## Heating at Constant Pressure

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Curtis Tam 1J
Posts: 105
Joined: Thu Jul 13, 2017 3:00 am

### Heating at Constant Pressure

I'm slightly confused by what the book means when it says heating at constant pressure. Are they referring to the pressure of the system or the pressure of the surroundings (external pressure)? In other words, when you use deltaU=q+w, does q(p) refer to heat transferred when the system is at constant pressure or when there is a constant external pressure. Thanks!

Matthew Lin 2C
Posts: 30
Joined: Fri Sep 29, 2017 7:06 am

### Re: Heating at Constant Pressure

When it says heating at a constant pressure, it refers to the pressure of the system. However, if it is an open system (such as a flask or test tube), then the pressure of the system is the same as the pressure of the surroundings.

Curtis Tam 1J
Posts: 105
Joined: Thu Jul 13, 2017 3:00 am

### Re: Heating at Constant Pressure

Matthew Lin 2C wrote:When it says heating at a constant pressure, it refers to the pressure of the system. However, if it is an open system (such as a flask or test tube), then the pressure of the system is the same as the pressure of the surroundings.

Yea I think I understand that a bit better now. I looked online and it said it has something to do with pressure. When we have a system at constant pressure being heated, you can break the process down into two processes. Heating at constant volume and then isothermal expansion. Heating at constant volume would lead to a higher pressure with no change in volume. Isothermal expansion would then take place using some of that heat to expand the system so that the pressure of the system matches the pressure of the surroundings, meaning there's no net change of pressure and therefore this is called "heating at constant pressure."