Home » Ask & Discuss » Physics. » Thermal Physics « Back to Discussion

Thermal Physics

New kid on the Block

Joined:	16 Mar 2010
Post:	5

16 Mar 2010 18:26:00 IST

0 People liked this

465

how to use formulas of latent heat and specific heat in problems?

None

how to use formulas of latent heat and specific heat in problems?

Share this article on:

Comments (4)

edison

Forum Expert

Joined: 19 Oct 2006

Posts: 7452

16 Mar 2010 21:48:42 IST

Like 0 people liked this

Whenever the change in temperature is accompanied by change in state too then the expression for latent heat is used i.e. the heat gained or lost to change the state at same temperature.

Question: There is no rise in temperature of a melting solid despite applying heat to it. Explain.

Answer: The energy that is being supplied for melting the solid is used to reduce the attractive forces between molecules of the solid. In addition, the excessive heat energy is stored as potential energy in the resulting liquid in the form of latent heat energy. As a result there is no rise in temperature.

edison

Forum Expert

Joined: 19 Oct 2006

Posts: 7452

16 Mar 2010 21:50:57 IST

Like 0 people liked this

Experiment 1

Take a few pieces of ice in a beaker. Insert a thermometer. Note the temperature. You will observe it to be 0⁰C.

Now place the beaker on a flame and heat it. Ice starts melting. As you supply heat to ice, its temperature should rise. But you will find that the temperature of ice remains at 0⁰C until all the ice melts into water. If heating is continued you will notice that the temperature starts rising. Why does the temperature not rise until all the ice melted into water?

The heat supplied to a solid at its melting points is used up by the solid molecules for the change of state process, i.e., the solid molecules use the heat supplied to break free from intermolecular attraction so that the solid melts into liquid form.

The heat supplied to a solid at its melting point is used up for change of state process and is known as the latent heat of fusion.

Latent heat of fusion of a solid is defined as the heat energy required by 1 kg of the solid at its melting point to completely melt into 1 kg of liquid

Latent heat of fusion of ice = 3.35 x 10⁵J/kg

Experiment 2

Now, heat the water at 0⁰C further. Observe that the temperature of water rises till the water starts boiling. At sea level the water boils at 100⁰C. As soon as water starts boiling, its temperature remains constant, even if heating is continued. 'The temperature at which a liquid starts boiling is known as boiling point'. Boiling point is a characteristic property of a given liquid.

What happens when a liquid starts boiling? The liquid molecules escape from the surface of the liquid and move into vapor or gaseous state. The process in which liquid molecules escape from the surface of the liquid to become vapor is known as evaporation. At boiling point evaporation is maximum.

Heat supplied to a liquid at its boiling point is utilized by liquid molecules to turn into vapor form. 'The heat energy required by 1 kg of the liquid at its boiling point to completely turn into vapor form is known as the Latent heat of Vaporization'.

Latent heat of vaporization of water = 2.26x10⁶J/kg.

We have seen that during the process of change of state from liquid to vapor state, the liquid absorbs heat energy. In the reverse process i.e. when the vapors are cooled to condense into liquid state, they liberate the same amount of energy. This is why steam burns are more severe than burns due to boiling water.

Similarly, when liquids are cooled sufficiently to become solids, they liberate energy.

ThIrStY of IIT

Blazing goIITian

Joined: 6 Mar 2010

Posts: 380

19 Mar 2010 13:41:42 IST

Like 0 people liked this

In problems if there is only change the in the state of the substance then you have to latent heat formula; If there is only change in the temperature occure you have to use the specific heat formula;

. .

Blazing goIITian

Joined: 11 Apr 2009

Posts: 672

19 Mar 2010 13:44:24 IST

Like 0 people liked this

BUT YOU SHOULD KEEP IN MIND THAT WHENEVER THERE IS CHANGE OF STATE GOING ON THE TEMPERATURE REMAINS CONSTANT......YOU CAN ALSO SEE THE GRAPH WHCIH IS GIVEN IM MOST BOOKS ......U WILL GET A BETTER IDEA FROM THERE!

Quick Reply

Reply

	Some HTML allowed.
	Keep your comments above the belt or risk having them deleted.
	Signup for a avatar to have your pictures show up by your comment
	If Members see a thread that violates the Posting Rules, bring it to the attention of the Moderator Team