Electricity and circuits

• Q = I X t charge= current X time
• Energy transferred = charge X voltage

  $E =$$Q X V$
• Voltage = current X resistance

  $V =$$I X R$
• Power = current X voltage

  $P =$$I XV$
• The cell provides the circuit with a source of potential difference like energy. A battery is two or more cells
• A switch turns the circuit on (closed) or off (open)
• A resistor limits the flow of current. A fixed resistor has a resistance it cannot change
• what is a Variable resistor?

  a resistor with a slider that can be used to change its resistance
• Thermistor: a resistor with a slider that can be used to change it’s resistance. used as temperature sensors, allows temperature to be controlled. higher the temperature the less resistance
• Light dependant resistor (LDR): the resistance of an LDR depends on light intensity. As light Intensity increases, its resistance decreases, dimmer the light the more resistance
• Motor: a device that converts electrical energy to mechanical energy
• Diode: a diode allows current to flow in one direction only as it has high resistance the other way. they are used to convert AC to DC current. non-ohmic
• Light emitting diode (LED): equivalent to a diode and emits light when a current passes through it. Used for aviation lighting and displays
• Ammeter: used to measure the current in a circuit, connected in series with other components
• Voltmeter: used to measure the potential difference of an electrical component. connected in parallel with the relevant component
• Electron flow: negative to positive
• Conventional flow: positive to negative
• In a series circuit current is the same everywhere, only one loop
• In a parallel circuit more current will flow on an easier route
• Potential difference: energy transferred per unit charge passed
• a Volt is a joule per coulomb
• Voltage across two components in parallel is the same
• Series circuit:

  • both lamps have the same brightness
  • both lamps are dim
  • higher resistance, less current flows each second
  • lamps can’t be switched on independently
  • if one lamp blows then the circuit is broken
  • easy to make
  • battery will last longer
• Parallel circuit:

  • Both lamps have the same brightness
  • both lamps are bright
  • two alternative pathways, less resistance and higher current
  • lamps can be switched on independently by putting switches in parallel branches
  • less resistance
  • battery won’t last as long
  • if one lamp blows the other will stay lit
• Resistance: anything that reduces the rate of flow of charge, measured in ohms

  lower the current = higher resistance
• Ohms law: at constant temperature, the voltage given to a component will be directly proportional to the current through a component
• Resistance in a wire increases as the length + thinness of the wire increases, current is higher when wire is shorter/ thicker as more space for electrons to move
• Resistance wire: current is directly proportional to voltage as temperature is constant, ohmic
• Filament lamp: temperature is not constant (since lamp must get hot to give out light), filament wire heats up and increases in resistance which makes voltage and current not proportional to each other, non-ohmic
• Ways to protect from electric shocks:

  1. Double insulation: plastic cover not metal so current can’t flow through person, no earth wire needed
  2. fuses: fuse melts if there is a fault (too much current) in a circuit, current can’t flow so protects device & stops person getting shocked