Sunday, 21 July 2019

# Identify the signs of heat and work interaction in the following cases?

heat and work interaction in the following cases:A system has energy in different forms.Whenever a energy transfer occurs across a system boundary,there will be a heat or it may become a work.

• Heat and Work are boundary phenomenon.
• Heat and Work are transient in nature.
• Heat and Work  are path functions.
• Heat and Work are in-exact differentials. Identify the signs of heat and work interaction in the following cases

Explanation about the signs of Heat and Work Interaction:

Note:

• If heat is supplied to a system                    —-Take ‘+ve’sign
• If heat is withdrawn from the system        —-Take ‘-ve’sign
• If the system volume expands,the work is said to be done by the system.

-Here,the work done by the system is given a ‘+ve’sign.

• If the system volume contracts,the work is said to be done on the system.

– Here,the work done on the system is given a ‘-ve’sign.

Note:

Wsystem+Wsurroundings  =     0

### Identify the signs of heat and work interaction in the following cases:

1.A resistor is kept in flowing water and it is connected to external mains(Power Supply).What are the signs of heat and work interaction?

Sol:

• Workdone(W.D) = ‘-ve’sign
• Heat Transfer (Q)= ‘+ve’sign

2.In a RIGID insulated cylinder,a battery is kept across the terminals of a resistor and temperature of air increases increases from 25˚C to 200˚C.What are the signs of heat and work interaction?

Sol:

• Work done = Zero
• Heat Transfer Q= Zero(Insulated)

But,the temperature increases because the chemical potential of the battery is converted into internal energy(heat).

3.One end of the resistor is kept in Rigid Insulated cylinder and the other end is connected to the battery which is outside of the insulated cylinder.What are the signs of heat and work interaction for this environment?

Sol:

• Work done = ‘-ve’ (W.D on the system)
• Heat Transfer Q= Zero (Insulated)

4.A Piston-Cylinder mechanism has water and ice in it.It is insulated on all sides except the bottom and it is heated by means of a flame till the ice completely melts. What are the signs of heat and work interaction for this environment?

Sol:

• Workdone =Zero (No movement of piston in Upward or Downward at 0˚C)
• As the Volume decreases,the piston moves in downward direction.
• Thereby workdone is ‘-ve’ and Q= ‘+ve’(because heat is given externally)
• Min Volume & Max Density of water is at 4˚C.