Changes in Energy (OCR GCSE Physics A (Gateway))

Changes in Energy

• A thermodynamic system can be isolated, closed or open

  • An open system allows the exchange of energy and matter to or from its surroundings

  • A closed system can exchange energy but not matter to or from its surroundings

  • An isolated system does not allow the transfer of matter or energy to or from its surroundings

A system can be open, closed or isolated

• This means that for a closed system, the total amount of energy is constant

• The total amount of energy transferred into the system must be equal to the total amount of energy transferred away from the system

• Therefore, energy cannot be ‘lost’, but it can be 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 (eg. 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

Energy is transferred by heating from the thermal store of the hob 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

Energy transfers taking place when a box is pushed across the floor

• 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) and by 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)

Energy transfers taking place in an electrical circuit

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

• Or 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 energy in homes