TOPIC 7:ELECTRICITY AT

HOMEPages 319-331

In this PowerPoint, write down everything that is RED!!

QUESTIONS TO PONDER... How does power reach your home from

an electricity-generating power station? How is the power distributed through

your home? How do you pay for the electricity you

use?

https://www.youtube.com/watch?v=20Vb6hlLQSg

HOW DOES THE ELECTRICITY TURN ON MY STUFF?1. Power generating stations

produce large amounts of energy2. Transformers “step up” the

voltage for efficient transmission over long distances

3. Once at the spot, other transformers “step down” the voltage to about 240V/120V used in homes and factories.

4. Distribution lines connect your home to the power grid through a step-down transformer and a power meter.Meter is mounted on outside

wall of buildingsRecords total electric energy

supplied to electric devices in the home

5. Electricity now goes to service panelCircuit breaker: at the top of

panel, where the wires from the power meter are connected.

6. Main breaker acts as a switch that can cut off all power if the current exceeds the safe level. Bimetallic strip in the breaker heats up, bends,

and opens the circuit Older homes have fuse box

Metallic conductor that melts when excessive current heats up.

7. Lower part of service panel contains additional circuit breakers for each branch circuit in your home Each branch circuit supplies power to 1 or more

wall plugs or lights

Draw

this!

WHAT HAPPENS WHEN THE CIRCUIT HEATS UP? Current flow increases when more loads

are plugged in. The small resistance of the cable wires

causes them to heat up Before it gets TOO hot, the branch

breaker cuts off current to all loads in that circuit.

Pg. 320, Figure 4.45

Would other devices turn off if this happened in a parallel circuit? NO, other parallel circuit pathways stay on!

READ PAGE 320, 2ND PARAGRAPH: What is the difference between:

Neutral wire

Ground wire

Hot wire

Electrical code: a set of standards for electrical work that all

electricians must follow

DIGITAL DEVICES Digital electronic devices

are based on:

Transistors: Electronic switches turned on

and off by electric signals

Integrated circuits: Some have several transistors

in the same case

Microprocessors: chips

MEASURING ELECTRIC POWER Power (P):

Energy per unit timeMeasured in watts (W)

Electric power describes the amount of electric energy that is converted into other forms of energy every second.

POWER (in watts (W)) = Energy (in joules (J))

Time (in seconds (s))

P= E t

1 J/s = 1 W

A 100W light bulb converts 100 J of electric energy into light and heat EVERY second.

1 kilowatt = 1000 watts (like kilometres!!)

It is more common to measure the voltage and current in a circuit!!

POWER = current X voltage watts amps volts W A V

SAMPLE PROBLEM:

A current of 13.6A passes through an electric baseboard heater when it is connected to a 110V wall outlet. What is the power of the heater?

P=I=V=

QUESTIONS!Calculating POWER worksheet

Remember, for each question, I want to see:

- Formula- Work- Answer (in a box!)- /3 marks-

PAYING FOR ELECTRIC ENERGY Producing and distributing electricity is

expensive!Measured by electric meters.Many power companies measure electric

energy in kilowatt hour (kWh) 1 kWh is the total energy supplied to a 1000W

load during 1 hour of use

TOTAL COST = energy (kWh) X unit cost (cents

per kWh)

EXAMPLE A family uses 3000kWh of electric

energy in a two-month period. If the energy costs 11.0 cents per kilowatt hour, what is the electric bill for the period?

Total cost =energy =Unit cost =

To calculate kWh… Energy = Power (W) x Time (h)

For example: If a 120W dryer was left on for 3 hours,

it would use… 120 W x 3 h = 360Wh = 0.360 kWh 1000 W

EXAMPLE #2 A 1500W electric heater in a garage is

accidentally left on for 5 full days. If the price of electrical energy is 9 cents per kilowatt hour, how much money was wasted by the running heater unnecessarily?

POWER RATING Remember:

1W= 1 J/sEnerGuide labels are on appliances at home

Ex: refrigerators, washers, dryers, air conditioners, etc

1.This large number indicates the approximate amount of energy the appliance will use in one year of use.

2. The coloured bar shows how this appliance compares with others on the market

3.The number gives the yearly energy use of the most efficient and least efficient appliances of similar type

4.Shows the classification of appliance

EFFICIENCY No device is completely efficient!

Some input energy is converted into waste heat!

Energy taken in is input energy. Energy that is released is output

energy.

Efficiency = useful energy output X100%

total energy input

Read page 328 on how efficient different types of light bulbs are

EXAMPLE: A 1000W electric kettle takes 4.00min to

boil some water. If it takes 1.96 X 105J (196 000J) of energy to heat the water, what is the efficiency of the kettle?

Find input value ( E=Pt) Find output value (E=Pt) Then solve with efficiency equation