CBSE Class 7 Science Advanced Physics Heat Very Short Answer Questions Set 02

Very Short Answer Questions

Question 1. State whether the following statement is true or false.
On heating, iron expands more than copper.
Answer:
False. Copper expands more than iron for the same rise in temperature.
In simple words: Copper expands more.

Teacher's Note:
1. This is why a copper-iron bimetallic strip bends towards iron on heating.
2. Different solids expand by different amounts.

Question 2. State whether the following statement is true or false.
To transmit heat from one object to another by conduction, the two objects should be in contact.
Answer:
True. In conduction heat passes from particle to particle, so the objects must touch.
In simple words: Conduction needs touching.

Teacher's Note:
1. Radiation needs no contact.
2. A hot rod passes heat to your hand only when touched.

Question 3. State whether the following statement is true or false.
Black substances absorb and lose heat radiations faster.
Answer:
True. Black is a good absorber and also a good radiator of heat.
In simple words: Black takes in and gives out heat quickly.

Teacher's Note:
1. White and shiny surfaces are poor absorbers and poor radiators.
2. Remember: good absorber means good radiator.

Question 4. State whether the following statement is true or false.
No medium is required for transfer of heat by the process of convection.
Answer:
False. Convection needs a medium (a liquid or a gas) because the heat is carried by the moving particles of that medium. Only radiation needs no medium.
In simple words: Convection happens only in liquids and gases.

Teacher's Note:
1. No particles, no convection.
2. Heat from the Sun reaches us by radiation.

Question 5. Fill in the blank.
Water is a ________ conductor of heat.
Answer:
bad
In simple words: Water passes heat on very slowly.

Teacher's Note:
1. Water gets heated mainly by convection.
2. Ice stays solid at the bottom of a tube while the top water boils.

Question 6. Fill in the blank.
If temperature of a substance increases, the average kinetic energy of molecules of the substance ________.
Answer:
increases
In simple words: Hotter means the molecules move faster.

Teacher's Note:
1. Temperature depends on average kinetic energy.
2. So they are directly related.

Question 7. Fill in the blank.
Fastest mode of transmission of heat is ________.
Answer:
radiation
In simple words: Radiation moves at the speed of light.

Teacher's Note:
1. It needs no medium.
2. Conduction is the slowest.

Question 8. Fill in the blank.
Calorific value of ________ fuels is high.
Answer:
gaseous
In simple words: Gas fuels like LPG give more heat per gram.

Teacher's Note:
1. Gaseous fuels burn completely and cleanly.
2. Calorific value = Q ÷ M.

Question 9. Fill in the blank.
The substances which do not allow heat to travel through them easily are said to be ________ of heat.
Answer:
bad conductors (insulators)
In simple words: Wood and plastic are bad conductors.

Teacher's Note:
1. Bad conductors are used for handles.
2. Another name is insulators.

Question 10. Fill in the blank.
Change in temperature of a body is 57 °C. The equivalent change in temperature in Kelvin scale is ________.
Answer:
57 K. Since K = °C + 273, a change of 1 °C equals a change of 1 K.
In simple words: The size of one degree is the same on both scales.

Teacher's Note:
1. Only the zero point differs by 273.
2. So a change in °C equals the same change in K.

Question 11. Fill in the blank.
During sublimation, the solid substance, which is converting to gaseous state is called ________.
Answer:
sublimate
In simple words: The solid that changes straight to gas is the sublimate.

Teacher's Note:
1. Camphor and naphthalene sublime.
2. Its vapour is called the sublime.

Question 12. Fill in the blank.
Heat capacity of 250 g of water is ________.
Answer:
250 cal °C⁻¹. Heat capacity C = m s = 250 g × 1 cal g⁻¹ °C⁻¹ = 250 cal °C⁻¹.
In simple words: It takes 250 cal to warm this water by 1 °C.

Teacher's Note:
1. Heat capacity = mass × specific heat.
2. Its unit is cal °C⁻¹.

Question 13. Fill in the blank.
100 g of ice at 0 °C is mixed with 0.25 kg of water at 0 °C. The net transfer of heat is ________.
Answer:
zero. Both are at the same temperature (0 °C), so there is no flow of heat between them.
In simple words: No temperature difference, so no heat flow.

Teacher's Note:
1. Heat flows only when temperatures differ.
2. The masses do not matter.

Question 14. Fill in the blank.
Condensation point of a substance is numerically equal to ________.
Answer:
boiling point. A vapour condenses at the same temperature at which the liquid boils.
In simple words: Boiling and condensation happen at the same temperature.

Teacher's Note:
1. Similarly, melting point equals freezing point.
2. For water both are 100 °C.

Teacher's Note:
1. 37 + 273 = 310 K.
2. Learn the temperatures of the Moon and Mercury.

Teacher's Note:
1. Convert kcal to cal first.
2. Use Q = m s Δt for D.

Teacher's Note:
1. 0 °C + 273 = 273 K.
2. Six's thermometer gives both maximum and minimum.

Question 18. Why white clothes are comfortable in summer?
Answer:
White is a good reflector of heat. White clothes reflect most of the heat falling on them and absorb very little, so we feel cooler.
In simple words: White clothes bounce the heat away instead of taking it in.

Teacher's Note:
1. Dark clothes absorb more heat.
2. This is an application of radiation.

Question 19. Define calorie.
Answer:
One calorie is the amount of heat energy required to raise the temperature of 1 g of water from 14.5 °C to 15.5 °C, that is, through 1 °C.
In simple words: A calorie is the heat that warms one gram of water by one degree.

Teacher's Note:
1. 1 cal = 4.2 J.
2. Larger unit: 1 kcal = 1000 cal.