Chapter 1

Electricity

Before the strips of newspaper were in contact with the plastic bag: they hung straight down while touching each other

After sliding the plastic bag over them: they REPELLED each other

WHY? Rubbing the plastic bag charges the strips with like

charges. Like charges REPEL (try to get as far away from each other as possible)

Newspaper Demonstration

AtomSmallest unit of

material that still has the characteristics of that material

What makes up an atom?Protons: positiveNeutrons: neutralElectrons: negative

What is the overall charge on an atom?NEUTRAL (ZERO)

Electric Charge

Particles with unlike charges attract - pull on each other

Particles with like charges repel – push each other awayCan be 2 positive or

2 negative charges

Attraction Repulsion

A buildup of electric charge in an object caused by the presence of many particles with the same charge

Materials can have:Positive charge: more protons than electronsNegative charge: more electrons than protons

Electrons move more easily than protons – charges result from their movement

2 Types of static charge:Charging by contactCharging by induction

Static Charge

Charging by Contact

At first, a glass rod and

balloon each have balanced, neutral charge

When they touch,

electrons move from the rod to

the balloon

Afterwards, the balloon

has a negative charge and the

rod has a positive charge

How does it work?Builds up STRONG

electric charge through contact

As the sphere takes on negative charge electrons spread over the students skin and hair

Hairs now all have same charge and repel one another

Van de Graaff Generator

Charging By Contact

SKIN

GLASS

HAIR

NYLON

WOOL

FUR

SILK

PAPER

RUBBER

POLYESTER

How materials affect static charge

Materials higher on the list tend to give up electrons to materials lower on the list

Charging by Induction

At first, the rod has a negative

charge and the balloon has a

balanced charge

When the rod come close to the balloon, electrons in the balloon move away

from the rod

When the rod is removed, electrons in the balloon spread out evenly as

before

Draw a diagram of what is occurring between the newspaper strips and the plastic bag.

Revisiting the Newspaper Demo

What is occurring in this situation?Before the charged balloon comes near the

wall, the atoms in the surface of the wall are neutral

As the balloon nears the wall, the electrons move deeper into the wall leaving the wall closest to the balloon with a positive charge

Balloon – Wall Demonstration

Charges can move from one place to anotherSection 1.2

The force of attraction or repulsion between charged particles.

What causes a charge to move?

Energy a charged particle has due to its position in an electric fieldLike charges repel: EPE increases when

particles with like charges are pushed closer together

Opposite charges attract: EPE decreases when particles with like charges move away from one another

EPE is measured in volts

Electrical Potential Energy (EPE)

What happened in the bulb when the spark jumped?The bulb lit up briefly

How might you explain this observation?A static charge moved from the balloon to the

bulb and lit the bulb

Fluorescent Bulb Demonstration

How Lightning FormsCharge Separation: Collisions between particles in storm clouds separate charges. Negatively charged particles collect at the bottom of the cloud

Charge Buildup: The negatively charged bottom of the cloud induces a positive charge in the surface of the ground.

Static Discharge: The charge jumps through the air to the ground. The energy released by the discharge causes thunder and lightning.

ConductorsMaterial that allows electric charge to pass

through easilyExamples: iron, steel, copper, aluminum

InsulatorsMaterial that does not allow electric charge to

pass through itImportant in electrical safety because they keep

charge away from the bodyExamples: plastic, rubber

Materials Affect Charge Movement

Materials resist the movement of a charge in different amounts

Electrical resistance: the property of a material that determines how easily a charge can move through it

Units: ohms (Ω)Factors that affect resistance:

Type of material – conductors have LOW resistance; insulators have HIGH resistance

Amount – a long wire has more resistance than a short wire

Shape – a thin wire has more resistance than a thick wire

Resistance

Providing a harmless, low resistance path (a ground) for electricity to follow.

In many cases this path actually leads to the ground

EXAMPLE:Protect buildings from damage by lightning :

Most building have a lightning rod located high up on the building, which is made of a material that is a good conductor

The rod is connected to a conductor cable, which is connected into the ground.

Lightning hits the rod and passes harmlessly through the cable into the ground

Grounding

Electric current is a flow of chargeSECTION 1.3

Static charge cannot make your TV or other electronics play. Static charge contains a limited amount of

chargeStatic charges moves from higher to lower

potential

Electric current: flow of chargeSome electrical pathways receive a continuous

supply of charge (the difference between the 2 ends of the pathway are the same)

Electric charge can flow continuously

Can only flow if it has a pathway to follow – a material to conduct it

Charge flows from higher to lower potentialCharge that flows steadily has a certain rate

of flow.Standard unit of measure: ampere (amp)

Amount of charge that flows past a given point per unit of time

Electric Current

Low Pressure High Pressure

Low rate of flow High rate of flow

Recall: measured in ohms (Ω)Current decreases as resistance increases

Resistance

Low Resistance High Resistance

High rate of flow Low rate of flow

Ohm’s Law

Current = ____Voltage______ I = __V__ Resistance R

What is the current in an electrical pathway with an electrical potential of 120 volts and a resistance of

60 Ω?

Example Problem

V = 120 V

R = 60 Ω

I = ?

LIST

I = _V_ R

Plan

I = 120 V 60Ω

I = 2 A

Work

What is the voltage of a circuit with 15 amps of current and toaster

with 8 ohms of resistance?

YOU TRY!

Produces electric current using the chemical or physical properties of different materials

Example: batteries2 Types:

Primary cellsStorage cells

Electrochemical cells

Produce electric current through chemical reactions

Reactions continue until the chemicals are used up

Example: dry cell battery

Primary cell

Storage cellsProduce current through chemical

reactions that can be reversed inside the battery

Example: Car Battery Discharging: when storage cells are

producing currentBattery discharges to operate the motorOperate lights when car is off

Charging: sending current through the battery in the opposite direction (reverses reaction)When car is running, battery is continually

being chargedCar’s alternator produces the current.

Current from alternator runs through battery in reverse to recharge