Energy Stores & Transfers
Energy Stores
- In physics, a system is defined as:
An object or group of objects
- Defining the system, in physics, is a way of narrowing the parameters to focus only on what is relevant to the situation being observed
- A system could be large or small, incorporating just one object, or a whole group of objects and their surroundings
- When a system is in equilibrium, nothing changes, and so nothing happens
- When there is a change to a system, energy is transferred
- If an apple sits on a table, and that table is suddenly removed, the apple will fall
- As the apple falls, energy is transferred
- Energy is stored in objects in different energy stores
Energy Stores Table
Energy Transfers
- Energy is transferred between stores by different energy transfer pathways
- The energy transfer pathways are:
- Mechanical
- Electrical
- Heating
- Radiation
- These are described in the table below:
Energy Transfer Pathways Table
- An example of an energy transfer by heating is a hot coffee heating up cold hands
Energy is transferred by heating from the hot coffee to the mug to the cold hands
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
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- 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
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Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system
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- 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
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Step 3: Determine the transfer pathway
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- Energy is transferred by the flow of charge around the circuit
- Therefore, the transfer pathway is electrical
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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
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- 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
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Step 2: Determine the store that energy is transferred to, within the parameters described by the defined system
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- As the object falls, it is moving
- Therefore, energy is being transferred to its kinetic store
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Step 3: Determine the transfer pathway
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- 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
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Energy is transferred from the gravitational store to the kinetic store of the object via a mechanical transfer pathway
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.
- Sankey diagrams can be used to represent energy transfers
- Sankey diagrams are characterised by the splitting arrows that show the proportions of the energy transfers taking place
- The different parts of the arrow in a Sankey diagram represent the different energy transfers:
- The left-hand side of the arrow (the flat end) represents the energy transferred into the system
- The straight arrow pointing to the right represents the energy that ends up in the desired store; this is the useful energy output
- The arrows that bend away represent the wasted energy
Total energy in, wasted energy and useful energy out shown on a Sankey diagram
- The width of each arrow is proportional to the amount of energy being transferred
- As a result of the conversation of energy:
Total energy in = Useful energy out + Wasted energy
- A Sankey diagram for a modern efficient light bulb will look very different from that for an old filament light bulb
- A more efficient light bulb has less wasted energy
- This is shown by the smaller arrow downwards representing the heat energy
Sankey diagram for modern vs. old filament light bulb
Worked example
An electric motor is used to lift a weight. The diagram represents the energy transfers in the motor.
Calculate the amount of wasted energy.
Step 1: State the conservation of energy
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- Energy cannot be created or destroyed, it can only be transferred from one store to another
- This means that:
total energy in = useful energy out + wasted energy out
Step 2: Rearrange the equation for the wasted energy
wasted energy = total energy in – useful energy out
Step 3: Substitute the values from the diagram
500 – 120 = 380 J
