CBSE Class 7 Science Advanced Physics Heat MCQs Set 03

Answer: (c) When two bodies at the same temperature are in contact, no heat flows from one body to another.

Explanation:
1. Heat from the sun reaches us by radiation, not conduction, so (a) is wrong.
2. Sand heats up faster than water in the day, so their temperatures differ: (b) is wrong.
3. Heat flows only from a hotter body to a colder body, so (d) is wrong.
4. Two bodies at the same temperature are in thermal equilibrium and no heat flows between them: (c).

Teacher's Note:
1. Heat always flows from high temperature to low temperature.
2. No temperature difference means no net flow of heat.

Answer: (a) Conduction

Explanation:
1. Steel is a solid metal, and in solids heat passes from particle to particle without the particles moving from their places.
2. This is conduction. Convection needs a fluid, and radiation does not need a solid at all.

Teacher's Note:
1. Conduction: solids. Convection: liquids and gases. Radiation: no medium needed.
2. A spoon in hot tea gets hot at the handle by conduction.

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

Explanation:
1. Steam exists at 100° C or above at normal atmospheric pressure.
2. 100° C and 110° C are both possible, but 75° C is too low for steam.
3. Answer (d).

Teacher's Note:
1. Steam can be heated above 100° C once all the water has changed to steam.
2. Water boils at 100° C at normal pressure.

Answer: (a) Pyrometer

Explanation:
1. A laboratory thermometer reads up to about 110° C, a clinical thermometer from 35° C to 42° C, and Six's thermometer measures the highest and lowest temperatures of the day.
2. A pyrometer measures very high temperatures without touching the hot body: (a).

Teacher's Note:
1. Mercury would boil long before 1000° C.
2. Pyrometers are used for furnaces and molten metals.

Answer: (d) Wooden container painted white outside.

Explanation:
1. To keep the temperature steady, heat must not enter or leave easily.
2. Wood is a poor conductor (stops conduction) and white paint is a poor absorber and poor emitter of radiation.
3. So a wooden container painted white outside is the best: (d).

Teacher's Note:
1. Metals conduct heat and black surfaces absorb and emit heat easily.
2. Think of the two things to block: conduction and radiation.

Answer: (a) ABC

Explanation:
1. Ice must first melt, which takes heat without a rise in temperature, so ice stays coolest: A.
2. Water has a very large specific heat capacity, so it warms slowly: B.
3. Iron has a small specific heat capacity, so it warms up the most: C.
4. Increasing order of temperature: ice < water < iron, that is ABC.

Teacher's Note:
1. Lower specific heat capacity means a bigger temperature rise for the same heat.
2. Melting uses heat without changing the temperature.

Answer: (a) Calorie

Explanation:
1. Kelvin, Celsius and Fahrenheit are units (scales) of temperature.
2. The calorie is a unit of heat energy.

Teacher's Note:
1. Heat is energy and temperature is hotness; their units are different.
2. SI unit of heat: joule. 1 cal = 4.18 J.

Answer: (a) A → d; B → a; C → b; D → c

Explanation:
1. Metals expand by absorbing heat energy (A → d).
2. Conduction needs a medium (B → a).
3. Radiation does not need a medium (C → b).
4. The melting point of a pure substance equals its freezing point (D → c).
5. Answer (a).

Teacher's Note:
1. Heat transfer by radiation works in a vacuum.
2. Melting and freezing happen at the same temperature.

Answer: (a) cal g⁻¹

Explanation:
1. Calorific value is the heat produced by burning 1 g (or 1 kg) of a fuel completely.
2. So it is measured in heat per mass, for example cal g⁻¹.

Teacher's Note:
1. Calorific value = heat ÷ mass of fuel.
2. In SI it is J kg⁻¹.

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

Explanation:
1. In thermal equilibrium the temperatures of the two bodies are the same, so (b) is true.
2. The heat given out by the hotter body while reaching equilibrium equals the heat taken in by the other body, so (a) is true.
3. The external (kinetic or potential) energy has nothing to do with it.
4. Answer (d).

Teacher's Note:
1. Heat lost = heat gained is the law of conservation of energy for the heat exchange.
2. Equal temperature is the sign of equilibrium.

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

Explanation:
1. During a change of state the temperature stays constant while the heat goes into changing the state.
2. The mass of the substance remains the same.
3. The internal energy changes because heat is absorbed or given out.
4. Quantities that do not change: temperature and mass, (d).

Teacher's Note:
1. Melting ice stays at 0° C until all of it has melted.
2. Mass is never changed by heating or cooling.

Answer: (c) 375 cal

Explanation:
1. Change in temperature = 10 − (−5) = 15° C.
2. Q = m s Δt = 50 × 0.5 × 15 = 375 cal.

Teacher's Note:
1. Subtracting a negative number means adding: 10 + 5 = 15.
2. Q = m × s × Δt.

Answer: (b) 14.8 °C

Explanation:
1. Heat absorbed = 50% of the kinetic energy: \( \frac{1}{2} \times \frac{1}{2}mv^2 = m s \Delta t \).
2. The mass cancels: \( \Delta t = \frac{v^2}{4s} = \frac{50 \times 50}{4 \times 42} \) = 14.88 °C.
3. This is the value given in option (b) 14.8 °C.

Teacher's Note:
1. The mass of the bullet cancels out, so you do not need it.
2. The exact value is 14.88; the book gives 14.8.

Answer: (d) The temperature of ice can be less than 0° C.

Explanation:
1. (a) 100 cal s⁻¹ × 10 s = 1000 cal = 1 kcal, not 100 kcal: wrong.
2. (b) During a change of state the temperature stays the same although heat is supplied: wrong.
3. (c) A solid may only get hotter without melting: wrong.
4. (d) Ice can be cooled below 0° C, for example in a freezer: correct.

Teacher's Note:
1. Check numbers and units in the first option first.
2. Ice at −10° C is common in a deep freezer.

Answer: (b) Convection

Explanation:
1. Air is a fluid. Warm air rises and cool air sinks, and this carries heat upwards.
2. This process is convection.

Teacher's Note:
1. Convection currents are set up in liquids and gases.
2. Air is a poor conductor, so conduction is small.

Answer: (d) All the above

Explanation:
1. Water is liquid between 0° C and 100° C at normal pressure.
2. 0° C and 100° C are the freezing and boiling points, and 70° C is in between, so all can be the temperature of water: (d).

Teacher's Note:
1. Water can exist at 0° C and 100° C as well.
2. Freezing and boiling points are the limits of liquid water.

Answer: (a) 70° C

Explanation:
1. Mercury is a liquid between about −39° C and 357° C.
2. 500° C is above its boiling point and −273° C is below its freezing point, so only 70° C can be measured: (a).

Teacher's Note:
1. A thermometer liquid must stay liquid in the range measured.
2. −273° C is absolute zero, the lowest possible temperature.

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

Explanation:
1. Conduction and convection need a medium, and a vacuum has none.
2. So the vacuum stops both the loss of heat from hot contents and the gain of heat by cold contents through convection (and conduction): (d).
3. Radiation is reduced by the silvered walls, not by the vacuum.

Teacher's Note:
1. A vacuum cannot stop radiation; silvering does that.
2. A flask keeps hot things hot and cold things cold.

Answer: (d) DCBA

Explanation:
1. (D) 100 × 1 × 80 = 8000 cal.
2. (C) 100 × 1 × 100 = 10,000 cal.
3. (B) melting 100 × 80 = 8000 cal plus 10,000 cal = 18,000 cal.
4. (A) warming ice 100 × 0.5 × 5 = 250 cal plus 18,000 cal = 18,250 cal.
5. Increasing order: D < C < B < A, that is DCBA.

Teacher's Note:
1. Melting ice takes heat (80 cal per gram) without changing the temperature.
2. Add up all the steps each substance goes through.

Answer: (b) Joule

Explanation:
1. Heat is a form of energy, so its SI unit is the joule.
2. Calorie is not an SI unit, and Celsius and kelvin are units of temperature.

Teacher's Note:
1. Joule is the SI unit of all kinds of energy.
2. 1 calorie = 4.18 joule.

Answer: (a) A → d; B → c; C → b; D → a

Explanation:
1. Normal human body temperature is 37°C (A → d).
2. The hottest part of Mercury reaches about 460°C (B → c).
3. Venus is hotter on average, about 465°C (C → b).
4. The Moon's surface reaches about 110°C in the day (D → a).
5. Answer (a).

Teacher's Note:
1. Venus is the hottest planet because of its thick atmosphere.
2. Moon temperatures swing from very hot to very cold.

Answer: (d) All of these

Explanation:
1. Specific heat capacity = heat ÷ (mass × change in temperature).
2. All three options have this form (heat per mass per degree), so each is a valid unit: (d).

Teacher's Note:
1. The SI unit is J kg⁻¹ K⁻¹.
2. A change of 1° C equals a change of 1 K.

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

Explanation:
1. The sphere loses potential energy as it falls, and on hitting the ground this energy is changed into heat energy.
2. So the sphere gains heat energy and its temperature increases: (d).

Teacher's Note:
1. Energy is conserved; it only changes form.
2. The rise in temperature is very small.

Answer: (b) The final temperature t = t₁ + t₂

Explanation:
1. Mass is conserved, so m = m₁ + m₂ is correct.
2. The volumes of the same liquid add up, so v = v₁ + v₂ is correct (to a good approximation).
3. Temperatures do not add. The final temperature lies between t₁ and t₂, so t = t₁ + t₂ is incorrect: (b).

Teacher's Note:
1. The final temperature of a mixture lies between the two temperatures.
2. Mass and volume are additive; temperature is not.

Answer: (c) 1

Explanation:
1. Q = m s Δt, so 1000 = 100 × s × (20 − 10).
2. s = 1000 ÷ 1000 = 1 cal g⁻¹ °C⁻¹.

Teacher's Note:
1. Rearrange as s = Q ÷ (m Δt).
2. 1 cal g⁻¹ °C⁻¹ is the value for water.

Answer: (b) 140

Explanation:
1. Heat gained = m s Δt = 5% of ½mv².
2. \( \frac{5}{100} \times \frac{1}{2}v^2 = 490 \times 1 \), so \( v^2 = \frac{490 \times 200}{5} = 19600 \).
3. v = √19600 = 140 m s⁻¹.

Teacher's Note:
1. The mass cancels from both sides.
2. Take the square root at the end.

Answer: (d) 31°C

Explanation:
1. A clinical thermometer reads from 35°C to 42°C.
2. 36°C, 38°C and 40°C are in this range, but 31°C is below it: (d).

Teacher's Note:
1. Clinical thermometers are made only for body temperatures.
2. Range: 35°C to 42°C.

Answer: (a) Internal energy

Explanation:
1. Heat given to a body is stored as internal energy, so it always increases.
2. Density can decrease on heating, volume may stay the same or even shrink (water from 0° C to 4° C), and mass does not change: (a).

Teacher's Note:
1. Only internal energy is certain to rise.
2. Mass never changes with heat.

Answer: (b) \( \frac{1}{3} \)

Explanation:
1. For S: Q = 2500 cal, m = 500 g, Δt = 15 °C.
2. s = Q ÷ (m Δt) = 2500 ÷ (500 × 15) = \( \frac{1}{3} \) cal g⁻¹ °C⁻¹: (b).

Teacher's Note:
1. Use the 'Change in Temperature' column, not the final temperature.
2. Cancel zeros before dividing.

Answer: (c) CAB

Explanation:
1. First note the final temperatures: P is at 20°C and Q is at 85°C (C).
2. Then recall that heat flows from the hotter to the colder body (A).
3. Finally decide: heat flows from Q to P (B).
4. The order is CAB.

Teacher's Note:
1. Data first, rule second, conclusion last.
2. Heat flows from Q (85°C) to P (20°C).

Answer: (d) The specific heat capacity of Q is \( \frac{1}{6} \) cal g⁻¹ °C⁻¹.

Explanation:
1. s_P = 2000 ÷ (200 × 10) = 1 cal g⁻¹ °C⁻¹, so (b) is wrong.
2. s_R = 8000 ÷ (400 × 40) = 0.5 cal g⁻¹ °C⁻¹, so (c) is wrong.
3. m s for P = 200 × 1 = 200 and for R = 400 × 0.5 = 200, so equal heat gives equal rise in temperature: (a) is wrong.
4. s_Q = 1000 ÷ (300 × 20) = 1/6 cal g⁻¹ °C⁻¹: (d) is correct.

Teacher's Note:
1. Find all four specific heat capacities with s = Q/(m Δt).
2. The rise in temperature for a given heat depends on the product m × s.

Answer: (b) Radiation

Explanation:
1. Radiation travels at the speed of light, which is far faster than conduction or convection.
2. (b) Radiation.

Teacher's Note:
1. Sunlight reaches us in about 8 minutes across empty space.
2. Conduction and convection depend on particles moving or vibrating.

Answer: (d) Mass

Explanation:
1. Heating may change the temperature, density and volume.
2. The amount of matter does not change, so the mass remains the same.

Teacher's Note:
1. Heat is energy, and energy has (almost) no mass.
2. Mass is conserved.

Answer: (a) Conduction

Explanation:
1. A copper rod is a solid, and heat travels along it from particle to particle.
2. This is conduction.

Teacher's Note:
1. Metals are good conductors, copper especially.
2. Convection does not happen in solids.

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

Explanation:
1. Heat flows only while there is a temperature difference, so it stops when the final temperatures are equal: (c) is true.
2. The heat lost by the hot body equals the heat gained by the cold body: (a) is true.
3. The initial temperatures must have been different for heat to flow: (b) is false.
4. Answer (d).

Teacher's Note:
1. Initial and final temperatures are different things.
2. Heat lost = heat gained applies in an isolated exchange.

Answer: (d) 40°C

Explanation:
1. The graph has two flat parts, where heat is supplied but the temperature does not change.
2. The lower flat part (20°C) is melting and the upper flat part (40°C) is boiling.
3. The boiling point is 40°C.

Teacher's Note:
1. Flat parts of a heating graph are changes of state.
2. The higher flat part is boiling.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Work done by an external force (for example rubbing or hammering) is changed into heat energy, so the temperature rises. A is true.
2. Heat is a form of energy, which is why work (energy) can change into heat. R is true and explains A.
3. Answer (a).

Teacher's Note:
1. Rubbing your palms together warms them: work turns into heat.
2. Energy can change from one form to another.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Space between the sun and the earth is almost a vacuum, and heat still reaches us. A is true.
2. Heat radiations are electromagnetic waves, which do not need a medium. R is true and explains A.
3. Answer (a).

Teacher's Note:
1. Radiation is the only mode of heat transfer that works in vacuum.
2. Electromagnetic waves need no material medium.

Answer: (a) Both A and R are true, and R is the correct explanation of A.

Explanation:
1. Sound energy can be converted into heat energy in a medium, so the temperature can rise a little. A is true.
2. Heat and sound are different forms of energy, and one form can change into another. R is true and explains A.
3. Answer (a).

Teacher's Note:
1. Energy changes from one form to another.
2. The rise from sound is very small, but it does happen.

Answer: (d) A is false, but R is true.

Explanation:
1. In metals heat is transferred by conduction, not convection, so A is false.
2. The atoms of a metal vibrate about fixed positions and are not free to move from place to place, so R is true.
3. A is false but R is true: (d).

Teacher's Note:
1. Convection needs particles that can flow, which only fluids have.
2. Metals conduct heat very well.