Current and Resistance Physics 014 Current and Resistance

Currents and Resistors The electric current (i) is the amount of charge moving past a point or through a region in an amount of time.

Currents and Resistors The SI unit of charge is the ampere (A) 1 A = 1 coulomb per second

Charge is conserved i0=i1+i2

Currents and Resistors A current arrow is drawn in the direction in which positive charge carriers would move, even if the actual charge carriers are negative and move in the opposite direction.

??? The figure shows a portion of a circuit. What are the magnitude and direction of the current i in the lower right-hand wire? 1. 4A 2. 6A 3. 8A 4. 7A

Currents and Resistors The current density, , has a magnitude equal to current per unit area. SI unit of A/m2.

Currents and Resistors Current i is constant. What about J?

Currents and Resistors We see then that for current density

Currents and Resistors We get resistance by applying a potential difference V between two points and measuring the resultant current i. (Ohms law)

Currents and Resistors The SI unit for resistance is the ohm (Ω). 1 ohm = 1 volt per ampere

Currents and Resistors We can also calculate resistance from properties of materials. If we know the resistivity ρ or conductivity σ=1/ρ of a material, length Area

Currents and Resistors Resistances connected in series can be replaced with an equivalent resistance Req that has the same current I and the same total potential difference V as the actual resistance.

Circuits For n resistors in series,

Currents and Resistors Can resistors in parallel be reduced to an equivalent circuit?

Currents and Resistors Resistances in parallel can be replaced with an equivalent resistance Req that has the same potential difference V and same total current i as the actual resistances.

Currents and Resistors For n resistors in parallel, Proof: use the junction rule