CBSE Class 7 Science Advanced Physics Heat Very Short Answer Questions Set 03

Very Short Answer Questions

Question 1. Define temperature.
Answer:
The degree of hotness or coldness of a body is called its temperature. It depends on the average kinetic energy of its molecules and is measured with a thermometer.
In simple words: Temperature tells how hot or cold a body is.

Teacher's Note:
1. Units: °C, °F and K.
2. Heat is energy; temperature is the effect.

Question 2. Define melting and boiling points.
Answer:
Melting point: the constant temperature at which a solid changes into a liquid.
Boiling point: the constant temperature at which a liquid starts to boil and change into gas.
For water they are 0 °C and 100 °C.
In simple words: The melting point is where a solid melts and the boiling point is where a liquid boils.

Teacher's Note:
1. The temperature stays constant during both changes.
2. Melting point equals freezing point.

Question 3. What is specific heat capacity?
Answer:
The specific heat capacity of a substance is the quantity of heat required to raise the temperature of unit mass (1 g or 1 kg) of the substance by 1 °C. Its value is fixed for a given material. For water it is 1 cal g⁻¹ °C⁻¹.
In simple words: It tells how much heat 1 g of a material needs to warm up by 1 °C.

Teacher's Note:
1. s = Q ÷ (m Δt).
2. Water has a very high specific heat.

Question 4. Give three examples each of a good and bad conductor of heat.
Answer:
Good conductors: copper, aluminium, silver.
Bad conductors: wood, plastic, glass.
In simple words: Metals pass heat quickly; wood, plastic and glass do not.

Teacher's Note:
1. Bad conductors are also called insulators.
2. Other bad conductors: wool, cotton, asbestos.

Question 5. Why do we cover our bodies with blankets at night in winter?
Answer:
A blanket contains a lot of trapped air, which is a bad conductor of heat. It stops heat from flowing out of our body, so the body stays warm.
In simple words: The blanket keeps our own heat from escaping.

Teacher's Note:
1. Blankets do not produce heat.
2. The same reason applies to woollen clothes.

Question 6. What is evaporation?
Answer:
The change of a liquid into vapour at any temperature below its boiling point is called evaporation. It takes place from the surface of the liquid and is a slow process.
In simple words: Water slowly turns into vapour from the surface without boiling.

Teacher's Note:
1. Boiling is quick and happens throughout the liquid.
2. Wet clothes dry by evaporation.

Question 7. Define heat capacity.
Answer:
The heat required to raise the temperature of a given body (a certain mass of a substance) by 1 °C is called its heat capacity. Heat capacity = mass × specific heat (C = m s), and its unit is cal °C⁻¹.
In simple words: It is the heat needed to warm the whole body by one degree.

Teacher's Note:
1. It depends on how much substance there is.
2. Specific heat does not.

Question 8. What is heat?
Answer:
Heat is a form of energy, which cannot be seen but produces the sensation of hotness or coldness. It flows from a body at higher temperature to one at lower temperature.
In simple words: Heat is energy that makes things hot or cold.

Teacher's Note:
1. Its SI unit is the joule.
2. It can change into mechanical, electrical or light energy.

Question 9. What is the thermometer used to measure the temperature of a furnace?
Answer:
A pyrometer. It is made of two wires of different metals joined together; the electric current produced when one end is placed in the furnace gives the temperature.
In simple words: A pyrometer reads very high temperatures above 1000 °C.

Teacher's Note:
1. Laboratory thermometers cannot measure above about 110 °C.
2. The size of the current shows the temperature.

Question 10. What is the physical quantity that decides the flow of heat energy?
Answer:
Temperature. Heat flows from the body at the higher temperature to the body at the lower temperature.
In simple words: Heat flows downhill in temperature.

Teacher's Note:
1. Mass and heat content do not decide the direction.
2. Flow stops when temperatures become equal.

Question 11. What is the principle of thermometer?
Answer:
A thermometer works on the uniform expansion of the volume of a liquid (such as mercury) with the rise in temperature.
In simple words: The liquid expands evenly as it gets hotter, so its level shows the temperature.

Teacher's Note:
1. The expansion should be uniform.
2. Mercury is the commonly used liquid.

Question 12. Why is the handle of a kettle covered with plastic?
Answer:
Plastic is a bad conductor of heat. It does not allow the heat of the hot kettle to reach our hand, so we can hold the handle safely.
In simple words: Plastic stops the heat from burning our hand.

Teacher's Note:
1. Wood and ebonite are also used for handles.
2. The metal body is a good conductor.

Question 13. Define calorific value of a fuel.
Answer:
The total amount of heat energy produced by the complete burning (combustion) of a unit mass of a fuel is its calorific value. Calorific value = Q ÷ M, in J kg⁻¹ or cal g⁻¹.
In simple words: It tells how much heat 1 g of a fuel gives when burnt completely.

Teacher's Note:
1. Gaseous fuels usually have higher values.
2. Q = calorific value × mass.

Question 14. Define sublimation.
Answer:
The process in which a solid changes directly into a gas without becoming a liquid in between is called sublimation. The solid is called the sublimate and the gas the sublime.
In simple words: Some solids, like camphor, turn straight into vapour.

Teacher's Note:
1. Naphthalene balls are another example.
2. No liquid stage appears.

Question 15. What are the animals and plants living in polar region?
Answer:
Plants: lichens. Animals: polar bears, polar pigs and reindeer. They survive the extreme cold by adapting themselves.
In simple words: Only a few well-adapted living things survive the polar cold.

Teacher's Note:
1. Adaptations include thick fur and fat.
2. Few plants grow because it is so cold.

Question 16. Why boiling water is not used to sterilize a clinical thermometer?
Answer:
The clinical thermometer reads only up to about 43 °C (110 °F). Boiling water is at 100 °C, so the mercury would expand too much and the thermometer would break.
In simple words: Too much heat bursts the thermometer.

Teacher's Note:
1. It is cleaned with an antiseptic or cold water.
2. The constriction also prevents mercury from falling back.

Question 17. What is the range of temperatures, that can be measured using clinical thermometer in Fahrenheit scale?
Answer:
From 95 °F to about 110 °F (35 °C = 95 °F and 43 °C = 109.4 °F).
In simple words: It starts at 95 °F and ends near 110 °F.

Teacher's Note:
1. °F = (9/5) °C + 32.
2. Normal body temperature is 98.4 °F.

Question 18. What is the specific heat capacity of water in different system of units?
Answer:
CGS: 1 cal g⁻¹ °C⁻¹.
Also 1 kcal kg⁻¹ °C⁻¹.
SI: 4200 J kg⁻¹ K⁻¹ (1 cal = 4.2 J).
In simple words: The same number 1 in calories becomes 4200 in joules per kilogram.

Teacher's Note:
1. 1 kcal = 4200 J.
2. Multiply the CGS value by 4200 for SI.

Question 19. Two iron spheres of equal mass having temperatures 20 °C and 48 °C are brought into contact with each other. What is the equilibrium temperature? Assume there is no heat loss to surroundings.
Answer:
Equal masses of the same material have the same heat capacity.
Heat lost by the hot sphere = heat gained by the cold sphere: m s (48 − t) = m s (t − 20).
48 − t = t − 20, so 2t = 68.
Equilibrium temperature t = 34 °C.
In simple words: With equal masses of the same metal, the final temperature is the average of the two.

Teacher's Note:
1. (20 + 48) ÷ 2 = 34.
2. The answer lies between the two temperatures.