ELECTRICITY AND
MAGNETISM
CHAPTER 7
Electric Charge and Forces
Section 1
Electric Charges Atoms are made of electrons moving around
a nucleus (protons and neutrons).Protons have a positive charge.Neutrons have a neutral or no charge.Electrons have a negative charge.
Atoms are electrically neutral in charge having equal protons and electrons.Losing an electron makes the atom positively
charged.Gaining an electron makes the atom negatively
charged.
Forces Between Charges
Like charges repel and unlike charges attract
Distance and amount of charge determines the force between objectsGreater distance = less forceGreater amount = greater force
Electric Field and Forces
Objects do not have to touch to exert electric forces on each other
Electric fields surround every proton and electronElectric fields are invisible and at every point
in space surrounding a charge.
Electrically Charging Objects Charging by contact – objects must
come into direct contact with each otherElectrons are gained by one object and lost
by the other Charging by induction – rearranging
electric charges due to the presence of an electric fieldOne part of the object is positively charged
while another part is negatively charged
Conductors and Insulators Conductors hold electrons so loosely
that they can move through the material easily
○ Silver, gold, copper
Insulators hold electrons so tightly that they cannot move through the material easily
○ Plastic, glass, rubber, wood
Static Charge
The imbalance of electric charges on an object is called static charge.Occurs during charging by contact
The movement of static charge from one place to another is called electric discharge.Lightning – may occur between cloud and
ground, within cloud, or between clouds
Electrostatic potential 30,000 volts removing sweater 10,000 volts using a comb 3,000 volts walking on carpet 1,000 volts putting on coat LUCKY FOR US, these high volts have
very low amperage.
Lightning Safety 30-30 rule
If there is 30 seconds between thunder and lightning, seek shelter and stay away from metals.
Wait 30 minutes after last lightning strike before leaving shelter.
Storm clouds need not be present for lightning to strike.
Grounding is providing a path to drain excess electric charges into EarthLightning rods
FUN FACTS 50-100 lightning bolts hit the ground every
second on Earth. Lightning discharge 100 million volts of
electricity, heating the air to 60,000 degrees F.
Bolts can be up to 100 miles long Steel turns into a magnet if struck be
lightning.
Electric CurrentSection 2
Electric Current
The flow of electric charges Electrons in a wire moving along the
wireEqual numbers of protons and electrons in
wire making it electrically neutralEqual numbers of electrons entering wire
and exiting the other end Measured in amperes (A) = the amount
of electrons flowing in and out of wire every second [about 6 billion billion]
Simple Electric Circuit Electricity is either static or current
Static - electrons accumulate and "stand" on the surface
Current - electrons must "flow" through something○ Direct current (DC) - flow in one direction (battery)○ Alternating current (AC) - reverses its flow many
times a second (household power)
Electric current will flow continually on a closed path known as an electric circuitBroken wires, opened switch, broken filament will
stop current flowing
Flowing Electric Charges
A force must be exerted on electric charges to make them flowAn electric field in the circuit will move
electrons in a single direction A battery will produce an electric field
when a chemical reaction in the battery occurs.Charges terminals of battery – and +Electrons flow from – to +
Electrical Resistance
Electrons have trillions of collisions every second inside batteries changing their direction and speedThe electric field keeps the electrons flowing
in the direction of the current The measure of how difficult it is for an
electron to flow in an object is known as electric resistance.Measured using ohms (Ω)
Electrical Energy Transfer Electrons flowing through a circuit have
kinetic energyEvery collision causes some energy to be
converted into heat and light As the strength of the electric field
increases so does the kinetic energy of the flowing electrons
Voltage
The measure of electrical energy transferred by an electron as it moves from one point to another in a circuitMeasured using a voltmeter
Battery voltage indicates that there is more electrical energy transferred in higher voltage
Ohm’s Law
Shows relationship between voltage, current, and resistanceAs voltage increases, current in the circuit
increases.Increasing resistance slows down the
current flow voltage(V) = current(A) X resistance(Ω)
V = IR
Series and Parallel Circuits Devices connected so there is only one
closed path for current to follow is a series circuit.Any part of path broken, current will not flow.
Devices connected so there is more than one closed path for current to follow is called a parallel circuit.If one path is broken, current will continue to
flow through another path.
Electric Cost Companies charge by the kilowatt-hour
(kWh) That is the amount of energy equal to
1kW of power continuously for 1 hour About ten 100-W light bulbs for 1 hour
MagnetismSection 3
Magnetic Poles
Every magnet has a north and a south pole.
Magnetic poles are like electric charges.Like poles repel; unlike poles attract.Forces are stronger the closer they are to
each other. Magnetic fields are like electrical fields.
Fields are stronger at the poles of magnets.
Magnetic Materials
Metals that contain iron, nickel, cobalt and a few other rare-earth elements
Objects that do not contain the above elements are not effected by magnetic fields.
Magnetic Domain
A group of atoms that have their magnetic poles pointing in the same direction has magnetic domain.
With magnetic domains aligned in a single direction, the magnetic field is stronger.
Materials become magnetized when magnets exert forces on the materials magnetic domains.
Electromagnetism
Discovered in early 1800s, a wire carrying an electric current is surrounded by a magnetic field
Magnetic fields can be made stronger by wrapping the wire around an iron core.
Increasing current, increases the magnetic field.
Electromagnets are temporary and can be controlled.
Generating Electric Current Moving a magnet through a wire loop
that is part of a circuit makes an electric current flow – electromagnetic induction
Generators convert mechanical energy to electrical energy.
Power plants use steam or moving water to convert mechanical energy to electrical energy.