Exercises: 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 traditional thermometer has a bulb containing a liquid (formerly mercury, nowadays coloured alcohol) that rises up a very thin tube when it is hot, as shown in the diagram.

On which effect of heat is its operation based? Explain it.

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.

Explain in your own words what thermal expansion is and give an everyday example in which it is noticeable, other than a thermometer.

Why does a fleece or a woollen jumper keep us warm? Does the garment "give heat" to the body or do something else? Explain.

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?

Double-glazed windows have a layer of air trapped between two panes of glass, as shown in the diagram.

Knowing that still air is a poor conductor of heat, explain what is achieved with this layer in winter.

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?

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.

The metal lid of a jam jar cannot be opened. You hold it under a hot water tap for a few seconds and, when trying again, it opens easily. Explain what has happened, taking into account that metal expands more than glass.

In a pressure cooker, steam cannot escape and the internal pressure increases, so water boils above $100\ ^\circ\text{C}$.

1. a) Explain why food cooks faster than in a normal pot.
2. b) What is the relationship between pressure and the boiling temperature of water?

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.

In homes with underfloor heating, hot water pipes run beneath the floor, heating the house very comfortably.

1. a) By which mechanism is heat distributed from the floor to the rest of the room?
2. b) Why is it more comfortable to heat from the floor than with a small, very hot radiator in a corner?

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.

The liquid in a thermometer expands very little, yet the column rises significantly up the tube. Knowing that the tube is a very fine capillary, explain how a small increase in volume is converted into a large rise that is easy to read.

On modern high-speed railway lines, the rails are welded together forming continuous sections kilometres long, without joints, despite thermal expansion.

1. a) What problem did traditional joints between rails solve?
2. b) Why do you think a high-speed train prefers continuous rails?
3. c) Reason how the rail can be attached to the sleepers to withstand thermal expansion: when installing it, is it better to leave it under more tension in summer or in winter? Justify 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.

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 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.

An igloo is made of blocks of compacted snow filled with small air pockets, and inside, the temperature remains close to $0\ ^\circ\text{C}$ even if it is $-20\ ^\circ\text{C}$ outside, as shown in the diagram.

1. a) Why is compacted snow a good insulator despite being cold?
2. b) Which mechanisms of heat loss does the closed shape of the igloo reduce?
3. c) If the temperature inside rises above $0\ ^\circ\text{C}$, does the igloo not melt? Reason all three answers.

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 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.

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.

Topic challenge. As a domestic energy auditor, design an insulation prototype to keep a hot drink from cooling down quickly.

1. Environmental conditions: Check the current temperature of your town or city on the AEMET website and record it alongside the indoor temperature of your home, expressing both in degrees Celsius and in kelvins ($T(\text{K}) = T(^\circ\text{C}) + 273,15$).

2. Home experiment: Pour hot water into two identical glasses: one left open to the air and the other covered with aluminium foil. Measure the temperature every $3\text{ min}$ for $15\text{ min}$, taking into account instrumental uncertainty (thermometer $\pm 1\,^\circ\text{C}$ and stopwatch $\pm 1\text{ s}$).

3. Data processing: Construct a table with your measurements and plot the cooling curves ($T$ versus $t$). Identify the areas where conduction, convection, and radiation act, analysing thermal equilibrium with the surroundings.

4. Reasoned proposal: Design a brief report with a proposal for an efficient homemade thermos using everyday recycled materials.

Assessment criteria:

• Correct conversion between temperature scales and appropriate expression of uncertainties ($\pm 1\,^\circ\text{C}$, $\pm 1\text{ s}$).

• Rigorous construction of the data table and comparative cooling graph.

• Accurate physical identification and justification of conduction, convection, and radiation.

• Scientific consistency, creativity, and feasibility in the homemade insulator proposal.