Energy, Density and Kinetic Theory

Energy, density and particles

Three big ideas in physics come together in this unit. Energy is never made or destroyed; it is only stored and transferred from one form to another, and how much useful energy you get out compared with what you put in is the efficiency. Density describes how much mass is packed into a space, worked out from mass and volume. Kinetic theory explains matter as tiny particles in constant motion, which is why a solid, a liquid and a gas behave so differently. Together these ideas explain everything from why a ship floats to why ice melts.

Energy and matter words

Learn these before you start. They name the energy ideas and the particle model at the heart of this unit.

Match the store to its example

A solid against a gas

Kinetic theory says all matter is made of particles, but how those particles are arranged and how they move is what makes a solid so different from a gas.

Which store is it?

A ball is held still at the top of a tall tower. Before it is dropped, which energy store does it mainly have because of its height?

• A gravitational potential store
• A kinetic store
• A thermal store
• An elastic store

Track where the energy goes

In any energy question, follow the energy from store to store. Name the store it starts in, name the store it ends in, and name how it was transferred. For example, do not just say the ball falls. Say the gravitational potential store empties as the ball falls and fills the kinetic store, transferred mechanically. Remember conservation: the total stays the same, though some energy spreads to the surroundings and warms them. Start store, end store, transfer: that is a full answer.

Pick the true energy facts

Select every statement about energy that is true.

• Energy cannot be made or destroyed, only transferred
• Efficiency compares useful energy out with total energy in
• Some energy usually spreads to the surroundings
• Energy can simply disappear
• A real machine can waste no energy at all

Order heating ice to steam

Put these stages in order as ice is heated steadily until it becomes steam.

• Solid ice, with particles vibrating in fixed places
• The ice melts as particles gain energy and break free
• Liquid water, with particles close but free to move
• The water boils as particles gain still more energy
• Steam, with particles far apart and moving fast

Complete the energy facts

The idea that energy cannot be made or destroyed is the _____ of energy. The mass packed into a volume is the _____. The fraction of supplied energy that is transferred usefully is the _____. The model of matter as moving particles is the _____.

Why a heavy ship floats

Density explains a puzzle: how can a steel ship float when steel is denser than water? Density is mass divided by volume, so it is about how spread out the mass is, not just how heavy something is. A solid block of steel sinks because its mass is packed into a small volume, making it denser than water. But a ship is mostly hollow and full of air, so the same mass is spread across a far larger volume. That lowers the overall density below that of water, and the ship floats. Change the volume and you change the density, even when the mass stays the same. That is the power of the density idea.

Spot the correct physics

Tap the TWO statements that are correct physics.

• Energy is only transferred, never made or destroyed
• In a gas the particles are far apart and move quickly
• Density is found by adding the mass and the volume
• Heating a solid destroys its particles

Work out the density

A block has a mass of 200 grams and a volume of 50 cubic centimetres. Density is mass divided by volume. What is its density, in grams per cubic centimetre?

Apply the physics

Use energy, density and kinetic theory to answer each problem.

• A machine takes in energy but wastes some as heat to the surroundings. What describes how good it is?
• You want to compare which of two blocks is denser. What must you know for each block?
• A liquid is heated until its particles move fast and spread far apart. What has happened?

Build a physics point

Choose the word for each gap to complete one correct physics point.

Explain energy, density and particles

A friend cannot see how energy, density and kinetic theory connect. Explain each idea and how physicists use it.

• Explain what conservation of energy means
• Explain what efficiency measures
• Explain how density is found and why a ship can float
• Explain how kinetic theory describes solids, liquids and gases
• Explain what happens to particles during a change of state