1. Ohm’s Law and Electrical Power

2. Reminders
Resistance ( R ): “Electrical friction”, opposition to current
Measured in ohms
Current ( I): How fast the electricity is traveling
Measured in amps

3. How do resistance( R ) & current (I) relate to each other?
Inverse relationship As resistance increases, current decreases As resistance decreases, current increases

4. Formulas for Ohm’s Law Resistance = Voltage/ current
R = V/I
Voltage = Current X Resistance
V = I X R

5. Practice Problems
A flashlight has 1.5 ohms of resistance and uses 5 amps of current. How many volts does the flashlight need?
1. Write the equation V = I X R
2. Fill in the numbers and units
V = 5 amps X 1.5 ohms
3. Solve:
V = 7.5 volts

6. Practice on Your Own
If a lightning bolt has 1000 ohms of resistance and a current of 150 A how much voltage of electricity does it have?

7. Solved 1. Write the equation V = I X R
2. Fill in the numbers and units
V = 150 amps X ohms
3. Solve:
V = 150,000 volts

8. Electrical Power
The rate at which electrical energy is changed into other forms of energy
Formula for calculating power
Power = voltage X current
P = V X I
Unit for power is Watts (W) and Kilowatts (kW)
1 kW = 1000 W

9. Practice Problems
A toaster draws about 10 A of current. A home receives 120 V at each outlet. What is the power of the toaster?
1. Write the equation P = V X I
2. Fill in the numbers and units
P = 120 V X 10 A
3. Multiply to solve:
P = 1,200 W

10. 
Practice on Your Own 
An overhead projector draws about 4.5 A of current from a 120 V outlet. What is the power needs for the overhead projector?


11. Solved
P = V X I
P = 120 V X 4.5 A
P = 540 watts