Energy

Welcome to the World of Energy!

Energy is a bit of a superhero in Physics. It can’t be created out of nothing, and it can’t be destroyed; it just changes form. In this chapter, we are going to learn how to track energy as it moves through "systems," calculate how much energy is being stored, and look at how the world gets the energy it needs to keep the lights on. Don't worry if it seems like a lot of formulas at first—we will break them down step-by-step!

1. Energy Stores and Systems

In Physics, a system is just a fancy word for an object or a group of objects that we are interested in. When a system changes, energy is transferred.

Common Energy Stores

Think of energy stores like different bank accounts where energy can be kept:

• Kinetic Energy: Stored in moving objects.
• Gravitational Potential Energy: Stored in objects raised above the ground.
• Elastic Potential Energy: Stored in stretched or squashed objects (like springs).
• Thermal Energy: Stored in hot objects.

How Energy Moves

Energy doesn't just sit still; it moves between stores. For example:
Example: When you drop a ball, the energy moves from its gravitational potential store to its kinetic store. When it hits the floor and makes a "thud," some energy is transferred to the thermal store of the floor and the surroundings as sound.

Quick Review: The Big Rule

Energy can be transferred usefully, stored, or dissipated, but it cannot be created or destroyed. This is the Law of Conservation of Energy.

Key Takeaway: A system is whatever we are studying. When something happens in that system, energy moves from one store to another.

2. Calculating Energy (The "Big Three" Equations)

To succeed in AQA Physics, you need to be comfortable with these three equations. Let's look at them simply.

1. Kinetic Energy (\(E_k\))

This is the energy of movement. If it’s moving, it has \(E_k\).
\(E_k = 0.5 \times m \times v^2\)
(\(m\) is mass in kg, \(v\) is speed in m/s)

2. Elastic Potential Energy (\(E_e\))

This is for things that stretch or squash.
\(E_e = 0.5 \times k \times e^2\)
(\(k\) is the spring constant in N/m, \(e\) is the extension in metres)

3. Gravitational Potential Energy (\(E_p\))

The higher you go, the more you have!
\(E_p = m \times g \times h\)
(\(m\) is mass in kg, \(g\) is gravitational field strength in N/kg, \(h\) is height in metres)

Did you know? On Earth, the gravitational field strength (\(g\)) is usually 9.8 N/kg. You will always be given this value in the exam!

Key Takeaway: Kinetic is for speed, Elastic is for stretch, and Gravitational is for height. Always check that your units are in kg and metres before calculating!

3. Temperature and Specific Heat Capacity

Different materials need different amounts of energy to heat up. Think about a metal spoon versus a wooden spoon in boiling water—the metal one gets hot much faster!

What is Specific Heat Capacity (SHC)?

The specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1°C.

The Equation

\(\Delta E = m \times c \times \Delta\theta\)
\(\Delta E\) = Change in thermal energy (Joules, J)
\(m\) = Mass (kg)
\(c\) = Specific heat capacity (J/kg°C)
\(\Delta\theta\) = Temperature change (°C)

Analogy: Heating water is like filling a bucket. The SHC is the "size" of the bucket. Water has a very high SHC (a huge bucket), so it takes a lot of energy to raise its temperature.

Key Takeaway: Substances with high SHC take a long time to heat up but also stay warm for a long time.

4. Power

Power is the "speed" of energy. It tells us how fast work is being done or how fast energy is being transferred.

The Equations

\(P = \frac{E}{t}\) (Power = Energy / Time)
\(P = \frac{W}{t}\) (Power = Work Done / Time)

Power is measured in Watts (W). 1 Watt means 1 Joule of energy is transferred every second.

Example: If two motors both lift a heavy box, the more powerful motor is the one that lifts it the fastest.

Key Takeaway: Power = Energy per second. Faster energy transfer means higher power.

5. Efficiency and Wasted Energy

In the real world, no machine is perfect. Some energy is always "dissipated" (spread out) to the surroundings. We often call this wasted energy, and it’s usually in the form of heat.

How to reduce waste

• Lubrication: Reduces friction (so less energy is wasted as heat).
• Thermal Insulation: Reduces the rate at which heat escapes from a building.

Calculating Efficiency

Efficiency is a measure of how much energy put into a machine actually comes out as useful work.
Efficiency = Useful output / Total input

You can give this as a decimal (e.g., 0.35) or a percentage (35%). It can never be more than 1 (or 100%) because you can't get more energy out than you put in!

Key Takeaway: High efficiency means less energy is wasted. We use insulation and lubrication to keep efficiency high.

6. Energy Resources

The world needs energy for transport, heating, and generating electricity. We group our sources into two types:

1. Non-renewable

These will run out one day and they damage the environment (usually by releasing \(CO_2\)).

• Fossil Fuels: Coal, Oil, and Gas. Reliable but cause global warming.
• Nuclear Fuel: Very clean but produces dangerous radioactive waste.

2. Renewable

These can be replenished as they are used and generally do less damage to the environment.

• Solar and Wind: Great but unreliable (the sun doesn't always shine).
• Hydroelectric and Tides: Very reliable but can hurt local habitats.
• Bio-fuel: Carbon neutral because the plants take in \(CO_2\) while they grow.

Quick Review: Why don't we use renewables for everything? Usually, it's about reliability or cost. A wind farm won't help you if the wind stops blowing!

Key Takeaway: We have to balance our need for energy with the environmental impact and the reliability of the source.

Final Summary Tip:

When answering exam questions, always check your units. If you see grams, change to kg. If you see minutes, change to seconds. You've got this!