Energy Transfer and Efficiency
Designed by Dr T. Marlow
U-values and Specific Heat Capacity
a) U-values measure how effective a material is as an
b) The lower the U-value, the better the material is as an insulator.
c) Solar panels may contain water that is heated by radiation from the Sun. This water may then be used to heat buildings or provide domestic hot water.
d) The specific heat capacity of a substance is the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius.
E = Mass x Specific heat capacity x Temperature change
a) Energy can be transferred usefully, stored, or dissipated, but cannot be created or destroyed.
b) When energy is transferred, only part of it may be usefully transferred; the rest is ‘wasted’.
c) Wasted energy is eventually transferred to the surroundings, which become warmer. The wasted energy becomes increasingly spread out and so becomes less useful.
d) To calculate the efficiency of a device using:
efficiency = useful energy out
total energy in
efficiency = useful power out
total power in
Types of Energy and Power Transfer
a) Examples of energy transfers that everyday electrical appliances are designed to bring about.
b) The amount of energy an appliance transfers depends on how long the appliance is switched on and its power.
c) To calculate the amount of energy transferred from the mains using:
E = P t
d) To calculate the cost of mains electricity given the cost kilowatt-hour.