Sample Questions


Ans: B.
Copper is a good conductor of heat and conducts the heat away from the flame, causing the flame to be extinguished.


Ans: A.
Heating element is placed at the bottom to set up the convection current in order to heat up the water in electric kettle. With the heating element at bottom, water at the bottom warms up, becomes less dense and rises, and push down the cooler (denser) water to the bottom.


Ans: C.
Both air and glass are poor conductors of heat, this reduces heat flowing through window by conduction. Heat flowing through window by radiation is hard to prevent as radiation does not require any medium, hence heat flow through window mainly by radiation.


Ans: B.
Astronaut in space should wear highly reflective surface so as to not absorb the thermal energy in space and to reflect it away.


Ans: D.
Vacuum is a perfect insulator. Both conduction and convection requires medium and vacuum is not a medium, this prevents heat transfer by conduction and convection. Radiation can transfer heat in vacuum as it does not require a medium. Heat can still be lost through radiation in a thermo flask.


Ans:
a) The cup is expanded which takes a longer time for heat to be transferred into surroundings by conduction. It is also made of polysterine material, which is a poor conductor of heat, hence reduces thermal energy loss. Air trapped in expanded polysterine cannot flow and air is a poor conductor of heat, hence thermal energy loss is reduced. It is also white in colour, which reduces heat loss through radiation.

b) The cover prevents heat loss from surface of drink by convection and evaporation.


Ans:
a) Copper is a metal which contains many free electrons in it. Thermal energy is transferred through molecular vibrations and free electron diffusion. When heated, particles in copper gain kinetic energy and start to vibrate vigorously. Particles start to transfer thermal energy to neighbouring particles through vibrations When heated, the free electrons, which are able to move freely, gain kinetic energy and carry thermal energy in them, diffuse to the cooler part of copper. Electrons move very fast and transfer their thermal energy through collisions with other particles.

b) In air, particles are spread far apart from each other and are able to move about freely, thermal energy transfer is by molecular collisions. Unlike in solids, particles are closely packed together and not able to move about freely, thermal energy transfer is by vibrations. In air, due to the large distance between particles, molecular collisions is less frequent so transfer of thermal energy is slower, hence air is a poor conductor of heat.

Experiments
Exp 1:

1. Light up a match
2. Place flame onto wooden block
3. Place flame onto ashtray

Observations:

Flame went off, wooden block got scorched


Flame immediately went off, ashtray was not scorched

This proves that ashtray (stainless steel) is a good conductor of heat and it conducts the heat away quickly from the lighted matchstick. The wooden block, being a poor conductor of heat, conducts the heat away slowly, hence got scorched by the flame.

Exp 2:
[self made video]

As seen from the video, the flame of match connected to the glass goes off when comes into contact with the glass, even though glass is a poor conductor of heat. Glass still conducts heat, but at a slower rate compared to the steel (spoon), hence flame goes off as heat is conducted away by the glass. Also seen from video, flame goes off when comes into contact with steel as heat is conducted away quickly, this proves that steel is a good conductor of heat. The spoon also feels hot after the flame goes off, this shows that heat was transferred to the steel.

Conclusion: Poor conductors still conduct heat, just at a slower rate. Both glass and steel conducts heat. Steel is a better conductor than glass.

Conduction videos
Video 1: Test on rate of conductions of different metals

This video proves that different materials have different rate of conduction. In the video, the different metals conduct heat from flame to the end of the rod, the wax placed at end of each rod gains heat from the metals and melts. The wax attaching the thumbnail to the copper rod melts the fastest, which proves that copper is better conductor when compared to the other metals. Wax on the steel rod took the longest time to melt, which shows that steel is not a better conductor when compared to other mteals.

This is the conductivity of metals in descending order [better--->good].
Copper, aluminium, bronze, nickel, steel.

Video 2: Test on conduction of wire gauze

This video shows that flame is not able to burn through a wire gauze. As seen, the wire gauze is made of metal, which conducts heat [by molecular vibration & electron diffusion] away from the flame, that is why the flame is not seen burning through the gauze. If gauze is pressed fully down onto the flame, it will extinguish as the wire gauze conducts all the heat away from the flame.

Video 3: Test on conduction of fork [similar to my self made video]

As seen from video, the flame goes off as soon as it comes into contact with the fork. The metal fork is a good conductor of heat. All the heat from the flame is conducted away by the metal fork, which causes the flame to go off.

Convection videos
Video 1: Convection in clouds

As seen in the video, the lower part of the clouds rose up and pushed the upper part of the clouds down.

The Sun heats up the ground and the air will be heated up and a parcel of warm air forms. Once the warm air gets hot enough, it becomes less dense and rise quickly upwards in the atmosphere. When air rises upwards, it will cool due to the decrease in pressure and the change of density. This forms the convection current.

Something interesting
If the air cools enough, some of the water in the air will start to change from gas phase to a liquids phase. They will form as tiny droplets of water and form a convective cloud!


This shows how convection rain is formed! [due to convection currents]

Video 2: Convection currents in liquid

The cooler dyed water is denser than the surrounding water sinks as the ice melts and the warmer water, which is less dense, starts to rise. This sets up the convection current.

Video 3: Convection currents in liquid 2

As heat source is placed at the right arm, water in the right arm becomes warmer and less dense, and starts to rise. The warm water rising up the arm pushes the cooler water, which is denser, and cause it to sink down the left arm. The dyed cooler water is pushed down towards the heat source by the warm water and eventually will become less dense and rise up the right arm after being heated. A convection current is set up.

Radiation
Definition:
Radiation is the continual emission of infrered waves from surface of all bodies, transmitted without aid of medium.

• does not require medium
• all objects emit some radiant heat
• the hotter an object, the greater amount of radiant heat emitted

Absorption of infrared radiation
Infrared radiation is absorbed by all objects and surfaces, which cause a temperature rise.

Emission of infrared radiation
Infrared radiation is emitted by all objects and surfaces, which cause a temperature fall.

Factors affecting rate of infrared radiation

• Colour, texture of surface
• Surface temperature
• Surface area

Colour, texture of surface
Dull coloured, rough surfaces are better absorber and emitter
Shiny, white coloured, smooth surfaces are poorer absorber and emitter



Surface temperature
"The higher the temperature of surface of object relative to surrounding temperature, the higher the rate of infrared radiation."

Meaning of statement: The larger the temperature difference between surface and surroundings, the higher the rate of radiation of object.

Rate of emission

As seen from the picture above, the cup on the left side has a temperature difference with the surroundings of 50°C, while the cup on the right side has a temperature difference with surroundings of 10°C. The cup on the left side has a larger temperature difference, hence it will emit heat faster than the cup on the right.

Rate of absorption

As seen from the picture above, cup on the left side has a temperature difference with surroundings of 20°C, while cup on the right side has a temperature difference with surroundings of 10°C. The cup on the left side has a larger temperature difference, hence the cup on the left will absorb heat faster than the cup on right.

Graph

The high temperature at A shows a steep gradient, which proves that the rate of temperature decrease is high, which means the object is emitting heat, the rate of infrared radiation is high.

The lower temperature at B shows a gentle gradient, which proves that the rate of temperature decrease is low, which means the object is emitting heat at a slower speed than before, the rate of infrared radiation is low as the surface temperature gets nearer to room temperature.

Surface area
The larger the surface area of object, the higher the rate of radiation (emission, absorption).

Both cups have the same mass and material. The larger in size cup radiate heat faster than the smaller cup as it has a larger surface area, it will emit and absorb heat faster than the smaller cup.

Applications of radiation
Cooking utensils (eg. pots)

Cooking utensils are usually sliver in colour, polished and smooth, hence it is a bad absorber and emitter of heat. This helps to keep food cold when taken out of fridge, as it does not absorb much heat. This also helps to keep food warm, as it also does not emit much heat.

Cooling fins

The cooling fins are painted black as black colour is a good emitter of heat. It is able to radiate heat to the surroundings quickly and allow the equipment to be cooled.

Greenhouse

Solar radiation from the Sun enters greenhouse through the glass roof. This warms up the plants and soil, which cause them to start to emit infrared radiation. This infrared radiation emitted by plants is different from the solar radiation, hence it is unable to pass through the roof and gets trapped inside. The infrared radiation trapped inside gets built up and cause a temperature increase. This trapped heat helps plants grow better in cold climates.

Convection
Definition:
It is the main mode of heat transfer in fluids- liquids and gases (does not occur in solids!), which involves movement of hotter fluids from hot region to cool region, with the medium moving.

Why only occur in liquids/gases, not in solids?
In liquids and gases, thermal energy is transferred through the movement of particles, while in solids, thermal energy can only be transferred through vibrations of particles and particles are not able to move. Convection requires the movement of medium, particles in solids are not able to move, hence it only occurs in liquids and gases.

Formation of Convection Current
Convection Current is set up due to the difference in density.

Convection in liquids

Diagram shows direction of convection current in liquids.

1) When heated, water at the bottom expands.
2) Expanded water becomes less dense than cooler water.
3) Hotter, expanded water becomes lighter and starts to rise.
4) Cooler water is denser than the hotter water.
5) Cooler water starts to sink.
6) An upward current moves in the direction as shown, current forces cooler liquid to replace the hotter liquid.

The same concept applies to convection in gases.

Warm air rises, cool air sinks.

Main concept from both diagrams:
Convection current is set up by the difference in density,
with less dense warm fluids carrying thermal energy upwards.
hot liquid/gas rises, cool liquid/gas sinks.

Applications of convection
1) Land & Sea Breeze

Land is heated faster in the day, warm air rises, cooler air sinks and moves in.
Convection Current is set up.


Land is cooled faster in the night, sea becomes warmer,
warm air from sea rises, cooler air on land sinks.

2) Cooking appliance (eg oven)

Heating element is found at the bottom, so that convection current can be set up to increase temperature of appliance, warms up food. Heating element causes air at the bottom to be warmer. Warm air rises, cool air sinks. Convection currents help to heat up appliance.

What happens if heating element is placed on top?
Convection current is not set up. Heating element causes air at the top to be warmer. Hot air unable to rise, cool air remains, temperature in appliance remains the same, appliance is unable to heat up.

3) Refridgerator

Freezing unit is placed on top, so as to set up convection current. Air on top is cooled by the cooling unit, becomes denser, hence sinks. Warm air at the bottom is less dense, hence rise. Cooler air on top pushes the warm air at bottom to the top. The entire space in the refidgerator is cooled.

Conduction
Definition:
It is the transfer of thermal energy through a medium, without medium moving.

• Main mode of heat transfer in solids, also occur in liquids and gases.
• Conductors: Metals (eg. Silver, gold, copper, brass)
• Insulators (poor conductors): Glass, rubber, plastic, wood, wool, cotton, water, air
• Different materials conduct heat at different rates (depends on whether material is conductor or insulator)

How does conduction work?

• Molecular vibration
• Free electron diffusion

Molecular vibration

• slow to transfer thermal energy
• no transfer of particles
• occurs in both conductors and insulators

How does molecular vibration work?
1) Heat supplies thermal energy to particles
2) Thermal energy is transferred from one particle to another by vibration of particles
3) Thermal energy is converted to kinetic energy
4) Kinetic energy of vibrating particles at hot end is transferred to neighbouring particles

Free electron diffusion

• increase speed of transfer of thermal energy
• involves transfer of particles
• occurs in conductors only

How does free electron diffusion work?
1) Only conductors contain free electrons
2) Thermal energy in heat converted to kinetic energy
3) Free electrons gain kinetic energy and move faster
4) Electrons diffuse into cool end
5) Electrons collide with particles and transfer kinetic energy into them

Conduction in liquids and gases
Liquids and gases are POOR conductors of heat.



As shown, water at the top of test tube boils, but ice remains. The rate of heat transfer from top to bottom is extremely slow as water is a poor conductor. Hence, ice melts extremely slowly.


Bunsen burner does not light up match when it is placed a distance away from Bunsen burner, hence air is a poor conductor. The rate of heat transfer from flame to the match is extremely slow. Heat is trasnferred by conduction and also radiation.

Which is a poorer conductor? Air or Water?
Air.
In air, particles are spaced further apart than those in solids. Collisions between molecules are less frequent in liquids, lesser in gases. Transfer of kinetic energy from fast-moving particles to neighbouring particles is slower.

Applications of conduction

• Conductors
• Insulators

Conductors
1) Cooking utensils made of metals (eg. aluminium, stainless steel) are able to conduct heat and cook food faster.
2) Soldering iron rods are made of iron with tip made of copper. Copper is a much better conductor of heat than iron

Insulators
1) Wool is used as winter clothings. Wool, being an insulator, cause heat to be trapped in wool, unable to get out of wool, keeps body warm.
2) Handles of cooking utensils made of insulators so that hand will not get scalded while using it.

Transfer of Thermal Energy

What is Transfer of Thermal Energy?
It is the transfer of thermal energy ONLY when there is a difference in temperature.

How does it occur?
Thermal energy always flows from region of higher temperature to a region of lower temperature.

When will it stop?
Transfer of thermal energy will only stop when a state of thermal equilibrium is reached between two bodies, when the temperatures are same, there is no net flow of thermal energy between them.

Thermal energy is transferred through:
-> Conduction
-> Convection
-> Radiation


Heat transfer video

This video shows how thermal energy can be transferred in 3 ways.[conduction, convection, radiation]. As the moving black part on the object shows the thermal energy being transferred from heat source onto the object.