• amimo

  • Kandungan

  • Kategori

  • Archive

  • Flickr Photos

    Photonic Symphony

    Kuifmees / Crested Tit / Mésange huppée

    6922

    More Photos

4.2 Specific Heat Capacity,c

Definition and the S.I unit of Specific Heat Capacity ,c

The specific heat capacity of a material is defined as the amount of heat  required to raise the temperature of 1 kg of the substance through a temperature of 1°C.

The S.I. unit for c is  J kg-1 °C-1

A  substance with a large specific heat capacity( able to store a lot of heat ) will experience a smaller temperature rise when absorbing heat and vice versa.

The relationship between  m,c, θ  and Q

clip_image001

Where,

Q  =  the heat energy transferred to the substance

m =   the  mass of the substance

c   =   the  specific heat  capacity of the substance 

θ =   the temperature change

Example  1

A metal of mass 2 kg. Calculate the amount of heat that must be transferred to the  metal to raise the temperature from 30 °C  to 70o

(specific capacity of the metal = 500 J kg -1°C-1 )

Solution

Example 2

8.4 x 10 5 J  of heat energy raises the temperature of 4 kg of water from  40oC  to  90oC . What is the specific heat capacity of the water?

Solution

Mixtures

 

Whenever two materials are at different temperatures and in thermal contact, the hotter material will lose energy by transfer to the colder one until both come to the same temperature. They are now in thermal equilibrium.

Example 3

0.2 kg  of water at 100oC  is mixed 0.25 kg of water at 10oC. What is the maximum temperature reached by the mixture?

Solution

To determine the specific heat capacity of a solid ( aluminium cylinder)

clip_image003

The electrical power of the heater is recorded = P

The mass of the aluminium cylinder is recorded = m

The initial temperature of the aluminium is read off the thermometer = q1

The electric heater is switched on and the stopwatch  is started simultaneously.

After heating for a  t ime , t  , the heater is switched off.

The maximum reading  on the thermometer is recorded = q2

Calculate the heat supplied  by the heater = Pt

Calculate the heat absorbed by the aluminium cylinder = mc ( q2q1)

On the assumption that there is no heat loss to surroundings;,

 Pt = mc (q2q1)

  c  =         Pt       .

              m (q2q1)

Precautions

(1)     Insulates the aluminium cylinder with felt cloth or wool to decrease the loss of heat to the surroundings.

(2)     Some oil is poured into the hole before the thermometer is inserted to ensure more perfect heat conduction.

Discussions

The value of the specific heat capacity of aluminium ,c determined in the experiment is larger than the standard value of c.

This is because the experimental value of the temperature rise ,q less than the expected temperature rise due to some heat loss to the surroundings.

The smaller the temperature q, the greater the

specific heat capacity because  c­  =   Pt

                                                                 mq¯

To determine the specific heat capacity of a solid ( aluminium cylinder)

clip_image005

The electrical power of the heater is recorded = P

The empty beaker is weighed and its mass is recorded =  m1

The beaker is filled with water  and the beaker is weighed again and it mass is recorded = m2

The initial temperature of the water is read off the thermometer = q1

The electric heater is switched on and the stopwatch  is started simultaneously.

After heating for a  time , t , the heater is switched off.

The maximum reading  on the thermometer is recorded = q2

Calculate the heat supplied  by the heater = Pt

Calculate the heat absorbed by the water

= ( m2 – m1)c (q2q1)

On the assumption that there is no heat loss to surroundings;,

 Pt = ( m2 – m1)c (q2q1)

  c  =               Pt           .  

          ( m2 – m1) (q2q1)

Precautions

(1)     The water must be stirred continuously to ensure its temperature is uniform.

(2)     The beaker should be wrapped with a tissue or felt cloth and placed on a polystyrene sheet to prevent heat loss to the surroundings and the table.

Discussions

The value of the specific heat capacity of water  ,c determined in the experiment is larger than the standard value of c.

This is because the experimental value of the temperature rise ,q less than the expected temperature rise due to some heat loss to the surroundings.

The smaller the temperature q, the greater the

specific heat capacity because  c­  =   Pt

                                                                 mq¯

Example  4

A  2 kW,240 V  electric heater is used to raise the temperature of 3 kg  of water. If power is supplied for  8 minutes, what is the increase in temperature of the water?  

(The specific heat capacity of water is 4200  J kg -1 o C-1)

Solution

Example 5

A 1.2 kW immersion heater is used to raise the 0.2 kg water in a copper container of mass 0.05 kg . Calculate the time taken so that the temperature of the water and the container are increased by  20oC.

(The specific capacity of water =  4200  J kg -1 o C-1The specific of cooper =  400  J kg -1 o C-1)

Solution

Applications of specific heat capacity of a substance.

1           Cooking instruments

     clip_image007

Cooking instruments such as frying pans, pots,kettles, electric iron and so on

made of  substances with low specific heat capacities . This is because they can quickly  heated up when there is only small heat absorption.

The  handle of  the cooking instruments  are made by the substances with high  specific  heat capacities. This is because  these materials undergo a small heat  change while heat is released or absorbed.

 So , the handles are not too hot to be held  by the bare hands.

2           Water

Water has a very high specific heat capacity. This makes it very useful for storing energy. For a given rise in temperature , water can store more energy than most other substances; as it cools , it releases this energy again. It is fortunate that water is cheap , safe and readily available.

clip_image009

In a central heating system water is good at storing energy and carrying it from the boiler to the radiators, as it cools there it releases a lot of energy into the room.The system is commonly used in cold countries

to heat and keep houses warm.

clip_image011

In a car engine cooling system , water is circulated through pipes around the engine block to absorb energy from the hot engine and so to keep it cool. From the cylinder  block ,the water passes into radiator where it is cooled by air drawn in by radiator fan.

The cool water is re-circulated  through the engine to absorb the heat and this cycle is repeated continuously while the engine is running.

               

3    Sea breeze and Land breeze

             clip_image013

          clip_image015

In daytime the sun warms the land to higher temperature than the sea.

It is because land has a lower specific heat capacity than sea-water.

The air above the land is heated and rises,and its place is taken by cooler air above the sea moving inland (convection currents)

Air higher in the atmosphere completes the circulation , and hence a  sea-breeze is obtained.

At night the sea temperature drops only slightly , since it is warmed to a considerable depth during the day. On the other hand , the land temperature drops considerably at night. This time, therefore , a convection current is obtained in the opposite direction to daytime, and this is a land-breeze.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: