Chapter 10
Solution for
Transfer of
Thermal Energy
Worksheet
Sec 3E Physics
MCQ
 Q1
–C
 Q2 – B
 Q3 – D
 Q4 – A
 Q5 – B
 Q6 – A
 Q7 - D
Section B
 Q1
Conduction, Convection and Radiation.
bi) Radiation. It does not require a material
medium to transfer thermal energy.
bii) Convection. When heat is lost from the surface,
the coffee at the surface contracts. Since density
equals mass per unit volume, the cooler coffee on
top becomes more dense than the surrounding
coffee, hence it sinks. Coffee from the lower region
rises as it is hotter and less dense. The difference in
the densities of the coffee sets up a convection
current, allowing the rest of the coffee to cool
down.
a)
Section B
 Q2
a)
b)
c)
The polished surface is shiny and therefore a poor emitter
of infra red radiation. Thus reduces the rate of thermal
energy lost to the surroundings, thus reducing the
amount of electrical energy and time required to boil the
water.
When the kettle is switched on, the water near the
heating elements expands. Since density equals mass
per unit volume, the warmer water becomes less dense
than the surrounding water, hence it rises. Water from the
higher region sinks as it is cooler and denser. The
difference in the densities of the water sets up a
convection current, allowing the rest of the water to
warmed up.
Plastic is a poor conductor of heat. It protects the user
from the heat generated by the kettle.
Section B
 Q3
a)
b)
Birds fluff up their feathers to trap air. Air is a poor
conductor of heat, hence it reduce heat loss from
the body of the birds to the cold surroundings.
Black surfaces are better absorber of heat than
white surfaces. Hence, the black car will be hotter
as it has gained more heat through radiation.
Section B

4a)Conduction

4b) Convection. During the day, the land heats up faster
than the sea. The air above the land will expands,
become less dense (density = mass/volume) than the
surrounding air and rises. The cooler air above the sea
will move towards the land to replace the warm air, and
in turn is being heated. This forms the sea breeze. At
night, the land becomes cooler than the sea. The air
above the sea will expands, become less dense than
the surrounding air and rises. The cooler air above the
land will move towards the sea to replace the warm air.
This forms the land breeze. The difference in the densities
of the air sets up a convection current, hence the
breezes.
Section B
 5a)
Conduction occurs in copper. On top of the
heat transfer through the vibration of copper
atoms, the presence of free electrons allow the
heat to be transferred quickly. When the free
electrons gained energy, they will move at great
speed towards the cooler region, where they
collide with the copper atoms, making them vibrate
more vigorously. Hence heat is transferred to the
cooler region. This process is known as free electron
diffusion.
Section B
 5b)
As copper is a good conductor of heat, it
quickly conducts the heat away from the lamp to
the surroundings, preventing it from reaching the
ignition temperature.
 6a) When the air at the top near the cooling unit is
cooled, it contracts. Since density = mass/volume,
the density of the cooled air will increase. When the
density of the cooled air is higher than the
surrounding air, it will sink. The warmer air at the
bottom, being less dense, will rise to take the space.
This difference in density of the air results in the
convection current, which allows the air to circulate
in the refrigerator.
Section B
 6b)
The warm air that moves into the refrigerator is
less dense and thus rise to the top of the
refrigerator. The cooler air at the top will continue to
sink towards the contents, therefore keeping them
cool.
 7a) Convection
 7b) Pipe P. As the water in the boiler is heated, it
expands and becomes less dense. Hence it rises
towards pipe P and flows through it into the hot
water tank.
Section B
 7c)
This connection keeps the cold, denser water
from pipe R below the hot water in the hot water
tank, minimising heat loss by convection from the
hot water to the cold water.
 7d) In the event that the temperature of the water
in the hot water tank becomes too high and causes
large expansion of the hot water, pipe S allows the
water to overflow into the cistern.
Section C
 1a)
Aluminium and stainless steel are good
conductors of heat. Thus, thermal enrgy from stoves
is transferred quickly through the cooking pots or
pans to cook the food.
 1b) Sawdust is an insulator and traps air, which is
also an insulator. This slows down the rate at which
thermal energy is transferred from the surroundings
to the ice blocks, preventing them from melting
quickly.
Section C
 1c)
The aluminium foil helps to cook the potatoes
faster as aluminium is a good heat conductor.
Thermal energy is transferred quickly through the
aluminium foil to cook the potatoes. The foil is shiny
and therefore a poor emitter of infrared radiation. It
slows down the rate at which thermal energy is lost
from the potatoes, thus keeping them for a longer
time after they are removed from the barbecue pit.
Section C
 2a)
In both metal and wood, the atoms or
molecules that are heated vibrates vigorously
Thermal energy is transferred when they collide with
neighbouring atoms or molecules, making them
vibrate more vigorously. This process of atomic or
molecular vibration is slow. In metal, an additional
mechanism called the free electron diffusion takes
place. The free electrons in the metal gains kinetic
energy upon heating, moves at great speeds
towards the cooler regions. As the electrons move,
they collide with the atoms in the cooler regions,
making them vibrate more vigorously, hence
energy is transferred. Metal is therefore a better
conductor than wood.
Section C
 2b)
As the water at the
bottom is heated, it expands.
Since density equal
mass/volume, the expanded
water is less dense than the
surrounding water and rises.
The cooler water at the top,
being denser, will sink. The
difference in the densities of
water in the upper and lower
region sets up a convection
current, which transfer the
heat to all parts of the water.
Data Based Question
 3a)
A material of lower thermal conductivity. It
greatly reduces the rate of conduction between
the interior and the exterior of the house, thus
helping to maintain the desired temperature inside
the house.
 3b) Expanded polystyrene foam. It has the lowest
thermal conductivity of 0.012W/m/K.
Data Based Question
 2500J/s
= 2500 joules per second
 Let t taken to be 1s, Q = 2500 J
 Using the formula provided,
 d = (thermal conductivity x A x ∆ θx t)/Q
 = (0.8 x 2 x 3 x 1)/2500
 = 0.00192 m
 = 1.92 mm