First law of thermodynamics rate problem

[tron_2.0]

Homework Statement

consider a room that is initially at the outdoor temperature of 20 degrees C. The room contains a 100-W lightbulb, a 110-W TV set, a 200-W refrigerator, and a 1000-W iron. Assuming no heat transfer through the walls, determine the rate of increase of the energy content of the room when all of these electric devices are on.

Homework Equations

dE/dt (of the control mass)= -Q(convection)+W(electrical)-Q(conduction)
W(electrical)=Q(convection)?

The Attempt at a Solution

I tried to take into account that the electrical work is given, but I am unsure on how this correlates to the rate of increase of energy. am i solving the first equation i wrote for dE/dt? and if that's the case, how would i find the average heat transfer coefficiant for Q(conv) since i can cancel out Q(conduction)?

thanks a lot

 

[Borek]

You are told to assume no heat is lost.

Honestly, I have no idea what to make out of your analysis of the problem, it seems to be completely off.

 

[Blueshift5]

I would approach this problem by first defining the system and the control mass. In this case, the room would be the system and the control mass would be all the objects inside the room (lightbulb, TV set, refrigerator, iron).

Next, I would use the first law of thermodynamics, which states that the change in internal energy of a system is equal to the net heat transfer into the system minus the net work done by the system. In this case, the internal energy of the room would increase due to the heat transfer from the electrical devices and decrease due to the heat transfer through the walls.

Since there is no heat transfer through the walls, the rate of increase of energy content of the room would be equal to the sum of the electrical work done by the devices, which is 1410 W (100 W + 110 W + 200 W + 1000 W). This means that the energy content of the room would increase by 1410 Joules every second.

To find the average heat transfer coefficient for convection, you would need to know the surface area of the walls, the temperature difference between the room and the outside, and the thermal conductivity of the walls. This information is not given in the problem, so it is not possible to calculate the average heat transfer coefficient. However, since we are assuming no heat transfer through the walls, this value would not affect the final answer.