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Electricity
A. Electric Charge
1. Static electricity is the accumulation of excess electric
charges on an object.
a. More e¯ = negative charge
b. More protons = + charge
2. Charge is conserved (e¯ move from one object to another).
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3. Law of Charges
a. Opposite charges attract.
b. Like charges repel.
4. Electric fields
a. Electric fields exert force on objects within the field.
b. Weaker with distance.
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5. Transferring electric charge
a. Conductors: e¯ move easily.
b. Insulators: hold e¯ tightly.
c. Contact charging is done when two
materials are rubbed together (best with
insulators).
d. Charging by induction is done when one charged
object induces a charge on another.
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6. Lightning
a. Large static discharge between the
earth and clouds.
b. Lightning was found to be static
electricity by Ben Franklin.
7. Grounding
a. Conductive path to Earth.
b. Lightning rods & plumbing.
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8. The electroscope can be used to
detect electric charge
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B. Electric Current
1. The reason electric charge flows from one place to
another is voltage.
HIGH
LOW
a. Voltage is the difference in electrical potential
between two places where e¯ are flowing.
b. Voltage is the “push” that makes electric charges
move.
c. Measured in volts (V).
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2. The flow of electric charge is called current.
a. Current is measured in amperes, or amps (A).
b. Voltage causes current.
3. The amount of electric charge is measured in coulombs.
a. 1 coulomb is the charge carried by 6.24 x 10^18 e¯.
b. 1 amp is 1 coulomb per sec.
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4. Batteries are e¯ pumps.
a. They provide a voltage difference to a circuit.
b. Types: wet-cells & dry-cells
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5. Resistance
a. Opposition to the flow of e¯.
b. It changes electrical energy
into thermal energy and/or light.
c. Measured in ohms.
d. Conductors have less
resistance than insulators.
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e. Wire resistance greater for:
1) Longer wires
2) Thinner wires
3) Higher temperatures
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6. Ohm’s law
Voltage Difference
Current =
Resistance
V
I
R
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C. Electrical Circuits
1. A circuit is a conducting path.
2. Series circuit
a. One path
b. Any break & all devices go
out
c. Current is the same throughout the circuit
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Series Circuit Concepts
(assuming 3 resistors)
RTOTAL = R1 + R2 + R3
V drop for each R is VX = I • RX
so, VTOTAL = I • R1 + I • R2 + I • R3
or,
VTOTAL = I • (R1 + R2 + R3)
since I = V ÷ R
then… ICIRCUIT = V ÷ (R1 + R2 + R3)
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3. Parallel circuit
a. Multiple paths
b. A break in one branch & the
other branches stay on
c. Voltage is the same in each branch, but current and
resistance may be different
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Parallel Circuit Concepts
(assuming 3 paths)
V is the same everywhere in the circuit
ITOTAL = I1 + I2 + I3
since ITOTAL = V ÷ R and I1 = V ÷ R1
then V ÷ R = V ÷ R1 + V ÷ R2 + V ÷ R3
1
1 1 1
RTOTAL R1 R2 R3
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Comparison of series and parallel
circuits
The same
voltage battery
Notice the
brightness of
the bulbs
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4. Household circuits
a. Mostly parallel.
b. 120 V in the U.S.
c. More devices plugged in a circuit mean more
current.
d. More current means more heat in the wiring.
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5. Electrical safety devices
a. Fuses - one time use.
b. Circuit breakers - can be reset and reused.
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D. Electric Power and Energy
1. Electrical power is the rate at which electricity is converted into
another form of energy.
a. Power = current x voltage
b. Unit is the watt or kilowatt.
2. Electrical energy
a. Energy = power x time
b. Unit is the kilowatt-hour.
(1000 watts for 1 hour)
c. This is what we buy from the electric company.
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E. Light Bulbs
1. Incandescent
a. Have a tiny filament that
resists the flow of electrons.
b. The filament gets hot and glows
to produce light.
c. Very hot and inefficient.
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2. Fluorescent
a. Filled with gas and coated
with phosphor.
b. Electricity excites the
gas
making it give off U-V rays.
c. Phosphor absorbs U-V rays and glows to make light.
d. Very cool, efficient, and last a long time.
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Thanks
Submitted by
Deepanka Mishra
DAV Public School
Chandigarh
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