Energy - Forces doing work

Welcome to Topic 8: Energy - Forces Doing Work!

In this chapter, we are going to explore how energy makes things happen. Whether you are lifting a heavy box, driving a car, or boiling a kettle, energy is being moved from one place to another. We call this "doing work." Don't worry if Physics feels like a bit of a workout—by the end of these notes, you’ll understand exactly how forces and energy work together to power our world!

1. Energy Stores and Transfers

Energy is a bit like money: it can be stored in different "bank accounts" and moved between them, but it cannot be created or destroyed. This is the Law of Conservation of Energy.

What is a System?

In Physics, a system is just a fancy word for the object or group of objects we are looking at. When a system changes, energy is transferred. For example, when you throw a ball, the system changes from the ball being still in your hand to the ball moving through the air.

The Three Main Ways to Transfer Energy

Energy can be moved into or out of a system in three main ways:
1. Through work done by forces: Pushing or pulling an object.
2. In electrical equipment: Using a battery or the mains to power a device.
3. In heating: Moving energy from a hotter object to a colder one.

Did you know? In a closed system, no energy can enter or leave. This means the total amount of energy inside stays exactly the same, even if it moves between different stores!

Quick Review: Energy cannot be created or destroyed. It only moves between stores. In a closed system, the total energy is constant.

2. Forces Doing Work

In everyday life, "work" means a job. In Physics, work done has a very specific meaning: it is the energy transferred when a force moves an object through a distance.

The Rule of Work

If you push a wall as hard as you can but the wall doesn't move, you haven't done any "work" in Physics terms! To do work, the object must move in the direction of the force.

The Work Done Equation

You can calculate the amount of energy transferred using this formula:
Work done (joule, J) = force (newton, N) × distance moved in the direction of the force (metre, m)
\(E = F \times d\)

Memory Aid: Remember that 1 Joule = 1 Newton-Metre. If you see "Newton-metres" (Nm) on an exam, it’s just another way of saying Joules (J)!

Example: If you push a shopping trolley with a force of 10 N for a distance of 5 m, the work done is:
\(10 \times 5 = 50\) J.

Key Takeaway: Work done is simply a measure of energy transferred. If \(E = F \times d\), then no movement (\(d = 0\)) means no work is done.

3. Gravitational Potential and Kinetic Energy

When forces do work on an object, the energy usually ends up in one of two main mechanical stores: Gravitational Potential Energy (GPE) or Kinetic Energy (KE).

Gravitational Potential Energy (GPE)

This is the energy an object has because of its height. When you lift an object, you are doing work against gravity. That energy is "stored" as GPE.
Change in GPE (joule, J) = mass (kg) × gravitational field strength (N/kg) × change in vertical height (m)
\(\Delta GPE = m \times g \times \Delta h\)

Note: On Earth, the gravitational field strength (\(g\)) is usually taken as 10 N/kg.

Kinetic Energy (KE)

This is the energy of a moving object. Anything that has mass and is moving has kinetic energy.
Kinetic energy (joule, J) = 0.5 × mass (kg) × (speed)^2 (m/s)
\(KE = \frac{1}{2} \times m \times v^{2}\)

Common Mistake to Avoid: Don't forget to square the speed (\(v^{2}\)) before multiplying it by the mass and 0.5! This is the most common place students lose marks.

Quick Review: Lift an object up? You increase its GPE. Speed an object up? You increase its KE.

4. Wasted Energy and Dissipation

Whenever energy is transferred, some of it always ends up in a "less useful" store. We usually call this wasted energy.

Dissipation

When energy "spreads out" and becomes less useful, we say it is dissipated. For example, in a car, some energy is transferred to the thermal energy store of the brakes and the surroundings due to friction. This energy isn't "lost" (remember the conservation law!), but it is no longer useful for making the car move.

Mechanical Waste

Mechanical processes (like gears turning or wheels spinning) become wasteful when they cause a rise in temperature. This happens because of friction. We can reduce this waste by using lubrication (like oil) to help parts slide more easily.

Key Takeaway: No system is 100% efficient because energy is always dissipated (usually as heat) to the surroundings.

5. Power: How Fast is the Transfer?

Power is not about how much energy you have; it’s about how fast you can move it. Think of two cars: they both have enough energy to reach the top of a hill, but the car with the more powerful engine gets there faster.

Defining Power

Power is the rate at which energy is transferred (or work is done).
Power (watt, W) = work done (joule, J) ÷ time taken (second, s)
\(P = \frac{E}{t}\)

The Watt

The unit for power is the Watt (W).
1 Watt = 1 Joule per second (J/s).

Analogy: Imagine two students carrying bags of flour up a flight of stairs.
- Student A walks up slowly.
- Student B runs up quickly.
Both students do the same amount of work (because the bags and the stairs are the same), but Student B has more power because they did the work in less time.

Quick Review: Power is "energy per second." To find Power, divide the energy by the time it took to move it.

6. Efficiency: How Good is the Device?

Efficiency tells us how much of the energy we put into a device actually comes out as useful energy.

The Efficiency Equation

Efficiency = (useful energy transferred by the device) ÷ (total energy supplied to the device)

Because you can't get more energy out than you put in, your answer will always be a decimal between 0 and 1 (or a percentage between 0% and 100%).

Step-by-Step Example:
1. An electric motor is supplied with 100 J of energy.
2. It uses 80 J to lift a weight (useful work).
3. The other 20 J is wasted as heat.
4. Efficiency = \(80 \div 100 = 0.8\) (or 80%).

Common Mistake: If your efficiency answer is greater than 1, you have accidentally swapped the numbers! Remember: Small number ÷ Big number.

Key Takeaway: Efficiency is a way of measuring how much energy is "wasted." Higher efficiency means less energy is dissipated as heat.

Summary of Topic 8

• Energy: Cannot be created or destroyed, only moved between stores.
• Work Done: The energy transferred when a force moves an object (\(E = F \times d\)).
• GPE: Energy from height (\(m \times g \times h\)).
• KE: Energy from movement (\(0.5 \times m \times v^{2}\)).
• Power: The speed of energy transfer (\(P = E \div t\)).
• Efficiency: The ratio of useful energy to total energy supplied.