Download Electricity 7-1, 7-2

Electricity 7-1, 7-2
Electric Charge and Electric
Current
Static Electricity
Static Electricity- accumulation of excess
electric charges on an obj. pg. 194
 Protons = + electrical charge
 Electrons= - electrical charge
 Example of electron transfer:
Walk on carpet, the electrons are transferred from
carpet to your shoes
 Shoes have a more - charge and carpet is
more + = static electricity

Conservation of Charge
When an object becomes charged the
charge is neither created nor destroyed
(Law of Conservation of Charge)
 Electrons move from one object to another

Opposites Attract
**Opposite charges attract- like charges
repel
 (+) charges are attracted to (–) charges.

Electric Field

Surrounds every electric charge- force that
causes electrons to move
Conductors and Insulators
Conductors- material in which electrons
move freely ex. Metal, skin
 Insulators- material that doesn’t allow
electrons to flow smoothly. Ex. Plastics,
wood, glass, rubber
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Transfer of Charge
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Charging by contact- process of transferring charge by
touching or rubbing (rubbing balloon on head)
Fig. 7 pg. 198- Charging by induction- the
rearrangement of electrons on a neutral object (static
cling)
Lightning- large amount of static discharge Fig. 8
Detecting Electric Charge
Electroscope Fig. 10 pg. 199
 No charge- leaves hang straight down
 Object is charged- leaves repel (move
apart)

Electric Current 7-2 Notes 813

I. Voltage Difference- the push that causes charges to
move and is measured in volts (V). (move from areas
of high voltage to low voltage)
1.
An object may lose voltage as it flows through a circuit.

II. Circuit- a CLOSED, conducting path (must be closed
to have a circuit)

III. Electric Current- a flow of charges through a wire or
any conductor.
Current is measured in Amperes (A) Current is the flow
of electrons.
Most wall sockets are 120 V in our homes.

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Batteries

Batteries keep current flowing in a circuit.
Dry Cell (D, C batteries)- electrons flow from
positive to negative terminal.
Positive terminal is where carbon rod is located.

Wet Cell- car battery is a wet cell battery.

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Resistance
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The tendency for a material to oppose the
flow of electrons, changing electrical energy
into thermal energy and light.
All materials have SOME electrical resistance.
Resistance is measured in ohms pg. 205
Metals (conductors) have low resistance.
Insulators have high resistance.
Resistance as temperature
Thick v. Thin wire= Thin wire has a greater
resistance.
Ohm’s Law

Increasing resistance causes the current to decrease. Increasing the
pressure causes the current to increase.

Ohm’s Law- the current in a circuit equals the voltage difference divided by
the resistance.
I = V / R current = volts/ resistance
or
V = IR volts= current x resistance
Practice Problems from Ohm’s Law
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1. What is the voltage difference across a 70 ohm resistor if a .9A current is
flowing through it?
2. A current of .24 A flows through a wire. What’s the resistance if the voltage
is 70V?