3.3 Radiation In cold countries, solar hot water systems are often used to heat a house. In a typical solar hot water system, water from a storage tank is pumped through a solar panel. In the solar panel, the water passes through copper pipes that are painted black. The water is then returned to the storage tank. Heating a houseExample 4 solar panel water pipe pump radiator storage tank

3.3 Radiation A black surface is a good absorber of radiation. It is more effective in absorbing radiation from the Sun. (a) Explain why the pipes in the solar panel are painted black. (b) Plastic pipes are cheaper than copper pipes. Explain why copper pipes are used in the panel rather than plastic ones. Copper conducts heat better than plastic. Copper pipes are used to facilitate heat exchange between the pipes and the water.

3.3 Radiation The density of cold water is higher than that of hot water. Cold water sinks to the bottom of the storage tank. Hot water rises and distributes at the top of the storage tank. The solar panel takes cold water from the bottom of the tank in order to heat the water up while the radiator takes hot water at the top of the tank to heat the house. (c) Explain why the solar panel takes water from the bottom of the storage tank while the radiator takes water from the top of the storage tank.

3.3 Radiation (d) The house is heated by pumping hot water from the storage tank through a radiator. Explain why the radiator is installed near the floor of the room. When the air near the radiator is heated, it expands and become less dense. The less dense air rises. Colder and denser air sinks and flows in to take its place. This can setup a convection current that flows around the room: