# Heat & thermo

1. Jul 22, 2007

### kthouz

1. for fluids,why is the specific heat capacity at constan pressure is greater than that at constant volume

2. Can some one explain me about the use of air as an insulator in windows, they say it is coz of its low conductivity, how?

3. i've a got confusions about the word " net" that usually precedes some other words.eg: Net force, net thermal energy. So what is the difference between "force shortly and net force" or "thermal energy and net thermal energy?"

2. Jul 22, 2007

### cesiumfrog

1. is it?
2. compare holding your hand 10 cm from a stove flame, to holding the end of a 10cm spoon that is touching a stove flame.
3. dictionary: net versus gross

3. Jul 22, 2007

### chaoseverlasting

1. Cp=p/n*dv/dt + Cv. Its greater than or equal to.

4. Jul 22, 2007

### Staff: Mentor

5. Jul 22, 2007

### Andrew Mason

At constant pressure the same amount of heat flow into the gas does work as well as increasing the internal energy (temperature) of the gas. So the same amount of heat cannot increase the internal energy of the gas, dU, by as much as in the constant volume (no work done) situation.

Think of a constant pressure change as a series infinitessimal heat flows of dQ into the gas at constant volume followed by an infinitessimal adiabatic (dQ=0) expansion to reduce the pressure to the original pressure.

When the gas receives heat at constant volume, no work is done. Applying the first law of thermodynamics, dQ = dU + dW this means that dQ = dU.

When the gas then expands (adiabatically: dQ=0), it does work. Applying the first law: dQ = dU + dW = 0. So dW = -dU. This means that the internal energy, hence temperature, decreases.

So at the end of the constant pressure process there is a lower temperature than in the constant volume process, with the same amount of heat flow into the gas. $dT_{cp} < dT_{cv}$. This means that Cp = dQ/dTcp > Cv = dQ/dTcv.

An insulator is something that does not conduct heat very well ie. low heat conductivity. Since air does not conduct heat very well, it is a good insulator.

. If you have several forces applied to the same object, the total force is the vector sum of all the forces. That is the net force.

Heat energy is a scalar (non-vector) quantity. Heat flows into or out of a body. If it flows into a body, the convention is that dQ is positive. If heat flows out of a body, dQ is negative. Net heat flow or net thermal energy flow, is the sum of all the heat flows into/out of the body.

AM