Three ways to change a system

1. The energy stored in a system changes only when energy is transferred into it, out of it or between its parts.
2. There are three main ways this happens: by heating, by work done by forces and by work done when a current flows.
3. Each of these is a transfer pathway, which describes how energy moves rather than where it is held.
4. A pathway always links a store that decreases to one or more stores that increase, so naming the pathway completes the description of a change.

Changes caused by heating

1. Heating transfers energy from a hotter object to a cooler one whenever there is a temperature difference between them.
2. Energy keeps flowing from hot to cold until both objects reach the same temperature, so a radiator warms a room and an ice pack cools a sprained ankle.
3. Energy transferred to an object by heating increases its internal energy, which is the total energy held by its particles.
4. If the object does not change state, its particles move or vibrate faster, so its temperature rises and its thermal store increases.
5. The size of the temperature rise depends on three things: the energy transferred, the mass of the object and the material it is made from.
6. For the same energy, a larger mass gives a smaller temperature rise, because the energy is shared among more particles.
7. The specific heat capacity of a material is the energy needed to raise the temperature of $1kg$ of it by $1{}^{\circ }C$, measured in $J/kg{}^{\circ }C$.
8. A material with a high specific heat capacity, such as water at about $4200J/kg{}^{\circ }C$, needs a lot of energy for each degree of temperature rise, so it warms up and cools down slowly.
9. A material with a low specific heat capacity, such as copper at about $385J/kg{}^{\circ }C$, needs far less energy for each degree, so the same energy gives it a much larger temperature rise.
10. This is why a metal saucepan on a hob is hot to the touch within seconds, while the water inside takes several minutes to boil.
11. It is also why water is used in central heating systems and hot water bottles, because each kilogram carries a large amount of energy for a small temperature change.
12. Heating does not always raise the temperature, because at a melting point or boiling point the energy transferred changes the state of the substance instead.
13. Objects can also lose internal energy by heating, so a cup of tea left on a table cools as energy is transferred from it to the cooler air and table.

Changes caused by forces

1. A force does work when it moves an object through a distance along the line of action of the force.
2. The work done by the force equals the energy it transfers, so $1J$ of work done means $1J$ of energy transferred.
3. Energy transferred by a force in this way is said to be transferred mechanically.
4. The store that increases depends on what the force does to the object.
5. A force that makes an object speed up increases its kinetic store, as when a footballer kicks a ball.
6. A force that lifts an object increases its gravitational potential energy store, as when a crane raises a steel beam.
7. A force that stretches, squashes or bends an elastic object increases its elastic potential energy store, as when an archer draws back a bowstring.
8. A force such as friction or air resistance that acts against the motion decreases the kinetic store and increases the thermal stores of the surfaces or the air.
9. Rubbing your hands together on a cold day is a simple example, because the work done against friction makes your palms warm.
10. The energy for the work always comes from another store, such as the chemical store of your muscles, of a fuel or of a battery.
11. If a force acts but the object does not move, as when you push hard against a brick wall, no work is done on the wall and no energy is transferred to it.

Changes caused by a current

1. An electric current is a flow of charge, and in a metal wire the charge is carried by electrons.
2. A cell, a battery or the mains supply provides a potential difference that drives the charge around the circuit.
3. As the charge flows through a component it transfers energy to that component, which is called work done by a current or energy transferred electrically.
4. In a battery-powered circuit the energy comes from the chemical store of the battery, which decreases while the current flows.
5. In a resistor or a heating element, the moving electrons collide with the vibrating ions of the metal, so the thermal store of the component increases.
6. In a motor, the current produces a turning effect, so energy is transferred mechanically to the kinetic store of the motor and whatever it drives.
7. In a filament lamp, the filament becomes hot enough to glow, so its thermal store increases and energy is carried to the surroundings by light and infrared radiation.
8. The more charge that flows and the larger the potential difference driving it, the more energy is transferred to the component.
9. In every electrical device some energy goes into thermal stores that you did not want, which is why phone chargers, laptops and hairdryer motors feel warm while they run.