Heat energy

Outcomes:

Relate changes in the heat energy of a substance to changes in the movement of particles.

The state of a sample of matter depends on temperature:

Temperature: related to the average kinetic energy of an objects atoms or molecules.
Thermal expansion: increase in the size of a substance when the temperature increases and contracts when cooled. (Exception to the rule: water- when cooled it expands)

Heat

Most of us use the word ‘heat’ to mean something that feels warm, but science defines heat as the flow of energy from a warm object to a cooler object.

Actually, heat energy is all around us – in volcanoes, in icebergs and in your body. All matter contains heat energy.

Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat energy can be transferred from one object to another. The transfer or flow due to the difference in temperature between the two objects is called heat.

For example, an ice cube has heat energy and so does a glass of lemonade. If you put the ice in the lemonade, the lemonade (which is warmer) will transfer some of its heat energy to the ice. In other words, it will heat up the ice. Eventually, the ice will melt and the lemonade and water from the ice will be the same temperature. This is known as reaching a state of thermal equilibrium.

Transferring heat energy – particles in collision

At higher temperatures, particles have more energy. Some of this energy can be transmitted to other particles that are at a lower temperature. For example, in the gas state, when a fast moving particle collides with a slower moving particle, it transfers some of its energy to the slower moving particle, increasing the speed of that particle.

With billions of moving particles colliding into each other, an area of high energy will slowly transfer across the material until thermal equilibrium is reached (the temperature is the same across the material).

Changing states by heat transfer

Faster moving particles ‘excite’ nearby particles. If heated sufficiently, the movement of particles in a solid increases and overcomes the bonds that hold the particles together. The substance changes its state from a solid to a liquid (melting). If the movement of the particles increases further in the liquid, then a stage is reached where the substance changes into a gas (evaporation).

Activities:

Define heat.
Describe how thermal energy is transferred.
Hot cocoa is poured into a cold mug. Apply the concept of heat to explain what happens next.

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