Physics question...

FluffySheep

FluffySheep

FluffySheep

Associate
Joined
24 May 2006
Posts
754
Location
North Wales
I am a physics teacher and I have taken over part of the course from another teacher who is on long term sick and they have given a question to the pupils. I *think* they have given a COMPLETELY wrong solution, but I need to double check before I look like a pleb, so can people answer this for me:

A copper rod and an aluminium rod are in thermal equilibrium with each other. Copper rod = 50cm long and the aluminium rod is 20cm. Radius of pair is 8cm.
Copper end is heated (and maintained) at 80C , other end of pairing is 20C. k of copper is 390 and k aluminium is 210. Calculate the rate of heat flow through the pairing.

(side note: drawing shows 2 rods in series, touching only on ends)

Equation is:

Q/t = kA.(temperature difference, theta)/l

I can get A = 0.02m^2 no probs

I do have a worked solution, but I would like the forum to attack this cold without me polluting your thought with either my rubbish idea, or my colleagues rubbish idea, whichever is wrong.

Fluffy
 
delta Q = delta T / SUM ( R )

=> DQ = DT / R1+R2....+...R0

R1 = 1 / k1A , R2 = 1 / k2A ,....., R0 = 1 / k0A
 
Last edited:
BigglesPiP said:
:rolleyes:

Got to love the quality of teaching in schools today. Did they get rid of the real physics teacher because of the credit crunch?
Click to expand...


I just cannot see the other teacher's direction of thought to work out the system. Basically, they worked out the system as 2 parts each with a theta of 60C and added the two together. I *think* that solves the system if the 2 bars were in parallel each going from 80C to 20C, but not if they are connected in series.

Fluffy
 
FluffySheep said:
I just cannot see the other teacher's direction of thought to work out the system. Basically, they worked out the system as 2 parts each with a theta of 60C and added the two together. I *think* that solves the system if the 2 bars were in parallel each going from 80C to 20C, but not if they are connected in series.

Fluffy
Click to expand...

So the resistance at the join is ignored?
 
It's a trick question. Copper and Aluminum are incompatible metals and the joint will experience galvanic corrosion so rapidly that there can't be any measurable heat exchange.
 
Why do people think teachers are gods?

Like they are to know everything, There is some brilliant people out there who are not teachers and would make crap teachers as well.
 
Normally these type of threads come across as pretentious and a show off by the OP, very rarely seeking genuine help. As it happens, i've nothing to contribute. :p
 
Last edited:
I got a solution and it involved simultanious equations then substituting this solution into the origional equation...which is well above A-level standard requirement.

I've spoke to the powers that be about the situation and the other teacher's poor teaching, but Im not expecting them to do anything about it...

Thanks for the help to some,
Fluffy
 
FluffySheep said:
I got a solution and it involved simultanious equations then substituting this solution into the origional equation...which is well above A-level standard requirement.
Click to expand...


You've got to be kidding me? :eek: That was GCSE level a few years back (last time I taught it).

Anyway, to solve the (steady) heat-transfer problem (assuming no adverse effects at the interface):

* You know a linear distribution of temperature is present in each material
* You know an end-temperature for each rod (fixes two of the unknown coefficients in terms of the other two)
* You know that the heat flux must match. So, by taking the first derivative in each direction, you have the other two coefficients in terms of the first two.

You then have a temperature profile over the bar, and can extract any information you want (like the flux at the join).
 
You must log in or register to reply here.
Back
Top Bottom