Question

The air contained in a room loses heat to the surroundings at a rate of 60 kJ/min while work is supplied to the room by computer, TV, and lights at a rate of 1.2 kW. What is the net amount of energy change of the air in the room during a 10-min period? (Hint: Draw a sketch showing the energy flow directions and magnitudes.)

Answer 1

Answer:

The net amount of energy change of the air in the room during a 10-min period is 120 KJ.

Explanation:

Given that

Heat loss from room (Q)= 60 KJ/min

Work supplied to the room(W) = 1.2 KW = 1.2 KJ/s

We know that  1 W = 1 J/s

Sign convention for heat and work:

1. If heat is added to the system then it is taken as positive and if heat is rejected from the system then it is taken as negative.

2. If work is done by the system then it is taken as positive and if work is done on the system then it is taken as negative.

So

Q = -60 KJ/min

In 10 min Q = -600 KJ

W = -1.2 KJ/s

We know that

1 min = 60 s

10 min = 600 s

So   W = -1.2 x 600 KJ

W = -720 KJ

WE know that ,first law of thermodynamics

Q = W + ΔU

-600  =  - 720 + ΔU

ΔU = 120 KJ

The net amount of energy change of the air in the room during a 10-min period is 120 KJ.

Answer 2

Answer:

Net amount of energy during 10 min is 120 kJ

Explanation:

Given data:

Heat loss = 60 kJ/min

Work rate $= 1.2 kW = 1.2 \times 60 kJ/min = 72 kJ/min$

We know that net amount of work done is computed as

E = W -Q

where W is work done

Q is heat energy

E = 72 - 60

E = 12 kJ/min

Energy change in 10 min is computed as

$E' = E\times t$

$E' = 12\times 10$

E' = 120 kJ

Thus net amount of energy during 10 min is 120 kJ