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• Units in electricity:

  • The ampere (A) is the unit of current
  • The coulomb (C) is the unit of charge
  • The joule (J) is the unit of energy
  • The ohm (Ω) is the unit of resistance
  • The second (s) is the unit of time
  • The volt (V) is the unit of voltage
  • The watt (W) is the unit of power
• Electric current is when electrons flow through a conductor
• An electric circuit consists of a power supply, various components, and connectors
• In a closed circuit with a power supply, there is current in a conductor
• Moving electrons take energy from the power supply to circuit components, where it is transferred into a useful form of energy
• Examples of electrical devices transferring energy:

  • Electric motor transfers energy electrically to kinetic energy, heating, and radiation (sound)
  • Television transfers energy by radiation (light and sound) and heating to the surroundings
  • Smartphone transfers energy by radiation (light and sound) to the user and heating to the surroundings
  • Immersion heater transfers energy by heating to the water and surroundings
• In all energy transfers, some energy is dissipated to the surroundings
• Resistance in electrical components is like 'electrical friction'
• Direct current (d.c.) is the flow of charge in one direction
• Alternating current (a.c.) is the flow of charge backwards and forwards, changing direction many times every second
• The mains supply is always a.c., while a battery supply is always d.c.
• Energy transfer is represented by the symbol E and is measured in joules (J)
• Power is represented by the symbol P and is the rate of transfer of energy, measured in watts (W)
• The power of an electrical component is calculated by multiplying the current by the voltage: power = current × voltage
• The power of an electrical component can also be calculated by multiplying the energy transferred by the time taken: power = energy transferred / time
• The power of an electrical component depends on the current and voltage applied to it
• The voltage of an electricity supply is measured in volts (V), with the mains supply in many countries at 230V
• The equation linking power, energy transferred, and time is: energy transferred = power × time
• The equation linking power, current, and voltage is: power = current × voltage
• The equation linking energy transferred to voltage, current, and time is: energy transferred = current × voltage × time
• Energy transferred by a 100W lamp in 2.0 minutes is 12,000 J
• The power of an electric cooker connected to a 230V mains supply with 8.0A current is 1840W
• The energy transferred by a hairdryer running on 230V and 7.5A for 8.0 minutes is 830,000 J
• Energy transferred (E) = Power (P) × time (t)
• P = 1840 W
• t = 30 minutes = 1800 s
• E = 3,300,000 J (to 2 sig. figs)
• Energy transferred (E) = Current (I) × Voltage (V) × time (t)
• V = 230 V
• I = 7.5 A
• t = 8.0 minutes = 480 s
• E = 830,000 J (to 2 sig. figs)
• Potential hazards of mains electricity:
• Electric shock from touching a live wire
• Frayed cables can expose live wires
• Exposed cables can cause short circuits and overheating leading to fires
• Using electrical appliances in damp conditions can be fatal
• Safety measures:
• Fuses and circuit breakers break the circuit in case of excess current
• Fuse rating should be slightly higher than the appliance's current