IGCSE Thermal - Conduction, Convection, Radiation, Evaporation

A solar water heater is a sustainable alternative to gas water heaters in homes. It uses sunlight to heat up large panels. These are connected to a liquid that then heats the metal container.

The large panels (pictured) are painted black inside some glass. Why are they painted black?

• Black absorbs more radiation than other colours.

• Black looks better for neighbours.

• Black prevents rusting.

• Black reflects more radiation than other colours.

Answer explanation

Black objects both absorb and emit the most radiation. In this case, their absorption properties are useful for absorbing sunlight and heating up.

In the solar water heater, the heated black plate under the glass then warms a cool liquid in a pipe that is in contact with the black plate.

What is the likely method of thermal energy transfer occurring from the black plate to the cool liquid?

• Conduction

• Convection

• Radiation

• Evaporation

Answer explanation

Since there is direct contact between the black plate and the pipe with water, the most likely method of thermal energy transfer is CONDUCTION. This is because conduction primarily occurs between objects that are in contact.

Convection only happens within a fluid.

Radiation is less important when objects are in contact compared to conduction.

In the heater, the pipes containing the heated liquid flow into contact with the water tank which is always ABOVE the black plates.

Why is it best to place the water tank above the black plates?

• The hot liquid has enough time to cool down by the time it reaches the water tank.

• The hot liquid will have enough pressure to flow up the pipe to the water tank.

• Because the hot liquid is less dense than cool liquid, so it rises and reaches the water tank.

• Because it is easier to install

Finally, the hot liquid in the pipe passes by the water tank. The full system is known as a "heat exchanger", which is used to pass thermal energy between objects and repeat.

The hot liquid cools down as it passes energy to the water tank. By the time it gets back to the black plates, the liquid is cold and the whole cycle repeats. (This is using the convection of a fluid to transfer energy between objects that are not connected!)

The question is, why is the water tank made of a shiny metal?

• Metal conducts the energy from the hot liquid well and it reflects radiation from the hot water inside to not cool too quickly overnight.

• The shiny metal is heated directly by sunlight in addition to the hot liquid so it gets even warmer.

How does evaporation lead to a liquid cooling down?

• The highest kinetic energy particles near the surface of the liquid escape, leaving behind particles with a lower average kinetic energy.

• The highest kinetic energy particles in any part of the liquid escape, leaving behind particles with a lower average kinetic energy.

• The highest kinetic energy particles near the surface of the liquid escape, leaving behind only lower energy particles.

• The highest kinetic energy particles in any part of the liquid escape, leaving behind only lower energy particles.

When a fluid is heated, the reason it rises is because the [BLANK] expands and becomes less dense than its surroundings.

• fluid particle

• fluid

• density

• system

When does evaporation occur?

• Below the boiling point of a liquid

• Above the boiling point of a liquid

• At any temperature of a liquid

• At the boiling point of a liquid

A metal rod is painted black on one half and a shiny silver on the other side. The metal bar is left in an ice-cold compartment for a few hours and then returned back into a normal room.

Which side of the metal bar will warm up fastest, and why?

• The black side will warm up faster because black is better at absorbing radiation than shiny silver.

• The silver side will warm up faster because silver is a better conductor.

• Both sides will warm up at the same rate because they are made of the same material.
• The black side will warm up faster because black is a better conductor.

Which of these actions will accelerate the evaporation of a lovely glass of hot chocolate?

• Sitting in a humid room

• Blowing over the surface of the liquid

• It being at a high temperature

• Putting a lid over it