# Difference between Isothermal and Adiabatic?

• Saippo
In summary, the conversation explains that for isothermal processes, there is no change in temperature (delta T), so the change in internal energy (delta U) is 0. However, there can still be a change in heat (Q) without a change in temperature, as work can also cause a change in internal energy and temperature. This is similar for adiabatic processes, where the change in temperature must be negative, but this is due to work being done on the gas instead of heat. The first law of thermodynamics states that both heat and work can cause changes in internal energy and temperature.

#### Saippo

So for Isothermal, there is no change of T so hence delta U is 0. However, if there isn't a change of temperature, how is there a change of Q? I thought heat was the flow of energy of different temperatures. This confuses me for adiabatic as well. With no heat, how is there a temperature change. Also, apparently adiabatic processes, the change of temperature must be negative. However, why? For a second, I thought it may be because there's no added heat but there shouldn't be heat leaving either.
I feel like this is all tangled up in my head and I would appreciate some clarification!

Do you think that work being done by the gas or on the gas has anything to do with this?

I did think about that. So if Q is 0 all the internal energy would go to work. However, I thought temperature change was required for there to be a change of internal energy. Then, I ended up back to, how is there no heat when there is a temperature change when heat is involved when there's a change of temperature. Even if I think that all the energy went to work, because of what I believed in the previous statement, it doesn't make sense to me.

Saippo said:
I did think about that. So if Q is 0 all the internal energy would go to work. However, I thought temperature change was required for there to be a change of internal energy.
Yes. That's correct for an ideal gas.
Then, I ended up back to, how is there no heat when there is a temperature change when heat is involved when there's a change of temperature. Even if I think that all the energy went to work, because of what I believed in the previous statement, it doesn't make sense to me.
Heat is not the only thing that can cause a change in internal energy and, along with it, a temperature change. Work can also cause a change in internal energy (even without heat), and, along with it, a temperature change. This is the whole idea behind the first law of thermodynamics ##\Delta U=Q-W##, which is basically a statement of conservation of energy. Both Q (heat) and W (work) can cause the internal energy (and temperature) to change. Joule proved this experimentally when he did mechanical work on a liquid and its temperature increased. This basically showed that doing work on a system is equivalent to adding heat to the system.

## 1. What is the main difference between isothermal and adiabatic processes?

The main difference between isothermal and adiabatic processes is the change in temperature. In an isothermal process, the temperature remains constant, while in an adiabatic process, there is no heat exchange and therefore the temperature can change.

## 2. How do you calculate the work done in an isothermal process?

The work done in an isothermal process can be calculated using the equation W = nRT ln(V2/V1), where n is the number of moles of gas, R is the gas constant, T is the temperature, and V1 and V2 are the initial and final volumes, respectively.

## 3. Why is an adiabatic process considered to be more efficient than an isothermal process?

An adiabatic process is considered more efficient because it does not involve any heat transfer, so all the energy is converted into work. In an isothermal process, some energy is lost as heat, making the process less efficient.

## 4. Can an isothermal process also be adiabatic?

No, an isothermal process cannot also be adiabatic. An isothermal process requires the temperature to remain constant, while an adiabatic process does not allow for any heat exchange, which would result in a change in temperature.

## 5. How are isothermal and adiabatic processes used in real-world applications?

Isothermal processes are commonly used in refrigeration systems, while adiabatic processes are used in engines and compressors. In both cases, these processes are used to control temperature and energy transfer to achieve specific goals.