1. Unit 2: Electricity Lesson 7: Power
Science 9H
Unit 2: Electricity
Lesson 7: Power

2. Objectives By the end of the lesson you should be able to:
Use the relationship between voltage, current and power to solve more complex electrical problems

3. WORK: Work is the force used to move something some distance
Work = Force x Distance
Force: units are Newtons (N)
1 N = force that accelerates a 1 kg mass at 1 m/s2
Distance: units meters (m)
Work: units Joules (J)
1 J = 1 N·m
(amount of force over a distance)

4. Practice Questions:
If a 1N force is used to move an object 1m how much work is done?
If you push a book 0.5m across a desk with a force of 8N, how much work is done?
(ENERGY: the ability to do work)

5. Power Power is the rate of doing work
- units Watts (W) or (Joules per second)
A Watt is relatively small so we use kiloWatts (kW)
So we need to do conversions again!

6. Watts and kiloWatts There are 1000 Watts in 1 kiloWatt
EX kW = 30,000 W
- 14 W = kW

7. Practice 125 W = 34,798 W = 67 W = 5 kW = 32 kW = 0.98 kW =
0.125 kW kW kW 5000 W 320 W 980 W

8. Joules’ Law
There is a relationship between voltage, current and power
YAY! Another triangle!!
P = V x I
I = P V
V = P I P
I V

9. Examples:
What is the power of a light that draws 2 A of current from 100 V?
A transistor draw 5 mA from 12 V. What is the power?
A toaster used 500 W of power, if the voltage is 100 V, what is the current?
A resistor draws 3 kW of power on a current of 6 mA, what is the voltage?
Draw 4 dry cells in series, a 100 Ω resistor and an ammeter. What is the power drawn by this resistor? (Each cell is 1.5 V, so you need to find the ammeter reading first!)

10. Power Consumption
Power consumption is the amount of power used over a period of time
After you have calculate the power you can determine the total amount used by multiplying it by the length of time it was used
POWER MUST BE IN KILOWATTS!!
Power Consumption = power x time

11. Practice:
1. A 300 mA current runs through a resistor connected to 100 V.
What is the power?
It runs for 100 hrs, how much electricity is used?
Electricity costs 7 cents per kW·hr, how much did it cost to run this resistor?

12. Practice:
2. Mom is yelling at her teenager for leaving the lights on in the basement for one week (7 days!!). There are 3 lights each drawing 75 W. What did it cost to run the lights for the week if electricity costs 7 cents a kW·hr?

13. Practice:
3a. An arc welder used 100 A of current on 220 V, what is the resistance?
b. If the arc welder is used for 6 hrs, how much electricity did it use?
c. If electricity costs 7 cents per kW·hr, how much did it cost to run that arc welder?