Chapter 11 – Energy

Consider the following statements and click to reveal the answer.

1. How much work is done if a pram is pushed with a force of 60N for 50 metres?

3000 Joules.

2. How much work is done keeping a 500W security lamp on for two hours?

500 × 7200 = 3 600 000 joules.

3. Will a red-hot drawing pin have more heat energy than a large pan of boiling water? Explain your answer.

No, the pan will have more heat energy. If you dropped the very hot drawing pin into a pan of warm water, there would not be enough energy transferred from the pin to the water to cause it to boil.

4. What form of energy is associated with lying on a mattress?

Strain energy – the springs are compressed and release their energy when you get off the bed.

5. What is the liquid that drips out of car exhausts on cold days?

Water. It is formed when the hydrogen in the petrol burns. On cold days, it condenses in the cool parts of the exhaust.

6. If digestion takes place in the gut, where does respiration take place?

In all the cells of the body. The sugars are transported there in the bloodstream.

7. Why does the growth of birds, animals and arthropods account for such a small portion of their food intake?

Much of it is converted to power the life processes and the heat and movement of the living things.

8. Thinking about converting energy in our bodies: how much chocolate would you need to replace the energy used in eight hours of sleeping?

Just over 100g.

9. How does a thermos flask keep tea hot for long periods?

The tea is kept hot because

1. conduction is kept to a minimum by the thin wall of the flask,
2. convection is minimised by having two layers of glass separated by a vacuum (convection needs a gas to transfer the heat from one wall to the other),
3. radiation is minimised by silvering the glass so reflecting much of the heat back into the flask.

10. Would a fridge with the door left open cool a perfectly insulated room?

No, it would warm it, since there is an input of electrical energy extracted from the body of the fridge which is simply released into the room.

11. Imagine that we were able to collect all of the gases given off by a burnt candle. Would the mass of the gases be less than the original candle, more than the original candle or the same as the original candle? Explain your answer.

More than the mass of the original candle because burning the wax is a constructive process in which the oxygen is combined with the elements which make up the wax.

12. When plants photosynthesise, what chemical bonds are they breaking in the materials that they use?

They are pulling apart the oxygen and carbon from the carbon dioxide and they are separating the oxygen from the hydrogen in the water molecules.