Adiabatic Thermodynamics

[wrathofgod64]

I had a few questions pertaining to such processes

1) In an adiabatic expansion, the formula is U = -PΔV, since there is no heat transfer. However, we also know that in an adiabatic expansion, temperature decreases, volume increases, and pressure decreases. Can someone explain how pressure decreases, when according to the formula for calculating work in an adiabatic process, -PΔV, pressure stays the same and only volume changes?

2) In an adiabatic process, why doesn't temperature remain constant? There is no heat transfer, and we know the heat = mcΔt. So if heat transfer equals 0, shouldn't Δt = 0 too?

---

Members don't see this ad.

 

[TieuBachHo]

1) Are you sure it's not the isobaric process where the pressure remains constant?
2) In an adiabatic process, no heat transfers in and out of a system. Heat and temperature are not the same. The temperature is the average kinetic energy of particles in a system, so it changes accordingly. When you add heat to a system, you actually increases the average kinetic energy of particles.

 

[wrathofgod64]

ah, i think ur right, so -PΔV is the formula only for an isobaric process, but if pressure changes, then u calculate work by area under the PΔV curve?

And secondly, i think i might have been confusing heat and internal energy. Internal energy is only dependent on temperature, but the reason why q=mcΔT is because it's only true for constant volume systems where work =0. Is this correct?

 

[TieuBachHo]

ah, i think ur right, so -PΔV is the formula only for an isobaric process, but if pressure changes, then u calculate work by area under the PΔV curve?

And secondly, i think i might have been confusing heat and internal energy. Internal energy is only dependent on temperature, but the reason why q=mcΔT is because it's only true for constant volume systems where work =0. Is this correct?

That's right. Work is the area under the P vs. V curve. 🙂

Internal evergy of a system is the energy that can be used to do work. Heat depends on temperature changes of a system (q=mcΔT). With constant volume sytems where no work is done, when you add heat into it, you increases its temperature for endothermic rxns and vice versa.

 

[lhenslee]

ah, i think ur right, so -PΔV is the formula only for an isobaric process, but if pressure changes, then u calculate work by area under the PΔV curve?

And secondly, i think i might have been confusing heat and internal energy. Internal energy is only dependent on temperature, but the reason why q=mcΔT is because it's only true for constant volume systems where work =0. Is this correct?

Don't forget that on the MCAT, they often don't specify between heat and internal energy. Assume that they are the same thing.

 

[TawMus]

Just to clarify, in an adiabatic process, no heat comes in and out of the system. So what causes the decrease in pressure, temperature, and volume when there is no heat causing change?