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Year 8 Science EOY: Physics - Coggle Diagram
Year 8 Science EOY: Physics
Chapter 2 Energy
2.1 Food and Fuels
We measure energy in joules (J) and kilojoules (kJ). A kilojoule is equivalent to 1000 joules. Energy is used in everything we do.
Food labels tell you how much energy in the store associated with food. Different foods contain different amounts of energy stores.
When you choose what foods to eat, you also need to consider the nutritional value of the food as well as the energy in the store.
You need oxygen to burn both food and fuels. People use the energy in fuels in order to do tasks that require heat, such as heating the house or cooking food. Electrical devices need an electrical current to work, which is produced when you burn fuel in a power station.
An adult should just take the energy they need for the activities they do. Any more will be stored as fat to be used in the future.
A person's intake of energy depends on three things:
their body mass (larger body mass means more cells to give energy to)
their level of activity (more activity means more energy needed to do those activities)
their age (children need more energy than adults as they are growing)
2.2 Energy Adds Up
The Law of Conservation of Energy is:
"Energy cannot be created nor destroyed, only transferred"
There are 5 energy stores:
chemical (food and fuels)
thermal (hot objects)
kinetic (moving objects)
gravitational potential (position in a gravitational field)
elastic (changing shape, squashing or stretching)
When energy is being transferred, it is moving from one of these stores to another.
You can transfer energy by one of four ways:
mechanically
electrically
by heating
by light/sound
2.3 Energy and Temperature
You use a thermometer to measure temperature. We measure temperature in degrees Celsius (°C)
The difference between temperature and energy is that temperature does not matter on the mass of the object while energy does. A swimming pool filled with water and a glass of water that are the same temperature can have different sizes of thermal energy store.
Hot objects cool down. Energy is only transferred from a hot object to a cooler object. Eventually, both objects will end up at the same temperature and be in thermal equilibrium, where no more energy will be transferred in their thermal stores.
Heating changes the movement and energy of particles. The particles aren't getting hotter, they are only moving/vibrating faster.
2.4 Energy transfer: Particles + 2.5 Energy transfer: Radiation
Energy can either be transferred by conduction, convection, radiation.
In conduction, particles transfer energy by colliding with each-other when they vibrate. Energy transfer happens until the two surfaces are at the same temperature. If you keep one surface warm, the temperature difference will carry on and the solid will still conduct.
Energy is not transferred easily through insulators, which don't conduct and have energy transfer through them very slowly. Liquids and gases are not very good at energy transfer either.
Convection works like this:
particles near the heat source get hotter and move faster
This results in them spreading further apart, becoming less dense and rising up.
Cooler particles take its place near the heat source and the cycle repeats.
This is called a convection current (only occurs in fluids)
Radiation is the transfer of energy through waves or particles. Both light and infrared radiation (heat) are types of radiation. Radiation does not need particles to transfer energy.
All objects emit and absorb infrared radiation. How much radiation something emits depends on the type of surface.
The colour of something affects how good it is at emitting, absorbing and reflecting radiation. In order from best to worst at emitting and absorbing, the colours are:
dark matte colours
dark shiny colours
light matte colours
light shiny colours
2.6 Energy Resources
Coal, oil and gas are called fossil fuels, as they were made from the compressed and heated remains fossilised creatures (coal is made from remains of trees, while oil and gas were made from remains of sea creatures.
Three disadvantages of fossil fuels are:
burning them releases carbon dioxide which is a greenhouse gas that contributes to climate change.
burning them also releases pollutants like sulfur dioxide, nitrogen oxides and particulates.
they are non-renewable.
Thermal power stations burn coal and gas, while oil is used to make petrol, plastics and other materials. A fuel is burned to heat water, generating steam. The steam drives a turbine, which turns a generator. The generator produces electricity that is carried along cables to where it's needed
Alternative (and renewable) sources of electricity include:
wind
tidal
biomass
solar
hydroelectric
geothermal
2.7 Energy and Power
Many electrical appliances have a power rating in watts. You can calculate power using this equation:
power (W) = energy (J) / time (s)
Energy companies use kilowatt hours (kWh) instead of joules (J). They use the equation:
energy (kWh) = power (kW) x time (h)
To reduce your energy bill, you could:
use fewer appliances
use appliances that use less power to get the same result
use appliances for fewer hours
2.8 Work, Energy and Machines
A way of transferring energy is by doing work. Work done = Energy transferred, so we measure work in joules.
The equation for measuring work done is:
work done (J) = force (N) x distance (m)
You can use simple machines to reduce the force needed to do work. Levers and gears are examples of simple machines.
When you use a screwdriver to open a tin of paint, your hand moves a small force over a large distance. The distance the end of the screwdriver moves is a small distance but the work is the same, resulting in a large force being applied to the lid of the tin. A lever is known as a force multiplier.
Similarly to levers, gears on the pedal of a bike are also a force multiplier as the force you pedal is small but is put over a large distance, resulting in a large force being applied to the gear at the wheel