electricity

ElectricityChapter 7 Section 1

Electric Charge

Sections• 1. Electric Charge slides 3 - 35

• 2. Electric Current slides 36 - 67

• 3. Electrical Energy slides 68 - 96

Electric Charge• The center of an atom is made of protons (+)

and neutrons (no charge).

Electric Charge• The center of an atom is made of protons (+)

and neutrons (no charge).

• Electrons (-) move around the center of the atom.

Electric Charge• The amount of +

Charge on a proton =

The amount of –

Charge on an electron.

Electric Charge• The amount of +

Charge on a proton =

The amount of –

Charge on an electron.• Each atom has the

same number of protons & electrons making them electrically neutral.

Electricity• Atoms have no charge.

Electricity• Atoms have no charge.• - ion if it gains

electrons


electrons• + ion if it loses

electrons


electrons• + ion if it loses

electrons• Electrons can move

from atom to atom or from object to object.

Static Electricity• Is the buildup of charges on an object.


• When there is static electricity, the electric charges are not balanced.


• When there is static electricity, the electric charges are not balanced.

• The Law of Conservation of charge states that charge can be transferred from object to object but it cannot be created or destroyed.

Electricity

• Opposite charges attract

• Like charges repel

+ - ++

- -

Electricity• The electric force between charged objects

depends on how far apart they are.



• The force decreases the farther apart the charges are.




• The electric force also depends upon the amount of charge on the objects.




• The electric force also depends upon the amount of charge on the objects.

• More charge = More force

Electric Field• Area around every electric charge that exerts

a force.

• Movement of a positive charge in electric fields.

+ -

Electric & Gravitational Force• Electric forces are stronger than gravity.


• Atoms are held together by electric forces.



• These forces cause chemical bonds to make a new substance.



• These forces cause chemical bonds to make a new substance.

• Electric forces between atoms are greater than the gravitational forces between atoms.

Electric & Gravitational Force• Electric forces are stronger than gravity.• Atoms are held together by electric forces.• These forces cause chemical bonds to make

a new substance.• Electric forces between atoms are greater

than the gravitational forces between atoms.• Electric forces between most objects are

less than the gravitational forces between them because most objects are elec. neutral.

Conductors & Insulators• Electrons move more easily in conductors.

Conductors & Insulators• Electrons move more easily in conductors.

Ex. Metals such as copper

• Insulators do not allow electrons to move as easily. Ex. Plastic, wood rubber and glass

Charging Objects• Charging by contact- transferring charges by

touching or rubbing ex. Clothes in a dryer

Charging Objects• Charging by contact- transferring charges by

touching or rubbing ex. Clothes in a dryer

• Charging by induction- charged object rearranges the electrons on a nearby neutral object.

Lightning• Large static discharge between clouds and

the ground that causes atoms & molecules to light up.



• Thunder- Electric energy in a lightning bolt rips electrons off atoms in the air causing great amounts of heat, up to 25,000 °C.



• Thunder- Electric energy in a lightning bolt rips electrons off atoms in the air causing great amounts of heat, up to 25,000 °C.

• The heat makes the air around the lightning bolt move faster creating sound waves.

Grounding• Grounding avoids damage by providing a

path for electric charge to move to earth.

Electroscope• Can detect when an

object has an electric charge.

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• The metal rod is a conductor.



• The metal rod is a conductor.

• When the leaves have a charge they repel each other and spread apart.

Electric CurrentChp 7 Section 2

Electric Current• The net movement of electric charges in one

direction.


direction.

• Measured in units called amperes or amps.


direction.


• Symbol for amperes is A.


direction.



• Measures the electrons that flow past one point.


direction.



• Measures the electrons that flow past one point.

• 1 A = 6,250 million billion electrons moving past a point every second.

Voltage• The electric force that makes charges move.


• Voltage is like the force that acts on water in a pipe.


• Voltage is like the force that acts on water in a pipe.

• Water flows from higher pressure to lower; likewise, electric charge flows from higher voltage to lower voltage.

Voltage difference• Related to the force that makes electric

charges flow.

Voltage difference• Related to the force that makes electric

charges flow.

• Measured in units called volts (V).

Electric Circuit• Closed loop-like path

that current must follow.

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Electric Circuit• Closed loop-like path

that current must follow.

• If the circuit is broken, current will not flow & bulb will not light.

Batteries• Provide the voltage difference that keeps

electric current flowing in a circuit.



• The positive & negative ends are called terminals.



• The positive & negative ends are called terminals.

• When a closed path connects the terminals, current will flow.

Types of Electric Cells• Electrochemical- electrons move b/w

different metals in an electrolyte solution– A. dry cell - use paste – flashlight battery– B. wet cell – use liquid – car battery



• Photovoltaic – electrons move when light hits a material – calculators, solar panels




• Thermoelectric – electrons move across two metals joined together & held at different temperatures – thermostats for ovens

Types of Electric Cells• Electrochemical- electrons move b/w different

metals in an electrolyte solution– A. dry cell - use paste – flashlight battery– B. wet cell – use liquid – car battery


• Thermoelectric – electrons move across two metals joined together & held at different temperatures – thermostats for ovens

• Piezoelectric – electrons move when opposite surfaces of crystals become charged under pressure – mikes, keyboards

Electrical outlets• Also give a voltage difference, usually higher

than a battery, such as 120 V or 240 V for large appliances.

Resistance• The tendency for a material to oppose the

flow of electrons, changing electric energy into heat and light


flow of electrons, changing electric energy into heat and light.

• Electrons flowing through the wire filament of a light bulb bump into metal atoms heating the filament until it glows lighting up…electric to thermal to radiant energy.


flow of electrons, changing electric energy into heat and light.

• Electrons flowing through the wire filament of a light bulb bump into metal atoms heating the filament until it glows lighting up.

• Resistance turns electric energy into heat & light.

Resistance• is measured in units called ohms (Ω) or the

Greek letter omega.


Greek letter omega.

• Temperature, length, & thickness of a material can affect its electrical resistance.


Greek letter omega.

• Temperature, length, & thickness of a material can affect its electrical resistance.

• Hotter, longer & thinner – more Ω

a simple circuit requires1. Source of voltage difference i.e. Battery

2. Device with resistance i.e. light bulb

3. Conductors i.e. wires connecting all three so that current can flow.

Ohm’s law• States that the current in a circuit equals the

voltage difference divided by the resistance.

Ohm’s law• States that the current in a circuit equals the

voltage difference divided by the resistance.

• If I stands for electric current, Ohm’s law can be written as :

current (in amps) = voltage difference

resistance (in Ω)

I = V/ R or R = V/I

Problem• Suppose a current of 0.5 A flows in a 75-W

light bulb. The voltage difference b/w the ends of the filament is 120 V. Find the resistance of the filament.

Problem• Suppose a current of 0.5 A flows in a 75-W

light bulb. The voltage difference b/w the ends of the filament is 120 V. Find the resistance of the filament.

• R = V/I = 120/0.5 = 240

• The resistance is 240 Ω.

Electrical EnergyChapter 7 section 3

Electrical energy• When you turn a hair dryer on, you close a

circuit. EE turns into thermal energy & ME.– Voltage difference –– Resistance –– Conductor -

Electrical energy• When you turn a hair dryer on, you close a

circuit. EE turns into thermal energy & ME.– Voltage difference – outlet– Resistance – hair dryer– Conductor - wires

Kinds of Circuits• Series –

• Parallel -

Kinds of Circuits• Series – current has only one loop to flow

through i.e. flashlights

• Parallel -

Kinds of Circuits• Series – current has only one loop to flow

through i.e. flashlights, some Christmas lights

• Parallel – have at least two circuits for current to move through i.e. houses

Circuits

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Circuits

SERIES

PARALLEL

Parallel Circuits• Break up current lowering resistance.


• More current flows through paths with less resistance.


• More current flows through paths with less resistance.

• Houses, cars and most electrical systems use parallel systems.

Household Circuits

Household Circuits• There is a main switch

& a circuit breaker or fuse box.



• Parallel circuits branch out from the circuit breaker or fuse box.



• Parallel circuits branch out from the circuit breaker or fuse box.

• The circuits run to wall outlets, appliances, & lights.

Fuses• Many appliances use current from the same

circuit causing more heat in the wires.


circuit causing more heat in the wires. To keep wires from causing a fire, household circuit include a fuse or circuit breaker.



• A fuse is a small glass tube with a piece of metal inside that melts if the current is too high breaking the circuit.



• A fuse is a small glass tube with a piece of metal inside that melts if the current is too high breaking the circuit.

• To restore current, the fuse must be replaced.

Fuses• Using too many appliances at once is the

main cause for a blown fuse.

• You should turn off or unplug some appliances before changing the fuse.

Circuit Breaker• Switching device that prevents circuits from

overheating by opening the circuit if the current is too high.

Circuit Breaker• Switching device that prevents circuits from

overheating by opening the circuit if the current is too high.

• You can reset the breaker by flipping the breaker box back to its original position but you should turn off or unplug some of the appliances first.

Electric Power• Rate at which electrical energy is changed

into another form of energy.



• Different appliances use different amounts of energy and are usually marked with a power rating.



• Different appliances use different amounts of energy and are usually marked with a power rating.

• Electric power (watts) = current (amps) X voltage difference (V) or P = IV

• Power is usually measured in kilowatts (kW)

Electric energy calculation• Electrical energy (kW) =

electric power (kW) X time (hours)

or E = Pt

A 100 W light bulb left on for 5 h uses:

E = Pt = (0.1kW) (5h) = 0.5 kWh

If the power company charges 10 cents per kW, the cost of using the light would be 5 cents.

Cost of using Home AppliancesHair dryer Stereo

Color TV

• Power rating 1,000 100 200

• Hours/day 0.25 2.0 4.0

• Kwh /month 7.5 6.0 24.0

• Cost / kWh $0.09 $0.09 $0.09

• Monthly cost $0.68 $0.54 $2.16

Electric Fuse Circuit Breaker

And makes the current

when when

contains contains

that that

Electric Fuse Circuit Breaker

A piece of metal

And makes the current

when when

contains contains

that that

Can melt

It gets too hot

STOP

Switches the circuit off

electricity

Documents

electric charge slides

electric chargesections1

electric fields

electricitythe electric

gravitational forces

electric chargethe center

electric current slides

positive charge