ElectricityChapter 16

Review Smallest particles of

matter are called atoms Electrons Protons Neutrons

electron

neutron

proton

…Protons Positive

charge

Electrons Negative

charge

**If atom has equal number of protons & electrons there is no net charge***

Charge & Force Charges produce a force

between objects› Opposite charges attract› Like charges repel

Moving Electrons Electrons can be moved

Rubbing hair against a balloon will move some electrons from the hair to the balloon

Both the hair and the balloon will have a charge.› What will the charge on

the balloon be?› What will the charge on

the hair be?

SI Unit for electric charge is the Coulomb, C

Electric Charge If there is an imbalance with the

number of electrons and protons, there is a net electric charge

If there is more electrons than protons, object is negative› Electron’s charge = -1.6 x 10-19C

If there is more protons than electrons, the object is positive› Proton’s charge = 1.6 x 10-19C

Two types of materials Conductor

› allows electrons to move through it easily

› e- are loosely held › ex: metals like copper and silver

…. Insulator

› material that doesn’t allow electrons to move through it easily

› e- are tightly held› ex: plastic, wood, rubber, glass

Electric Field

An electric field surrounds all charged objects.

Electric forces act at a distance because of this field.

Static Electricity Static means not moving

Static electricity is electricity at rest

Friction can cause it Objects rub together and

electrons move from one object another.

Static discharge Eventually static electric charge

will move. Slowly the electrons may move

into moisture in the air Or quickly in a spark.

Lightning Wind rubs particles in cloud

together Cloud gains charge Induce charge in ground Eventually a big charge jumps Lightning rod protects

buildings

Transfer of ElectronsCharging by

friction Electrons are

transferred when different materials are rubbed together

Depends on materials

Charging by contact (Conduction)

When negative object touches neutral object, electrons move objects

Charging by Induction• charging an object without actually touching the object to any other charged object

Electroscope Consists of:

› Flask› Metal bar (conductor)

through rubber stopper (insulator)

› 2 pieces of thin foil on the bottom

› Charge on the metal will push the foil apart because they have the same charge

…

No Charge- leaves hang straight down

Induction

Rod with negative charge

…

Rod with negative charge:•Pushes negative charges away - down electroscope

…

•Extra negative charges cause leaves to move apart

…

Remove rod everything returns

Conduction

Rod with negative charge

…

Rod with negative charge touches electroscope

Electrons are transferred from rod to electroscope

Extra negative charges cause the leaves to move apart.

Remove rod leaves stay apart.

Potential Difference

Often called voltage Change in the electrical potential

energy of a charged particle divided by its charge

Occurs when charge moves from one place to another

SI Unit for potential difference is the Volt (V)› 1 V = 1 J/C

Current Flow of electrons Number of electrons move

through a conductor Measured in Amperes of Amps

(A)

Resistance Internal friction, which slows the

movement of charges through a conducting material

A dim 40W bulb has a higher resistance than the filament of a bright 100W bulb

SI Unit is ohm (Ω)› Ω = Volts/Amps

Found by dividing the voltage across the conductor by the current

electrical energy is converted to thermal energy & light

Resistance depends on…› the conductor (low)› Wire thickness

Less resistance in thicker wires

› Wire length Less resistance in shorter wires

› Temperature Less resistance at low temps

Copper - low resistance

Tungsten - high resistance

Ohm’s Law The relationship among

current, voltage, and resistance.

Ohm’s law states that the current in a circuit is equal to the voltage divided by the resistance

I = V R

V

I R

Ohm’s Law

V = I × R

V: potential difference (V)

I: current (A)

R: resistance ()

• Voltage ↑ when current increases.• Voltage ↓ when resistance increases.

Ohm’s LawA light bulb with a resistance of 160 is

plugged into a 120-V outlet. What is the current flowing through the bulb?

GIVEN:

R = 160 V = 120 V

I = ?

WORK:

I = V ÷ R

I = (120 V) ÷ (160 )I = 0.75 A

I

V

R

Do the Math A car has a 12 volt system. The

headlights are on a 10 amp circuit. How much resistance do they have?

Your house uses 120 volts. What amount of current would flow through a 20 ohm resistor?

Circuit Circuit

› Path through which electrons can flow

Circuit Components

A - battery C - light bulbB - switch D - resistor

Circuits

For current to flow there must be a complete loop

Electrons flow from negative to positive terminal

Work is done if there is a resistance in the wire.

2 Types of Circuits Series Circuit

› current travels in a single path 1 break stops the flow of current

› current is the same throughout circuit lights are equal brightness

› each device receives a fraction of the total voltage get dimmer as lights are added

Series Circuits

Series Circuits

Series Circuits

Break in the wire turns off all the lights

Series Circuits

Parallel Circuits Parallel Circuits

› current travels in multiple paths one break doesn’t stop flow

› current varies in different branches takes path of least resistance “bigger” light would be dimmer

› each device receives the total voltage no change when lights are added

Parallel Circuit

Parallel Circuit

Electrical Power

P = I × VP: power (W)

I: current (A)

V: potential difference (V)

•rate at which electrical energy is converted to another form of energy

Electrical Power A calculator has a 0.01-A current flowing through it.

It operates with a potential difference of 9 V. How much power does it use?

GIVEN:

I = 0.01 A

V = 9 V

P = ?

WORK:

P = I · V

P = (0.01 A) (9 V)

P = 0.09 W

I

P

V

Electrical Energy energy use of an appliance

depends on power required and time used

E = P × tE: energy (kWh)

P: power (kW)

t: time (h)

The SI unit for energy is a joule. Kilowatt-hour meters measure

the electricity used in your home (kWh)

Electrical EnergyA refrigerator is a major user of electrical power. If it uses 700 W and runs 10 hrs each day, how much energy (in kWh) is used in 1 day?

GIVEN:

P = 700 W = 0.7 kW

t = 10 h

E = ?

WORK:

E = P · t

E = (0.7 kW) (10 h)

E = 7 kWh

P

E

t

Types of current Direct current: electrons that flow

in the same direction in a wire (DC) › From batteries

Alternating current: electrons that flow in different directions in a wire (AC)› From Genrators› Used in your home

Transformers change AC to DC

Household Circuits

Combination of parallel circuits› too many devices can cause wires to overheat

Safety Features:› fuse - metal melts, breaking circuit› circuit breaker - bimetallic strip bends when hot, breaking circuit

Electric Safety Many appliances are equipped with a “ground” wire on the plug.

The ground wire prevents electric shock. The rounded third prong of a three-way electric plug is attached to the ground wire.

It constantly moves static electricity from the appliance to the ground.

Broken wires or water can cause electric appliances to short-circuit.

A short circuit occurs when electricity takes a short path and bypasses the resistors in the circuit.

Because of this the resistance of the circuit is less and the circuit wire increases.

The increased current can produce enough heat to melt wires and start a fire, or cause serious electric shock.

Circuit Protectors

Fuses and circuit breakers protect against overloaded circuits.

A number on the fuse indicates the max. current that will flow through it.

Circuit breakers are often used in place of fuses. A circuit breaker is a switch that opens automatically when electric current in a circuit reaches its max.