Tranfer Of Thermal Energy

Question 1

What is the definition of thermal energy

Answer 1

Thermal energy always flows from a region of higher temperature to region of lower temperature. Net flow of thermal energy occurs only when there is a difference in temperature.

Question 2

What is conduction

Answer 2

Conduction is the transfer of thermal energy through a medium without any flow of the medium.

Question 3

How does conduction work(non metals)

Answer 3

The particles at the hot end vibrate vigorously about their fixed positions.
They collide with neighbouring particles, making them vibrate more vigorously
The neighbouring region of rod becomes hot.
Thermal energy has been transferred without transfer of particles.
Eventually, particles at cooler end also set into vigorous vibration.>cooler end becomes hot.

Question 4

How does conduction work(metals)

Answer 4

In addition to the process that takes place in non metals, free electron diffusion takes place in metals.
Free electrons at the heated end absorb thermal energy, hence gain kinetic energy
The free electrons that gain kinetic energy move at greater speeds, and move to the cooler regions of the rod.
As these electrons move, they collide with the atoms in the cooler parts of the rod, making them vibrate more vigorously.
Thermal energy>transferred via the motion of free electrons.
Cooler end becomes hot

Question 5

What is convection

Answer 5

Convection is the transfer of thermal energy by means of convection currents in a fluid(liquid or gas), due to a difference in density.

Question 6

Why does convection currents only occur in fluids and not solids?

Answer 6

This is as convection involves bulk movement of fluid that carries thermal energy.
For solids, thermal energy is transferred from one particle to another through vibrations, without any bulk movement of particles.

Question 7

What is radiation

Answer 7

Radiation is the transfer of thermal energy in the form of electromagnetic waves such as infrared radiation without the aid of a medium.

Question 7

Factors affecting rate of emission and absorption of infrared radiation

Answer 7

1) colour and texture of surface- Dull and black surfaces are better emitters and absorbers of infrared radiation than shiny and silver surfaces
2) surface temperature- The higher the temp, the higher the rate of emission of infrared radiation
3) surface area- object with larger surface area will absorb and emit infrared radiation at higher rate.

Question 8

Application of convection(electric kettles)

Answer 8

When power is switched on, water near heating coil heats up, expands and becomes less dense.
Heated water rises, while cooler water at upper part sinks to replace heated water>convection current set up

Question 9

Applications of radiation(greenhouses)

Answer 9

1) During the day, infrared radiation from the sun passes through the glass roof. This warms up the soil and plants in the greenhouse.
2) The soil and plants get warm and emit infrared radiation with longer wavelengths
3) Infrared radiation with longer wavelengths is unable to pass through glass roof. It gets trapped in greenhouse.

Question 10

Vacuum flasks

Answer 10

Plastic stopper- reduces heat transfer by conduction, convection and evaporation
Trapped air- Air is a poor conductor of heat and hence, conduction through trapped air above the liquid is minimised.
Vacuum- vacuum between the double glass walls prevents conduction and convection through sides of the flask.
Double-walled glass container with silvered surfaces- silvered surfaces so that infrared radiation is reflected back into hot liquid or back to surroundings if liquid is cold. Heat transfer through radiation is minimised.
Cork-Cork is a poor conductor of heat and is used to prevent heat transfer by conduction.