Story 1Story 1

En Halba who now has a microwave oven En Halba who now has a microwave oven puts in two bowls of soup. After heating for puts in two bowls of soup. After heating for one minute, he finds that the bowl with one minute, he finds that the bowl with more soup is just right for drinking but the more soup is just right for drinking but the bowl with small amount of soup is too hot. bowl with small amount of soup is too hot. Why is it that the two bowls of soup have Why is it that the two bowls of soup have different temperatures?different temperatures?

Story 2Story 2

Pn Halba is preparing the ingredients to Pn Halba is preparing the ingredients to make some bread. She puts equal masses make some bread. She puts equal masses of water and cooking oil into the oven and of water and cooking oil into the oven and turns on the heat for 2 minutes. She turns on the heat for 2 minutes. She discovers that the oil became very hot but discovers that the oil became very hot but the water was just warm to touch. Why the the water was just warm to touch. Why the oil hotter than the water?oil hotter than the water?

SPECIFIC HEAT CAPACITYSPECIFIC HEAT CAPACITY

At the end of the lesson, you should be At the end of the lesson, you should be able to:able to:Define specific heat capacityDefine specific heat capacityState that State that

Describe applications of specific heat capacityDescribe applications of specific heat capacitySolve problems involving specific heat Solve problems involving specific heat

capacity.capacity.

Explanation for story 1 Explanation for story 1

The The same amount of heatsame amount of heat was supplied to the was supplied to the soup in each bowl.soup in each bowl.

The The bigger mass of soupbigger mass of soup is said to have a is said to have a larger larger heat capacityheat capacity because it because it experiences a smaller experiences a smaller temperature increase.temperature increase.

This means that a larger amount of heat would This means that a larger amount of heat would be required to raise the temperature of the be required to raise the temperature of the bigger mass of soup to the same temperature as bigger mass of soup to the same temperature as the small mass of soup. the small mass of soup.

Heat capacityHeat capacity The The heat capacityheat capacity of an object is the of an object is the amount of amount of

heat required to increase the temperature of the heat required to increase the temperature of the object by 1object by 1°°C.C.

Heat capacity is expressed in unit Heat capacity is expressed in unit JJ°°CC-1-1 or JK or JK-1-1..

Heat capacity denoted as C (capital C).

Suppose the water in a kettle has a heat capacity of 9000 J°°CC-1-1. This means that when . This means that when the water absorbs 9000J of heat energy, its the water absorbs 9000J of heat energy, its temperature will increase by 1temperature will increase by 1°°C. C.

Heat capacity of an object is calculated in the following Heat capacity of an object is calculated in the following way:way:

Heat capacity = heat absorbed or released Heat capacity = heat absorbed or released by objectby object

change in temperaturechange in temperature

Heat capacity of an object depends on the:Heat capacity of an object depends on the:Mass of the objectMass of the objectTypes of materialTypes of materialThe amount of heat suppliedThe amount of heat supplied

An object with a An object with a larger masslarger mass will have a will have a larger heat larger heat capacitycapacity than an object with smaller mass of the same than an object with smaller mass of the same material.material.

Explanation for story 2Explanation for story 2The water and the oil were supplied with The water and the oil were supplied with

heat at the same rate.heat at the same rate.

The The oiloil has a has a smaller heat capacitysmaller heat capacity than than the water since it experiences a larger the water since it experiences a larger increase in temperature.increase in temperature.

Since both the oil and water have equal Since both the oil and water have equal masses, the masses, the oiloil is said to have a is said to have a smaller smaller specific heat capacityspecific heat capacity than water. than water.

When comparing the heat capacities of When comparing the heat capacities of different substances, we refer to their different substances, we refer to their specific heat capacities. The term specific heat capacities. The term “specific” means one unit of mass, that is 1 “specific” means one unit of mass, that is 1 kg of mass when SI units are used.kg of mass when SI units are used.

Specific heat capacitySpecific heat capacity The The specific heat capacityspecific heat capacity of a substance is of a substance is the the

amount of heat that must be supplied to increase amount of heat that must be supplied to increase the temperature by 1the temperature by 1°°C for a mass of 1 kg C for a mass of 1 kg of the of the substancesubstance..

Specific heat capacitySpecific heat capacity is expressed with units is expressed with units JkgJkg-1-1°°CC-1-1 or Jkg or Jkg-1-1KK-1-1..

The specific heat capacity of aluminum is 900 The specific heat capacity of aluminum is 900 JkgJkg-1-1°°CC-1-1. This means that 900 J of heat required . This means that 900 J of heat required to raise the temperature of a 1 kg block of to raise the temperature of a 1 kg block of aluminum through 1aluminum through 1°°C. C.

Specific heat capacitySpecific heat capacity, , cc can be calculated from the can be calculated from the amount of amount of heat suppliedheat supplied, , QQ to a to a massmass, , mm of a substance of a substance and the and the change in temperaturechange in temperature, , Ư.Ư. Specific heat Specific heat capacity:capacity:

Therefore, the Therefore, the quantity of heat absorbed or lostquantity of heat absorbed or lost from a from a body is given by:body is given by:

Q=mcƯ Q=mcƯ

Heat absorbedHeat absorbed increase in temperatureincrease in temperature

Heat lost Heat lost decrease in temperaturedecrease in temperature

Other forms of energy such as electrical Other forms of energy such as electrical energy, potential energy and kinetic energy, potential energy and kinetic energy can be converted to heat energy. energy can be converted to heat energy.

Electrical energy Heat energy

Pt = mcØ

Heater

Power , P

Potential energy

Object falls from a high position

Heat energy

mgh = mcØ

Kinetic energy

Moving object stopped due to friction

Heat energy

½ mv2 = mcØ

List of specific heat capacities for List of specific heat capacities for some common substancessome common substances

When two objects of equal mass are When two objects of equal mass are heated at equal rates, the object with the heated at equal rates, the object with the smaller specific heat capacitysmaller specific heat capacity will have a will have a faster temperature increasefaster temperature increase..

When two hot objects of equal mass are When two hot objects of equal mass are left to cool down, the left to cool down, the temperaturetemperature of the of the object with object with smaller heat capacitysmaller heat capacity will will drop drop faster.faster.

Applications of specific heat Applications of specific heat capacitycapacity

Cooking pot Cooking pot The cooling system of car engineThe cooling system of car engineSea breezeSea breezeLand breezeLand breeze

Problem solving…..Problem solving…..

1.1. How much heat energy is required to raise the How much heat energy is required to raise the temperature of a 4 kg iron bar from 32°C to temperature of a 4 kg iron bar from 32°C to 52°C? [ Specific heat capacity of iron = 452 52°C? [ Specific heat capacity of iron = 452 JkgJkg-1-1°C°C-1-1]]

2.2. A bottle containing 1.5 kg of water at 34°C is A bottle containing 1.5 kg of water at 34°C is put into a refrigerator. What is the temperature put into a refrigerator. What is the temperature of the water after 1.575 x 10of the water after 1.575 x 1055 J of heat has J of heat has been removed from the water? [Specific heat been removed from the water? [Specific heat capacity of water = 4200 Jkgcapacity of water = 4200 Jkg-1-1°C°C-1-1]]

3.3. An electric kettle has a power rating of 2.4 kW. An electric kettle has a power rating of 2.4 kW. How long does it take the kettle to heat up 4.5 How long does it take the kettle to heat up 4.5 kg of tap water from 28°C to 100°C? [Specific kg of tap water from 28°C to 100°C? [Specific heat capacity of water = 4200 Jkgheat capacity of water = 4200 Jkg-1-1°C°C-1-1]]

1.1. Amount of heat required,Amount of heat required,

Q = mcQ = mcØØ

= 4kg x = 4kg x 452 Jkg452 Jkg-1-1°C°C-1-1 x (52-32) x (52-32)°C°C

= 36160 J= 36160 J

2.2. Q = mcQ = mcØØ

1.575 x 101.575 x 1055 J = 1.5 kg x 4200 Jkg J = 1.5 kg x 4200 Jkg-1-1°C°C-1 -1 x (34 – x (34 –

t ) °Ct ) °C t t = 9°C= 9°C

Solution Solution

3.3. Q = mcQ = mcØØ

Pt = Pt = mcmcØØ

2.4 kW x t = 2.4 kW x t = 4.5 kg x 4200 Jkg4.5 kg x 4200 Jkg-1-1°C°C-1 -1 x (100-x (100-28) °C28) °C

t = 567 st = 567 s