1. Power Grid and Your Home

2. Transmission of Electricity Through the Power Grid
Power travels from the power plant to your house through an amazing system called the POWER DISTRIBUTION GRID.
Voltage at the generation plant is stepped up by a transformer to travel through the grid

3. AC = alternating current which is how power travels from the generating plant to your home. AC can be stepped up and stepped down easier than DC
Alternating current means the current switches direction many times per minute
DC = direct current which is how batteries supply power
Direct current means that the current moves in the same direction

4. From the Grid Into Your Home
Distribution lines connect your home to the power grid with a step-down transformer and a power meter
The meter measures the electricity supplied to your house
Then goes to your service panel and a circuit breaker
which is a safety device that cuts off all power
(bimetallic strip) greater than 20A of current
Branch circuits (parallel) let other branches work when
1 is overloaded
Neutral wire is white, hot wire is black, ground wire is green

5. Circuit Breakers Absolutely + +
HowStuffWorks "How Circuit Breakers Work" + +
30 A
15 A
10 A
Circuit breaker will trip when 50 A is exceeded
Absolutely
Will this breaker trip?

6. Short Circuits
The hot wire is connected to the ground wire via all the loads in your circuit, they never normally touch
The ground wire completes your circuit
If you accidentally hammered a nail between the two or a motor burned out inside an appliance and melted between the two, then POW! The ground wire is now directly connected to the hot wire and pulls enormous amounts of current through the circuit, the circuit overheats massively and starts a fire

7. Home Wiring Electricians need a license and must meet electrical codes
In our homes there is a 120V potential difference
For stoves/ovens there is a 220V potential (step-up and step-down transformers)

8. Digital Devices
Digital electronic technology = TINY switches that are ‘on’ or ‘off’ which is the binary code
The tiny switches are transistors which, when ‘on’ or ‘off’ control other signals
Examples are: CD player, calculator, cell phone, iPod….

9. Measuring Power
Power is energy per unit of time and is amount of electricity that is converted to heat, light, sound or motion over time
Power (in watts) = Energy (Joules) P = E
Time (seconds) t
Power is more commonly measured with voltage and current
Power (watts) = Current (amps) x Voltage (volts) P=I x V
By the way…..James Watt invented the steam engine

10. Power Problems
A current of 14.2 A passes through a heater when connected to 110 V outlet. What is the power of the heater?
- formula is: P = I x V
A microwave oven requires 7.5 A of current to run. What is the power rating of this appliance? (hint: What is the normal voltage running into all North American homes?)
14.2 A x 110 V = 1562 W
(1.6 x 103)
7.5 A x 120 V = 900 W

11. What is a kiloWatt? A MegaWatt is a kiloWatt divided by 1000
A Watt divided by a thousand
1000 Watts =
1 kiloWatt
2000 Watts =
2 kiloWatts
43210 Watts =
kiloWatts
A MegaWatt is a kiloWatt divided by 1000

12. Paying For Electric Power
Power companies record household power use on their meters … (kWh) kilowatt hours
Always read the lowest number when the dial is between numbers – five digits
What is the reading on these dials?
kWh

13. Multiply kWh by the cost per kWh
How Much Will it Cost?
Multiply kWh by the cost per kWh
Example: A family uses 3000kWh of electric energy in a two month period. If the energy costs 11.0 cents per kWh, how much will it cost them?
Solution: Cost = 3000 kWh x 11.0 cents per kWh
= cents
= $330 because there are 100 cents in every $

14. Electric Devices and Efficiency
No device is 100% efficient – some input energy is always converted to waste heat
Efficiency = useful energy output x 100%
total energy output
Memory strategy: Little Joule divided by big Joule x 100
Ex: Find the efficiency of a 23 W fluorescent bulb that is used 4 hours a day and produces J of useful light energy.
Useful energy output = J
Total energy output = P x t
= 23 W x 4(60 min x 60 sec)
= J
Efficiency = J x 100% = 20% efficient (not very!!!) J
Why is an incandescent light bulb so inefficient?

15. Practice power problems Practice efficiency problems
Your Job
Practice power problems
Practice cost problems
Practice efficiency problems
Know how to read an electricity meter
Become familiar with all vocabulary terms in this topic