Changes in Energy
- Changes make things happen. When there is a change in a system, energy is transferred
- Energy can be transferred via different pathways:
- Heating by particles
- Heating by radiation
- Mechanical work done by forces
- Electrical work done when a current flows
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
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
Examiner Tip
Don't worry too much about the parameters of the system. They are there to help you keep your answers concise so you don't end up wasting time in your exam.
If you follow any process back far enough, you would get many energy transfers taking place. For example, an electric kettle heating water. The relevant energy transfer is from the thermal store of the kettle to the thermal store of the water, with some energy dissipated to the surroundings. But you could take it all the way back to how the electricity was generated in the first place. This is beyond the scope of the question. Defining the system gives you a starting point and a stopping point for the energy transfers you need to consider.
Work is done whenever energy is transferred, so whenever you see this phrase, it just means that energy has been transferred. Electrical work done just means that energy has been transferred when a current flows. This also means that the unit of work done is the same as energy, so is measured in joules. You will meet the concept of work done again in the next topic on Power.
