[Physics Class Notes] on Unit of Heat Pdf for Exam

Generally, heat is anything that provides warmth but scientifically, heat is the flow of energy from a warmer object to a cooler object in comparison till both the objects attain equilibrium. Every matter on earth has some amount of heat energy stored in it. Heat energy flows due to the difference in temperature of the two bodies. In this article, students will learn about the units and conversions of heat energy but first let’s look at a few terms, definitions and concepts.

Heat: Scientifically, heat is defined as the energy that is spontaneously transferred from one object to another due to differences in temperatures. Heat transfer occurs until the bodies attain equilibrium.

Temperature: Temperature is defined as the kinetic energy of molecules of a body.

Internal Energy: The total energy of all the molecules of a body is the internal energy within the object.

Specific Heat: Specific heat, also known as heat capacity, is the amount of energy required to produce a unit change in its temperature.

Difference between Temperature, Heat, and Internal Energy

• Temperature is the kinetic energy of the molecules of a body. The average kinetic energy of individual molecules is termed temperature.

• The total energy of all the molecules is the internal energy within the object. Internal energy is an extensive property.

• Heat is defined as the energy that is spontaneously transferred from one body to another due to its temperature difference.

For example, if a 5 kg of steel, at 100°C, is placed in contact with a 500 kg of steel at 20°C, heat flows from the cube at 300°C to the cube at 20°C, even though, the internal energy of the 20°C cubes is much greater because there is so much more of it. Mathematically heat can be expressed as:

[C=frac{Q}{mtimes Delta T}]

Where m = mass of the body,

C = specific heat,

Δ T = temperature difference.

Q = heat

 

SI Unit of Heat:

As all the energy is represented in Joules (J), therefore, heat is also represented in Joules. Hence, the SI unit of heat is Joules. Joules can be defined as the amount of energy required to raise the temperature of a given mass by one degree. To increase the temperature of one unit weight of water by one degree, we require 4.184 joules of heat.

 

Other Heat Units:

Other heat units are:

BTU:

BTU is a British thermal unit. It is the amount of energy required to raise the temperature of one pound of water by 10 F at sea level.

 

Conversion:

1 BTU = 1055.06 J = 2.931 x 10-4 kWh = 0.252 kcal = 778.16 ft lbf = 1.055 x 1010 ergs = 252 cal = 0.293 watt-hours

 

Calorie:

The amount of energy required to raise the temperature of one gram of water by 10 C.

 

Conversion: 

1 kcal = 4186.8 J = 426.9 kp m = 1.163 x 10-3 kWh = 3.088 ft lbf = 3.9683 BTU = 1000 cal

 

Joule:

Joule is the SI unit of heat. 

 

Conversion:

1 J = 0.1020 kpm = 2.778 x 10-7 kcal = 0.7376 ft lb = 1 kg m2 / s2 = 1 watt second = 1 Nm = 9.478 x 10-4 BTU

Conversion Table:

 

Temperature Conversion :

Celsius to Kelvin

K = C+273 

For Example:

1000C = 100+273 = 373 K

Kelvin to Celsius

C = K – 273

For Example:

273 K = 273 – 273 = 00C

Celsius To Fahrenheit

0F = 9/5 (0C ) + 32

Kelvin to Fahrenheit

0F = 9/5 (K-273) +32

Fahrenheit to Celsius

0C = 5/9 (0F-32)

Fahrenheit to Kelvin

K = 5/9 (0F-32) + 273

Example 1:  An electric kettle contains 1.5 kg of water. The specific heat capacity of water is 4180 J kg-1 K-1. Calculate the amount of energy required to raise the temperature of the water from 15 0C to 100 0C.

Solution: 

Given:

Specific heat (C) = 4180 J kg-1 K-1

T₁ = 15 0C = 15+273 = 288 K

T₂ = 100 0C = 100+273 = 373 K

m = 1.5 kg

Q = m x Δ T x C

Q = 1.5 x 4180 x (373-288)

    = 533 kJ 

Example 2:  Calculate the energy needed to raise the temperature of the water from 20 0C to 90 0C. 

Solution: Q = mcΔθ

= (0.7) (4200) (90-20) = 205.8 kJ

Methods of Transfer of Heat Energy

Convection:  Transfer of heat energy via fluids. When fluids get heated, they form vapours and rise higher up in the environment.

Conduction: Transfer of heat energy through direct contact between two bodies. Such a method of transfer of heat is generally observed in solids.

Radiation: Radiation from hot objects (such as the sun) warms up the air in all directions which are absorbed by molecules all around.

Try Yourself:

1. Calculate heat required to evaporate 1kg of water at the atmospheric pressure (p = 1.0133 bar) also at the temperature of 100°C.

2. Calculate heat required to evaporate 1 kg of feed water at the pressure of 6 MPa (p = 60 bar) and the temperature of 275.6°C.

3. Calculate the specific heat of a 100 kg mass of water if the temperature changes from 150 C to 1000 C. Heat required is 130 BTU.

4. Calculate the heat required to raise the temperature of 60-milligram mass from 22 K to 273 K. Specific heat given 223 J/K.

5. Calculate the specific heat of a 20 dkg mass of water if the temperature changes from 150 C to 260 C. Heat required is 137 BTU.

6. Calculate the heat required to raise the temperature of 200 kg mass from 2320 C to 300 K. Specific heat given 203 J/K.

7. Calculate the specific heat of a 1000 kg mass of water if the temperature changes from 15 K to 100 K. Assume the rest data.

8. Calculate the heat required to raise the temperature of 29 kg mass from 220 C to 273 K. Assume the rest data.

9. Calculate the specific heat of a 20 kg mass of water if the temperature changes from 1500 C to 1000 C. Heat required is 130 cal.

10. Calculate the heat required to raise the temperature of 505 kg mass from 320 C to 273 K. Specific heat given 320 J/K.

11. Explain how heat is transferred in the body?

12.  Name the other methods for transferring heat.

13.  What is the SI unit of heat?