Current Electricity

Applied Physics and Chemistry Circuits Lecture 2

Ohm's Law

Resistance: slowing of current due to atomic structure

Things that affect resistance: Length: longer wire has more resistance Area: smaller cross section has more

resistance Temperature: higher temperature has more

resistance Material: different materials have different

resistance

Resistances of different materials

• Pt

• Fe

• Al

• Au

• Cu

• Ag

Res

ista

nce

Incr

ease

s

Ohm's Law

Relates potential, current and resistance V = I R Resistance measured in Ohms (Ω)

1 Ω = 1 Js/C2

V measured in Volts (V) 1 V = 1 J/C

Current measured in Amps (A) 1 A = 1 C/s

Ohm's Law

Many typical metals and circuit components obey Ohm's Law

Some materials (like transistors and diodes) do not (non-Ohmic)

Ohm's Law

A 30 V battery is connected to a 10 Ω resistor. What is the current through the circuit?

Known: V=30V R=10 Ω Equation: V=IR or I=V/R I=30V/10Ω so I=3A

Ohm’s LawWhat is the current through a 30 Ω resistance that has

a potential difference of 120 V?

What we know:

R = 30 Ω V = 120 V

Equation:

V = IR so I = V/R

Substitute:

I = (120V)/(30 Ω)

Solve!

I = 4A