Electrical Energy and Power

• Everyday appliances transfer electrical energy from the mains to other forms of energy in the appliance

  • e.g. heater, this will transfer electrical energy into a thermal energy store
• The amount of energy an appliance transfers depends on:

  • How long the appliance is switched on for
  • The power of the appliance
  • e.g. A 1 kW iron uses the same amount of energy in 1 hour as a 2 kW iron would use in 30 minutes or A 100 W heater uses the same amount of energy in 30 hours as a 3000 W heater does in 1 hour
• To calculate electrical energy use the equation: E = VIt

  • E = energy (joules, J)
  • V = voltage (volts, V)
  • I = current (amps, A)
  • t = time (seconds, s)
• 1 kW = 1000 W
• Electric circuits transfer energy from the source to the circuit components and then into the surroundings
• As electricity passes around a circuit, energy is transferred from the power source to the various components (which may then transfer energy to the surroundings)

  • As charge passes through the power supply it is given energy
  • As it passes through each component it loses some energy (transferring that energy to the component)
• Energy transfer in a circuit

  A) gained

  B) current

  C) lost

  D) lost

  
• Different domestic appliances transfer energy from batteries, such as a remote control
• Formula for energy transferred is E = Pt

  • can also be written as IVt because power is related to voltage and currents
• Most household appliances transfer energy from the AC mains

  • The AC main can be to the kinetic energy of an electric motor or in heating devices
• Households appliances and energy

  A) energy

  B) main

  C) kinetic

  D) thermal

  E) electrical

  F) mains

  
• Motors are used in:

  • Vacuum cleaners - to create the suction to suck in dust and dirt off carpets
  • Washing machines - to rotate the drum to wash (or dry) clothes
  • Refrigerators - to compress the refrigerant chemical into a liquid to reduce the temperature
• Heating is used in:

  • Toasters - to toast bread
  • Kettles - to boil hot water
  • Radiators - hot water is pumped from the boiler so the radiator can heat up a room
• In mechanics, power (P) is defined as the rate of doing work

  • The potential difference is the work done per unit charge
  • Current is the rate of flow of charge
• Electrical power - the rate of change of work done

  • The work done is the energy transferred so the power is the energy transferred per second in an electrical component
• Electrical power and work done

  A) energy

  B) j

  C) power

  D) w

  E) time

  F) s

  G) work done

  H) j

  
• Formula for electrical power

  • P = IV
• The power dissipated (produced) by an electrical device can also be written as P = IV

  • P = power
  • I = current
  • V = potential difference
• • Power can be written in terms of resistance (R)

  A) resistance

  
• For a given resistor, if the current or voltage doubles the power will be four times as great

  • which equation to use will depend on whether the value of current or voltage has been given in the question
• Rearranging the energy and power equation, the energy can be written as E= VIt

  • E = energy transferred (J)
  • V = potential difference (V)
  • I = current (A)
  • t = time (s)
• The unit for power is Watts (W)
• When doing calculations involving electrical power, remember the unit is Watts W, therefore, you should always make sure that the time is in seconds
• Energy usage in homes and businesses is calculated and compared using the kilowatt hour
• The kilowatt hour - A unit of energy equivalent to one kilowatt of power expended for one hour
• All electrical equipments have a power rating which indicates how many joules of energy are supplied every second for something to work
• Appliances are given power ratings, which tell consumers the amount of energy transferred (by electrical work) to the device every second

  • commonly measured in kilowatt-hour (kW h), which is then used to calculate the cost of energy used
• The kilowatt hour can also be defined using an equation: E = Pt

  • E = energy (kWh)
  • P = power (kW)
  • t = time (h)
  • This equation is unusual because S.I. unit are not used, both energy and power are × 10^3, and time is in hours, not seconds
• In a joule, there is 1W in 1s so 1kWh = 1000W x 3600s = 3.6 x 10^6 J
• Converting between joules and kWh:

  • kWh x (3.6 x 10^6) = J
  • J / (3.6 x 10^6) = kWh
• The kWh is a large unit of energy, and mostly used for energy in homes, businesses, factories and so on
• Electricity companies charge a set amount per kWh for the energy they supply
• What is the formula for power in terms of potential difference and current?

  Power = Voltage x Current
• Define power


  Work done per unit time i.e. energy transferred per unit time.