1. Ohm’s Law and Resistance. Resistivity.
Today’s agenda:
Electric Current.
You must know the definition of current, and be able to use it in solving problems.
Current Density.
You must understand the difference between current and current density, and be able to use current density in solving problems.
Ohm’s Law and Resistance.
You must be able to use Ohm’s Law and electrical resistance in solving circuit problems.
Resistivity.
You must understand the relationship between resistance and resistivity, and be able to calculate resistivity and associated quantities.
Temperature Dependence of Resistivity.
You must be able to use the temperature coefficient of resistivity to solve problems involving changing temperatures.

2. Electric Current Definition of Electric Current
The average current that passes any point in a conductor during a time t is defined as
where Q is the amount of charge passing the point.
The instantaneous current is
One ampere of current is one coulomb per second:

3. Here’s a really simple circuit:
Currents in battery-operated devices are often in the milliamp range: 1 mA = 10-3 A.
“m” for milli—another abbreviation to remember!
Here’s a really simple circuit: + -
current
Don’t try that at home! (Why not?)
The current is in the direction of flow of positive charge…
…opposite to the flow of electrons, which are usually the charge carriers.

4. An electron flowing from – to + -
current
electrons
An electron flowing from – to +
An electron flowing from – to + gives rise to the same “conventional current” as a proton flowing from + to -.
“Conventional” refers to our convention, which is always to consider the effect of + charges (for example, electric field direction is defined relative to + charges).

5. “Hey, that figure you just showed me is confusing.
“Hey, that figure you just showed me is confusing. Why don’t electrons flow like this?”
“Hey, that figure you just showed me is confusing. + -
current
electrons
Good question.

6. Electrons “want” to get away from - and go to +.
current
electrons
Electrons “want” to get away from - and go to +.
Chemical reactions (or whatever energy mechanism the battery uses) “force” electrons to the negative terminal. The battery won’t “let” electrons flow the wrong way inside it. So electrons pick the easiest path—through the external wires towards the + terminal.
Of course, real electrons don’t “want” anything.

7. Note!
Current is a scalar quantity, and it has a sign associated with it.
In diagrams, assume that a current indicated by a symbol and an arrow is the conventional current. I1
If your calculation produces a negative value for the current, that means the conventional current actually flows opposite to the direction indicated by the arrow.

8. Example: 3.8x1021 electrons pass through a point in a wire in 4 minutes. What was the average current?