CBSE Class 7 Science Advanced Physics Heat MCQs Set 04

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

Explanation:
1. At 0°C ice (solid) and water (liquid) can exist together.
2. At 100°C water (liquid) and steam (gas) can exist together.
3. Answer (d).

Teacher's Note:
1. Melting point and boiling point are the temperatures where two states meet.
2. A solid and a gas of water do not co-exist at one temperature at normal pressure.

Answer: (b) Erg

Explanation:
1. Heat is energy, and the CGS unit of energy is the erg.
2. Joule is the SI unit, and watt is the unit of power.

Teacher's Note:
1. 1 J = 10⁷ erg.
2. Calorie is a practical unit of heat, not a CGS unit.

Answer: (c) 30°K

Explanation:
1. Celsius, Fahrenheit and Réaumur temperatures are written with a degree sign: 20°C, 40°F, 35°R.
2. On the kelvin scale no degree sign is used, so 30°K is wrong; it should be 30 K.

Teacher's Note:
1. Write kelvin as K without the degree sign.
2. Degree sign for °C and °F only.

Answer: (a) Undergoing a change of state

Explanation:
1. Heat supplied with no change in temperature means the heat is being used to change the state of the substance.
2. Answer (a).

Teacher's Note:
1. Melting and boiling happen at constant temperature.
2. Latent heat does not change the temperature.

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

Explanation:
1. A black surface absorbs most of the radiation falling on it and also emits radiation well.
2. So it is a good absorber and a good emitter: (d).

Teacher's Note:
1. Good absorbers are good emitters.
2. Black reflects very little.

Answer: (c) 42°C

Explanation:
1. A clinical thermometer has a range of 35°C to 42°C.
2. 34°C is below the range, and 44°C and 45°C are above it. 42°C is the top of the range: (c).

Teacher's Note:
1. Check the limits: 35°C to 42°C.
2. Body temperature never goes above about 42°C.

Answer: (a) Average kinetic energy

Explanation:
1. Temperature is a measure of the average kinetic energy of the particles.
2. So a rise in temperature always means a rise in average kinetic energy: (a).

Teacher's Note:
1. Volume and density may or may not change.
2. Kinetic energy is linked to temperature.

Answer: (c) 25

Explanation:
1. Q = m s Δt gives 2000 = 800 × 0.5 × Δt, so Δt = 2000 ÷ 400 = 5°C.
2. Final temperature = 20 + 5 = 25°C.

Teacher's Note:
1. Add the rise to the initial temperature.
2. m × s = 800 × 0.5 = 400 cal per °C.

Answer: (b) BCA

Explanation:
1. First note the specific heat capacities (B): A = 0.2 and B = 0.8.
2. Then identify which is larger (C): B.
3. Then use the fact (A): A reaches the higher temperature, so heat flows from A to B.
4. The order is BCA.

Teacher's Note:
1. Look at the data, compare, then reason.
2. The body with less specific heat capacity becomes hotter.

Answer: (b) 8000 cal

Explanation:
1. Q = m s Δt = 500 × 0.8 × 20 = 8000 cal.

Teacher's Note:
1. Multiply mass, specific heat capacity and rise in temperature.
2. 500 × 0.8 = 400 cal per °C.

Answer: (c) Radiation

Explanation:
1. Space is a vacuum, so conduction and convection cannot work.
2. Heat reaches the earth as electromagnetic radiation: (c).

Teacher's Note:
1. Radiation does not need a medium.
2. This is also how you feel warm near a fire without touching it.

Answer: (d) All of these

Explanation:
1. Heating raises the temperature.
2. Most substances expand on heating, so the volume increases and the density decreases.
3. All three can change: (d).

Teacher's Note:
1. Density = mass ÷ volume, and heating changes the volume.
2. Mass is the one quantity that does not change.

Answer: (b) Convection

Explanation:
1. The cool air from the AC moves through the room and warm air moves to the AC, setting up air currents.
2. These currents carry heat, which is convection.

Teacher's Note:
1. Air currents mean convection.
2. Hot air rises and cold air sinks.

Answer: (b) The temperature of both the bodies is equal.

Explanation:
1. Heat flows only because of a temperature difference.
2. It stops when the temperatures of both the bodies become equal: (b).
3. Equal heat loss and gain is always true during the exchange and is not the condition that stops it.

Teacher's Note:
1. Think of what drives the flow of heat: a temperature difference.
2. No difference, no flow.

Answer: (b) 20° C

Explanation:
1. The graph has two flat parts, at 20°C and at 45°C.
2. The first flat part is the solid melting, so the melting point is 20°C.
3. The second flat part (45°C) is the boiling point.

Teacher's Note:
1. First plateau: melting. Second plateau: boiling.
2. 5°C and 35°C are only reference marks on the axis.

Answer: (b) Heat flows from gaseous to solid state.

Explanation:
1. The gaseous state of a substance is at a higher temperature than its solid state (the substance must be heated through melting and boiling to become a gas).
2. Heat flows from the hotter body to the colder one, so it flows from the gaseous state to the solid state: (b).

Teacher's Note:
1. A gas is always hotter than the solid of the same substance in normal conditions.
2. Heat flows from hot to cold.

Answer: (c) K

Explanation:
1. The SI unit of temperature is the kelvin (K).
2. °C and °F are other scales and joule is the unit of energy.

Teacher's Note:
1. There are 7 SI base units; kelvin is the one for temperature.
2. K = °C + 273.

Answer: (c) 30 C

Explanation:
1. Temperatures on the Fahrenheit and Celsius scales are written with the degree sign: 20° F and 30° C.
2. Kelvin is written without the degree sign: 30 K.
3. 30 C has no degree sign, so it is incorrect: (c).

Teacher's Note:
1. Degree sign for °C and °F; none for K.
2. The unit symbol must be written properly.

Answer: (a) Change of size

Explanation:
1. A substance usually expands when heated, so its size changes.
2. A change in state happens at constant temperature, and the mass never changes.
3. The possible change is (a).

Teacher's Note:
1. Thermal expansion is the normal effect of heating.
2. Mass stays the same.

Answer: (c) Reflector

Explanation:
1. A polished, shiny surface reflects most of the radiation falling on it.
2. It is a poor absorber and poor emitter, but a good reflector.

Teacher's Note:
1. Shiny means reflecting.
2. That is why a thermos flask has a silvered surface.