bit stuck

hi, i'm a bit stuck on a few of these questions.
it was a homework sheet and i've done most of the questions but these ones just completely blew me.

4, A student thinks that the formula for the current I in a conductor of length l, is: I = (TT Q V / l)
where q is the charge on each charge carrier and v is their average drift velocity. Show whether or not the equation is homogeneous with respect to units. (2 marks)
i know what homogeneous means but i dont really understand how i answer the question.

5ii) The unit of PD is the volt. Express the volt in terms of base units only. (3 marks)
volt = joule per coulomb
m^2
kg ^ -3
a ^ -1
not sure

6b)A student has a metal conductor and a plastic insulator of the same dimensions. He applies the same PD across each. Explain how the relative values of n for the metal conductor and plastic insulator affect the current in each. (2 marks)
completely lost!

c) The student connects two pieces of copper wire, A and B, in series with each other and a battery. The diameter of the wire A is twice that of wire B. Calculate the ratio of the drift velocity in wire A to the drift velocity in wire B and explain your answer. (3 marks)
completely lost!

Reply 1

Henerz

4) OK, so to show that an equation is homogeneous, you have to break both sides down into their base units and show that the two sides are equal. If you can do question 5, you shouldn't have any problem with that, so we'll consider Q5

5) Right, you've got volt = joule per coulomb which is a really good starting point, but neither joules nor coulombs are base units. Youve got to then break these up into base units with formulea you know to be true. You're on the right lines here, so just keep going. Unfortunatly, your answer isn't right, but you are going about it the right way.

Good Luck. Will check back with help for other questions later...but must go to work now :sigh:

(Or mabee someone else will :smile:

)

EDIT: OK, a bit more before I go, Q6:

Do you understand what "n" is? It's used in many sences, but here its the "number of charge carriers per unit volume". I hope thats enough to get you on the road for answer the question. You'll see its really quite simply once you've done it :cool:

ANOTHER EDIT: 6c) You are familliar with the forumla I=vane ? Well, by the looks of it, you really should be. If in doubt, I'd recommend skimming through this:

http://en.wikipedia.org/wiki/Drift_velocity

So imagine (or better draw) the diagram of the circuit. Two wires (just think of them as resistors) in series....now remember the formula, I=vane. What are the constans between the two wires? There are in series, made from the same material and the charge on the electron is a universal constant. So what does drift velocity depend upon? What will changing this do to drift velocity?...think of the formula. You should be able to answer the question if you can answer all those mini-questions.

Hope thats of use...any other queries, dont hesitate to ask :smile:

thanks!

Ok, I'll take off from where Henerz stopped:

6b) n is the number of free electrons in the medium. The difference between insulators and conductors is that:

Insulators: Bad conductors - low number of free electrons, less current flows. Therefore low n, low I
Conductors: Good conductors - high number of free electrons (metals especially so, since they have a 'sea' of free conduction electrons), more current flows. Therefore high n, high I

bit stuck

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