Transcript Slide 1

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 repel
b) 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)
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.
Aligned atoms: Magnetized Material
http://www.howmagnetswork.com/
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 magnet
2) Electromagnet
3) 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 energy
b) thermal energy to electric energy
c) 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