Unit 2 electricity q

Could someone pls help me out?
In this mcq could someone explain to me how we got the ratio of the drift speeds? I know it's from the I = navq formula but how? TT
The answer is A
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Reply 1

phys981

I would rearrange so I can see more clearly what the drift speed depends on

v = I/naq

The n and q don't change so essentially we have v α I/a. The thing to watch is that if A has twice the diameter of B, then it has area a 4x as big.

Does that help? Look for a row in the table which would fit the v α I/a relationship.

Reply 2

Presto

That clears things up a bit. Thank you =D I just don't understand one thing though. How would the area of A be 4 times as big as area of B? Why not twice?

Reply 3

phys981

Yikes, and having read it properly this time, if they're in series, the current must be the same in both, so that knocks out C and D and gives you a 50/50 chance of guessing :smile: