# QUANTITY OF HEAT.

Heat.

• A form of energy that flows from one body to another due to temperature difference between them, hence quantity of heat is quantity of energy.
• It is measured in joules (J).
• Temperature is the degree of hotness or coldness of a body or place.
• It is measured in kelvin (K) and Celsius ().
• When a body absorbs heat energy
1. Its temperature rises. This is called heat capacity.
2. Its state changes. This is called latent heat.

Change of temperature.

Heat capacity.

• The quantity of heat required to raise the temperature of a body without change of state.
• When the temperature of a body changes, the state remains constant.
• The quantity of heat energy required to raise the temperature of a given mass of a body by 1.

Heat capacity =        C = J/ or J/K

Q M      is change in temperature.

Q  KM

K =                       K = C       C is a constant called specific heat capacity.

C =                       Q = MC

• the amount of heat energy required by a body to change its temperature depends on
1. Mass of the body.
2. Change in temperature.

Specific capacity.

• The quantity of heat energy required to raise the temperature of a unit mass of a body by 1or kelvin.
• If two substances of the same mass are subjected to the same amount of heat, they will acquire different temperature changes. That is, they have two different heat capacities.

Determination of specific heat capacitor.

• Can be determined by using one of the following methods.
1. Method of mixture.
2. Electric method.
3. Mechanical method.
4. Continuous flow method.

1. Method of mixture.
• In this method, a body at higher temperature is mixed with a body at a lower temperature. The one at higher temperature supplies heat to the one at lower temperature until the temperature is the same. This is thermal equilibrium.
• At this state, heat lost is equal to heat gained.

Heat lost = heat gained

M1C1= M2C2

C2 =

• The two substances mixed
1. Should not react.
2. Should not dissolve in each other.

EXP: TO DETERMINE THE SPECIFIC HEAT OF A SOLID (Aluminium).

Requirements.

1. Metal block.
2. Electric heater. .
3. Thermometer.
4. Ammeter.
5. Voltmeter.
6. Stop watch.
7. Rheostat.
8. Connecting wires.
9. Power source.
10. Weighing balance

Procedure.

1. Weigh the metal block and note its mass.
2. Record the initial temperature of the metal block.
3. Set up the apparatus as shown.
4. Switch on the current and start the stop watch.
5. Record the voltmeter and ammeter reading.
6. Record the time taken for temperature to rise by 20.

Results.

• The mass of the block m ____________ kg.
• Initial temperature Q1 ______________ .
• Voltmeter reading V ______________ V.
• Ammeter reading I ______________ A.
• Final temperature Q2 _____________ .
• Time taken to heat T ______________ min.
• If no heat lost to the surrounding.

VIT = MC

C =

1. Electrical method.
• In this method, the heat energy is supplied by electricity. Electric energy is converted to heat energy and raises the temperature of a body.

VIT = MC

I2RT = MC

=  MC

Pt = MC

Precautions.

1. The metal block must be highly polished and highly lagged.
2. The two holes for heater and thermometer should be filled with a light oil to improve thermal contact.
3. Mechanical method.
• Mechanical or potential energy is used to raise the temperature of a body.

p.e = MC

Mgh = MC

C =

C =

• In this method the mass of the substance is not required.

EXP: TO MEASURE THE SPECIFIC HEAT CAPACITY OF LEAD SHOTS.

Requirements.

1. Glass or cardboard tube.
2. Meter rule.
3. Stop watch.
5. Two rubber cork.

Procedure.

2. Note the initial temperature and then cork the tube both sides.
3. Turn the tube upside down 50 times and make sure all the shots fall the full length of the tube.
4. Note the final temperature.
5. Repeat the experiment and find the average.

Calculation.

• Energy lost by lead shots in falling N times, the p.e will be

p.e = Nmgh

• heat gained by lead shots.

H = MC

• If no heat lost to the surrounding.

p.e = H

Nmgh = MC

C =

Precautions.

1. The tube should be of material with negligible heat capacity e.g. wooden, plastic or polysterene.
2. Ensure that all the lead shots fall the full length of the tube.
3. Put some oil on the lead shots or side of the tube to prevent friction which may cause heating.

Errors.

1. The effect of air in the tube is assumed to be negligible.
2. Heat lost in form of sound as lead shots strike the bottom is negligible.

Change of state.

• When a body is heated, the internal energy increases and the vibration of the particles decreases.
• This may lead to the structure of the substance being destroyed resulting to a new state.
• During this change, there is no observable change in temperature. The substance may change from solid to liquid or from liquid to vapor.
• The heat applied or absorbed by the body is called latent heat.

Latent heat.

• It is the heat required to change the state of a substance without change in temperature.

Q  M

Q = M

Q = KM

Q = Ml           where l is a constant called specific latent heat. It depends on the change of the body.

• It is directly proportional to the mass of the body and depends on the state being changed.
• It depends on the change of the body if the state change from solid to liquid, the constant specific heat of fusion lf.
• If from liquid to vapor, the constant is specific latent heat of vaporization lv.

Change from solid to liquid.

Specific latent heat of fusion lf.

• Quantity of heat required to change a unit mass of solid to liquid without changing temperature.
• It is given out when liquid turns to solid.

Q = M lf

lf=

• it is measured by
1. Method of mixture.
2. Electrical method.

1. Method of mixture.

EXP: TO DETERMINE THE SPECIFIC LATENT HEAT OF FUSION OF ICE.

Requirements.

1. Ice cube.
2. Calorimeter.
3. Compression balance.
4. Water.
5. Thermometer/stirrer.

Procedure.

1. Measure some water in a calorimeter and warm it to about 5 degrees room temperature.
2. Find the mass of the calorimeter.
3. Pour the water into the calorimeter.
4. Add pieces of dry melting ice, one at a time, each time turning until the pieces melt before.
5. Continue adding until the temperature falls to about 5 degrees below room temperature.
6. Find the mass of the calorimeter with the mixture.

Results.

1. Mass of calorimeter and stirrer   MC ____________
2. Mass of water    MW ______________
3. Mass of ice added Mi _______________
4. Temperature of water in calorimeter 1 ______________
5. Final temperature of the mixture 2 ________________
6. Change in temperature 2–  1 _____________-

Observation.

• Heat lost = heat gained

MCCC+MWCW = MiCi+Mlf

Mlf = (MC CC+  MW CW) – MiCi

• NOTE: The water was warmed above room temperature and then cooled below room temperature so that it balances the heat exchanged by the calorimeter content and the surrounding.
• The net heat gained or lost is zero.
• Dry ice is used as it has minimum water, moisture and any heat absorbed is utilized in changing solid to liquid but not in warming the water.
1. Electrical method.
• Electric energy is used to change the state of

VIT = MlF