Download electric circuit - Madison County Schools

Electric Charge
and Static
Electricity
Notes
Electric Charge
• Electric charge is a property of protons
and electrons.
• Protons have a positive charge.
Electrons have a negative charge.
Proton
Electro
n
Electric
Charge
• Two charges that are the same push away from
each other. Two charges that are different pull
toward each other.
• If a proton and an electron come close together,
they attract each other.
• Attraction (pull) and repulsion (push) between
electric charges is known as interaction between
charges. The interaction between charges is
called electricity.
Repulsion
Attraction
Electric Charge
• This is different from the interaction
between magnetic poles, which is
known as magnetism.
• The terms “positive” and “negative”
were given to charges by Benjamin
Franklin in the 1700s.
Electric Charge
• Balloon + Aluminum Can
• Comb + Paper
• Balloon + Water
Electric Force
• Electric force is the attraction or
repulsion between electric charges.
• A magnetic field surrounds a charged
object. An electric field is a region
around a charged object in which
electric force occurs.
Electric Force
• Suppose one charged object is placed
in the electric field of a second charged
object. The first charged object is either
pushed or pulled -- repelled or
attracted.
• The strength of an electric field
depends on how far away the charged
object is. The farther away a charged
object is, the weaker the electric field is.
The strength of an electric field is represented by how
close the electric field lines are to each other.
Static Electricity
• Most objects usually have no charge.
However, objects can become charged.
• If an object loses electrons, it has more
protons than electrons. Therefore, it has
a positive charge.
• If an object gains electrons, it has more
electrons than protons. Therefore is has
a negative charge.
- +
- +
+
+
- - +
- +
+
+
- + - - - -
Before Rubbing
(uncharged)
After Rubbing
(Negatively Charged)
+
Static Electricity
• Static Electricity is the buildup of
charges on an object. Static means “not
moving.” In static electricity, the
charges do not flow or move.
Transferring Charge
• An object becomes charged when
electrons move from one place to
another place.
• Charging by friction is when electrons
move from one uncharged object to
another object by rubbing. For
example, a girl charges by friction when
she runs her socks on the carpet.
Transferring Charge
• Charging by conduction is when
electrons move from a charged object
to another object by direct contact. You
can charge yourself by conduction
when you touch a charged object.
• Think: carpet > socks = friction
• Then: socks > feet = conduction
Transferring Charge
• Charging by induction is when
electrons move to one part of an object
due to the electric field of another
object. There is no touching in charging
by induction.
Transferring Charge
• You can find out if an object is charged
by using an instrument called an
electroscope.
When the object is uncharged, the leaves hang
down, but if a charged object gets close, the leaves
repel each other and spread apart.
Static Discharge
• Charges may build up as static
electricity on an object. But the charges
do not stay on that object forever.
• The loss of static electricity as charges
move from one object to another is
called static discharge.
Static Discharge
• A static discharge often produces a
spark. For example, there may be a tiny
spark when you touch a metal
doorknob. Lightning is another example
of static discharge.
Section 6:2
Electric Current
Flow of Electric Charges
•
•
•
Electric current is the flow of electric charges
through a material. The charges must flow
continuously, or without stopping.
The rate of electric current through a wire is how
much charge passes a place in a certain amount of
time (One coulomb per second = 1 amp).
The unit used for the rate of electric current is the
ampere. The name can be shortened to amp or A.
Flow of Electric Charges
• A current needs a path to follow. An electric
circuit is an unbroken path through which
electric charges flow. An electric circuit is
always a complete loop with no breaks in the
loop.
• If an electric circuit is complete, charges can
flow continuously. If an electric circuit is
broken, charges will stop flowing.
Open
Circuit
Closed Circuit
Conductors and Insulators
• Any material that an electric charge can go
through easily is called a conductor. Metals
are good conductors. Silver, copper,
aluminum, and iron.
• In a conductor, atoms contain electrons that
are bound loosely. These electrons, called
conduction electrons, are able to move
throughout the conductor.
Conductors and Insulators
• Any material that an electric charge has a
hard time going through is called an
insulator. Rubber, glass, plastic, and wood.
• The rubber coating on an electric cord is an
example of an insulator; allowing electrons
to flow though the copper wiring inside, but
preventing the electrons from flowing into
your hand.
Voltage
• Charges need energy to flow through a wire.
The energy that makes charges flow is
called electrical potential energy. A battery
for example, creates an electrical potential
energy in an electric circuit.
Voltage
• Voltage is the difference in electrical
potential energy between two places in a
circuit. Another name for voltage is potential
difference. The unit of measure of voltage is
volt, which is abbreviated as V.
Voltage
• An electric circuit needs a source of energy
to have voltage. A voltage source creates a
potential difference, or voltage, in an electric
circuit. A battery is an example of a voltage
source. An electric generator is also a
voltage source.
Resistance
• Resistance is the measure of how hard it is
for charges to flow through a material. The
unit for resistance is the ohm the symbol 
stands for “ohms.”
• How much current there is through a circuit
depends on how much resistance there is.
The more resistance there is, the less
current there will be.
Resistance
• How much resistance there is in a wire
depends on these four factors:
- material the wire is made of
- length of the wire
- diameter of the wire
- temperature of the wire
Resistance
• Material
- Conductor - Low Resistance
- Insulator - High Resistance
• Length
- Short - Low Resistance
- Long - High Resistance
Resistance, cont.
• Diameter
- Wide - Low Resistance
- Narrow - High Resistance
• Temperature
- Hot - High Resistance
- Cold - Low Resistance
Resistance
• If an electric charge can flow through one or
more paths with varying resistances, it will
flow through the path with the least
resistance.
Bird on an Electric Wire
The bird has more
resistance than the wire.
Section 6:4
Electric Circuits
Ohm’s Law
• Current, voltage, and resistance are related
to one another. The relationship among
resistance, voltage, and current is summed
up in Ohm’s Law.
Ohm’s Law
•
•
•
Ohm’s Law states that the resistance of most
conductors does not depend on the voltage across
them.
Changing the voltage in a circuit changes the
current but will not change the resistance.
Ohm concluded that conductors and most other
devices have a constant resistance regardless of
the applied voltage.
Ohm’s Law
•The equation for Ohm’s Law is:
Voltage
Resistance =
Current
or R=V/I
• The units in the equation are:
• Ohms (Ω) = Volts (V) ÷ Amps (A)
Ohm’s Law
•You can rearrange Ohm’s Law to find voltage
with this equation:
•Voltage
•or
•V=IR
= Current X Resistance
Ohm’s Law
• If a circuit has a resistance of 30.5 ohms and
a current of .05 amps, what is it’s voltage?
1.52 volts
Ohm’s Law
• The brake light on an automobile is
connected to a 12-volt battery. If the
resulting current is 0.40 amps, what is the
resistance of the brake light?
30 Ω
Features of a Circuit
• Circuits have devices that are run by
electrical energy.
• A circuit has a source of electrical energy.
• Electric circuits are connected by conducting
wires.
Features of a Circuit
• Radios, appliances, and light bulbs are
examples of electrical devices. All these
devices resist the flow of electrical energy.
As a result, electrical devices are known as
resistors.
Features of a Circuit
• Sources of electrical energy in a circuit
include batteries, generators, and electric
power plants. When you plug a radio into a
wall socket, the source of electrical energy is
your local electric plant.
Features of a Circuit
• Conducting wires complete the path of an
electric circuit. Wires allow charges to flow
from the energy source to the electric device
and back to the energy source.
Features of a Circuit
• Often, a switch is placed in an electric circuit.
With a switch, you can turn a device on or off
by opening or closing the circuit.
Circuit Diagram
Circuit Diagram
Electric Circuits
Part 2
Series Circuits
• A series circuit has all the parts of the
circuit connected on one path.
• An example of a series circuit is a circuit with
a battery and two light bulbs connected by a
single wire.
Series Circuits
• Suppose a series circuit has two light bulbs.
If one burns out, the second bulb goes out
because the circuit is broken when the first
bulb burns out.
Series Circuits
• If you add resistors to a series circuit, the
resistance of the circuit increases.
• This can be shown as:
•R =R +R +R ...
Total
1
2
3
•Remember, Resistance is measured in Ohms
and uses the symbol Ω.
Series Circuits
•
•
•
•
In a series circuit, the current throughout the circuit
is constant.
This can be shown as:
ITotal = I1 = I2 = I3...
Where current is represented by the letter I in the
equation, but is measured using the units amperes
or amps, and uses the units A.
Series Circuits
• Also, in a series circuit, as bulbs are added
in line, all of the bulbs become dimmer.
• Each bulb adds more resistance to the
circuit, therefore, for a constant voltage,
when resistance increases, current
decreases.
Series Circuits
• An ammeter is an instrument used to
measure current.
Parallel Circuits
• A parallel circuit has more than one path for
current to take. There are separate branches
in a parallel circuit. Each resistor may be on
its own branch.
Parallel Circuits
• Suppose a parallel circuit has two light
bulbs, each on its own branch. If one light
bulb burns out, the other light bulb will stay
lit. The second bulb stays lit because it is on
its own branch.
Parallel Circuits
• Because the voltage source (battery)
provides constant voltage, the only factors
that can change in a circuit are resistance
and current.
Parallel Circuits
• In a parallel circuit, when more branches are
added, the overall resistance decreases,
meaning that the current will increase.
• Think blowing through one straw versus
blowing through 2 straws. Two straws allow
twice as much air through, but your lungs
are only capable of blowing so much at a
time.
Parallel Circuits
•
The major disadvantage of a parallel circuit is that
as you add more things in parallel, the current draw
on the source goes up with each new branch. If the
source cannot supply the current that is demanded
by the multiple resistors of the circuit, the voltage
will (must!) decrease. This could be bad, as some
devices, notably motors, do not like to run at low
voltage and can actually be damaged if voltage
decreases too much.
Parallel Circuits
•A
voltmeter is a device used to measure
voltage, or electrical potential energy
difference.
Parallel Circuits
• The circuits in your home are parallel.
• Electrical energy enters your home through
heavy-duty, low resistance wires, and then
branch out in parallel to your wall sockets,
then to appliances and lights in each room.
• The voltage in most household circuits is
120 V.