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Heat Energy.

posted Nov 2, 2015, 2:35 PM by Upali Salpadoru   [ updated Oct 14, 2017, 11:56 AM ]




A Burning candle gives off heat.

But what is heat?


Heat is really the kinetic energy present in atoms and molecules due to their movements and vibrations. 

This is better described as Thermal energy.


The best interpretation to explain ‘Heat’  comes from the kinetic theory which in 1905 Albert Einstein used to explain ‘Brownian Motion’ which was observed in 1827.


Kinetic Theory

kinetic th.jpg

Fig.1. A solid can melt and evaporate by absorbing Microwaves.


This is an illustration to explain heat energy using the kinetic theory. If you place a slab of hard chocolate in a microwave and expose it to the electromagnetic waves of the ‘micro wave’ range, the particles will absorb the energy. This will increase the kinetic energy in the particles and vibrate with a higher amplitude. This would create a greater space between the particles. This is what is generally known as expansion.

The greater distance between the particles will reduce the inter molecular forces. Then the hard solid will become soft ultimately melting down into a liquid.

The particles in the liquid will absorb more energy and gain velocity. Some of the faster moving particles escape from the liquid phase changing into a gaseous state. This reduces the average energy in the liquid.  This is why evaporation brings cooling.

Once the particles break the surface barrier and escape they are free to roam as a swarm of bees. In the absence of inter molecular attractions each particle will move at a certain velocity until it collides with another. Diffusion is the result of this kind of movement.

Heat and Temperature?

Temperature scale is a device to determine the average ‘kinetic energy’ (internal energy, or thermal energy) in a solid, liquid or a gas. Most thermometers that measure the temperature uses the expansion of mercury inside a glass tube. As the liquids expand more than solids, when the kinetic energy in the testing system is high the mercury level goes up.

In a thermometer there are two fixed points. The upper point is the temperature of pure boiling water at sea level and the lower point is that of freezing point of water. These are marked in the illustration.

Temperature gives us an idea of ‘how hot’ a material is; but it is not a measure of total energy. The quantity of heat energy absorbed or given out is measured in Joules.

Specific Heat Capacity

The specific heat capacity is the heat energy absorbed or given out during a change of 1 degree in Celsius or Kelvin scale of temperature.

If the temperature of water drops or rises by 1 degree Celsius,  1 gram of it gains or loses 4.18 Joules of heat energy.

Therefore SHC of water is 4.18 J per g.per .


Heat capacity.

Heat Capacity would mean the amount of heat required to raise the temperature of an object irrespective of mass.

.Latent Heat

This is the heat absorbed or given out without a change in temperature. When energy is used to make or break molecular bonds the temperature can remain unchanged. This type of energy change is called “Latent heat” Some latent heat values are shown in the above chart.


Heat absorbed by  1g. Of water.

Change

Ice to water.

at 0°C.

Water from per  1° C.


Water to vapour

At 100° C.

Joules per gram.

Absorb 334

Absorb 4.2

Absorb. 2230

Name used for change.

Latent heat of Melting.

Specific Heat capacity

Latent heat of vapourisation.

The heat of fusion (freezing) for water at 0 °C is 334 joules  per gram, and the heat of vaporization at 100 °C is 2,230 joules  per gram.

1. Conduction

In solids molecules do not travel but exhibit only vibrational movements. For the vibrations to pass on from one particle to another , they must be close to each other. In gases conduction is almost impossible as the particles are far apart. In liquids including water conduction does not take place but metallic liquids such as mercury can conduct.



Fig.4.  For conduction vibrations of one particle should reach the other.











2. Convection.
 In this mode particles travel taking the energy and unload it somewhere else. 

Fig.5 Convection currents in the air maintains the shape of the candle flame.



Fig. 6  Convection currents distribute the heat in water inside a kettle.


 3.Radiation.

The best example for thermal radiation is the Sun. The sun has a surface temperature of 5,800 C and emits the entire range of Electro magnetic waves ranging from very short wave length to long wave radio waves.  In the visible spectrum, just below the red waves are the Infra red waves. These are also called heat radiation.  
All molecules above -273 C radiate electromagnetic waves. For this to occur outside temperature is immaterial. They can travel to infinity in the absence of obstacles. Some matter can absorb these and change into internal energy.  
Formulae Connected with Quantity of heat

 Heat gained or lost =  mass x specific heat capacity x change in temperature 
                         Q = m c t.
 For a change of state   Q  =  mass x Latent heat.
                         Q = mL.


Worked out example. 

A heated block of copper weighing 12.5g was added to 50 g of water in an insulated container. The water temperature increased by  6 C. Assuming there were no heat losses calculate the temperature of the heated metal. (SHC of Cu =0.2J/g per ˚C)


  Working:-.

Let the heated temperature of Copper be  t. ˚C

Heat gained by water =  30 x 4.18 x 6 J

Heat gained by water = Heat lost by metal.˚

50x4.18x 6 =  12.5 x 0.2 x (t-36)

  t -36 =  50 x4.18 x6 / 12.5 x 0.2

  t     =  1254 / 2.5     -    36

  t  =  501.6 -36

  t = 465.6 ˚C


Q.1.0

1.1        A reading that indicates the average thermal or kinetic energy present in a sample of a substance.

1.2         A method by which heat can travel in metals.

1.3         What is the standard unit for measuring heat energy gained.

1.4          Heat energy required to evaporate 1 g of water.

1.5         The energy required to raise the temperature of 1 g. of iron from 20 to 21 ˚C.     

Select the answers from these.  Conduction, Specific heat capacity, Joule, Latent heat,  Temperature, 


4x 5= 20 Marks

Q 2.0

 2.1  How much energy is required to bring 25 g. of water at 20 ˚C to boiling point ?

2.2   If the kettle mass 150 g. made of aluminum, was used, how much more heat is necessary for the process? (Specific heat for Al is 0.4 J/g. per C.)

2.3   Could an electric kettle with a built in coil more efficient than keeping a kettle on a hot plate?Give a reason for the answer

2.4    What colour would be the most efficient for an electric kettle? Give a reason.                                                                                  5x 4 =20 Marks

Q. 3.0

3.1  An object of 20 g was taken from boiling water and added to 40g of water in a cup. The temperature of water increased from 20˚C to 28˚C. What is the Specific heat of the object.

3.2  Name an important error that may have caused during the experiment.

16 + 5 =20 Marks

Q.4.0


In order to determine Latent heat of melting of ice an electric current was passed into an insulated electric calorimeter. The temperature of ice remained at 0˚C while 41 g of ice melted in 5 minutes.(300 S) .The voltmeter read 1.5 and the ammeter 0.03 A. Calculate the latent heat of fusion of ice.

Electrical energy = volt x amperes x time (J =vit)   

 20 Marks








Q. 5.0

The diagram shows a Dewar flask used to keep hot water. What are the main functions of each numbered part.  

4 x5 = 20 Marks



 
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