Examples of Energy Transfers
- Energy transfers occur all the time in various everyday circumstances
- Some common situations include when:
- An object projected upwards or up a slope
- A moving object hitting an obstacle
- An object being accelerated by a constant force
- A vehicle slowing down
- Bringing water to a boil in an electric kettle
An Object Projected Upwards
- Before the ball is thrown upwards, the person holding the ball has energy in their chemical store
- When the ball is thrown, some of that energy is transferred to the kinetic store of the ball as it begins to move upwards
- As the height of the ball increases, energy from the kinetic store of the ball is transferred to its gravitational potential store
A Moving Object Hitting an Obstacle
- When an object, such as a car, is moving, energy in the chemical store of the fuel is transferred to the kinetic store of the car
- If the object hits an obstacle, such as a car hitting a wall, the speed of the car will decrease very quickly
- Therefore, the energy in its kinetic store will decrease
- In this scenario, most of the energy from its kinetic store is transferred to the thermal store of the surroundings (dissipated)
- Energy is transferred mechanically to the thermal store of the wall (the force of the car on the wall)
- Energy is also transferred by heating to the thermal store of the air as the sound waves transfer energy away from the system (causing the air particles to vibrate)
A Vehicle Being Accelerated by a Constant Force
- When an object, such as a vehicle is stationary, it has energy in the chemical store of the fuel
- When the vehicle speeds up or accelerates, the energy is transferred to the kinetic store of the car
A Vehicle Slowing Down
- When a vehicle is moving, it has energy in its kinetic store
- As it slows down or decelerates, energy is transferred to the thermal store of the surroundings (dissipated)
- This energy is transferred by heating due to friction between the tyres and the ground, and due to friction between the brakes and the brake pads
- Energy is also transferred by heating as the sound waves transfer energy away from the system (making the air particles vibrate)
Boiling Water in an Electric Kettle
- When an electric kettle boils water, energy is transferred electrically from the mains supply to the thermal store of the heating element inside the kettle
- As the heating element gets hotter, energy is transferred by heating to the thermal store of the water
Trampoline
- Whilst jumping, the person has energy in their kinetic store
- When the person lands on the trampoline, most of that energy is transferred to the elastic potential store of the trampoline
- That energy is transferred usefully back to the kinetic store of the person as they bounce upwards
- Energy is transferred from the kinetic store of the person to the gravitational potential store of the person as they gain height
- Some of the energy is dissipated by heating to the thermal store of the surroundings (the person, the trampoline and the air)
- The useful energy transfers taking place are:
elastic potential energy ➝ kinetic energy ➝ gravitational potential energy
Energy transfers taking place when a person jumps on a trampoline
Worked example
Describe the energy transfers in the following scenarios:
a) A battery powering a torch
b) A falling object
a)
Step 1: Determine the store that energy is being transferred away from, within the parameters described by the defined system
-
- For a battery powering a torch
- The system is defined as the energy transfer from the battery to the torch, so this is the transfer to focus on
- Therefore, the energy began in the chemical store of the cells of the battery
Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system
-
- When the circuit is closed, the bulb lights up
- Therefore, energy is transferred to the thermal store of the bulb
- Energy is then transferred from the bulb to the surroundings, but this is not described in the parameters of the system
Step 3: Determine the transfer pathway
-
- Energy is transferred by the flow of charge around the circuit
- Therefore, the transfer pathway is electrical
Energy is transferred electrically from the chemical store of the battery to the thermal store of the bulb
b)
Step 1: Determine the store that energy is being transferred away from, within the parameters described by the defined system
-
- For a falling object
- In order to fall, the object must have been raised to a height
- Therefore, it began with energy in its gravitational potential store
Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system
-
- As the object falls, it is moving
- Therefore, energy is being transferred to its kinetic store
Step 3: Determine the transfer pathway
-
- For an object to fall, a resultant force must be acting on it, and that force is weight, and it acts over a distance (the height of the fall)
- Therefore, the transfer pathway is mechanical
Energy is transferred from the gravitational store to the kinetic store of the object via a mechanical transfer pathway
Exam 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.
