Ohm’s Law &
• Resistance - the tendency for a
material to oppose the flow of
- All materials have some electrical
- Resistance depends on length, cross-
sectional area, material, and
Ex. Making wires thinner, longer, or
hotter increases the resistance.
• Resistors provide a specified
amount or resistance to a conductor.
• Used to regulate the amount of
current in a conductor.
• Ohm’s law - the current in a
circuit (I) equals the voltage
difference (V) divided by the
R = V/I
• Resistance is measured in
The resistance of a steam iron is 19.0
Ω. What is the current in the iron when
it is connected across a potential
difference of 120 V?
R = V/I V= 120 V R= 19 Ω
I = V/R = 120 V / 19 Ω = 6.32 A
• Electrical Power is the rate at which
electric charge converts electrical
potential energy to non-electrical
forms of energy.
P = IΔV = I²R = ΔV²/R
SI Unit = watt (W)
An electric space heater is connected
across a 120 V outlet. The heater
dissipates 1320 W of power in the form
of electromagnetic radiation and heat.
Calculate the resistance of the heater.
P = ΔV²/R P = 1320 W V= 120 V
R = ΔV²/P = (120 V)²/ 1320 W = 10.9 Ω
• Electric companies measure energy
consumed in kilowatt-hours (kW •h)
How much does it cost to operate a
100.0 W light bulb for 24 h if electrical
energy costs $0.080 per kW•h?
Total Energy Used = (0.100 kW)( 24 h) =
Cost = (2.4 kW•h)($ 0.080/ kW•h) = $0.19