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Electricity
Section 1: Electric Charge
What is electricity?
The collection or flow
of electrons in the
form of an electric
charge
What is static electricity?
When two objects rub against each other electrons
transfer and build up on an object causing it to have a
different charge from its surroundings.
Like the shoes rubbing against the carpet. Electrons
are transferred from the carpet to the shoes.
Conservation of Charge
Charge cannot be created or
destroyed. When an object
becomes charged, electric
charges have moved from one
place to another.
Charges Exert Forces
•Often when you remove
clothes from the clothes dryer,
they seem to stick together.
•This is because some of the
clothes have gained electrons
by rubbing against other
clothes.
•The clothes losing electrons
become positive. The negative
clothes are attracted to the
positive clothes.
Comparing Electric and Gravitational Forces
The electric force between protons and
electrons is 1039 times larger than the
force of gravity between those particles.
The electric force is so strong that if you
comb your hair and transfer 1 trillionth of
the electrons in a single hair to the comb,
the electric force will be strong enough to
overcome the force of gravity on the
strand of hair.
The Van De Graaf Generator
The van de Graaf generator (large silver ball)
deposits electrons on the ball. When a person
places their hand on the ball and the machine is
turned on, electrons are transferred to and
collected on the person touching the silver ball.
Why do you
think this
machine affects
the hair of the
children in the
picture?
What causes you to be shocked when you rub your
feet across carpet?
An electrical discharge is the passing of an
electric current through the air from a negatively
charged object to a positively charged object.
This is what causes lightning!
What are Conductors &
Insulators?
•A conductor is a material which
allows an electric current to pass.
Metals are good conductors of
electricity.
•An insulator is a material which does
not allow an electric current to pass.
Plastic, glass, wood, and rubber are
good insulators
Charging Objects
• Charging by Contact- the process of
transferring charge by touching or
rubbing.
• Charging by Induction- the
rearrangement of electrons on a neutral
object caused by a nearby charged
object.
Lightning & Thunder
• Lightning Video
Detecting Electric Charge
• A: Uncharged
• B: Negatively Charged
• C: Positively Charged
Section 2: Electric Current
Current and Voltage
Difference
• Electric Current- the net movement of
electric charges in a single direction.
– Measured in amperes.
• 1 ampere = 1 million billion electrons flowing past a
point each second!!!
Voltage Difference
• Electric charge flows from higher
voltage to lower voltage.
– A voltage difference is related to the
force that causes electric charges to flow.
• Measured in volts.
Electric Circuits
• Circuit- a closed path that electric
current flows.
– If the path opens, the current will not
flow.
What are batteries?
Batteries are composed of a chemical substance which can
generate voltage which can be used in a circuit.
There are two kinds of batteries: dry cell and wet cell
batteries. Below is an example of a dry cell.
The zinc container of the
dry cell
contains a moist chemical
paste surrounding a
carbon rod suspended in
the middle.
Wet cell batteries are most commonly
associated with automobile batteries.
A wet cell contains two
connected plates made of
different metals or metal
compounds in a
conducting solution. Most
car batteries have a
series of six cells, each
containing lead and lead
oxide in a sulfuric acid
solution.
What is electrical resistance?
Resistance (R) is the opposition to the flow
of an electric current, causing the electrical
energy to be converted to thermal energy or
light.
The metal which makes up a light
bulb filament or stovetop eye has a
high electrical resistance. This
causes light and heat to be given
off.
Temperature, Length, and Thickness
Resistance increases if:
•Temperature Increases
•Wire becomes longer
•Wire becomes thinner
Light Bulbs
• In a 60 Watt bulb, the filament is a
piece of tungsten wire coiled to be only
a few cm long. The uncoiled wire is 2 m
long, and only 0.25 mm thick, so the
resistance is large enough to cause the
bulb to glow!
What are electric circuits?
• Circuits typically contain a voltage
source, a wire conductor, and one or
more devices which use the electrical
energy.
Ohm’s Law
• According to Ohm’s Law, the current in
a circuit equals the voltage difference
divided by the resistance.
• Stuff you need to know:
Symbol
Unit
Current
I
A
(amperes)
Voltage
Difference
V
V
(Volts)
Resistance
R
Ω
(Ohms)
Formula
I= V/R
Ohm’s Law Practice #1
• A toy car with a resistance of 2Ω is
connected to a 3V battery. How much
current flows through the car???
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• R=
– Choose the equation you will use:
• I=V/R
– Solve!
Ohm’s Law Practice #2
• Calculate the voltage difference in a
circuit with a resistance of 25Ω if the
current in the circuit is 0.5A.
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• R=
– Choose the equation you will use:
• V= I * R
– Solve!
Ohm’s Law Practice #3
• A current of 0.5A flows in a lightbulb
when the voltage difference between
the ends of the filament is 120V. What
is the resistance of the filament?
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• R=
– Choose the equation you will use:
• R= V/I
– Solve!
Section 3: Electrical Energy
Circuits
• Circuits include 3 parts:
– 1. Source of voltage difference (battery,
power outlet).
– 2. A device that uses the electrical energy.
– 3. Conductors (wires) that connect the
devices to the source of voltage difference
– Forms a CLOSED PATH!
Series Circuits
• In a series circuit, the current only
has one loop to flow through.
– Examples: flashlights and some holiday
lights.
– Problems???
Parallel Circuits
• A parallel circuit has multiple pathways for the
current to flow. If the circuit is broken the
current may pass through other pathways and
other devices will continue to work.
What is the difference between an
open circuit and a closed circuit?
• A closed circuit is one in which the pathway
of the electrical current is complete and
unbroken.
• An open circuit is one in which the pathway
of the electrical current is broken. A switch is a
device in the circuit in which the circuit can be
closed (turned on) or open (turned off).
Fuses & Circuit Breakers
• Most household wiring is logically designed with a
combination of parallel circuits. Electrical energy
enters the home usually at a breaker box or fuse box
and distributes the electricity through multiple
circuits.
Electric Power
• Electric Power- the rate at which
electrical energy is converted into
another form of energy.
– Example:
• Hairdryer ~1875W
• Computer ~ 450 W
Calculating Electric Power
• Power (W) = Current (A) x Voltage Difference (V)
• P=IV
• The unit of electric power is the watt (W). Because
that is a very small unit electric power is expressed in
kilowatts (kW).
– 1 kilowatt = 1000 watts.
Electric Power Practice
Problem #1
How much power is used in a circuit which
is 110 volts and has a current of 1.36
amps?
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• P=
– Choose the equation you will use:
• P=IV
– Solve!
Electric Power Practice
Problem #2
• The current in a clothes dryer is 15A
when it is plugged into a 240-volt outlet.
How much electric power does the
clothes dryer use?
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• P=
– Choose the equation you will use:
• P=IV
– Solve!
Electric Power Practice
Problem #3
• A toaster oven is plugged into an outlet
where the voltage difference is 120V.
How much power does the toaster oven
use if the current in the oven is 10A?
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• P=
– Choose the equation you will use:
• P=IV
– Solve!
Electric Power Practice
Problem #4
• A VCR that is NOT playing still uses 10.0W of
power. What is the current in the VCR if it is
plugged into a 120V outlet?
– Step 1: WRITE WHAT YOU KNOW:
• I=
• V=
• P=
– Choose the equation you will use:
• P=IV
– Solve!
Electrical Energy
• Using electric power costs money!
• Electric companies charge by the amount
of electrical energy used, instead of the
electric power used.
• Electric Energy is measured in units of
kilowatt hours (kWh) and can be calculated
from this equation:
• Electrical Energy (kWh) = Power (kW) * Time (h)
• E= Pt
Electric Energy Practice
Problem #1
• A microwave oven with a power rating of
1,200W is used for 0.25h. How much
electrical energy is used by the
microwave?
– Step 1: WRITE WHAT YOU KNOW:
• P=
• t=
• E=
– Choose the equation you will use:
• E=Pt
– Solve!
Electric Energy Practice
Problem #2
• A refrigerator operates on average for
10.0h per day. If the power rating on the
refrigerator is 700W, how much electrical
energy does the refrigerator use in one
day?
– Step 1: WRITE WHAT YOU KNOW:
• P=
• t=
• E=
– Choose the equation you will use:
• E=Pt
– Solve!
Electric Energy Practice
Problem #3
• The current flowing through an appliance
connected to a 120-V source is 2A. How
many kilowatt hours of electrical energy
does the appliance use in 4 hours?
– Step 1: WRITE WHAT YOU KNOW:
• P=
• t=
• E=
• I=
• V=
– Choose the equation you will use:
• E=Pt AND: P=IV
– Solve!
The Cost of Using Electrical
Energy
• How do we find out how much we spend on each appliance
we use???
– Electrical energy x $ per kilowatt hour
• Example: If a 100W lightbulb is left on for 5h, the
amount of electrical energy is:
– E=Pt
– E= (0.1kW) (5h) = 0.5kWh
– If the power company charges $.10 per kWh, the cost
of using the bulb for 5 hours is:
– Cost= (kWh used)(cost per kWh)
– Cost= (0.5kWh) ($0.10/kWh) = $0.05
The Cost of Using Electrical
Energy~ Practice
• Use the following information to determine
the cost to run the following appliances.
Assume the cost per kWh is $0.09/kWh.
Appliance
Power Rating
Hours Used
Daily
Hair Dryer
1000W
0.25h
Stereo
100W
2.0h
Color TV
200W
4.0h
Clothes Dryer
1500W
2.0h
Daily Cost
Monthly Cost
(Assume 30
Days/Month)
Don’t be discouraged! You can do this!!!