Transcript 1.98 x 10 -2 V

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).
Law of Charges
a. Opposite charges attract.
b. Like charges repel.
Electric fields
a. Electric fields exert force on
objects within the field.
b. Weaken with distance.
Transferring electric charge
a.Conductors: electrons move
easily through them
b.Insulators: hold electrons
tightly
c.
Charging by Contact –
charging materials by
touching or rubbing (best
with insulators).
c.
Charging by induction –
when a charged object
induces a charge on
another.
How can a balloon be made to
stick to a wall?
When you rub a balloon on your
hair, electrons are transferred to
the balloon.
When the charged balloon is brought
near the neutral wall, it induces a
positive charge, and the balloon will
stick to the wall temporarily.
Lightning
- A very large discharge of static
electricity between the earth
and clouds.
-Lightning was found to be static
electricity by Ben Franklin.
-Grounding:
a. Conductive path to Earth.
b. Lightning rods & plumbing.
Why do you see lightning
before you hear thunder?
Although lightning and thunder occur
together, light travels about a million times
faster than sound, so you see it first.
Thunder is caused by the rapid expansion
of the air around the lightning bolt, which is
heated to an extremely high temperature.
The electroscope
can be used to
detect electric
charge
HIGH
B. Electric Current
LOW
1. The reason electric charge flows
from one place to another is voltage.
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.
Batteries are electron pumps
provide a voltage difference to a circuit
Types: wet-cells & dry-cells
5. Resistance- opposition to the
flow of electrons.
•It changes electrical
energy into thermal
energy and/or light.
•Measured in ohms (Ω)
•Conductors have less
resistance than
insulators.
Resistance in wire is greater for:
1) Longer wires
2) Thinner wires
3) Higher temperatures
Ohm’s Law
Voltage Difference
Current =
Resistance
V = Potential Difference or
Voltage Difference in volts (V)
V
I = current in
amperes (A)
I
R
R = Resistance in Ohms (Ω)
Ohm’s Law Example 1:
Silver has a resistance of 1.98 x
10-4 ohms. What voltage would
produce a current of 100 amps?
V= IxR
= (1 x 102 A)(1.98 x 10-4 Ω)
= 1.98 x 10-2 V
Ohm’s Law Example 2:
What resistance does a piece of
aluminum wire produce which
has a current of 150 amps and
120 V?
R= V =
I
120 V
150 A
= 0.80 Ω
Ohm’s Law Example 3:
Find the current when a 12 volt
battery is connected through a
resistance of 25 ohms.
I=
V = 12 V =
R
25 Ω
0.48 A
Electrical Circuits
A circuit is a conducting path.
Series circuit
a.One path
b.Any break & all devices go
out
c.Current is the same
throughout the circuit
d.Bulbs will become dimmer
as more and more bulbs are
added in series.
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
Comparison of series and parallel
circuits
The same
voltage battery
Notice the
brightness of
the bulbs
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.
Electrical safety devices
a. Fuses - one time use.
b. Circuit breakers - can be
reset and reused.
Electric Power
Electrical Power is the rate at which
electricity is converted into another
form of energy.
Unit is the watt or kilowatt
Electric Power = current x voltage
P = I x V
(watts) = (amps)(volts)
Electrical Energy
Electrical Energy = Power x time
E=P x t
(kWh) = kW x h
Unit is the kilowatt-hour (1000 watts for 1 hr)
This is what we buy from the electric
company.
Electrical Power Example
Problem:
Calculate the electrical power used if
20 A of current flows through a 120-V
outlet.
Electric Power = current x voltage
P = I x V
P = (20 A)(120 V)
P = 2400 Watts or 2.4 kW
Calculate electrical energy used if the
appliance (on the previous slide) runs for 50
hours in a month.
Electrical Energy = Power x time
E = P x t
= 2.4 kW x 50 h
= 120 kWh
Calculate the electric bill for running this
appliance if the rate is $0.05 per kWh.
120 kWh x $ 0.05 = $6.00
kWh
Light Bulbs
Incandescent
a.Have a tiny filament that
resists the flow of
electrons.
b.The filament gets hot
(electrical resistance) and
glows to produce light.
c.Very hot and inefficient.
Fluorescent
Lights
a. Filled with gas and coated with phosphor.
b. Electricity excites the gas making it give
off UV rays.
c. Phosphor absorbs UV rays and glows to
make light.
d. Very cool, efficient, and last a long time.