nature & background of electricity

What is Electricity?

• It is a form of energy,evident from the factthat it runs machineryand can be transformedinto other types ofenergy such as light andheat.

• It is invisible. During anelectrical storm, we donot see electricity. Weobserve the air beingionized when the

Where do charges come from?

Matter is made up of atoms.

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Proton (positive charge)

neutron (neutral)

electron (negative charge)

atom nucleus

Who Discovered Electricity?

The Greeks had some idea of electricity.

In the 18th century Franklin and otherEuropeans knew a great deal about it. Earlyin the century, Alessandro Volta inventedthe first source of continuous electriccurrent - the battery. Later, HansChristian Oersted discovered that anelectric current produced magnetism.

But it was Michael Faraday whodescribed the nature of the phenomena. Inhis electromagnetic induction theory hestated that an electric current flows in aconductor if that conductor is in a movingmagnetic field and is part of a circuit.

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The world is filled with electrical charges:

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Electrons…

Are located on the outer edges of atoms… They can be moved.A concentration of electrons in an

atom creates a net negative charge.If electrons are stripped away, the

atom becomes positively charged.

Charges interact with each other:

Coulomb’s law• The magnitude of the force between

2 equal sized objects

• Fe = K q1 q2

r2

K (electrostatic constant =9 x 109 N m2/C2)

q (charge in Coulombs)

r ( distance between the charges)

r

Fe

Coulomb’s Law Examples• A positive charge of 6.0 x 10 -6C is 0.030m from a second

positive charge of 3.0 x 10 -6C. Calculate the force between the charges.

Fe

= K q1

q2

r2

= (8.99 x 109

N m2/C

2) (6.0 x 10

-6C) (3.0 x 10

-6C)

( 0.030m )2

= (8.99 x 109

N m2/C

2) (18.0 x 10

-12C)

(9.0 x 10 -4

m2)

= + 1.8 x 10 -8

N

The interaction between static electric charges

van de Graaff

generator

What is Static Electricity?

Static electricity is electricity at rest. It isproduced by friction, by rubbing. All matter containspositively charged particles called protons andnegatively charged particles called electrons. In anuncharged atom, the protons and electrons balanceeach other and the atom is neutral. If this neutralatom loses an electron, because it has an excess ofprotons, it is said to be positively charged. If theneutral atom gains an electron, it is said to benegatively charged.

Rubbing can tear electrons loose from certainatoms. Some substances, because of the character oftheir atoms, tend to lose electrons and becomepositively charged; other substances gain electronseasily and become negatively charged.

Static Discharge• Occurs when there is a loss of

static electricity due to three possible things:

•Friction - rubbing

•Conduction – direct contact

•Induction – through an electrical field (not direct contact)

Electricity that moves…

• Current - The flow of electrons from one place to another.

• Measured in Amperes (Amps)

• Kinetic energy

Conductors vs. Insulators

Conductors – material through which electric current flows easily.

Insulators – materials through which electric current cannot move.

ExamplesConductors

CopperMercurySilverAluminumHuman BodyMetallic SaltsCharcoal

Insulators

WoodRubberGlassPorcelain

• The opposition to the flow of an electric current, producing heat.

• The greater the resistance, the less current gets through.

• Good conductors have lowresistance.

What Influences Resistance?

1. Material of wire –aluminum and copper have low resistance

2. Thickness – the thicker the wire the lower the resistance

3. Length – shorter wire has lower resistance

4. Temperature – lower temperature has lower

The unit for measuring resistance is the Ohm (Ω).

• The measure of energy given to the charge flowing in a circuit.

• The greater the voltage, the greater the force or “pressure”that drives the charge through the circuit.

Current is directly proportional to voltage and inversely proportional

to resistance.

Ohm’s Law Example 1

I = 1.5 AmpsI =

3 V

2 Ω

Example 2:A light bulb operates on a 110 volt circuit.

The bulb draws a current of .91 amps. What is the resistance of the light bulb?

V=IRR=V/IR=110V/.91A120.8 Ohms, Ω