Electromotive Force (CIE A Level Physics)

• When charge passes through a power supply such as a battery, it gains electrical energy
• The electromotive force (e.m.f) is the amount of chemical energy converted to electrical energy per coulomb of charge (C) when charge passes through a power supply
• e.m.f is measured in Volts (V)

 

• E.m.f is also the potential difference across the cell when no current is flowing
• E.m.f can be measured by connecting a high-resistance voltmeter around the terminals of the cell in an open circuit

Measuring e.m.f in a cell 

E.m.f is measured using a voltmeter connected in parallel with the cell

• The difference between potential difference and e.m.f is the type of energy transfer per unit charge

 

$potential difference = \frac{energy transformed from electrical to other forms }{charge}$

• When charge passes through a resistor, for example, its electrical energy is converted to heat in the resistor
  • The resistor therefore has a potential difference across it

• Potential difference describes the loss of energy from charges; ie. when electrical energy is transferred to other forms of energy in a component
• e.m.f. describes the transfer of energy from the power supply to electrical charges within the circuit