Unit of Heat

Introduction :

Heat is a form of energy. It gets transferred between the bodies having a temperature difference between them. If two bodies are in touch then heat flows from a hotter body to a colder body. For example, a metal rod is heated at high temperatures due to which atoms start oscillating at its mean position. As the temperature increases, atoms start oscillating faster and if this rod is kept near any low-temperature body, there will be a transfer of energy.

Heat Capacity or specific heat is the amount of energy that is required to raise a unit mass in a specified temperature interval.

In other words, if two objects are in thermal contact, heat will flow between them till the equilibrium is reached. Heat transfer is due to conduction or thermal radiation. The flow of heat stops as the heat between the bodies reaches the same temperature. This is how the thermal equilibrium is reached. 

 

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 as 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 their 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 cube is much greater because there is so much more of it. Mathematically heat can be expressed as:

C = \[\frac{Q}{m\times∆T }\] 

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

  • Calorie

  • Joules

 

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:

Units of Heat                                                         

Calorie

1 cal

4184 J

Joules

1 J

0.000239006 kcal / 0.000947817 Btu

BTU

1 Btu

1055.06 J

 

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

 

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?