Welcome to Energy!
Welcome to one of the most exciting and important topics in GCSE Physics: Energy! Energy is everywhere around us. It powers your smartphone, keeps your home warm, helps you run in PE, and lights up the night sky. In this chapter, we will break down what energy is, how it changes from one form to another, how we calculate it, and where we get our energy from on Earth.
Don't worry if physics formulas seem a little scary at first! We are going to take things step-by-step with clear examples, simple tricks, and helpful memory aids.
1. Forms of Energy and Conservation
The Different Forms of Energy
Energy can exist in many different "forms". You can remember them with the handy mnemonic: "Most Kids Hate Learning GCSE Energy Names" (or just learn them in related pairs!):
• Kinetic Energy (\(E_k\)): The energy of moving objects (e.g., a moving car, a flying football).
• Gravitational Potential Energy (\(E_p\)): Energy stored in an object because of its height above the ground (e.g., a book on a high shelf, a rollercoaster at the top of a hill).
• Chemical Energy: Energy stored in chemical bonds (e.g., food, batteries, fuels like coal and petrol).
• Elastic Potential (Strain) Energy: Energy stored in stretched or squashed objects (e.g., a stretched rubber band, a compressed spring).
• Thermal (Heat) Energy: Energy due to the temperature of an object (e.g., a hot cup of tea, a radiator).
• Electrical Energy: Energy transferred by moving electric charges (e.g., electricity flowing through wires).
• Sound Energy: Energy released by vibrating objects that travels in waves (e.g., a guitar strum, a loudspeaker).
• Light (Radiant) Energy: Energy that travels as electromagnetic waves that our eyes can detect (e.g., the Sun, a torch bulb).
• Nuclear Energy: Energy stored inside the nucleus of an atom (e.g., in uranium used in nuclear power stations).
The Principle of Conservation of Energy
This is one of the most fundamental laws in all of science. You must memorize this definition for your exam:
The Principle of Conservation of Energy states that: Energy cannot be created or destroyed; it can only be transferred from one form into another.
This means the total amount of energy in the universe stays exactly the same!
Did You Know?
When you drop a ball, its gravitational potential energy turns into kinetic energy. When it hits the floor, some of that energy turns into sound and heat (thermal energy), which warms up the floor by a tiny fraction of a degree!
Key Takeaway: Energy never disappears. It just changes "jackets" (forms), but the total amount at the start always equals the total amount at the end.
2. Energy Transfers and Sankey Diagrams
Useful vs. Wasted Energy
Whenever an energy transfer takes place, not all of the input energy ends up where we want it to go:
• Useful Energy: Energy transferred into the form we actually want (e.g., light from a light bulb).
• Wasted Energy: Energy transferred into forms we do not want, usually dissipated into the surroundings as heat (thermal) and sound.
Because energy is always conserved:
Total Energy Input = Useful Energy Output + Wasted Energy Output
Example: If a filament lamp takes in \(100\text{ J}\) of electrical energy, and produces \(10\text{ J}\) of useful light energy, it must produce \(90\text{ J}\) of wasted thermal energy (\(100 - 10 = 90\)).
Sankey Diagrams
A Sankey diagram is a special visual arrow diagram that shows energy transfers:
• The width of the arrow represents the amount of energy in Joules (\(\text{J}\)).
• The arrow starts on the left as the Total Input Energy.
• The arrow splits: the useful energy usually goes straight across to the right.
• The wasted energy bends downwards.
Top Tip: In exam questions, count the grid squares! If the input arrow is \(10\) squares wide, the sum of the useful arrow width and the wasted arrow width must equal exactly \(10\) squares.
Key Takeaway: Sankey diagrams use arrow thickness to show energy amounts. Width in = Width out!
3. Efficiency
What is Efficiency?
Efficiency is a measure of how good a device is at turning input energy into useful output energy. A very efficient device wastes very little energy.
The Efficiency Equation
You can calculate efficiency using either of these equations:
\(\text{Efficiency} = \frac{\text{Useful energy output}}{\text{Total energy input}}\)
Or, if using power:
\(\text{Efficiency} = \frac{\text{Useful power output}}{\text{Total power input}}\)
Percentage Efficiency
To write efficiency as a percentage, multiply your decimal answer by \(100\):
\(\text{Percentage Efficiency} = \frac{\text{Useful energy output}}{\text{Total energy input}} \times 100\%\)
Worked Example:
Question: An electric motor takes in \(500\text{ J}\) of electrical energy. It provides \(350\text{ J}\) of useful kinetic energy. Calculate its efficiency as a percentage.
Step 1: Identify the numbers: Total input = \(500\text{ J}\), Useful output = \(350\text{ J}\).
Step 2: Use the equation: \(\text{Efficiency} = \frac{350}{500} = 0.7\)
Step 3: Convert to percentage: \(0.7 \times 100\% = 70\%\)
Answer: \(70\%\) (or \(0.7\)).
Common Mistakes to Avoid:
• Never get an answer greater than 1 (or greater than 100%): If your calculation gives \(1.4\) or \(140\%\), you have accidentally divided the input by the output. Flip the numbers upside down!
• Efficiency has no units: It is written as a decimal (e.g., \(0.75\)) or with a \(\%\) sign (e.g., \(75\%\)).
Key Takeaway: Efficiency tells you what fraction of the energy you put in actually does the job you want.
4. Work Done
What is Work in Physics?
In everyday life, "work" means doing homework or going to a job. In physics, work is done whenever a force moves an object through a distance.
Work Done = Energy Transferred
Both work done and energy are measured in Joules (\(\text{J}\)).
The Work Done Equation
\(W = F \times d\)
Where:
• \(W\) = Work done in Joules (\(\text{J}\))
• \(F\) = Force applied in Newtons (\(\text{N}\))
• \(d\) = Distance moved in the direction of the force in metres (\(\text{m}\))
Worked Example:
Question: A student pushes a box with a force of \(40\text{ N}\) across a floor for a distance of \(3\text{ m}\). Calculate the work done.
Step 1: State the formula: \(W = F \times d\)
Step 2: Substitute the values: \(W = 40 \times 3\)
Step 3: Calculate the answer: \(W = 120\text{ J}\)
Key Takeaway: If you push something with a force and it moves, you do work on it: \(W = F \times d\).
5. Gravitational Potential Energy and Kinetic Energy
Gravitational Potential Energy (\(E_p\))
When you lift an object up, you do work against gravity. That work is stored as Gravitational Potential Energy (\(E_p\)).
\(E_p = m \times g \times h\)
Where:
• \(E_p\) = Gravitational potential energy in Joules (\(\text{J}\))
• \(m\) = Mass of the object in kilograms (\(\text{kg}\))
• \(g\) = Acceleration due to gravity (on Earth, \(g = 10\text{ N/kg}\) or \(9.8\text{ N/kg}\); use the value given on your exam paper, commonly \(10\text{ N/kg}\) in CCEA exams)
• \(h\) = Height in metres (\(\text{m}\))
Kinetic Energy (\(E_k\))
Any moving object has Kinetic Energy (\(E_k\)). The faster it moves and the heavier it is, the more kinetic energy it has.
\(E_k = \frac{1}{2} m v^2\)
Where:
• \(E_k\) = Kinetic energy in Joules (\(\text{J}\))
• \(m\) = Mass in kilograms (\(\text{kg}\))
• \(v\) = Velocity (speed) in metres per second (\(\text{m/s}\))
Important Maths Tip for \(E_k\):
Remember order of operations (BODMAS)! Only square the velocity (\(v\)), not the mass (\(m\)).
Step-by-step: First calculate \(v^2\), then multiply by \(m\), then multiply by \(0.5\) (or divide by \(2\)).
Energy Conservation: Falling Objects
When an object drops (ignoring air resistance):
Loss of \(E_p\) = Gain in \(E_k\)
Example: A roller coaster cart at the top of a track has maximum \(E_p\) and zero \(E_k\). At the very bottom of the dip, all that \(E_p\) has converted into maximum \(E_k\) (it is travelling at its fastest!).
Key Takeaway: Lift something up \(\rightarrow\) it gains \(E_p = mgh\). Let it fall \(\rightarrow\) \(E_p\) turns into \(E_k = \frac{1}{2}mv^2\).
6. Power
What is Power?
In physics, power is the rate of doing work or the rate of transferring energy. In simple words: power is how quickly energy is transferred!
The Power Equations
\(P = \frac{W}{t}\) or \(P = \frac{E}{t}\)
Where:
• \(P\) = Power in Watts (\(\text{W}\))
• \(W\) = Work done in Joules (\(\text{J}\))
• \(E\) = Energy transferred in Joules (\(\text{J}\))
• \(t\) = Time taken in seconds (\(\text{s}\))
Note: \(1\text{ Watt (W)} = 1\text{ Joule per second (J/s)}\). A \(60\text{ W}\) bulb transfers \(60\text{ J}\) of energy every second.
Real-World Analogy:
Imagine two people with the same weight running up the exact same flight of stairs. Both do the same amount of work because the force and distance are the same. But the person who runs up in \(5\text{ seconds}\) has more power than the person who walks up in \(15\text{ seconds}\), because they did the work much faster!
Common Mistake: Time Units
Time must always be in seconds! If an exam question gives time in minutes, multiply by \(60\). (e.g., \(2\text{ minutes} = 2 \times 60 = 120\text{ s}\)).
Key Takeaway: Power is speed of energy transfer: \(P = \frac{E}{t}\), measured in Watts (\(\text{W}\)).
7. Energy Resources
We need energy resources to generate electricity, heat our homes, and fuel our transport. Energy resources are split into two categories: renewable and non-renewable.
1. Non-Renewable Resources
Definition: Resources that are used up faster than they can be replaced. Once they run out, they cannot be replaced in our lifetime.
• Fossil Fuels (Coal, Oil, Natural Gas):
- Advantages: Reliable (can produce energy whenever needed), produce large amounts of energy, cheap infrastructure.
- Disadvantages: Release carbon dioxide (\(\text{CO}_2\)) which causes global warming; burning coal and oil releases sulfur dioxide (\(\text{SO}_2\)) causing acid rain; non-renewable.
• Nuclear Fuel (Uranium/Plutonium):
- Advantages: Does not produce greenhouse gases; produces enormous amounts of energy from a small amount of fuel.
- Disadvantages: Produces radioactive waste that is dangerous and difficult to store safely for thousands of years; high decommissioning costs; risk of rare catastrophic accidents.
2. Renewable Resources
Definition: Resources that can be replaced naturally within a short time and will not run out.
• Wind: Wind turns turbines directly to generate electricity. (Advantage: No emissions; Disadvantage: Unreliable / weather dependent, visual/noise impact).
• Solar: Photovoltaic cells convert sunlight into electricity; solar panels heat water. (Advantage: Clean, renewable; Disadvantage: Doesn't work at night, less power in winter/cloudy days).
• Hydroelectric: Water stored in a high reservoir flows downhill through turbines. (Advantage: Highly reliable, quick start-up time; Disadvantage: Flooding valleys destroys habitats).
• Tidal: Uses the natural movement of ocean tides to turn turbines. (Advantage: Highly predictable; Disadvantage: Can harm marine habitats, high setup cost).
• Waves: Uses the up-and-down motion of sea waves to generate electricity. (Advantage: Clean; Disadvantage: Unreliable in calm weather, can be damaged by storms).
• Geothermal: Uses heat from underground rocks to turn water into steam. (Advantage: Very reliable; Disadvantage: Only available in certain volcanic regions).
• Biomass: Burning plant or animal waste (e.g., wood pellets). (Advantage: Carbon neutral if replanted; Disadvantage: Requires large areas of land to grow crops).
Key Takeaway: Non-renewables (fossil fuels, nuclear) are reliable but harm the environment or run out. Renewables (wind, solar, hydro) won't run out and are cleaner, but many depend on the weather.
Quick Revision Checklist
Before your exam, make sure you can:
• State the Principle of Conservation of Energy word-for-word.
• Calculate efficiency using: \(\text{Efficiency} = \frac{\text{Useful energy output}}{\text{Total energy input}}\).
• Draw and interpret Sankey diagrams.
• Calculate work done: \(W = F \times d\).
• Calculate potential energy: \(E_p = mgh\).
• Calculate kinetic energy: \(E_k = \frac{1}{2}mv^2\).
• Calculate power: \(P = \frac{W}{t}\) and convert minutes into seconds.
• Compare the advantages and disadvantages of renewable and non-renewable energy resources.