Transcript No Slide Title

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).
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.
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.
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.
8. The electroscope
can be used to
detect electric
charge
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).
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.
4. Batteries are e¯ pumps.
a. They provide a voltage
difference to a circuit.
b. Types: wet-cells & dry-cells
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.
e. Wire resistance greater for:
1) Longer wires
2) Thinner wires
3) Higher temperatures
6. Ohm’s law
Voltage Difference
Current =
Resistance
V
I
R
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
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)
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
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
Comparison of series and parallel
circuits
The same
voltage battery
Notice the
brightness of
the bulbs
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.
5. Electrical safety devices
a. Fuses - one time use.
b. Circuit breakers - can be
reset and reused.
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.
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.
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.