UNIT 3. HEAT AND TEMPERATURE

1 WHAT IS THE HEAT? Heat is the thermal energy that is transferred from a body or

system to another. This transfer is caused by a different temperature. Heat flows in a direction from a higher temperature to a lower

temperature. A system releases heat and the other absorbs. The heat flow stops when temperatures are the same. For

example, if you place a iron rod heated to 80 ° C in a container with water at 15 º C, the iron rod will lose heat and the water absorbed.

The thermal transfer of heat stops when the iron rod and water are at the same temperature. This is called "Thermal equilibrium".

2. HOW THE HEAT IS MEASURED? There are two units of measurement for heat. In the

International System, IS, the unit of measurement is the joule (J) but the calorie is another unit of measurement. A Joule is 0.24 calories.

1 j = 0.24 cal

3. WHAT IS THE TEMPERATURE?

The temperature is the amount of heat from a system. Therefore, the temperature is the magnitude of heat. Heat and temperature are not the same. When a system releases heat, its temperature decreases.

A thermometer is an instrument that measures temperature. Most thermometers use liquids in its inner, because liquids expand when are heated. Mercury and alcohol are the most common liquid in thermometers because, when are heated, the expansion is very large and easy to see in the glass tube of thermometers.

4. SCALE OF TEMPERATURE MEASUREMENT

The main scales to measure the temperature are: • The Celsius scale (° C) also called centigrade scale. It is the

most common scale in Spain. This scale is based on the behavior of water. The melting point of water is 0 ° C and the boiling point is 100 ° C.

• The Kelvin scale (K) also called absolute scale. This scale is used by the International System. This scale is based on the behavior of matter. The 0 K is the temperature at which the particles are stopped, no kinetic energy.

• Transforming temperature measurements between the two scales:

K = 0 - 273 ° C

K = C + 273For example, how do you transform a temperature of 360 K to °

C?

360 K = º C + 273360-273 º C =

º C = 360-273 = 87 º C

5. QUESTIONS AND ANSWERS ON THE HEAT CONCEPT:

DO THEY CONTAIN MORE HEAT AT HIGHER TEMPERATURES?

Heat is not a form of energy that is stored, as is the case with potential energy. The heat is simply the transfer of energy between two bodies that have different temperatures.

THEN WHAT DO WE CALL THE ENERGY THAT THE BODIES OR SYSTEMS CONTAIN?

It is called internal energy because it comes from the internal particles of an object. This could be due to the relationship between the particles of the object and the kinetic energy of its particles.

PROVIDING THAT HEAT IS TRANSFERRED TO A BODY DOES IT INCREASE HIS TEMPERATURE?

Not always. When ice is warmed to -5°C, the temperature increases until it reaches 0ºC, this is the temperature which produces the change of state from solid to liquid. Until a total change of state takes place the temperature does not rise because the energy is being used to break the bonds between the particles. Once we have only liquid water, the temperature will begin to rise, because the particles are acquiring kinetic energy.

6 WHAT ARE THE EFFECTS OF HEAT?

The effects of heat are two: • Contraction: the decreasing in volume that occurs when a

system loses heat. The particles move slowly and thus fill less space. (Example: a balloon when we put it in the fridge).

• Expansion: is the increasing in volume that occurs when a system gains heat. The particles move faster and therefore take up more space. (Example, the balloon when we put on the radiator)

6.1 THE ABNORMAL BEHAVIOUR OF WATER All solids, liquids and gases change volume when exchange

energy with another system. Normally, liquids expand more than solids. However, there are exceptions. For example, the water volume and are more solid than liquid, since solid state ordered molecules along a geometric pattern which leaves voids. Therefore, ice floats on liquid water. This is called "abnormal dilation" water.

7 HOW THE HEAT IS TRANSFERRED.Heat is transferred by conduction, convection and radiation.

a) Conduction: occurs in most solids. The heat is transferred by direct contact. Heat flows from the hotter to the colder body. The heat flows when the substances to reach the same temperature. (Thermal equilibrium)

Depending on the behaviour of the object may be:

• Conductors: are substances that easily allow the flow of heat through them.

• Insulation: are substances that do not easily allow the flow of heat through them. Examples are plastic, wood, air, etc.

b) Convection only occurs in fluids and gases. The heat is transferred by the movement of convection currents.

c) Radiation: It is the transfer of heat through electromagnetic waves. No contact between the heat source and the heated substance is necessary. It is transferred in the vacuum too. It is the fastest way to transfer heat.