Help needed---an AS physics question

I've been revising physics and got stuck on one question in a past paper.
Here is the question:

The presense of people in a room warms the air in the room. Use these figures to answer the questions below:

mass of air in room 740kg
number of people in room 27
rate of thermal output from each person 80W
specific heat capacity of air 960J kg-1 C-1

The room can be maintained at a steady temperature by having a window open, so that warm air from inside changes places with colder air from outside. The desired temperture difference between the air inside and outside the room is 8 degree celsius. Show that the air must be exchanged through the open window at a rate of about 0.3kg per second to maintain this steady temperature difference within the room.

It's only 2 marks so it shouldn't be hard. But it's giving me a headache :frown:

anyone got any idea? :confused:

thank u!!

Reply 1

abshirf2

2 marks only! but i think i've got it, u may need to double check :biggrin:

formula is Q=mcT, energy transferred= mass x specific heat capacity x tempertaure

total enrgy transfered = Q = 27 people x 80 = 2160j

rearranging formula is m = Q/cT

so m= 2160 / (960 x 8) = 0.28125

round it up gives you 0.3kg!

hope it helps!

Reply 2

Robusty

i've checked and agree with abshirf2,

it wants you to find out the rate of the air flow , as unit of Power is give, which is Energy/time (Js-1), if you find the mass, kg, then eveything on the right hand side of equation will be divided by s, including mass, kg, which will then be kgs-1 --> answer!!

I hope you understand what i mean.