Power Supplies


  • How much electrical charge a battery has is called its capacity and is measured in ampere hours (Ah) and is dependent on the current drawn by the circuit.
    • Current x time = capacity:
      • You will notice that this formula is exactly the same as that used to work out electrical charge thus an ampere hour is equivalent to one coulomb of charge.
    • It describes the length of time for which a battery will be able to supply a steady amount of voltage.


  • When cells are connected in series the voltage increases and, if the resistance of the circuit does not change, then current will also increase. V = v1+v2+v3… Current increases can then be calculated using V=IR.
    • Capacity may decrease because of the current increase.


  • When cells are connected in parallel the voltage remains constant and, if the resistance of the circuit also remains constant, then the current will also remain constant. The only difference is that the capacity becomes the sum of the capacities of the two batteries.
    • I know that capacities will increase in this case but I do not know whether the new capacity will be the sum or something else.
    • Batteries connected like this must supply the same voltage at all times or else there will be potential difference between the anode of the battery supplying the highest voltage and that of the other batteries.


  • DC flows in only 1 direction and is used in most electronic devices.
  • Steady DC supply has one value.
  • Smooth DC supply has a very small variation called ripple.
  • Cells and batteries provide steady DC.


  • AC means alternating current and DC direct current.
  • AC flows in one direction then reverses and flows in the other direction, during the transition the current is shut down for a moment allowing the wire to cool.
    • This, however, would fry polarized components. The rate at which AC changes direction is called the frequency and is measured in hertz or forward-backward cycles per second.
  • RMS (root mean square) voltage of an AC current indicates the average power dissipated over time usually as thermal energy.


    • The above formula does not reveal how much heat energy is dissipated but allows one to comprehend what DC current would dissipate the same amount of heat as a certain AC current. With this it is then possible to calculate the amount of heat energy dissipated given that one knows the current (P=IV).
    • It is for this reason that AC dissipates much less heat energy than DC current of a similar voltage.
  • Mains electricity is AC.
  • Oscilloscopes show how voltage varies over time.
  • The shorter the time for a cycle the higher the frequency (AC).
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