CBSE Class 10 Physics Heat MCQs Set 08

Answer: (b) mercury

Explanation:
1. Air, water and cork are poor conductors of heat.
2. Mercury is a metal and is a good conductor, unlike most liquids.

Teacher's Note:
1. Liquids except mercury, and gases, are poor conductors.
2. Metals conduct heat well because of their free electrons.

Answer: (a) silver

Explanation:
1. Among common metals silver has the highest thermal conductivity.
2. Copper is next, then aluminium, and iron is much lower.

Teacher's Note:
1. Approximate values in W/m K: silver 430, copper 400, aluminium 240, iron 80.
2. Copper is preferred for utensils because it is cheaper than silver.

Answer: (b) all of these: the cross-sectional area, the temperature difference between the ends and the time of flow

Explanation:
1. Q = K A (θ1 - θ2) t / d.
2. Q is proportional to area A, to temperature difference θ1 - θ2 and to time t.
3. It is inversely proportional to the length d.

Teacher's Note:
1. Doubling the length halves the heat conducted.
2. K is the constant of proportionality.

Answer: (b) W m⁻¹ K⁻¹ (equal to J s⁻¹ m⁻¹ °C⁻¹)

Explanation:
1. From Q/t = K A (θ1 - θ2) / d, K = (Q/t) d / [A (θ1 - θ2)].
2. Units: W × m / (m² × °C) = W m⁻¹ °C⁻¹.
3. A difference of 1 °C equals a difference of 1 K.

Teacher's Note:
1. J/s per metre per °C is the same thing as W/m °C.
2. In CGS units K is in cal/s cm °C.

Answer: (c) copper and iron

Explanation:
1. A thermocouple consists of two dissimilar metals joined at their ends.
2. Copper and iron are dissimilar metals.
3. The same metal at both ends gives no emf.

Teacher's Note:
1. Other pairs used are antimony-bismuth and platinum-rhodium.
2. The emf depends on the temperature difference of the two junctions.

Answer: (c) thermal radiation

Explanation:
1. A thermopile is a number of thermocouples joined in series.
2. The hot junctions are blackened to absorb radiation.
3. A larger galvanometer current means more intense radiation.

Teacher's Note:
1. A single thermocouple would give only a very small emf.
2. Thermopile is more sensitive because the emfs add up.

Answer: (d) It discharges more heat at the higher temperature than it takes from the cold space, using external mechanical work.

Explanation:
1. A refrigerator is a heat engine working in reverse.
2. A compressor does work, so the heat discharged outside equals the heat taken from inside plus the work done.

Teacher's Note:
1. This is why a closed refrigerator in a room warms it slightly.
2. Freon or ammonia is the working substance.

Answer: (b) the relative humidity of air

Explanation:
1. Hygrometers measure the moisture content of air.
2. The wet and dry bulb hygrometer uses the difference between the two thermometers readings.

Teacher's Note:
1. A barometer measures pressure and an anemometer wind speed.
2. A small difference in readings means high humidity.

Answer: (d) more moisture

Explanation:
1. The saturated vapour pressure rises with temperature.
2. So air at a higher temperature can hold more water vapour per unit volume.

Teacher's Note:
1. Cooling air can bring it to saturation and form dew.
2. This is why dew forms at night.

Answer: (c) good absorbers and good emitters of heat radiation

Explanation:
1. A good absorber of radiation is also a good emitter.
2. So a black surface absorbs more and also cools faster when hot.

Teacher's Note:
1. Polished surfaces reflect more and emit less.
2. A thermos flask is silvered to reduce loss by radiation.

Answer: (d) the temperature difference between the two junctions is greater

Explanation:
1. The emf produced increases with the difference of temperature of the junctions.
2. So the current increases too.

Teacher's Note:
1. The current is zero when both junctions are at the same temperature.
2. This is the Seebeck effect.

Answer: (c) the temperature gradient

Explanation:
1. The fall of temperature per unit distance along the conductor is the temperature gradient.
2. It is measured in °C per metre.

Teacher's Note:
1. K is defined for unit temperature gradient.
2. Steady state means the gradient does not change with time.

Answer: (c) thermocouple

Explanation:
1. A thermocouple is a circuit of two dissimilar metals with its junctions at different temperatures.
2. An emf and so a current is produced.

Teacher's Note:
1. A thermopile is several thermocouples in series.
2. The effect is named after Seebeck.

Answer: (b) thermopile

Explanation:
1. A thermopile uses many thermocouples in series with all the cold junctions on one side and all the hot junctions on the other.
2. The emfs add up.

Teacher's Note:
1. It is used to detect heat radiation.
2. The hot junctions are coated with lamp black.

Answer: (b) the coefficient of thermal conductivity

Explanation:
1. From Q/t = K A (temperature gradient), K = (Q/t) / [A × gradient].
2. This is the definition of the coefficient of thermal conductivity.

Teacher's Note:
1. It is a property of the material.
2. Its SI unit is W/m K.

Answer: (d) watt per metre per °C

Explanation:
1. K = (Q/t) d / [A Δθ], so the unit is W m / (m² °C) = W m⁻¹ °C⁻¹.
2. Watt per metre per °C is the SI unit.

Teacher's Note:
1. Do not drop the area factor when working out the unit.
2. In CGS it is cal/s cm °C.

Answer: (c) dew starts to form because the air becomes saturated

Explanation:
1. When moist air cools, a stage is reached at which it is saturated with the water vapour it contains.
2. Further cooling deposits dew.
3. That temperature is the dew point.

Teacher's Note:
1. Dew point depends on how much water vapour the air holds.
2. At dew point the relative humidity is 100 per cent.