ELECTRICITY @ HOME

S1-3-18 Explain parallel circuits, components, and safety of house wiring.

S1-3-21 Develop a formula for power consumption and solve related problems.

S1-3-22 Analyze the electrical energy consumption of a household appliance.

KEY WORDSOverload Circuit breaker FusePower Efficiency

House circuits are different than the circuits made with batteries:

1. Uses alternating current (AC) instead of direct current (DC)

2. Operates at a higher voltage - 120 V or 240 V3. Electricity controlled and moved through a

main panel (fuse box)4. All branches off the main line are in parallel

Common household circuits are 120 volts and 15 amps

Remember this?

Generators produce A/C current for the home

DC: electrons travel in one direction – (-) to (+)AC: e- move back and forth many times a second

DC is like rolling balls down a hill – need to keep adding more to the top (or run out)

AC is like balls rolling along a teeter totter – don’t need more since they will just move back and forth

There are 3 wires that bring electricity into a home:• Neutral wire (white)• Two “Hot” wires (black) – carrying 120 V each

There is also a safety “ground” wire - green or bare

copper - removes stray current safely

Normal use:1 hot connected to neutral creates 120 V potential

Larger appliances: Both hot wires connect to neutral creates 240 V

• each branch delivers typical 120 volts• total current of all branches equals 15 A• break any branch without affecting any other

All house branch lines link from the main panel.

Outlets and switches are all connected in parallel

Remember what parallel circuits offer…

Safety issues:

Short circuit: current bypasses the appliance. • less resistance – increases branch current• Branch heats up – risk of fire

There are two components that

safeguard against an increase in current

Overload: too many things plugged into one outlet. • increased branching – increases total current• Main heats up – risk of fire

A A

AA

main

Circuit Breakers: • contain either magnet / metal / semiconductor• all are altered by excess current - breaks circuit

(kill switch)

Fuses: • uses a strip of metal that easily melts• excess current heats metal – breaks circuit

Breakers can be reset

Fuses “burn out” and need to be replaced

A A

AAmain

Power and Energy Consumption

Power is a measure of energy output over time:• running the same distance faster – more power• lifting heavier weight in the same time – more power

1 Watt is the power of completing 1 joule of work in 1 second

• E is energy/work done in joules• t is the time in seconds

P =tE

Power – measured in watts – is the energy used or work done every second

An iPhone charger moves 72 000 joules of energy in the 4 hours to fully charge a drained battery. What is the power of the charger?

P =tE Formula must be in seconds to work properly

– time must be converted

4 h · 60 min · 60 sec = 1 h 1 min

14 400 sec

P =14400 seconds

72000 joules= 5 watts (W)

Rarely do we know energy values, but we can find voltage and current requirements - sometimes they

are written on the component.

P =tE

Remember: Voltage = energy per coulomb of electrons

Current = coulombs of electrons per second

P = V · I

tQ

QE

·P = V · I

A 10 amp car heater is plugged in to a standard house outlet. What is the power of the heater?

Large power measurements are converted to kilowatts

P = V · I Standard home outlet is 120V

P = 120 V · 10 A P = 1200 W

1200 W · 1 kW = 1000 W

1.2 kW

Energy companies charge a fee for total energy consumption (usually) per month – “hydro” bill

Total energy is calculated by multiplying power by time used

Purchasing Electrical Energy

• P is the power in kilowatts (kW)• t is the time in hoursE = P · tP =

t

E

Notice the units: Time is in hours – seconds are not useful for householdsPower is in kilowatts – watts are too small to be useful

tt

You make smoothies every day in September (30 days) using a 900 W blender for 15 min. How much energy did the blender use that month?

E = P · t900 W · 1 kW =

1000 W 0.9 kW

15 min · 1 h · 30 (days) = 60 min

7.5 hours

E = 0.9 kW · 7.5 h

E = 6.75 kWh

Energy usage is measured in kilowatt-

hours (kWh) instead of joules (too small)

Reading a Hydro Meter

Hydro meter uses 5 dials to measure energy usage:• far right dial is “ones” - far left dial is “ten thousands”• a reading is taken every 1-2 months

Reading

23 930 (kWh)

20 769 (kWh)

3161 (kWh) energy used this month

Hydro charges about $0.08 for every kilowatt-hour (kWh) used = $253 this month

This household uses natural gas for heat and has a second bill for the total amount of gas used.

Sample Hydro bill:

kWh usedTime

rate

Efficiency measures how well a device converts energy into work:• every device wastes some energy • lost as heat, sound, leakage…

If it is more efficient it does the same work with

less input energy

EnerGuide labels give consumers an idea of the efficiency of an appliance:• gives energy usage in regular use over time (month/year) • allows consumer to estimate cost of using the device• efficient devices will have a lower energy usage

Energy star: government-regulated symbol used to identify most efficient devices (based on tests).

This appliance will use 125 kWh of electricity per month of normal use

CAN YOU ANSWER THESE QUESTIONS?

S1-3-18:How is a household wired to ensure safety?

S1-3-21:What is power consumption and how does it affect your electricity bill?

S1-3-18:How can you lower electrical energy use of a household?

KEY WORDSOverload Circuit breaker FusePower Efficiency