CBSE Class 7 Science Advanced Physics Heat MCQs Set 01

Answer: (b) temperature

Explanation:
1. Heat always flows from the body at the higher temperature to the body at the lower temperature.
2. So the direction of flow depends only on the temperature of the bodies, not on their mass, density or heat capacity.

Teacher's Note:
1. Heat does not care how big or heavy a body is; only the temperature difference matters.
2. Flow stops when both bodies reach the same temperature.

Answer: (a) melting point of the substance.

Explanation:
1. Solid to liquid at a fixed temperature is melting.
2. That fixed temperature is the melting point. The boiling point is for liquid to gas.

Teacher's Note:
1. Match the change of state with its name: solid to liquid is melting.
2. Ice melts at 0 °C, which is its melting point.

Answer: (c) J kg⁻¹ K⁻¹

Explanation:
1. Specific heat capacity = Q ÷ (m Δt).
2. In the SI system Q is in joule, m in kg and temperature in kelvin, so the unit is J kg⁻¹ K⁻¹.
3. Option (b) is a CGS-type unit and option (a) uses °C, so only (c) is the SI unit.

Teacher's Note:
1. SI uses joule, kilogram and kelvin.
2. A change of 1 °C equals a change of 1 K, so the numbers are the same.

Answer: (d) All the above

Explanation:
1. Temperature can be given in the Celsius, Kelvin or Fahrenheit scale.
2. So all the above are units of temperature.

Teacher's Note:
1. Kelvin is the SI unit; Celsius is used in CGS.
2. Fahrenheit is used in clinical thermometers along with Celsius.

Answer: (c) 37 °C

Explanation:
1. Normal body temperature is 37 °C, which is 98.4 °F or 310 K.
2. 87 K, 273 K and 82 °F do not match this value, so the answer is 37 °C.

Teacher's Note:
1. Remember the three forms: 37 °C, 98.4 °F and 310 K.
2. 273 K is the freezing point of water.

Answer: (d) Both (a) and (b)

Explanation:
1. A clinical thermometer has two scales: Celsius (35 °C to 43 °C) and Fahrenheit.
2. So it is calibrated in both Celsius and Fahrenheit.
3. It has no absolute (kelvin) scale.

Teacher's Note:
1. The printed answer key gives (c), which is wrong because the thermometer has no kelvin scale.
2. Look at a real clinical thermometer: °C on one side and °F on the other.

Answer: (b) convection

Explanation:
1. The engine heats the water in the radiator circuit.
2. Hot water rises and cooler water sinks (circulates), carrying heat away. This movement of the liquid is convection.

Teacher's Note:
1. Convection means heat carried by the moving liquid or gas itself.
2. Copper tubes help, but the circulation of water does the main work.

Answer: (c) radiation

Explanation:
1. Conduction and convection need a material medium.
2. Heat that travels without any medium (like sunlight through space) is radiation.

Teacher's Note:
1. Think of the Sun: there is no medium in space, yet heat reaches us.
2. Only radiation can cross a vacuum.

Answer: (a) conduction

Explanation:
1. The particles of a solid cannot move from their places, so they cannot carry heat along (no convection).
2. They vibrate and pass the energy to neighbours, which is conduction.

Teacher's Note:
1. Particles of a solid only vibrate in place.
2. Convection needs particles to move from place to place.

Answer: (d) G > L > S

Explanation:
1. Gases expand the most, liquids expand less and solids expand the least for the same rise in temperature.
2. So the order is G > L > S.

Teacher's Note:
1. Particles of a gas are far apart and free to move, so it expands the most.
2. Solids hold their particles tightly, so they expand very little.

Answer: (d) Both (a) and (c)

Explanation:
1. On heating, the volume changes (expansion).
2. Mass stays the same, so density = mass ÷ volume also changes.
3. So density and volume change.

Teacher's Note:
1. Mass is the quantity of matter and does not change on heating.
2. Whenever volume changes with fixed mass, density changes too.

Answer: (a) Only A

Explanation:
1. Two window panes with a gap between them trap air, and air is a bad conductor of heat.
2. So a set of double windows is a better insulator (A is correct); the statement about single-double thickness glass (B) does not hold.
3. The answer is only A.

Teacher's Note:
1. Trapped air is the real insulator in double windows.
2. Same idea as two thin blankets being warmer than one thick one.

Answer: (a) glass, iron, copper

Explanation:
1. Glass is a bad conductor, iron is a fairly good conductor and copper is a very good conductor.
2. In increasing order of conductivity: glass, iron, copper.

Teacher's Note:
1. Metals conduct better than glass, and copper is better than iron.
2. Increasing order means lowest first.

Answer: (c) uniform expansion of volume of liquid or gas with temperature.

Explanation:
1. A thermometer uses the rise of a liquid (mercury) in a narrow tube.
2. The liquid expands uniformly in volume as the temperature rises. So the principle is uniform expansion of volume of a liquid or gas.
3. Solid expansion is not used, so (b) and (d) are wrong.

Teacher's Note:
1. The liquid thread rises because its volume increases.
2. Option (d) is wrong because it includes the solid in (b).

Answer: (c) Both A and B

Explanation:
1. At 0 °C the heat given to ice goes into pulling the molecules apart as the ice melts, so their potential energy increases (A is correct).
2. The kinetic energy does not change, so the temperature stays at 0 °C (B is correct).
3. The answer is both A and B.

Teacher's Note:
1. Temperature depends on the average kinetic energy of molecules.
2. During melting the heat changes the state, not the temperature.

Answer: (c) Both C and D

Explanation:
1. A is false: boiling takes place only at the boiling point.
2. B is false: evaporation takes place at any temperature below the boiling point.
3. C is true (bubbles form throughout the liquid) and D is true (evaporation is a surface process).
4. So the answer is both C and D.

Teacher's Note:
1. Boiling: fixed temperature, whole liquid. Evaporation: any temperature, surface only.
2. Check each statement one by one.

Answer: (c) Both A and B

Explanation:
1. A solid has length, area and volume, so it can expand in length (linear), area (areal) and volume (cubical).
2. Liquids and gases have no fixed shape, so they show only volume (cubical) expansion.
3. Both are true, so the answer is both A and B.

Teacher's Note:
1. Remember: only solids show all three types of expansion.
2. The word printed as arial in the book should read areal.

Answer: (d) Both (a) and (b)

Explanation:
1. Two different metals expand and contract by different amounts.
2. This unequal expansion on heating and unequal contraction on cooling makes the strip bend, so the answer is both (a) and (b).

Teacher's Note:
1. If both metals expanded equally the strip would not bend.
2. Used in fire alarms and thermostats.

Answer: (c) There is no effect on volume during cooling or heating a gas.

Explanation:
1. (a) and (b) are normally true: most solids expand on melting and most liquids contract on freezing (water is an exception).
2. (c) is wrong, because a gas expands on heating and contracts on cooling.
3. So the wrong statement is (c).

Teacher's Note:
1. Gases show the largest change in volume with temperature.
2. Read the question again: it asks for the wrong statement.

Answer: (a) the cool air below the lighted end moves to take the place of hot air above the lighted end.

Explanation:
1. The air at the lighted end becomes hot, lighter and rises.
2. Cooler air from below moves in to take its place, so a convection current carries the smoke up. The answer is (a).

Teacher's Note:
1. Hot air is less dense, not more dense.
2. This is convection in a gas.

Answer: (b) 15

Explanation:
1. Melting is the flat part BC of the graph, from 40 J to 340 J.
2. Heat absorbed in melting 20 g = 340 − 40 = 300 J.
3. Heat for 1 g = 300 ÷ 20 = 15 J.

Teacher's Note:
1. Read the heat values at the two ends of the flat part.
2. Divide by the mass to get the heat per gram. Option (c) was printed as 113, 50 in the book.

Answer: (d) Both (b) and (c)

Explanation:
1. A change of state takes place at constant temperature, shown by a horizontal part of the graph.
2. BC (melting at 0 °C) and DE (boiling at 100 °C) are horizontal, so the answer is both (b) and (c).

Teacher's Note:
1. Sloping parts mean temperature is changing; flat parts mean the state is changing.
2. Here there are two flat parts.

Answer: (d) CBDA

Explanation:
1. First, the room has ventilators at the top (C).
2. Hot air, being lighter, rises (B).
3. It passes out through the ventilators, leaving low pressure in the room (D).
4. Cold air then enters through windows (A). The order is CBDA.

Teacher's Note:
1. Follow the air: ventilator, hot air rises, leaves, cold air enters.
2. Cold air comes in last to fill the low pressure region.

Answer: (a) BEFADC

Explanation:
1. Take the capillary tube (B).
2. Fill with mercury while warming the bulb to remove air (E).
3. Cut the funnel and seal (F).
4. Mark the lower fixed point (A) and the upper fixed point (D).
5. Divide the gap into 100 parts (C). The order is BEFADC.

Teacher's Note:
1. Building comes first, marking the scale comes after.
2. Sealing must be done before the fixed points are marked.

Answer: (d) Both (b) and (c)

Explanation:
1. A change of state needs heat to be given to the substance (melting, boiling) or taken away from it (freezing, condensation).
2. So the answer is both (b) and (c).

Teacher's Note:
1. Heat gain or heat loss is the real cause of a change of state.
2. Merely 'decreasing temperature' is a result of losing heat, so (b) and (c) are the complete answer.

Answer: (d) All the above

Explanation:
1. Heating makes a body expand (size changes), raises its temperature and can change its state.
2. So all the above can change.

Teacher's Note:
1. Heat can bring about three kinds of change: size, temperature and state.
2. It may also change the chemical nature.

Answer: (a) temperature

Explanation:
1. Heat flows only because of a temperature difference.
2. When both bodies are at the same temperature, there is no net flow.

Teacher's Note:
1. Equal heat energy is not needed; equal temperature is.
2. This state is called thermal equilibrium.

Answer: (c) heat does not flow between oil and iron.

Explanation:
1. Both are at 40 °C, so there is no temperature difference.
2. Hence heat does not flow between them.

Teacher's Note:
1. Heat needs a temperature difference to flow.
2. Their temperatures stay unchanged.

Answer: (d) mechanical energy

Explanation:
1. Steam pushes the piston and turns the wheels.
2. Heat is mainly changed into mechanical energy.

Teacher's Note:
1. A steam engine does work, which is mechanical energy.
2. A little heat is wasted as sound and heat loss.

Answer: (a) 35 °C to 43 °C

Explanation:
1. Human body temperature lies between about 35 °C and 43 °C.
2. The clinical thermometer is marked over this range, so the answer is 35 °C to 43 °C.

Teacher's Note:
1. The same numbers in °F or K would be wrong temperatures for the body.
2. Normal value 37 °C lies in this range.

Answer: (d) Both (a) and (b)

Explanation:
1. (a) is true: convection needs particles that can flow, so only liquids and gases.
2. (b) is true: radiation travels at the speed of light.
3. (c) is false: conduction takes place mainly in solids but also, slowly, in liquids and gases.
4. So the answer is both (a) and (b).

Teacher's Note:
1. The book prints (d) as All the above, which is wrong because (c) is false; option (d) is corrected here.
2. Conduction is not limited to solids, though it is weak in liquids and gases.

Answer: (b) convection

Explanation:
1. Land and sea warm and cool at different rates, which sets up air currents.
2. Hot air rises and cooler air flows in: this is convection.

Teacher's Note:
1. Breezes are convection currents in air.
2. Sea breeze blows by day and land breeze by night.

Answer: (d) Both (a) and (c)

Explanation:
1. Wool is a bad conductor, and the air trapped between its fibres is also a bad conductor.
2. So body heat cannot escape. The answer is both (a) and (c).

Teacher's Note:
1. Trapped air is the key insulator.
2. Wool does not produce heat; it only stops its loss.

Answer: (d) All the above

Explanation:
1. The vacuum stops conduction and convection.
2. The shiny walls reflect radiation.
3. So all the three losses are minimized.

Teacher's Note:
1. A flask is built to stop heat entering or leaving.
2. Vacuum + cork + shiny walls.

Answer: (d) All the above

Explanation:
1. Heat can cause chemical changes (cooking), change in size (expansion) and change in temperature.
2. So the answer is all the above.

Teacher's Note:
1. Think of boiling an egg, an expanding rail and a warming room.
2. All three are effects of heat.

Answer: (a) Only A

Explanation:
1. A is true: the air layer between two blankets is an extra insulator.
2. B is false: mud houses with thatched roofs stay cool in summer and warm in winter, not the other way round.
3. So the answer is only A.

Teacher's Note:
1. Read statement B carefully: summer and winter are reversed.
2. Trapped air and mud both resist heat flow.

Answer: (d) All the above

Explanation:
1. A vacuum has no particles to pass heat on; water and air are bad conductors.
2. So all the above are poor conductors.

Teacher's Note:
1. Gases and liquids conduct poorly compared with metals.
2. A vacuum cannot conduct at all.

Answer: (d) All the above

Explanation:
1. (a) True: the CGS unit of temperature is °C.
2. (b) True: K = 30 + 273 = 303 K.
3. (c) True: the temperature changes depend on the masses and specific heats of the bodies.
4. So all the above are true.

Teacher's Note:
1. Kelvin = Celsius + 273.
2. If a small mass of hot water mixes with a large mass of cold water, the two temperature changes are not equal.

Answer: (c) Both A and B

Explanation:
1. Boiling takes place at a fixed temperature and the liquid absorbs heat (A true).
2. Solidification (freezing) also takes place at a fixed temperature, and heat is released (B true).
3. So both A and B are correct.

Teacher's Note:
1. Heat is absorbed on going solid to liquid to gas, and released in the reverse direction.
2. Temperature stays fixed in both cases.

Answer: (a) evaporation of water is more at higher temperature.

Explanation:
1. Hot water evaporates faster, so the air has much more water vapour.
2. The vapour condenses into tiny droplets on the cool mirror, forming fog. So the answer is (a).

Teacher's Note:
1. Fog needs plenty of vapour, and hot water supplies it.
2. Options (b) and (c) are false.