Physics Unit 2: Charge, Current and Potential Difference

The current in a wire is like water flowing in a pipe. The amount of water that flows depends on the flow rate and the time. It's the same with electricity - current is the rate of flow of charge.

I = Change in Charge / Change in Time

The Coulomb is the Unit of Charge: One coulomb (C) is defined as the amount of charge that passes in 1 second when the current is 1 ampere.

Current can be measured with an ammeter.

When current flows through a wire, you might imagine the electrons all moving in the same direction in an orderly manner. Nope. In fact, the move randomly in all directions, but tend to drift one way. The drift velocity is just the average velocity and its much much less than the electrons actualy speed.

Current Depends on drift velocity: I = nqvA

I =  Current

n = Charge Carrier Density

A = Cross Sectional Area

v = Drift Velocity

q = charge in C carried by each charge carrier

In a metal, the charge carriers are free electrons = they're the ones from the outer shell of each atom. Thinking about the formula I = nAvq, there are loads of charge carriers, making n big. The drift velocity only needs to be small, even for a high current.

Semiconductors have fewer charge carriers than metals, so the drift velocity will need to be higher is you're going to have the same current.

A perfect insulator wouldn't have any charge carriers, so n = 0 in the formaula and you'd get no current. Real insulators have a very small n.

Ionic crystals like sodium chloride are insulatos. Once molten, though, the liquid conducts. Positive and negative ions are the charge carriers. The same thing happens in an ionic solution like copper sulphate solution.

Gases are insulators, but is you apply a high enough voltage, electrons get ripped out of atoms, giving you ions along a path. You get a spark.

To make electric charge through a conductor, you need to do work on it. Potential difference or voltage, is defined as the energy converted per unit charge moved.

Definition of the Volt: The potential differenc eacross a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component.