Effects of heat and everyday applications

Small gaps are left at regular intervals between train tracks, and the same occurs between sections of many bridges, as shown in the diagram.

What effect of heat are these joints preventing? What could happen if they did not exist?

A concrete bridge of $300\ \text{m}$ can lengthen by several centimetres in summer.

1. a) Why does it lengthen?
2. b) What would happen at the ends of the bridge if it were built without expansion joints?
3. c) Will the joints be more open or more closed in winter? Reason your answer.

A bimetallic strip is made of two different metals joined together; when heated, one expands more than the other and the strip bends.

1. a) Which way does it bend: towards the metal that expands more or towards the one that expands less? Justify your answer.
2. b) Thermostats in irons and heating systems use this effect to switch the device on and off. Explain how this idea works.

When ice is heated, it first turns into liquid water and, if we continue heating, into steam. Observe the diagram:

1. a) What are these two changes of state called?
2. b) What are the opposite changes called, those that occur upon cooling?
3. c) In each of the four changes, indicate whether water absorbs or releases energy.

You hang out wet clothes to dry and, after a few hours, they are dry.

1. a) What change of state has the water in the clothes undergone?
2. b) Where did it get the energy required for this change?

When it is very hot we sweat, and as the sweat evaporates, we feel relief.

1. a) What change of state does sweat undergo?
2. b) This change requires energy: where does it come from?
3. c) Use the above to explain why a fan cools us down even though it does not cool the air.

You place two ice cubes in a glass of drink that is at room temperature.

1. a) What change of state do the ice cubes undergo?
2. b) Where do they get the energy required for this change?
3. c) Using the previous answers, explain why the drink cools down.

A burn from steam at $100\ ^\circ\text{C}$ is more severe than a burn from boiling water at $100\ ^\circ\text{C}$.

1. a) What change of state does steam undergo upon touching skin?
2. b) Taking into account that this change of state releases energy, explain why steam causes a worse burn even though it is at the same temperature as boiling water.

At the summit of a $3000\ \text{m}$ peak, where atmospheric pressure is significantly lower than at sea level, water boils at around $90\ ^\circ\text{C}$.

1. a) Explain the relationship between pressure and the boiling temperature of water.
2. b) Why is it harder to hard-boil an egg properly on a mountain than at the beach, even if the water is boiling in both places?
3. c) What would happen if you used a pressure cooker on the mountain? Justify your answer.

A vacuum flask keeps coffee hot for hours, but it is also used to carry cold water in summer. Explain what the flask does with heat in each case: does it heat up, cool down, or prevent heat from passing through?

A vacuum flask consists of a double-walled container with a vacuum between the walls, silvered surfaces, and an airtight stopper, as shown in the diagram.

Explain which mechanism of heat transfer is blocked by each of these three features.

A house has double-glazed windows, external shutters, and thick curtains.

1. a) Explain what each of the three elements contributes to reducing heat loss in winter.
2. b) In summer, what combination of shutters and curtains would you use during the day and at night to keep the house cool? Justify your decisions considering the three heat transfer mechanisms.

You forget a glass bottle completely full of water and tightly sealed in the freezer, and the next day it is broken.

1. a) Water is an anomalous substance: when it freezes, it expands instead of contracting. Using this fact, explain why the bottle broke.
2. b) Predict what would happen if the bottle were made of soft plastic and were not completely full. Reason your answer.

A hot-air balloon rises when a burner heats the air inside it, as shown in the diagram.

1. a) Upon heating, does the air expand or contract?
2. b) How does its density change compared to the air outside?
3. c) Explain why the balloon rises and what the pilot must do to descend, relating everything to the effects of heat.