Power Grid and Your Home

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

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 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

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?

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 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)

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….

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

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

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 = 43.210 kiloWatts A MegaWatt is a kiloWatt divided by 1000

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? 27 038 kWh

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 = 33000 cents = $330 because there are 100 cents in every $

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 66 240 J of useful light energy. Useful energy output = 66 240 J Total energy output = P x t = 23 W x 4(60 min x 60 sec) = 331 200 J Efficiency = 66240 J x 100% = 20% efficient (not very!!!) 331200 J Why is an incandescent light bulb so inefficient?

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