ELECTRICITY AND MAGNETISM

INSULATORS AND CONDUCTORS• Conductors are materials that are good at

carrying an electric charge.

• Good conductors of electricity include metals, salt water, electrolytes (solutions containing charged atoms – ions), and the human body.

• Insulators keep an electric charge from flowing.

• Good insulators include nonmetals, plastic, wood, and rubber.

ELECTRICITY• Electricity is a phenomenon caused by the action of

charged particles (electrons: negatively charged; protons: positively charged).

• The Law of Electric Charges:a) Like charges repelb) Opposite charges attract

• There are 2 main types of Electricity:a) Static Electricity (does not flow)b) Current Electricity (flow of charges through a circuit)

The physicsclassroom

STATIC ELECTRICITY• Static electricity results from the

buildup of electric charges on the surface of objects.

• The buildup is due to the exchange of electrons from one object to another

• Objects may acquire static charge by friction (rubbing to objects together), conduction (transfer due to direct contact), or induction (transfer without direct contact).

• Ex) Generate static electricity by rubbing silk and glass together or plastic and wool.

The physicsclasroom

ELECTRIC CURRENT• Current is the flow of charged particles (ex:

electrons) through a conductor (like a wire). • There are 5 ways to produce an electric

current:1) Electromagnetic generators (a coil of wire moves through a magnetic field)2) Electrochemical batteries (chemical reactions)3) Solar cells (sunlight)4) Thermocouples (temperature changes)5) Piezoelectric crystals (the stress of some materials)

ELECTRICAL UNITS• Current (I): rate at which electric charges flow

through a wire (the number of electrons that pass by a specific point in a circuit in one second). SI unit: Amps (A)

• Voltage (V): Electrons need energy to force the electrons through the wire. Voltage is the amount of energy available to move the electrons. The higher the voltage, the more work the electrons can do. SI unit: Volts (V)

• Resistance (R): The measure of how difficult it is to move electrons through a circuit (the force opposing the flow of electrons) Good conductors = R and poor conductors = R. Resistance depends on the material’s length, thickness, and temperature. SI unit: Ohms ()

OHM’S LAW

• Ohm’s Law relates current, resistance, and voltage.

• Current = (Voltage)/(Resistance)

• I = V/R

MAGNETISM• Magnetism is a force produced by the motion

of charged particles.

• Within a magnet, groups of atoms have electrons spinning in the same direction. This creates a magnetic field in the material. Not all materials can become magnetized.

• A magnetic field is the invisible lines of force that runs from one pole to the other.

• A magnet always has two poles – north and south.

• Like poles repel and opposite poles attract.

http://www.howmagnetswork.com/

Aligned atoms: Magnetized Material

Unaligned atoms: Unmagnetized Material

http://www.geocities.com/SunsetStrip/Palms/8423/domain.htm

ELECTROMAGNETISM• A magnetic field can be produced using a

current through a wire and a piece of metal that can be magnetized.

• An electromagnet is a temporary magnet Benefits: can be turned on and off, and can be made stronger than permanent magnets

ELECTROMAGNETISM

• A simple electromagnet consists of a battery, wire, and an iron nail.

• The strength of the electromagnet depends on the number of turns in the wire coil and the size of the iron core. turns = stronger the magnetic field

MOTORS• Magnets are used in electric motors.• Motors convert electrical energy into

mechanical energy.• Motors consist of 3 main parts:

1) Permanent magnet2) Electromagnet3) Commutator

• The poles repel and attract on the permanent and electro- magnets which causes the electromagnet to spin. The commutator changes the direction of the poles on the electromagnet to keep it spinning.

MOTORS

OTHER USES FOR ELECTROMAGNETS

• A generator is the opposite of a motor. It converts mechanical energy to electrical energy.

• A transformer is a device that uses electromagnetic induction to change the voltage of a current. These allow for high voltage from power plants to be used in our homes (require much lower voltage).

• They are also used in telephone receivers, radio and television speakers, tape recorders, and many other items. Electromagnets help control the intensity of signals as well as being a method of transmittal.

GHSGT PRACTICE QUESTIONS• 1) The difference in energy carried by electrons at

different points in a circuit will determine the

a) voltage b) resistance c) current d) power

• 2) What is created when a coil of wire is moved through a magnetic field?

a) heat b) an electric current

c) a nuclear reaction d) a chemical reaction• 3) The amount of current that will flow through a

10-ohm light bulb operating at 5.0 volts is

a) 2.0 A b) 0.5 A c) 5.0 A d) 1.0 A

ANSWERS• 1) A: VOLTAGE (amount of energy

available to move the electrons)

• 2) B: AN ELECTRIC CURRENT

• 3) B: 0.5 A

(I = V/R I = (5 volts)/(10 ohms) = 0.5 A

GHSGT PRACTICE QUESTIONS• 4) An electric generator converts

a) solar energy to electric energyb) thermal energy to electric energyc) chemical energy to electric energy

d) mechanical energy to electric energy• 5) In which way do permanent magnets and

electromagnets differ?a) Electromagnets have fixed magnetic strength.b) Permanent magnets can only be used in fixed positions.c) Electromagnets can attract other substances besides metals.d) The largest permanent magnets are weaker than the largest electromagnets.

ANSWERS

• 4) D: Mechanical Electrical

• 5) D: The largest permanent magnets are weaker than the largest electromagnets

GHSGT PRACTICE QUESTIONS• 6) Which of the following is common to all electric

motors?

a) battery power

b) magnetic forces

c) hydroelectric power

d) internal combustion engines• 7) Which of these electrical sources produces

electric current by chemical reaction?

a) thermocouple

b) electromagnetic generation

c) nickel-cadmium battery

d) solar cell

ANSWERS

• 6) B: Magnetic Forces

• 7) C: Nickel-Cadmium Battery

GHSGT PRACTICE QUESTIONS• 8) Which of the following is the source of

electricity produced at electric power plants?

a) batteries

b) static electricity

c) electromagnetic generators

d) thermocouples• 9) Electric motors produce mechanical energy

from the interaction of

a) electric and magnetic fields.

b) gravitational and electric fields.

c) magnetic and gravitational fields.

d) two thermodynamic fields

ANSWERS

• 8) C: Electromagnetic Generators

• 9) A: Electric and Magnetic Fields

GHSGT PRACTICE QUESTIONS• An electromagnet can be made by coiling a

wire around an iron nail and connecting the wire to a battery. Which of the following would cause the strength of the magnetic field to increase?

a) decreasing the length of the nail

b) increasing the number of coils around the nail

c) increasing the temperature of the nail

d) replacing the iron nail with a copper nail

ANSWER

• 10) B: Increasing the number of coils around the nail