CBSE Class 7 Science Advanced Physics Heat Short Answer Questions Set 01

Short Answer Questions

Teacher's Note:
1. Draw a two-column table.
2. Always give the units of both.

Question 2. What are the applications of expansion of solids?
Answer:
1. Iron rims are fitted on wooden cart wheels (fixed in the hot state, they grip tightly on cooling).
2. Metal plates are joined by riveting.
3. A small gap is left between rails when a railway track is laid.
4. Bridge girders are mounted on rollers, leaving a little space.
5. A sag is left in telephone and electric wires.
6. Small gaps are left between concrete road slabs.
7. Pyrex or Borosil glass, which expands very little, is used so that it does not break when hot water is poured.
In simple words: Because solids expand and contract with temperature, designers leave room for the change.

Teacher's Note:
1. Link each use to the idea of leaving room for expansion.
2. Give at least four points.

Question 3. Describe about clinical thermometer.
Answer:
A clinical thermometer is used to measure the temperature of the human body.
It has a long, narrow glass tube of uniform bore with a mercury bulb at one end.
A constriction (kink) near the bulb prevents the mercury from falling back, so the reading stays after the thermometer is removed.
It has two scales: Celsius (°C) and Fahrenheit (°F), marked from 35 °C to 43 °C (about 95 °F to 110 °F).
Normal body temperature, 37 °C or 98.4 °F, is marked with a red arrow.
In simple words: It is a special thermometer with a kink that holds the reading, for body temperature only.

Teacher's Note:
1. Mention the constriction; it is the key feature.
2. Draw a neat labelled sketch.

Question 4. Write the applications of good conductors of heat.
Answer:
1. Cooking vessels are made of metals so that heat passes to the food; copper-bottom vessels save time and fuel.
2. Mercury is used in thermometers partly because it is a good conductor.
3. Copper tubes are used in vehicle radiators, air conditioners and refrigerators to pass heat quickly.
4. The tip of a soldering iron is copper so that heat reaches the solder.
In simple words: Wherever heat has to move quickly, a good conductor is used.

Teacher's Note:
1. Mention copper often.
2. Give the reason for each use.

Question 5. Find the heat energy required to raise the temperature of 5 kg of water at 80 °C to 100 °C. (Specific heat of water is 1 cal g⁻¹ °C⁻¹)
Answer:
m = 5 kg = 5000 g, s = 1 cal g⁻¹ °C⁻¹, Δt = 100 − 80 = 20 °C
Q = m s Δt = 5000 × 1 × 20 = 1,00,000 cal = 100 kcal
In joule: 1,00,000 × 4.2 = 4,20,000 J
In simple words: Change kg to g first, then use Q = m s Δt.

Teacher's Note:
1. Convert 5 kg to 5000 g because s is per gram.
2. Give the answer in cal or kcal.

Question 6. Explain how land and sea breezes occur.
Answer:
Sea breeze (day): land heats up faster than the sea. The air above the land becomes hot and rises. Cooler air from the sea moves in to take its place, forming the sea breeze blowing from sea to land.
Land breeze (night): land cools faster than the sea. The air above the sea is now warmer and rises, and cooler air from the land moves towards the sea, forming the land breeze from land to sea.
Both are convection currents in air.
In simple words: Hot air rises and cool air rushes in; by day the land is hotter, by night the sea is hotter.

Teacher's Note:
1. State the direction of each breeze.
2. Mention convection.

Question 7. Why is mercury used in thermometers?
Answer:
1. It expands uniformly over a wide range of temperature.
2. It is a good conductor of heat.
3. It has a low specific heat capacity.
4. It is opaque, shiny and easily visible.
5. It does not wet the glass.
6. It is available in pure form.
7. It has a high boiling point and low melting point.
8. It has a very low vapour pressure.
In simple words: Mercury has all the qualities that make a good thermometer liquid.

Teacher's Note:
1. Give at least four reasons.
2. Also remember its drawback: it freezes at about −40 °C.

Question 8. Define conduction, convection and radiation.
Answer:
Conduction: the transfer of heat from the hotter end to the colder end of a body through the vibration of its particles, without the particles moving from their places. It occurs mainly in solids.
Convection: the transfer of heat in liquids and gases by the movement of the heated particles of the medium from one place to another.
Radiation: the transfer of heat without the help of any medium.
In simple words: Conduction passes heat particle to particle, convection carries it with moving fluid, radiation crosses empty space.

Teacher's Note:
1. Say whether a medium is needed.
2. Give one example of each.

Question 9. Write the applications of bad conductor of heat.
Answer:
1. Woollen clothes and blankets trap air and keep us warm in winter; animal fur does the same.
2. Mud houses with thatched roofs stay cool in summer and warm in winter.
3. Air between the double walls of cold storage stops heat from entering.
4. Double walls of an ice box are filled with glass wool so that the ice does not melt.
5. Handles of pressure cookers, electric irons and ovens are made of wood, plastic or ebonite.
6. Steam pipes are covered with asbestos or glass wool to avoid heat loss.
In simple words: Insulators are used to stop heat from going where we do not want it.

Teacher's Note:
1. Mention trapped air.
2. Write at least four uses.

Question 10. Why is an iron tyre heated before fitting it on a cart wheel?
Answer:
The iron tyre is made slightly smaller than the wooden wheel.
On heating, the tyre expands and becomes large enough to slip over the wheel.
On cooling it contracts and grips the wheel tightly.
In simple words: A hot tyre is bigger and slips on; cooling makes it shrink and hold tight.

Teacher's Note:
1. Linked to expansion of solids.
2. The fit becomes very firm.

Question 11. What are the factors affecting the heat lost or gained by a body?
Answer:
The heat gained or lost by a body depends on:
1. the mass of the body (m),
2. the specific heat of its material (s),
3. the change in temperature (Δt).
Q = m s Δt
In simple words: More mass, higher specific heat or a bigger temperature change means more heat gained or lost.

Teacher's Note:
1. Write the formula.
2. Q is directly proportional to each factor.

Question 12. Write the applications of radiation.
Answer:
1. The bottom of cooking vessels is coated with mud or painted dull black to absorb more heat.
2. White clothes are comfortable in summer because white reflects heat; dark clothes are comfortable in winter because black absorbs and radiates heat well and keeps the body warm.
3. Tankers carrying inflammable substances such as petrol and LPG are painted white so that they reflect heat.
In simple words: Light colours reflect heat and dark colours absorb it, and we choose accordingly.

Teacher's Note:
1. Black: good absorber; white: good reflector.
2. Mention clothes and tankers.

Question 13. What is the function of a thermos flask, how does it reduce heat loss?
Answer:
A thermos flask keeps hot liquids hot and cold liquids cold for a long time.
It has a double-walled glass bottle with a vacuum between the walls, resting on a rubber cork inside a case with the gap packed with cork.
Conduction loss: vacuum cannot conduct; cork and glass wool are bad conductors.
Convection loss: no air between the walls, so no convection.
Radiation loss: the shiny surface reflects heat.
In simple words: A vacuum and shiny walls stop all three ways of heat transfer.

Teacher's Note:
1. Name all three losses.
2. Draw the diagram.

Question 14. Explain thermal expansion in solids by using Gravesand's ring and ball experiment.
Answer:
The apparatus has a metal ball and a ring; the internal diameter of the ring equals the external diameter of the ball, so the cold ball just slips through the ring.
After heating the ball with a spirit lamp, it no longer passes through the ring because it has expanded.
When the ball cools to room temperature it slips through again, showing that solids contract on cooling.
In simple words: A heated ball gets bigger, and a cooled ball returns to its size.

Teacher's Note:
1. Give three observations.
2. Draw the ball and ring.

Question 15. Why does the air blown from a fan produce a cooling effect?
Answer:
The moving air increases the rate of evaporation of sweat from our skin. Evaporation takes heat from the body, which lowers the temperature and produces the cooling effect.
In simple words: Moving air dries sweat faster, and drying takes heat away.

Teacher's Note:
1. Evaporation causes cooling.
2. A fan does not lower air temperature.