1.
Which
types of surface are the best absorbers and the best emitters of infra‑red
radiation?
Correct Answer
A. Best absorber - black and dull
Best emitter - black and dull
Explanation
Black and dull surfaces are the best absorbers and emitters of infra-red radiation because they have a higher ability to absorb and emit radiation compared to other surfaces. Black surfaces absorb more radiation because they have a higher absorption coefficient, meaning they can absorb a larger amount of radiation that falls on them. Dull surfaces, on the other hand, have a higher emissivity, which allows them to emit a larger amount of radiation. This combination of high absorption and high emission makes black and dull surfaces the best absorbers and emitters of infra-red radiation.
2.
A small piece of aluminium is placed in your hand.
After a short while, your hand feels warm. Explain why this happens.
Correct Answer
A. Aluminium reflects the thermal energy radiated by the hand.
Explanation
Aluminium is a good reflector of thermal energy. When the hand comes in contact with the aluminium, the thermal energy radiated by the hand is reflected back towards the hand instead of being absorbed by the aluminium. This reflection of thermal energy causes the hand to feel warm.
3.
In the following set-up, equal volumes of hot water were poured into
the containers and the temperatures of both containers and contents were the
same. After an hour, the temperature of container P was found to be lower than that of container Q. This shows that P is
a better ____ of heat than Q.
Correct Answer
C. Radiator
Explanation
Thermal energy needs to be radiated out of the container into the surroundings. For P to be at a lower temperature, it has to be a better radiator.
4.
A small cork is fixed with wax to a metal plate. An electric heater is
placed close to the plate. After a time, the wax melts and the cork drops off.
How does thermal energy reach the wax?
Correct Answer
C. By radiation and conduction
Explanation
Thermal radiation radiates from the electric heater and conducts through the air. Convection cannot occur because the metal plate is not located above the heater.
5.
A copper plate is heated to 100oC and then allowed to cool.
It cools by emitting __________.
Correct Answer
C. Infra-red radiation
Explanation
When a copper plate is heated to 100oC and then allowed to cool, it cools by emitting infra-red radiation. Infra-red radiation is a form of electromagnetic radiation with longer wavelengths than visible light. As the copper plate cools, the atoms and molecules in the plate lose energy and emit this radiation. This radiation carries away the excess heat energy from the plate, causing it to cool down.
6.
A vacuum will prevent
heat transfer by ___________.
Correct Answer(s)
B. Conduction and convection only
D. Conduction, convection and radiation
Explanation
A vacuum will prevent heat transfer by conduction and convection only because there are no particles or molecules present in a vacuum to transfer heat through direct contact or through the movement of fluids. However, a vacuum does not prevent heat transfer by radiation, as radiation can occur through the emission and absorption of electromagnetic waves, which do not require a medium to travel through.
7.
A solar heater uses energy from the Sun to heat water. The panels of
the heater are painted black. Why is this?
Correct Answer
A. To improve absorption of infra-red radiation
Explanation
The panels of a solar heater are painted black to improve absorption of infra-red radiation. Black surfaces have a higher absorption rate for radiation compared to other colors, allowing the panels to absorb more of the Sun's energy and convert it into heat. This helps to maximize the efficiency of the solar heater in converting sunlight into thermal energy for heating water.
8.
Which of the following will be the best absorber of infra-red
radiation?
Correct Answer
A. Dark animal fur
Explanation
Black coloured objects are better absorbers of thermal energy.
9.
When metal hot-water pipes are used to heat air in a room, how is most
of the energy transferred through the metal and through the air?
Correct Answer
B. Through metal - conduction
through air - radiation
Explanation
Radiation is a more efficient method of heat transfer than convection.
10.
The thermal
energy from a campfire during a scout camp reaches the scouts’ faces mainly by __________.
Correct Answer
A. Radiation only
Explanation
Air is a bad conductor of thermal energy. So most of the thermal energy will reach the faces by radiation