Question

if the internal energy of a thermodynamic system is increased by 300 J while 75 J of expansion work is done, how much heat was transferred and in which direction, to or from the system

Answer 1

Hint: What is the 1st law of thermodynamics?

Answer 2

energy is conserved

Answer 3

Agreed...what is it mathematically in terms of internal energy, heat and work?

Answer 4

'triangle'=q+w?

Answer 5

"triangle E"

Answer 6

change in enthalpy=work + heat

Answer 7

so:
delta U = q + w (I use U you can use E if you like)

Answer 8

No not enthalpy...it say s change in internal energy = heat + work

Answer 9

ya, i know the equations, im asking an explanation of the energy transfer and yeah, my bad, its internal energy

Answer 10

In this problem which two values are you given?

Answer 11

three hunna and 75, both J, the system adds 300, buts 75 J of expansion work is done

i dont understand what they are saying with "75 J of expansion work"

Answer 12

ok...
by definition:
if heat flows into the system it is positive, if it flow out it is negative
if the system does work on the surroundings it is negative, if the surroundings does work on the system it is positive

so the 75J of expansion work means the system is doing work on the surroundings therefore by convention the sign for that is negative so:

delta E = q + w
300 J = q + (-75J)

Answer 13

Check your sign conventions in your notes.

Answer 14

so they expansion means the system is doing the work to the surroundings?

Answer 15

the* not they

Answer 16

exactly my friend...the system is doing work on the surroundings

Answer 17

ok, the "expansion" confused me

Answer 18

thank you

Answer 19

you are welcome...don't forget to check your notes for the work sign convention you are using in your class

Answer 20

k, got it