Changes in Energy (OCR GCSE Combined Science A (Gateway): Physics): Revision Note

Changes in Energy

• A system is an object or group of objects

• In a closed system, there is no net change to the total energy of the system

• Energy can be transferred between the stores of the objects in the system, but the total amount of energy stays constant

• This is the law of conservation of energy: energy cannot be created or destroyed, only transferred

• So energy is never ‘lost’ — when it is transferred out of a system, it is transferred to the surroundings

  • Energy can be dissipated (spread out) to the surroundings by heating and radiation

  • Dissipated energy transfers are often not useful, in which case they can be described as wasted energy

Heating

• Energy transfers by heating increase the energy in the kinetic store of the particles that make up that system, which increases the energy in the thermal store of the object

• This either raises the system's temperature or produces a change of state (e.g. solid to liquid)

• An example of an energy transfer by heating is warming a pan on a hob

  • Energy is transferred electrically from the mains supply to the thermal store of the hob, which is then transferred by heating to the thermal store of the pan

• The amount of energy required to heat an object depends on its specific heat capacity

  • This is a property of the material the object is made from

  • Specific heat capacity is the amount of energy required to raise the temperature of 1 kg of the substance by 1 °C

• Therefore, an object with a greater specific heat capacity will require more work to be done on it in order to increase its temperature

Work done by forces

• Mechanical work is done when a force acts over a distance

• For example, when a person pushes a box across the floor

• Energy is transferred mechanically from the kinetic store of the person to the kinetic store of the box

• If the system is defined as the man and the box, energy is transferred mechanically from the kinetic store of the person to the kinetic store of the box

• If the system is defined as the box and the floor, energy is transferred by:

  • heating from the kinetic store of the box to the thermal store of the floor (due to friction)

  • heating to the thermal store of the surroundings as the sound waves transfer energy away from the system and cause the air particles to vibrate

Work done when a current flows

• Current is the flow of charge

• A current flows when there is a potential difference applied to the circuit

  • This is provided by the power supply or a cell

• Energy is transferred electrically from the power supply to the components in the circuit 

  • This is the electrical work done by the power supply when a current flows

   

• Energy from the chemical store of the cell is transferred electrically to the thermal store of the lamp as the filament heats up

• Energy is transferred from the thermal store of the lamp by heating and by radiation (light) to the thermal store of the surroundings 

• Energy is also transferred by heating to the thermal store of the wires (due to resistance)

Calculations involving energy changes

Mechanical

• Mechanical energy transfers use the equation:

W = F × s

• Where:

  • W = work done in joules (J) 

  • F = force in Newtons (N)

  • s = distance in metres (m)

Electrical

• The amount of energy transferred by electrical work in a component (or appliance) depends upon:

  • The current, I

  • The potential difference, V

  • The amount of time the component is used for, t

• When charge flows through a resistor, for example, the energy transferred is what makes the resistor hot

• The energy transferred can be calculated using the equation:

E = P × t

• Where:

  • E = energy transferred in joules (J)

  • P = power in watts (W)

  • t = time in seconds (s)

• Since P = IV, this equation can also be written as:

E = I × V × t

• Where:

  • I = current in amperes (A)

  • V = potential difference in volts (V)

• The electrical energy transferred also depends on the charge and potential difference:

E = Q × V

• Where:

  • Q = charge in coulombs (C)

  • V = potential difference in volts (V)

• When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the energy transferred to all the components in the circuit

Thermal

• The equation for specific heat capacity is:

ΔE = mcΔθ

• Where:

  • ΔE = change in energy, in joules (J)

  • m = mass, in kilograms (kg)

  • c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg °C)

  • Δθ = change in temperature, in degrees Celsius (°C)

The Kilowatt-Hour

• Energy is commonly measured in kilowatt-hour (kW h), which is then used to calculate the cost of energy

  • This is used to calculate electricity bills

• A kilowatt-hour is defined as:

A unit of energy equal to 1 kW of power sustained for 1 hour

• It can also be written as an equation:

Energy (kW h) = Power (kW) × Time (h)

• Since the usual unit of energy is joules (J), this is the 1 W in 1 s

• Therefore:

1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J

• Since 1 kW = 1000 W and 1 h = 3600 s

• To convert between joules and kW h:

kW h  × (3.6 × 10⁶) = J

J  ÷ (3.6 × 10⁶) = kW h

• The kW h is a large unit of energy and is mostly used for calculating energy usage in homes