ELECTRICITY

Electric Charge• property of matter that creates electric

and magnetic forces and interactions.electric charge depends on the imbalance

of its protons and electrons.• Electrons negatively charged• protons positively charged• neutrons are neutral (no charge).• Like charges repel• opposite charges attract.

E l e c t r i c c h a r g e

• L i k e c h a r g e s r e p e l .

• O p p o s i t e c h a r g e s a t t r a c t .

Electric Charge

• Negatively charged objects have more electrons than protons.

• Positively charged objects have fewer electrons than protons.

SI unit:

• coulomb, C.

Electric Chargeelectrical conductor:• material in which charges can move freely and that can

carry an electric current.electrical insulator:• material that does not transfer current easily.

Objects can be charged by the transfer of electrons.• outermost electrons can be easily transferred from one

atom to another.Charging by friction• when one material gains electrons and becomes

negatively charged, and the other loses electrons and becomes positively charged.

INDUCED CHARGESpolarization • Alignment of charges at surface of

an object producing an electric charge

• Positive & negative charges on opposite sides

Charging by Contact

When a negative rod touches a neutral doorknob, electrons move from the rod to the doorknob.

The transfer of electrons to the metal doorknob gives the doorknob a net negative charge.

•charge a neutral object by touching it with a charged object.

Static Electricity

• Static Discharge• movement of

electrons to relieve a separation in charge

Electric Force• force of attraction or repulsion between

objects due to charge.• atomic and molecular level is

responsible for most of the common forces we can observe.

Electric force depends on charge and distance.

• is proportional to the product of charges on objects.

• is inversely proportional to square of distance between two objects.

Electric Force

• region in space around a charged object that causes a stationary charged object to experience an electric force.

ELECTRIC FIELD LINES

• point in direction of electric force on a positive charge.

• around a positive charge point outward.• around a negative charge point inward.

• Electric field lines never cross one another.

• show both direction of an electric field and relative strength due to a given charge.

• More lines are drawn for greater charges to indicate greater force.

Electric Field Lines

Electric Field Lines

CURRENT

ELECTRICAL POTENTIAL ENERGY

• ability to move an electric charge from one point to another.

• EPE of moving charge decreases because electric field does work on charge.

• EPE depends on distance between two charged objects for both an attractive and a repulsive electric force.

Electrical Potential Energy

The electrical potential energy between two negative charges decreases as the distance between

them increases.

Electrical Potential Energy and Relative Position

The electrical potential energy of a charge depends on its position in an electric field.

POTENTIAL DIFFERENCE

• work that must be done against electric forces to move a unit charge from one point to the other.

• measured in volts, V

VOLT

• equivalent to one joule per coulomb (1 J/C).

• aka voltage.

Voltage and Current

Cell:

• device that is a source of electric current because of a potential difference (voltage) b/w terminals.

• Positive & negative.

• Batteries convert chemical energy into electrical energy.

VOLTAGE

• Measure of energy available to move electrons.

• Electrical potential difference b/w two points.

• sets charges in motion.

CURRENT

• rate that electric charges move through a conductor.

SI unit:• ampere, A. • 1 amp = 1 C/s• battery is a direct current source b/c charges

always move from one terminal to other in same direction.

Conventional current • = movement of positive charge.• direction of current in a wire is opposite direction

that electrons move in that wire.

Electrical Resistance• Resistance:• opposition posed by a material or a device to

the flow of current.• caused by internal friction • which slows the movement of charges through

a conducting material.SI unit:• ohm, Ω. • 1 Ω = 1 V/AResistor: • special type of conductor used to control

current.

OHM’S LAW• Resistance = voltage/current• V: potential difference (V)• I: current (A)• R: resistance

R

V

I

A light bulb with a resistance of 160 Ω 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

Practice with Ohm’s Law

Ohms Volts Amps4 100 25

15 150 102 30 159 45 56 48 8

Electrical ResistanceConductors• have low resistances.Superconductors• Certain metals & compounds have zero

resistance when their temperature falls below a certain temperature (critical temperature).

Semiconductors • intermediate to conductors and

insulators.

CIRCUITS

What Are Circuits?

•Electric circuit:

• set of electrical components connected so that they provide one or more complete paths for the movement of charges.

• closed path through which electrons can flow

• Switches control the flow of charges in a circuit.

SCHEMATIC DIAGRAMS

• graphical representation of a circuit that uses lines to represent wires and different symbols to represent components.

SCHEMATIC DIAGRAMS

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

Series Circuits

– current travels in a single path•one 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 Circuit

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

Household Circuits

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

• Safety Features:fuse – electrical device that contains a metal strip that

melts when current in circuit becomes too great. metal melts, breaking circuit

circuit breaker – a switch that opens a circuit automatically

when the current exceeds a certain value;– bimetallic strip bends when hot, breaking circuit

Electric Power and Electrical Energy

• Electrical energy:• energy that is associated with charged

particles because of their positions.SI unit:• watt (W).

POWER EQUATION

Electrical Power

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

• P: power (W)• I: current (A)• V: potential difference (V)

P

I V

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: energy (kWh)• P: power (kW)• t: time (h)

E

P t

• A refrigerator is a major user of electrical power. If it uses 700 W and runs 10 hours each day, how much energy (in kWh) is used in one 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