emf internal resistance

• • 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 equal to 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
• • All power supplies have some resistance between their terminals
  • This is called internal resistance (r)
  • It is internal resistance that causes the charge circulating to dissipate some electrical energy from the power supply itself
  • This is why the cell becomes warm after a period of time
  • Therefore, over time the internal resistance causes loss of voltage or energy loss in a power supply
• • The terminal potential difference (p.d) is the potential difference across the terminals of a cell
  • If there was no internal resistance, the terminal p.d would be equal to the e.m.f
• • If a cell has internal resistance, the terminal p.d is always lower than the e.m.f
  • If you have a load resistor R across the cell's terminals, then the terminal p.d V alsothe p.d across the load resistor

   
• • In a closed circuit, current flows through a cell and a potential difference develops across the internal resistance
  • Since resistance opposes current, this reduces the energy per unit charge (voltage) available to the rest of the external circuit
  • This difference is called the ‘lost volts’
  • Lost volts is usually represented by little v
  • It is defined as the voltage lost in the cell due to internal resistance, so, from conservation of energy:
• • v = e.m.f − terminal p.d
  • lost volts is the differencebetween the e.m.f and the terminal p.d