emf and internal resistance

• resistance comes from electrons colliding with atoms and losing energy to other forms. in a battery, chemical energy is used to make electrons move. as they move, they collide with atoms inside the battery - so batteries must have resistance - this is called internal resistance.
• internal resistance is what makes batteries and cells warm up when they're used
• load resistance is the total resistance of all the components in the external circuit. it is also called external resistance
• electromotive force (or emf) is the amount of electrical energy the battery produces for each coulomb of charge.

  emf is meausred in volts
• terminal potential difference is the potential difference across the terminals of a cell. if there was no resistance, the terminal potential difference would be equal to the emf
• lost volts are the work done per unit charge to overcome the internal resistance when current flows
• conservation of energy tells us that:

  energy per coulomb supplied by the source = energy per coulomb transferred in load resistance + energy per coulomb wasted in internal resistance
• for cells in series in a circuit, you can calculate the total emf of the cells by adding their individual emfs
• for identical cells in parallel in a circuit, the total emf of the combination of cells is the same size as the individual cells. this is because the current will split equally between identical cells - the charge only gains emf from the cells it travels through - so they overall emf in the circuit doesn't increase
• to measure internal resistance and emf, vary the current by changing the value of the load resistance using a variable resistor. measure the potential difference for different values of current. plot a graph of current against potential difference...

  
• to find internal resistance and emf using a graph of current against potential difference:

  • V= E-Ir
  • rearrange to give V=-rI +E
  • so the y intercept = E
  • and the gradient is -r

  
• an easier way to measure the emf of a power source is by connecting a high resistance voltmeter across its terminals. however, a small current flows through the voltmeter, so there must be some lost volts, meaning the value you measure will be slightly less than the emf