# PHYsics as help

Could someone explain the following spec point |?

4.2 be able to use I = nqvA to explain the large range of resistivities of different materials 42
Sorry you've not had any responses about this. Are you sure you’ve posted in the right place? Posting in the specific Study Help forum should help get responses.

I'm going to quote in Tank Girl now so she can move your thread to the right place if it's needed.

Spoiler

Moved to physics.

I think it means you just need to be aware of and be able to use that equation.
Original post by Pure class
Could someone explain the following spec point |?

4.2 be able to use I = nqvA to explain the large range of resistivities of different materials 42

I = nqvA is also known as the transport equation.
I is the current. measured in Amps
n is the charge carrier density. measured in m^-3
q is the charge of one carrier. this value is a constant and is 1.6 x 10^-19. measured in C. This is the charge of one electron, but if the question gives a different charge, then we have to use that.
A is the cross sectional area of the material.

In some questions, we will be expected to calculate the area of a wire. A wire is a cylindrical object and therefore, we can apply these equations to find the area of the wire:

A =πr^2 or A = 1/4(πD^2)

Moving onto the next part of your question. Firstly, make sure you understand the definitions of resistivity and resistance.
Resistivity - A measure of the ability to resist a flow of charge
Resistance - An opposive force against a flow of current.

In the resistance equation below, we can re-arrange it to make resistivity the subject. Even though different materials give different resistance against current, two wires made from the same material such as copper would give different resistivity values.
How? Because the two materials would have different dimensions such as different lengths or diamaters. When the diameter varies, the resistivity would vary as well.

If charge carrier density is larger, then current will be larger as charge carrier density (n) is in the equation for current: I = nAqv. Larger current means smaller resistance for the same voltage applied to each material. Smaller resistance means smaller resistivity as resistivity and resistance are proportional as shown in the equation p=RA/L