AP Physics C 2017

Figure  19.1            Flow    of  charge  in  a   wire    connected   to  a   battery.

In general, the greater the potential difference between the terminals of the battery, the more current
flows.
The amount of current that flows in a circuit is also determined by the resistance of the circuit.

Resistance: A   property    of  a   circuit that    resists the flow    of  current

Resistance is measured in ohms. 1 ohm is abbreviated as 1 Ω.
If we have some length of wire, then the resistance of that wire can be calculated. Three physical
properties of the wire affect its resistance:

•           The material    the wire    is  made    out of: the resistivity ,   ρ   ,   of  a   material    is  an  intrinsic   property    of

that    material.   Good    conducting  materials,  like    gold,   have    low resistivities.^1

•           The length  of  the wire,   L   :   the longer  the wire,   the more    resistance  it  has.

•           The cross-sectional area    A of    the wire:   the wider   the wire,   the less    resistance  it  has.

We put all of these properties together in the equation for resistance of a wire:

Now, this equation is useful only when you need to calculate the resistance of a wire from scratch.
Usually, on the AP exam or in the laboratory, you will be using resistors that have a pre-measured
resistance.

Resistor: Something you put in  a   circuit to  change  the circuit’s   resistance

Resistors are typically ceramic, a material that doesn’t allow current to flow through it very easily.
Another common type of resistor is the filament in a light bulb. When current flows into a light bulb, it
gets held up in the filament. While it’s hanging out in the filament, it makes the filament extremely hot, and
the filament gives off light.
The way that a resistor (or a bunch of resistors) affects the current in a circuit is described by Ohm’s
law.