Heat vs Work: What is the Difference?
If you’re working with physics and thermodynamics, it’s usual to get this confusion. And it’s also important to know the difference.
Heat is the transfer of thermal energy between systems or objects, while work in physics it’s the transfer of mechanical energy. But the work in thermodynamics is different from the work in physics. The work in thermodynamics is the energy transferred by systems or objects to their surroundings.
In this article, you’ll learn the basics, similarities, and differences between each concept.
Heat
Heat is the form of energy that is transferred between systems or objects as the result of a difference in temperature. If we take two objects in different temperatures and make them in contact with each other, the heat is transferred. This will happen until the temperature of the two bodies become equal.
The heat energy is transferred from the hotter object to the colder object.
The heat absorbed by the object can be calculated using the heat equation.
Q = mc(ΔT)
Q is the heat energy in Joules (J)
m is the mass of the object in grams (g)
c the specific heat (J/g 0C)
ΔT is the change in temperature (0C)
Work in Physics
Work is also a form of energy that describe the energy transferred to or from an object via the application of force along a defined displacement. If we move an object a “d” distance in a horizontal plane, by using a horizontal force of “F”, the amount of work done is equal to F × d.
W = F × d
W is the work in Joules (J)
F is the force in Newton (N)
d is the distance in Meters (m)
Work in Thermodynamics
When it comes to thermodynamics, we have a different definition for the work. Work in thermodynamics is the energy transferred by systems or objects to their surroundings.
W = Qin – Qout
ΔU = Q – W
ΔU is the change in internal energy in Joules (J)
Q is the heat added to the system in Joules (J)
W is the work done by the system in Joules (J)
Both are forms of energies
As you can see both work and heat are forms of energy. Furthermore, both are energies that can flow.
Heat energy is the form of energy that can flow because of the temperature difference.
Work is any type of energy that can flow other than heat energy.
Note: In thermodynamics, there are two types of energies.
- Energy which can be stored (Eg: internal energy)
- Energy which can flow (Eg: heat, work)
Boundary Phenomena
Both the heat and the work are recognized at the boundaries of the system when they cross the boundaries.
Note: Boundary phenomena is when something is measured or recognized at the boundaries of the system when they cross the boundaries.
Similarities Between Heat and Work
- Both heat and work are forms of energy.
- Both the heat and work energies can flow.
- Joules are the units that use to measure both.
Differences Between Heat and Work
| Heat | Work (Thermodynamics) | Work (Physics) | |
| Energy transfer | Energy flows | Energy flows | Energy converts |
| Cause of transfer | Difference in temperatures | Difference in temperatures | Applied force |
| Subject area | Thermodynamics | Thermodynamics | Mechanics |
| Common symbol | Q | W | W |
| Boundary phenomena | Yes, recognized at boundaries | Yes, recognized at boundaries | No |
Special differences between heat and work in thermodynamics
| Heat | Work (Thermodynamics) | |
| Equation | Q = mc(ΔT) | W = F × d |
| When it’s positive | System gains energy | Work done on the system by surroundings (compress) |
| When it’s negative | System loses energy | Work done by the system on surroundings (expands) |
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