Temperature Control and Ventilation part 2

HEAT TRANSFER

Heat can be transferred in three ways:

(1)   Conduction

(2)   Convection

(3)   Radiation

CONDUCTION

When a hot object touches a cooler object, energy moves from the hot object to the cooler object.

The cooler object therefore gets warmer and the hotter object gets cooler. For examples when boiling water in a pan. The bottom of the pan touches the hot burner. Heat moves from the burner to the bottom of the pan. The water inside the pan touches the bottom of the pan. Heat then moves from the

pan to the water inside the pan.

Conductors are materials that allow heat to pass through them. Metals are good heat conductors.

Insulators are materials that do not allow heat to pass through them. Examples include plastic, rubber,

wood, liquid and air.

CONVECTION

Heat moves through a fluid. A fluid is something that flows such as liquid or air. The heat is carried from one place to the next. For example:

(1)   Air conditioning systems. The air conditioning unit is placed high up in a room. Cool air flows down from the unit. It picks up the warm air. This warm air is then carried back up into the unit. This warm air is cooled. The cool air flows down from the unit. The cycle continues.

(2)   Land and sea breezes. During the day warm air above the land rises. Cool air moves from the sea and comes over the land. The cool air picks up the heat from the land. This air now rises and moves back to the sea. At night, warm air over the sea rises. Cool air from the land comes over the sea. The cool air picks up heat from the sea. This air now rises and moves back to the land.

Convection takes places in the some appliances such as hot water systems, refrigerators, heaters,  radiators, and kettles.

RADIATION

A hot object gives off heat rays. These rays hit a cooler object. The cooler object absorbs the heat rays and becomes warmer. The two objects do not touch. There is an empty space/ vacuum between the two objects. For example:

(1)   A microwave is used at home to heat or cook food. The microwave gives off rays of heat. The rays of heat are absorbed by the food and the food cooks.

(2)   Walking in the sun. The rays of heat from the Sun hit the skin. The skin absorbs the heat and gets hot.

Dull, black, and rough surfaces absorb heat energy and radiate heat energy.

Shiny, silvery, and smooth surfaces do NOT absorb heat energy and do NOT radiate energy. Instead, they reflect heat energy.

The VACUUM/ THERMOS FLASK

The vacuum or thermos flask keeps food and drink close to their original temperature. Heat transfer is reduced. There is NO radiation, NO convection, NO conduction.

(1)   There is a vacuum/ an empty space – this reduces conduction and convection.

(2)   There are shiny, silver, and smooth glass walls – this reduces radiation.

(3)   The cover/ stopper is made from a heat insulator such as plastic – this reduces conduction and convection.

CONDITIONS THAT AFFECT THE RATE AT WHICH THE BODY LOSES HEAT

(1)   Wind

(2)   Temperature of the Air

(3)   Humidity

WIND

When the wind is blowing, the sweat easily evaporates from the skin. When it is windy we lose heat faster. The wind blows away the sweat. The sweat contains the heat.

TEMPERATURE OF THE AIR

When the temperature is hot (on a sunny day), our skin picks up heat. If it is cold, our skin gives off heat.

HUMIDITY

Humidity is the amount of water vapour in the air. If there is a lot of water vapour in the air, we do not sweat a lot because the air is saturated with water and can hold no more.

HOW WE KEEP OUR BODY COOL

(1)   We sweat a lot. The sweat is on the skin. The sweat picks up the heat on the skin by conduction. The sweat evaporates from the skin and carries the heat away by convection.

(2)   The hair on the skin lies flat. There is no layer of air close to the skin. No heat is trapped close to the skin.

(3)   Vasodilation – Capillaries get wider and carry more blood to the skin. The blood contains heat. The heat escapes by radiation from the surface of the skin.

The THERMOSTAT

A thermostat is used to control the temperature of appliances such as irons, kettles, and ovens. It contains a bimetallic strip. A bimetallic strip consists of two different metals such as iron and brass joined together. Each metal expands at a different temperature. Therefore, when the bimetallic strip is heated, the brass will expand more and become longer than the iron. The strip will therefore bend. This will stop the flow of electricity and the appliance will automatically switch off. It is used as a safety device to switch electrical devices off and on. The bimetallic strip bends and switches the appliance off if the temperature increases too much. When the temperature decreases and it is cool, the bimetallic strip straightens and turns the appliance on.