Changing the energy of a system
Definition
Energy transfer pathway
An energy transfer pathway is the route by which energy moves from one store to another, such as mechanically, electrically, by heating or by radiation.
- The energy of a system changes only when joules cross its boundary into or out of one of its stores.
- An energy transfer pathway is the route those joules take, so a pathway is how energy moves while a store is where it ends up.
- Three pathways change the energy of a system in this topic: work done by forces, electrical equipment and heating.
- Naming the pathway and naming the store that changes are separate marks, so a full answer needs both.
- No pathway makes or destroys joules, so the number leaving one store equals the number arriving in the others.
Work done by forces
- A force does work on an object when the object moves in the direction of that force, and the energy is then transferred mechanically.
- Lifting a crate off the floor transfers energy mechanically into the gravitational potential energy store of the crate and Earth.
- Pushing a supermarket trolley from rest transfers energy mechanically into the trolley's kinetic energy store.
- Drawing a bow transfers energy mechanically into the elastic potential energy store of the bow.
- Friction between brake pads and a wheel transfers energy mechanically out of the bicycle's kinetic energy store and into thermal energy stores.
- Holding a heavy bag still at one height transfers no energy mechanically, because the bag does not move.
- Carrying that bag horizontally at a steady speed adds nothing to its gravitational store either, because the upward force and the movement are at right angles.
Electrical equipment
- Energy is transferred electrically when charge is pushed through a component by a power supply.
- The store of the supply falls as this happens, so a torch battery's chemical energy store decreases while the torch is switched on.
- An electric motor transfers energy electrically and increases the kinetic energy store of its turning parts.
- A kettle element transfers energy electrically and increases the thermal energy store of the water.
- A filament lamp transfers energy electrically, then passes it on by light and by heating the surroundings.
- A loudspeaker transfers energy electrically and then carries it away by sound waves through the surrounding air.
- Electrical equipment never creates joules, so every joule delivered to the components has come out of the store of the supply.
Heating
Definition
Heating
Heating is the transfer of energy from a region at a higher temperature to a region at a lower temperature because of the temperature difference between them.
- Heating transfers energy whenever two regions are at different temperatures, and it always runs from the hotter region to the cooler one.
- The thermal energy store of the hotter region decreases while the thermal energy store of the cooler region increases.
- The transfer keeps going until the two temperatures are equal, and it then stops.
- A gas hob heats a pan, so the pan's thermal energy store increases and its temperature rises.
- A mug of coffee left on a desk cools down because energy is transferred by heating to the cooler air of the room.
- Energy transferred by heating can travel by conduction, by convection or by radiation.
- Burning gas in a boiler empties the chemical energy store of the fuel and then transfers energy by heating into the water in the radiators.
Example
Tracing the pathways in an electric kettle
- A kettle is plugged in and switched on to boil water for tea.
- Energy is first transferred electrically from the mains supply to the heating element.
- Energy is then transferred by heating from the hot element to the water, so the water's thermal energy store increases.
- Energy is also transferred by heating from the kettle body to the kitchen air, so the room's thermal energy store increases as well.
- Two pathways are named in this change, electrically and by heating, and each one is matched to the store that increases.
Example
Comparing a straight lift with a ramp
- A worker raises a 20 kg20\ \text{kg}20 kg box onto a shelf by lifting it straight up.
- The upward force moves the box in the direction of that force, so work is done and energy is transferred mechanically into the gravitational store.
- A second worker slides an identical box up a ramp onto the same shelf.
- The push moves the box along the ramp, so work is done mechanically again, and friction means part of that work also raises the thermal energy stores of the ramp and box.
- Both routes change the system's energy through work done by forces, so the pathway is the same even though the forces and distances differ.
Exam technique
Naming the pathway and the store
- Read the question for the words that fix the pathway, such as a force, a circuit or a temperature difference.
- Write the pathway in the board's own wording: mechanically, electrically or by heating.
- Follow the pathway with the store that changes, for example energy is transferred electrically, so the thermal energy store of the water increases.
- Give one pathway for each transfer the question asks about, because a two-mark question usually wants two separate transfers.
- The mark for direction is lost unless you say which store increases and which decreases.
Common Mistake
- Do not name a pathway when a store is asked for, because electrically and by heating are not stores.
- Do not write that a device makes or produces energy, because equipment only moves joules between stores.
- Do not say work is done when the object does not move, because a force on its own transfers no energy.
- Do not use heat as a store, because heating is the pathway and the thermal energy store is the destination.
Self review
- Name the three ways the energy of a system can be changed.
- Explain the difference between an energy store and an energy transfer pathway.
- State why holding a bag still at one height transfers no energy mechanically.
- Describe the pathway and the store that changes when a kettle heats water.
- Explain what decides the direction of an energy transfer by heating.
