CBSE Class 9 Physics Effect of Heat MCQs Set 07

Answer: (c) temperatures

Explanation:
1. Heat always flows from the body at the higher temperature to the body at the lower temperature.

Teacher's Note:
1. Heat flow stops when the temperatures become equal.
2. The amount of heat transferred depends on masses and specific heats.

Answer: (b) joule

Explanation:
1. Heat is a form of energy, so its SI unit is the joule.

Teacher's Note:
1. The calorie is a non-SI unit; 1 cal = 4.2 J.
2. The watt is the unit of power.

Answer: (d) all of the above

Explanation:
1. Mercury expands uniformly with temperature, boils at 357°C and freezes at -39°C, so it covers a wide range of temperatures.

Teacher's Note:
1. It also does not wet the glass and conducts heat well.
2. Alcohol is used for very low temperatures.

Answer: (c) its freezing point is very low

Explanation:
1. Alcohol freezes at about -115°C, far below the freezing point of mercury, so it can measure very low temperatures.

Teacher's Note:
1. Mercury would freeze in extreme cold at -39°C.
2. Colouring is a convenience, not the reason.

Answer: (c) expands on heating and contracts on cooling

Explanation:
1. The thermometer works on the expansion of mercury with temperature: the thread lengthens as the mercury is heated and shortens as it cools.

Teacher's Note:
1. Expansion with temperature is the basic principle.
2. Not wetting glass keeps the reading clean.

Answer: (d) depends on all the above

Explanation:
1. \( \Delta L = L\alpha\,\Delta T \): it depends on the original length L, the coefficient of linear expansion (the material) and the rise in temperature.

Teacher's Note:
1. A longer rod expands more for the same temperature rise.
2. Different materials have different coefficients.

Answer: (b) invar, steel, brass, aluminium

Explanation:
1. Approximate values of the coefficient of linear expansion: invar about 1 x 10⁻⁶, steel about 11 x 10⁻⁶, brass about 19 x 10⁻⁶ and aluminium about 23 x 10⁻⁶ per °C.

Teacher's Note:
1. Invar expands very little and is used in precision instruments.
2. Aluminium expands most among these.

Answer: (a) increase in size

Explanation:
1. Every linear dimension of the disc, including the circumference of the hole, increases on heating, so the hole expands as if it were made of the same metal.

Teacher's Note:
1. Imagine the hole filled with the same metal: it would expand.
2. The expansion is the same as for a solid disc.

Answer: (c) platinum resistance thermometers

Explanation:
1. The resistance of platinum varies smoothly and reproducibly over a wide temperature range, which makes platinum resistance thermometers accurate.

Teacher's Note:
1. Platinum has a very high melting point.
2. Alcohol and mercury thermometers are less precise.

Answer: (a) automatically maintaining a steady temperature

Explanation:
1. When the temperature exceeds the set value a bimetallic strip bends and breaks the circuit; when it cools the contact is restored.

Teacher's Note:
1. Refrigerators and irons use thermostats.
2. It does not itself produce heat.