Current and resistance

Electric Current


The direction of current flow is defined to be in the direction of positive charge flow
Conventional current flows from positive to negative, or from high potential to low potential
Current depends on the velocity of the moving charges, which are generally electrons




Resistance


Resistance is what limits current
If not for resistance, charges would accelerate indefinitely, continually increasing current
Resistance can be thought of as “rocks” in the “pipe” (wire) through which “water” (charge) flows as it is forced by a “pump”
Resistance is caused by collisions between charge carriers (usually electrons) and obstructions in the wire




Resistivity


Expect ρ to depend on any parameter used to describe a material, but temperature, T, is an important one.

Temperature coefficient, a, is different for
each material. It can be positive or negative



Loop Rule for Circuits


The sum of changes in the potential (V) encountered in a complete traversal of any loop of a circuit must be zero.





Internal Resistance

Real batteries, or other sources have an internal resistance that affects the amount of current that the device can deliver. The internal resistance is usually designated by the symbol, r.