?

  • Resistor R is the load resistor
  • r is the internal resistance
  • ε is the e.m.f
  • V is the lost volts
  • V is the p.d across the load resistor

  
• Given a battery with e.m.f 7.3 V and internal resistance 0.3 Ω, how do you find the current in the circuit?

  Use Ohm's law with total resistance

  
• How do you calculate lost volts from a battery?

  Use the formula v = Ir
• What should you assume if a battery has negligible internal resistance?

  e.m.f equals its voltage
• What must be used if the battery includes internal resistance in calculations?

  e.m.f equations must be used