Power Grid and Your Power Grid and Your HomeHome

Transmission of Electricity Transmission of Electricity Through the Power GridThrough the Power Grid

• Voltage at the Voltage at the generation generation plant is plant is stepped up by stepped up by a transformer a transformer to travel to travel through the through the grid grid

• Power travels from the power plant to your house through an Power travels from the power plant to your house through an amazing system called the POWER DISTRIBUTION GRID. amazing system called the POWER DISTRIBUTION GRID.

• 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

From the Grid From the Grid Into Your HomeInto Your Home

• Distribution lines connect your home to the power grid Distribution lines connect your home to the power grid with a with a step-down transformerstep-down transformer and a and a power meterpower meter

• The meter measures the electricity supplied to your The meter measures the electricity supplied to your househouse

• Then goes to your service panel and a Then goes to your service panel and a circuit breakercircuit breaker

which is a safety device that cuts off all power which is a safety device that cuts off all power

(bimetallic strip) greater than 20A of current(bimetallic strip) greater than 20A of current• Branch circuits (parallel) let other branches work whenBranch circuits (parallel) let other branches work when

1 is overloaded1 is overloaded• Neutral wire is white, hot wire is black, ground wire is Neutral wire is white, hot wire is black, ground wire is

greengreen

Circuit Breakers

HowStuffWorks "How Circuit Breakers Work"

+ +

30 A 15 A 10 A

Circuit breaker will trip when 50 A is exceededWill this breaker trip? Absolutely

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

Home WiringHome Wiring

• Electricians need a license and must meet Electricians need a license and must meet electrical codeselectrical codes

• In our homes there is a 120V potential differenceIn our homes there is a 120V potential difference• For stoves/ovens there is a 220V potential (step-For stoves/ovens there is a 220V potential (step-

up and step-down transformers)up and step-down transformers)

Digital DevicesDigital Devices

• Digital electronic technology = TINY switches Digital electronic technology = TINY switches that are that are ‘on’‘on’ or or ‘off’ ‘off’ which is thewhich is the binary code binary code

• The tiny switches are The tiny switches are transistorstransistors which, when which, when ‘on’ or ‘off’ control other signals‘on’ or ‘off’ control other signals

• Examples are: CD player, calculator, cell phone, Examples are: CD player, calculator, cell phone, iPod….iPod….

Measuring PowerMeasuring Power

• PowerPower is energy per unit of time and is amount of is energy per unit of time and is amount of electricity that is converted to heat, light, sound or electricity that is converted to heat, light, sound or motion over timemotion over time

• Power (in watts) = Power (in watts) = Energy (Joules)Energy (Joules) P = P = EE

Time (seconds) Time (seconds) tt

• Power is more commonly measured with voltage and Power is more commonly measured with voltage and currentcurrent

• Power (watts) = Current (amps) x Voltage (volts) Power (watts) = Current (amps) x Voltage (volts) P=I P=I x Vx V

• By the way…..James Watt invented the steam engineBy the way…..James Watt invented the steam engine

Power ProblemsPower Problems

• A current of 14.2 A passes through a heater A current of 14.2 A passes through a heater when connected to 110 V outlet. What is the when connected to 110 V outlet. What is the power of the heater?power of the heater?

- formula is: P = I x V- formula is: P = I x V

• A microwave oven requires 7.5 A of current to A microwave oven requires 7.5 A of current to run. What is the power rating of this appliance? run. What is the power rating of this appliance? (hint: What is the normal voltage running into (hint: What is the normal voltage running into all North American homes?)all North American homes?)

- formula is: P = I x V- formula is: P = I x V

14.2 A x 110 V = 1562 W 14.2 A x 110 V = 1562 W (1.6 x 103)(1.6 x 103)

7.5 A x 120 V = 900 W7.5 A x 120 V = 900 W

What is a kiloWatt?

A Watt divided by a thousand

1000 Watts =

1 kiloWatt

2000 Watts =

2 kiloWatts

43210 Watts =

43.210 kiloWatts

A MegaWatt is a kiloWatt divided by 1000

Paying For Electric Paying For Electric PowerPower

• Power companies record household power use on Power companies record household power use on their meters … (kWh) kilowatt hourstheir meters … (kWh) kilowatt hours

• Always read the lowest number when the dial is Always read the lowest number when the dial is between numbers – five digitsbetween numbers – five digits

• What is the reading on these dials? What is the reading on these dials? • 27 038 kWh27 038 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

= 33000 cents

= $330 because there are 100 cents in every $

Electric Devices and Electric Devices and EfficiencyEfficiency• No device is 100% efficient – some input energy is always No device is 100% efficient – some input energy is always

converted to waste heatconverted to waste heat• Efficiency = Efficiency = useful energy outputuseful energy output x 100% x 100% total energy outputtotal energy outputMemory strategy: Little Joule divided by big Joule x 100Memory strategy: Little Joule divided by big Joule x 100• Ex: Find the efficiency of a 23 W fluorescent bulb that is used 4 Ex: Find the efficiency of a 23 W fluorescent bulb that is used 4

hours a day and produces 66 240 J of useful light energy.hours a day and produces 66 240 J of useful light energy. Useful energy output = 66 240 JUseful energy output = 66 240 J Total energy output = P x tTotal energy output = P x t = 23 W x 4(60 min x 60 sec)= 23 W x 4(60 min x 60 sec) = 331 200 J= 331 200 J Efficiency = Efficiency = 66240 J66240 J x 100% = 20% efficient (not x 100% = 20% efficient (not

very!!!)very!!!) 331200 J331200 J

Why is an incandescent light bulb so inefficient?

Your Job

Practice power problems

Practice efficiency problems

Know how to read an electricity meter

Become familiar with all vocabulary terms in this topic

Practice cost problems