Heat transfer

HEAT TRANSFER

HEAT AND TEMPERATUREHeat is a form of energy which passes from a body at

higher temperature to a body at a lower temperature.

If two bodies at the same temperature are in contact, there is no net heat flow from one body to the other .This condition is known as thermal equilibrium.The SI unit of heat is the joule (J).

NOTE:Heat cannot be measured directly by an instrument as temperature is by a thermometer.

Modes of Heat Transfer

Heat can travel through a medium and also through vacuum.

There are three modes of heat transfer,

a) Conduction

b) Convection

c) Radiation

CONDUCTIONIf you stir hot tea using a metal spoon, you will observe

that the handle of the spoon becomes warm.

solids that are good conductors of heat (metals) use both atom vibration and free electrons to conduct heat.

EXPERIMENT 7.1:Comparing thermal conductivities of various conductors

Different materials have different thermal conductivities. Metals aregenerally good conductors of heat. Non-metals are poor conductors ofheat (insulator).

NOTE:Conduction is therefore the transfer of heat as a result of vibration of particles.

Factors Affecting Thermal Conductivity

Thermal conductivity in materials depends on the following factors:

(i) The temperature difference(AӨ) between

the ends of the conductor.

(ii) The length of the conductor (l).

(iii) The cross-section area(A) of the conductor.

(iv) The nature of the material (k).

In an equation form;

Thermal Conductivity in Liquids

Water at the top of the boiling tube boils while the ice remains unmelted.

Water is a poor conductor of heat.

Why Liquids are Poor Conductors of Heat ?

This is because there are large inter-molecular distances between liquid molecules.There are also fewer and rare collisions between the molecules.

Electrolytes, e.g., saltsolution,are better conductors of heat than pure liquids because of an increased compactness of the particles.

Thermal Conductivity in Gases

Gases are worse conductors of heat because of large inter-molecular distance.

Some applications of Good and Poor Conductors of Heat

• Good conductors are used to make cooking utensils ,soldering irons etc.

• Bad conductors are applied in, fire fighting and making handles for cooking pots.

CONVECTION

Convection is the process by which heat is transferred through fluids (liquids and gases).

(i) Natural convection; Involves change in density of the fluid with temperature.

(ii) Forced convection: Mixing of hot and cold parts of the fluid through some external stirring, like a fan or pump.

To demonstrate convection in liquids

When a liquid is heated ,it expands and this lowers its density.The less dense liquid rises and its place is taken by more dense colder liquid. This movement of liquid forms convection currents.

Convection Current in Gases

Convection currents are set up when air or gas is heated.

Molecular Explanation of Convection in Fluids

Molecules in fluids are further apart and have negligible cohesive force.

Convection currents are set up much faster in gases than in liquids because of the extremely low cohesive forces existing between the molecules of the gases.

Some Applications of Convection in Fluids

Domestic Hot Water System

A domestic hot water supply systemj works on the principle of convection current.

RADIATIONHeat transfer through vacuum is called thermal

radiation. All bodies absorb and emit radiation.

An electric bulb in a room produces both light and radiant heat. The radiant heat is absorbed by the materials in the room, which in turn give out radiant heat of lower energy, see figure.

Nature of Radiant Heat

Because of the nature of production, radiant heat is an electromagnetic wave that causes heating effect in objects that absorb it.

Radiation is also described as the flow of heat from one place to another by means of electromagnetic waves.

The paper burns out.

Emission and Absorption of Radiation

From the experiments above, black surfaces are better emitters of heatthan shiny surfaces.

Applications of Thermal Radiation1.Electric kettles ,iron boxes and cooking pans have polished surfaces to reduce heat loss through radiation.

2. Petrol tanks are painted silvery bright to reflect away as much heat as possible.

3. Houses in hot areas have their walls and roofs painted with bright colours to reflect away heat, while those in cold regions have walls and roofs painted with dull colours.

4. In solar concentrators, electromagnetic waves in form of radiant heat are reflected to a common point (focus) by a concave reflector.The temperature at this point can be sufficiently high to boil water.

Vacuum flask

A vacuum flask or thermos flask can keep the substance inside hot or cold for a long period of time.It designed such that the heat transfer by conduction ,convection and radiation between the contents of the flask and its surroundings is reduced to a minimum.

SUCCESS NOTES Heat is a form of energy which flows as a

result of temperature difference between two regions.

There are three modes of heat transfer namely conduction,convection and radiation.

In conduction and convection a material is required while in radiation,heat travels through a vacuum.

Thermal conductivity depends on:• Temperature difference-the greater the

temperature difference the higher the rate of conduction.

• Cross- sectional area-thicker materials conduct heat faster than thin ones.

• Length- rate of conduction decreases with increase in length of material.

• Nature of material-different materials conduct heat differently.

Metals are good conductors of heat than non-metals.

Water is not o good conductor of heat. The amount of radiation absorbed by a given

surface depends on its nature. Black/dull surfaces are better absorbers of radiation than polished surfaces.

QUESTIONS

1.Explain ?(a) Soup cools faster in a steel bowl

than in a glass bowl.

Since metals are better conductors of heat than non-metals,steel bowl, a metal alloy, conducts heat away faster from the soup than glass. Hence, the soup in steel bowl cools faster.

(b) Air conditioner unit is placed near the top of the ceiling in a room.

Convection currents occur when hot air rises and cold air sinks. The warmer air which is less dense rises to the top and after being cooled down by the air conditioner unit, the cooler air sinks. The convection currents cause the room to be cooled efficiently.

2.The figure shows the heating element of a heater being placed near the bottom of the liquid.

(a) Explain why the beaker is placed on a base made of saw dust.

Saw dust is a poor conductor of heat, so it can reduce heat loss from the beaker through conduction.

b.)What is the advantage of placing the heating element near the bottom of the liquid?

It allows convection currents to be set up quickly. The cooler liquid which sinks can be heated up quickly by the heating element placed near the bottom.

4) Mrs. Chen has two woks. A black one made of iron and a shiny one made of stainless steel. She prefers using the black one because she says it allows her to cook faster.

(a) Is she right? Explain your answer.

Yes. Black is a better absorber and emitter of heat so food cooks faster in black wok.

b)What forms of heat transfer take place when she fries vegetables in the wok?

Conduction and radiation from the flame to the wok.Conduction from the wok surface to the food.

5)The figure shows a beaker of water containing a small amount of blue dye crystals. When the container is heated, the blue dye crystals dissolve in the water and create a pattern in it.

Explain why wire gauze is used in this experiment.

It is used to spread heat of a Bunsen flame.

(i) Wire gauze to glass of beaker.

(ii) within water in the beaker

i) Conduction

ii) Convection

Hw.page 196

Revision Exercise 7