Thermal physics

Specific latent heat and radiation

What is specific latent heat?????

This is the energy absorbed or released when a substance changes state or phase e.g.) melting or freezing.

Specific latent heat of fusion

This is the heat required to convert 1kg of a substance at a fixed temperature from the solid to liquid state.

Specific latent heat of vaporization

This is the heat required to convert 1kg of a substance at a fixed temperature from a liquid to gaseous state.

Phase changesHere particles are rearranged when heat is added or removed from substances.

Example: when heat is supplied to change ice to water or water to water vapor.

PHASE CHANGE DEMONSTRATION

A THERMOMETER IS PLACED IN A BEAKER OF CRUSHED ICE, IT READS O DEGREES UNTIL THE ICE HAS COMPLETELY MELTED.A THERMOMETER PLACED IN BOILING WATER REGISTERS A CONSTANT TEMPERATURE OF 100 DEGREES………………………

Cooling curve

The above diagram is a demonstration of the cooling curve of naphthalene. At 80 degrees the temperature remains constant for around nine minutes, at this point in time the naphthalene is changing state. As demonstrated above temperature remains constant during a phase change.

The formulaBoth specific latent heat if fusion and

specific latent heat of vaporization are measured in J kg(-1).

The formula used is heat supplied or lost during a phase change = mass x

specific latent heat

E (H)= m x l (f) = m x l (v)

Examplemass of water evaporated =19.5g

Power rating of the immersion heater=50WTime for which the boiling water was heated=900s

Find a value for the specific latent heat of vaporization of water

The Heat supplied = power rating x time

=50 x 900 =45000J

45000J = mass of water evaporated x l (v)l (v)= ____45000___ x10(2)

1.95

=2.3 x 10(6) J Kg(-1)

Specific latent heat of vaporization using the electrical method

1) Place a beaker on a top pan balance.2) Switch on the immersion heater and bring the water

to a boil.3) Record the reading on the balance.4) Continue heating the water for a fixed time.5) Turn off the heater and at the same time take the

reading off the balance.6) Use the power rating off the heater and the time for

which the water was boiling to obtain a value for the heat supplied.

7) The difference of the two readings of the top pan balance gives the mass of water which turned to vapour.

8) Calculate a value for the specific latent heat of vaporization of water.

9) List possible sources of errors in the experiment as carried out.

Differences between evaporation and boiling

Boiling Evaporation

Takes place throughout the liquid

Takes place at one fixed temperature

Boiling temperature is not affected by change of surface area

Temperature is increased by increases in external pressure

Takes place at the surface of the liquid

Takes place at all temperatures

Rate increases if surface area increases

Rate decreases if external pressure increases

Rate is increased by increased movement of air over the surface of the liquid

Boiling and Evaporation

Radiation

What is radiation????????Nb) radiation takes place at the speed of light

and requires neither contact of mass flow. It can take place in a vacuum.

This is the process of heat transfer which does not require a medium. All objects radiate energy continuously in the form of electromagnetic waves. Objects also absorb electromagnetic radiation form their surroundings. An object whose temperature is steady is radiating and absorbing radiation at the same time.

The transfer of thermal energy by radiation continued

The radiation falling on an object may be partly reflected and partly absorbed . The absorbed radiation is converted to heat.

The greenhouse effectThe increasing use of fossil fuels have caused

a rise in the amount of carbon dioxide in the atmosphere. Neither carbon dioxide nor water vapour prevents the energy from the sun from reaching the surface of the earth. However, the radiant energy given off from the earth has a different wavelength and is absorbed by both the carbon dioxide and the water vapour. The absorption of the radiant energy causes the carbon dioxide molecules to vibrate and heat up. The heat so produced is reflected back to the earth which then heats up.

A vacuum flaskA vacuum flask is designed to reduce

heat loses by convection, conduction and radiation. The flask contains a double-walled glass vessel with a vacuum between the walls.

the silvering on the inner surfaces of the double wall reduces loss of heat by radiation.

The top and the support at the base are made of insulating materials. These reduce, but do not entirely cut out loss of heat by conduction. The flask hot liquids hot and cold liquids cold.

Picture of vacuum flask

The solar cookerThe solar cooker is a curved reflector with a

highly polished surface: it collects the sun’s rays and focuses them at a point. The pot or dish should be placed here.

Absorbers and Emitters

Good absorbers of radiant energy (black and rough surfaces) are also good emitters of radiant energy.

The end!!!!!!!!! *wink wink*