Thermal Energy: Heat Flow – JSS2 Basic Science Lesson Note (Third Term)

Heat flow is the transfer of energy from a place at a higher temperature to a place at a lower temperature.

Behavioural objectives

By the end of the lesson, students should be able to:

1. state what heat flow is;
2. state the three rules that describe the way heat flow happens;
3. explain why there is no heat flow between objects at the same temperature;
4. name the three ways heat is transferred;
5. give examples of heat flow in living things, in the home and in machines.

What is heat flow?

Heat flow is the movement of energy from one place to another because of a difference in temperature.

• Direction: energy moves from the higher temperature to the lower temperature.
• Speed: the bigger the temperature difference, the faster the heat flows.
• Same temperature: there is no change in energy due to heat flow.
• Heat: measured in joules (J), the unit of energy.
• Rate of heat flow: measured in watts (W), the unit of power.

Where does heat flow into and out of your body?

• Heat in: energy flows into your body from the surrounding air, nearby objects, the sun outdoors and light bulbs indoors.
• Heat out: energy flows out of your body into the air and nearby objects.
• Clear night: you lose the most heat to the sky when you are outside on a clear night.

Does heat flow inside the Earth?

• Earth’s interior: the inside of the Earth is very hot.
• Direction: heat flows from the hot interior to the cooler surface.

What are the three rules of heat flow?

There are three rules that describe heat flow.

1. Rule 1: there must be a temperature difference, because energy flows as heat only when there is one.
2. Rule 2: energy as heat flows from higher temperature to lower temperature.
3. Rule 3: the greater the temperature difference, the faster the energy flows.

Why is there no heat flow between objects at the same temperature?

There is no heat flow because heat flows only when there is a temperature difference.

• No difference: with equal temperatures, energy has no direction in which to flow.
• Cooling drink: a hot cup of tea loses heat to the room until it reaches room temperature.
• Cold drink: a cold bottle of water gains heat from the room until it reaches room temperature.
• Result: once both are at the same temperature, heat flow stops.

What are the ways heat flows?

Heat flows in three ways: conduction, convection and radiation.

• Conduction: heat passes through solids, or between objects in contact, as in a metal spoon heating up in hot soup.
• Convection: heat is carried by moving liquids or gases, as in water boiling in a pot.
• Radiation: heat travels as waves and needs no material, as in warmth from the sun reaching Earth.

Where can we see heat flow?

How does heat flow affect plants and animals?

• Plants: energy from the sun reaches plants, algae and some bacteria and supplies the energy for photosynthesis.
• Cold-blooded animals: reptiles and insects in cold climates only come out in summer.
• Climate and numbers: heat flow is why there are more reptiles and insects in equatorial climates than in arctic climates.
• Warm-blooded animals: mammals and birds have high metabolisms and warm bodies, so they can live in quite cold places.
• Needs of warm-blooded animals: good dry fur or feathers, a layer of blubber and plenty of high-energy food keep body heat in.
• Elephants: their large ears act like giant radiators that let body heat escape.

How does heat flow affect the human body?

• Body temperature: the body keeps its temperature very close to 37 °C (98.6 °F).
• Constant balance: energy as heat flows out of, and into, your body all the time.
• Jacket in winter: it slows heat flow out of the body, so you stay warm.
• Shorts on a hot day: they let heat leave the body more easily, so you stay cool.

How does heat flow work in machines and the home?

• Steam boiler: it changes the chemical energy in coal, fuel oil or natural gas into heat.
• Steam heating: the heat changes liquid water into steam, which flows up into the radiators.
• Radiators in a room: heat passes to the air by conduction and convection, and to objects in the room by radiation.
• Car radiator: it transfers heat out of the car engine so the engine does not overheat.

Evaluation

1. What is heat flow?
2. State the three rules that describe the way heat flows.
3. What happens to heat flow when two objects are at the same temperature?
4. Why are there more reptiles and insects in equatorial climates than in arctic climates?
5. Name two things that warm-blooded animals in cold places depend on to keep their body heat.

Assignment

Explain how wearing a jacket in winter and wearing shorts on a hot day help your body keep its temperature steady. Then give two examples of heat flow from the lesson and say in which direction the heat moves in each one.