P4 Electric circuits

• Switch (closed)

  
• Switch (open)

  
• Cell circuit symbol

  
• Battery circuit symbol

  
• Diode circuit symbol

  
• Resistor circuit symbol

  
• Variable resistor circuit symbol

  
• LED circuit symbol

  
• Lamp Circuit symbol

  
• Fuse circuit symbol

  
• Voltmeter circuit symbol

  
• Ammeter circuit symbol

  
• Thermistor circuit symbol

  
• LDR circuit symbol

  
• An electric current is a flow of electric charge around a circuit measured in amps
• In a series circuit

  • Current: the SAME through each component
  • Potential difference: the total p.d is SHARED between components
  • Resistance: the total resistance of two components is the sum of the resistance of each component (add resistance)
• In a parallel circuit:

  • Current: the total current through the whole circuit is the SUM of currents through separate components
  • Potential difference: the p.d across each component is the SAME
  • Resistance: the total resistance of two resistors is less than the resistance of the smallest individual resistor
• resistance is the opposition to the flow of charge in a circuit, measured in ohms
• Potential difference is a measure of energy per unit of charge transferred between two points in a circuit measured in volts
• Charge is the quantity of electricity measured in Coulombs
• Adding more resistors to a parallel circuit decreases the total resistance because it creates two pathways for the current to take therefore more total current will flow through the circuit
• Ohmic conductor - conductor where resistance remains constant as the current changes
• The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor
• The resistance of components such as lamps , diodes, thermistors and LDRs is not constant, it changes with the current through the component
• Filament lamp

  • the resistance of a filament lamp increases as the temperature of the filament increases
  • The current in a filament lamp is not directly proportional to potential difference because the filament gets hot which causes the resistance to increase
• The current through a diode flows in one direction only because the diode has a very high resistance in the reverse direction
• Diodes are extremely useful for controlling the flow of current in circuits
• LED (light emitting diode) :

  • a light emitting diode gives off light when a current flows through
  • LEDs are an extremely energy efficient source of light
• LDR (light dependent resistor) :

  • In dark conditions the LDR has a high resistance
  • In the light the resistance of the LDR is very low
• Thermistors :

  • The resistance of a thermistor decreases as the temperature increases
  • thermistors are really useful in thermostats