Electrical circuits

• Every component has its own agreed symbol. A circuit diagram shows how components are connected together.
• A battery consists of two or more cells connected together.
• The size of an electric current is the rate of flow of charge.
• The equation for the electric current of a circuit is I = Q/t
• Potential difference across a component: V = E/Q
• R = V/I
• Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.
• Reversing the potential difference across a resistor reverses the current through it.
• The resistance of an appliance is R = V/I
• A filaments lamp's resistance increases if the filament's temperature increases.
• Diode: forward resistance low; reverse resistance high.
• A thermistor's resistance decreases if its temperature increases.
• An LDR's resistance decreases if the light intensity on it increases.
• For components in series:

  • the current is the same in each component
  • the total potential difference is shared between the components
  • adding their resistances gives the total resistance
• For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences.
• Total resistance = R1 + R2
• Adding more resistors in series increases the total resistance because the current through the resistors is reduced and the total potential difference across them is unchanged.
• For components in parallel:

  • the total current is the sum of the currents through the separate components
  • the potential difference across each component is the same
• The bigger the resistance of a component, the smaller the current that will pass through that component.
• The current through a resistor in a parallel circuit is I = V/R
• Adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the total potential difference across them is unchanged.