Electricity

• Electric Charge Rules:– More protons than electrons: + charge– More electrons than protons: - charge– Like charges repel; opposite charges attract– In electricity, ONLY e- move, not protons!

• Strength of electric force due to amount of excess + or – charge

• Electric Field: region around charged particles in which others will attract or repel

• Definition: the buildup of –charge on an object

• Method of Charging:– Friction-movement of electrons from

one objects surface to another– Conduction-transferring electrons

from one object to another by direct contact

• Ex. Many metals allow electrons to flow

Static Electricity cont.,

• Voltage-– Energy or push that makes electrons move– The the voltage, the energy each

electrons carries– Measured in units called volts (V)

• Electric Current: flow of e- through a wire– Symbol for current is I– Current is measured in amperes, or

amps (A) for short• current = e-

Flow of Electricity cont.,• Resistance: opposition to flow of electricity

– Symbol for resistance is R– Resistance is measure in ohms, which is represented

by Ω– Poor conductors have high resistance (or low

conductivity)• Ex. Iron

– Good conductors have low resistance (or high conductivity)

• Ex. Copper

– Long and thin wires have more resistance than short and thick wires

– Any device that you plug in is called a resistor (load)

• Ohm’s Law: – current in a wire =voltage/resistance (I=V/R)

• See Ex. Problems on pg. Ω• 453

• Current Directions:– Direct Current (DC): electrons flow in the same

direction• Ex. Batteries (e- flow through and terminal)

– Alternating Current (AC): Electrons constantly change their direction of flow

• Ex. Electricity from power plants (via generators)

• What’s a Circuit?– Provides a complete, closed path for an electric

current to flow– Circuit works when path is closed; doesn’t work when

path is open

• Parts of a Circuit:– A source of e- (i.e. battery, outlet)– A load (resistance) which is the device that uses the

electricity – Wires carry the electrons– A switch, which opens or closes the circuit

•Only one path for a current to flow•All parts of the circuit are connected to one after the other

–Ex. Old Christmas Lights

•Same current (I) passes through each resistor•Current stays the same•Voltage drops after each resistor

•Different parts of circuit are on separate branches•Several paths for electrons can be takes

–Ex. New Christmas Lights

•Voltage stays the same•Current decreases through each branch

Electrical Power• Measure of the rate at which electricity

does work or provides energy• Power=voltage*amperes

– Units: Watts=volts*amperes– 1 kilowatt (kW)=1000 Watts

• Electric Energy: energy=power*time (E=P*t)– Units: kilowatt-hours=kilowatts*hours– Your electric meter at home reads in

kilowatt-hours; this number is multiplied by cost/kilowatt-hr to determine your bill

Power Distribution Grid• At the power plant:

– Generators produce 3-phase AC electricity via generators (electromagnetic conduction)

– Electric goes through a step-up transformer, were voltage is increased for travel over electric wires (transmission substation)

– Voltage is usually stepped up to between 155,000-765,000 volts!

Power Distribution Grid cont.,• Transmission of Electricity:

– High voltage lines are those huge steel towers (3 wires for 3 phases+ground)

– Electricity usually travels up to 300 miles from power plant

• Power Substation: can do several diff. things– Have ‘buses’: split the power off into

many directions– Decrease the voltage by using a step-down transformer

• Before Going Into the House:– Multi-phase wires are tapped

down to single phase (1 wire) @ 120 V•You often see three wires going to

houses (single phase electric + ground)

– Transformer drum step-down 7200 volts to 240 volts for house

– Some neighborhoods have underground wires, so you might see a green transformer box (these are step down voltage)

• At the House:– Electric wires travel through watt-hour

meter (charges you for electric)– Two 120-volts wires then travel to circuit

breaker (box in garage or basement), where it’s split and wired to all of the outlets in the house

Magnetism

Magnetism

• All magnets have North and South poles

• Rule for magnetic poles: like poles repel/opposites attract

• Magnetic Field=region in which magnetic forces can act– Earth has one around it; strongest at

poles– Magnetic lines of force define these areas– Electrons in atoms spin, causing the

magnetic field

Magnetic Materials

• Naturally occurring (ex. Magnetite)• Some materials that are non-magnetized can

become so: iron, nickel are examples

Earth as a Magnet• Magnetosphere: region of magnetic field

around the earth• Earth poles: Geographic north pole is actually

Magnetic south pole and vice versa

Compasses

• Electricity and magnetism are inseparable• Electromagnet: made by winding wire around an iron core

– Will have N and S poles– Polarity of magnet depends on polarity of electric current

• Magnetism can induce electricity• Electricity can induce magnetism

Electromagnetism

• Needle of a compass is actually magnetic• Compass needle points to north pole (magnetic)

Electric Motor

•Electric Wire wrapped around an iron core•These are temporary magnets; can be turned on and off

–Ex. Junkyards, doorbells, telephones, electric motors

Electromagnets

•Converts electric energy to mechanical energy (motion)•Operates on the idea of reversal of polarity•Uses a stationary permanent magnet, a rotating electromagnet, and a polarity-reversing switch (conductor)

Galvonometer

• Converts mechanical energy into electrical energy

• Loop of wire (conductor) spins inside a magnetic field to create electricity; known as an elector-magnetic induction

• Most of the power you use everyday comes from generators

Electric Generator

• Device that detects electric current (basically a coil of wire connected to an electric circuit and a needle)

• Current deflects needle towards direction of current