Chapter 11- ELECTRICITY

+

Rule 1 and 2

Rule 1: Like charges repel one another.

Rule 2: Unlike charges attract one another.

Electric charge is a basic characteristic of matter

Atomic Parts

• Every atom has a positively charge nucleus, surrounded by negatively charged electrons.

An atom in its normal state will have the same number of protons

and electrons

A charged atom is called an ion• When an atom loses one

or more electrons, it has

a positive net charge.

• When an atom gains

one or more electrons,

it has a negative net charge.

Electrons are transferred from the fur to the rod. The rod is then negatively charged. Is the fur charged? How much compared to the rod? Positively or negatively?

QUESTION: STATICElectric charge can be dangerous. Two hundred years ago, young boys called powder monkeys ran below the decks of warships to bring sacks of black gunpowder to the cannons above. It was ship law that this task be done barefoot. Why?

ANSWER

Because it was important that no static charge build up on the powder on their bodies as they ran to and fro. Bare feet scuffed the decks much less than shoes and assured no charge buildup that might produce an igniting spark and an explosion.

Coulomb’s LawThe electrical force between two charged bodies is directly proportional to the product of the charges and inversely proportional to the square of the distance between them: Formula:

F= k q1q2

d2

Charles Coulomb 18th century

Coulomb’s Law

• The unit of charge is the coulomb (C)

• Charge of 1 C is the charge on 6.25 billion electrons.

• k – constant (similar to constant G in law

of gravity), but it is very big number

9,000,000,000 N m2/C2

Gravity force Electrical force

Orbits of satellite, electron are similar

Electrically Polarized• Center of charge

moves.

• Fig a – center of negative cloud coincides with center of positive nucleus.

• Fig b – electron cloud is distorted, “polarized”

Balloon polarizes atoms in the wall

Electric Current• Current is the flow of electrons.

• Loose electrons in metals are responsible for good heat conduction. This is true for electrical conduction.

• The rate of flow is measured in amperes (A)

Voltage – Electric Pressure Voltage = Potential Energy

Charge

Current flows in a wire when there is a difference in voltage across the ends of the wire.

A steady current needs a pumping device to provide a difference in voltage.

Batteries, generators are “electrical pumps”

Electric Eel

• An unusual source of voltage. The electric potential between the head and tail of the electric eel (Electrophorus electricus) can be up to 600 V.

Electrical Resistance

• Measured in units called ohms Ω

• Named after Georg Simon Ohm, a German physicist in 1826.

Electrical Resistance

• More resistance– Narrow wire– Long wire– Higher temperature– Poor conductors

(rubber)

• Less Resistance– Wide wire– Short wire– Low temperature– Good conductors

(metals)

Van de Graaff Generator

OHM’S LAW

How much current?

• How much current flows through a lamp with a resistance of 60 Ω when the voltage across the lamp is 12 V?

• Using Ohms law,

• Current = Voltage/resistance

• So: Current =12 V/60 Ω = 0.2 A

ELECTRIC SHOCK

Tasers

A Taser works by delivering high voltage — but low amperage — to the human body. A Taser delivers a powerful but temporary shock rather than a sustained and deadly charge.

• To receive a shock, there must be a difference in voltage between one part of your body and another part. Electron flow will pass along the path of least electrical resistance connecting these two points.

• More info on electric shock on page 180

The bird can stand harmlessly on one wire of high voltage, but it had better not reach over and grab a neighboring wire! Why not?

Direct Current and Alternating Current

• Direct Current

• “dc”• Electrons flow

one direction

• Alternating Current

• “ac”• Electrons flow

initially one direction, then in the opposite direction

Electric Power

Electric Power

• Power is expressed in watts (when current is in amperes and voltage is in volts)

• Power is equal to current multiplied by voltage.

POWER = Current x voltage

WATTS = AMPERES x VOLTS

Superconductors

• Zero electrical resistance to the flow of charge.

• Very low temperatures (4K)

• No current is lost, no heat generated

• Energy saving potential!

Series and Parallel Circuits

Series Circuit• The current doesn’t pile up in any lamp but

flows through each lamp.

• The current has only one pathway.

• The resistance of the current starts in the first lamp in the circuit, then the second, then the third, and so on.

• If one device fails, current in the entire circuit stops.

Parallel Circuit• The total current in the circuit divides

• More branches means overall resistance of the circuit is lowered.

• When one bulb burns out, the other are not affected.

Fuses and Circuit Breakers

If a fuse is rated at 20 amperes, it will pass 20 amperes and not more. A current above 20 amperes will melt the fuse and breaks the circuit.