Electric Charges
A little chemistry
All matter Atoms are
is made up of atomsmade of protons,
neutrons, and electrons Protons: (+)
Neutrons ( no charge )
Electrons: (-)
Atoms can have a positive charge, anegative charge, or no charge at all.
Charges
Electrical charge – caused by animbalance of protons and electrons
Positive (more protons than electrons) Negative (more electrons than protons)
Neutral (same number of protons and (Electric Fields of each shown above…)
Opposites attract
electrons)
Likes repel
More Chemistry A
charge can never be created or destroyed.It is transferred from one object to another.
This happensto another
when electrons move from one object
Opposite Charge
Like Charge
Transferring ChargeElectric energy is never created or destroyed,only transferred
Objects (generally those that arecan become charged by one of 3
1. Induction
2. Conduction
3. Friction
conductors)ways:
Transferring ChargeInduction – Transferring a charge by bringing acharged object NEAR a neutral object withouttouching it.
The total charge on the door knob will still be neutral, but the opposite sides will have opposite charges
Transferring ChargesConduction - Electrons are transferred from oneobject to another by contact, or touching.
When a negatively charged rod touches a neutral door knob, electrons transfer from the rod to the doorknob to give the doorknob a negative charge
Transferring ChargeFriction - two objects rub up against each other,leaving electrons built up on one side. Bothobjects become charged.
i.e. rubbing a balloon on your head, walking acrosscarpet, wool rubbing on metal…
Static Discharge John Travoltage!!
http://phet.colorado.edu/en/simulation/travoltage
Controlling the Path of Electric Charge
Electrical Conductor: a material in which
charges move freely
i.e. metal (wires) and water
Electrical Insulator: a material in which
charges cannot move freely
i.e. plastic, wood, glass, foam, cloth, ceramic
Controlling the Path of Electric ChargeElectrical Conductor:
the metal inside the cord presents apath of little resistancecharge to flow
for the
Electrical Insulator:
the rubber/plastic has a highresistance, to keep the charge from leaving the metal path of wire
(and to ensure you don’t shock yourselfwhen you touch the cord!)
CURRENTS AND OHM’S LAW
What?
Electricity – flow of electric current Electric current –
the
movement
of anelectrical
charge.
In most cases, wethink of an electric current running through a circuit.
In comes Ohm’s Law.
Ohm’s Law
V = I x R
V = Voltage (units are volts, v)
I = Current (units are ampres, A)
R = Resistance (units are ohms, Ω)
Voltage
Voltage is the difference in
between two places where A negative charge (electron)
from other negative charges
electrical potentialelectrons are flowing.wants to move away(other electrons.)
These repulsive forces increase as electrons are closer to
each other. Electrons flow from high potential energy to low potential.
This potential difference is usually just called voltage
Voltage provides the energy that pushes and pulls
electrons through the circuit.
Voltage is measured in Volts (V)
Batteries
Batteries can have different
voltage, and therefore pushdifferent strength currents Range from 1.5 volts to 12 volts
Batteries have a positive terminal
and a negative terminal. Electrons are pushed from the
negative terminal and are pulled towards
the positive terminal
BUT current is in the opposite direction
Voltage
Basically…
Voltage is
difference
motionVoltage is
potential
electricalthat sets a charge
in
the PUSH and is measuredin Volts
(V)
Current: General Idea An object moving in a specific direction
Current can be water, air, cars, or charge
Current is caused because of a difference pressure on either side of an object
in
Once the difference in pressure is gone, current stops
Pumps & Batteries are used to maintain a difference in
pressure.
Electric
Current
Just as water current is flow of water molecules,
electric current is the flow of electric charge. Measured in Amperes (A)
In metal circuits, moving
flow of charge. Electrons travelnegative to positive Current travels in
electrons
make
up the
the OPPOSITE
as electrons do.
direction
Types of CurrentTHERE ARE TWO TYPES OF CURRENT:
In a direct current (DC) the current always movesterminal to the other in the same direction.
- example: battery
from
one
In an alternating current (AC) the current will
always alternate directions at regular intervals.
- example: appliances at homeRemember, direction of the current is
opposite the direction of electron flow.
Voltage
and Current
Voltage PUSHES charges through circuits. Current is how fast electrons FLOW thru the Example – you could say that…
circuit
Amps measure how much water comes out of
a hose.
Volts measure how hard the water
hose.
comes
out
of a
Basically: As Voltage Increases,Current Increases!
Practice!
So how does this relate to awesome computer simulations?
http://phet.colorado.edu/en/simulation/ohms-law
Resistance (R)
Objects use different amount of power
because they have different amounts ofcurrent running through them.
The difference in current is due to the
resistance.
Resistance (R)
Resistance: the tendency for a material to oppose the
flow of electrons, changing electrical energy intothermal energy and light.
Resistancemovement
is caused by internal friction, which slows theof charges through a conducting material.
Resistance
is measured in Ohms (Ω).
Resistance and Wires…
Conductors have low resistance
The better the conductor, the better that electrons willmove through the material in
thefield ie: metal wires
presence
of an electric
Insulators have high resistance
The better the insulator, the more that the substance will resist the movement of electrons.
ie: plastic or rubber
Resistance (R)
The resistance of a light bulb filament determines how
bright the bulb will be The filament of a 40watt bulb has a
much higher resistance than a 100watt bulb This is because more resistance = less current =
dimmer bulb
So being able to control resistance would be a
good thing…
a wire: 2 ways
make
make
that we can increase resistance in
the wire longer (ie: the loops)wire thinner
Thicker wire = less resistance
Thin wire = more resistance
Ohm’s Law
V = I x R
V = Voltage (units are volts, v)
I = Current (units are ampres, A)
R = Resistance (units are ohms, Ω)
Practice!
The headlights of a typical car are powered by a 12Vbattery
.What is the resistance of the headlights
ifthey draw 3.0 A of current
when
turned
on?
Use your Ohm’s Law Triangle-
--
Insert your values
Use unitsSolve!
SERIES AND PARALLELCIRCUITS
Circuits Provides a path for electricity to travelSimilar to water pipes in
your
Because of the voltage of an will travel through the circuit
house
outlet,
electrons
Electrons NEED to travel
around the ENTIRE pathfor anything to work
Open vs.
Closed Circuits
When there is
closed circuit.
When there is
a complete path, the circuit is considered
a
NOT a complete
path,
the
circuit
isconsidered
an open
circuit.
A switch allows you to open
and close a circuit
Conductive Ink!!!
The ink on this page conducts electricity
ink).
(yes, it is special
Whatover?
…
…
…
happens to the when
the
page is folded
to
to
to
the
the
the
circuit?
lightbulb?current?
Safety Precautions
If too many devices (tv, radio, hair
connected to an outlet, the overall circuit is lowered
This increases the current traveling
dryer, etc) areresistance of thethrough thecircuit, possibly more than a
safe level This is called an overloaded circuit.
of current.
Too much current traveling thougha wire can cause fires.
when too
Safety Precautions Both of these objects open the circuit by disrupting the complete
circle, preventing damage to the rest of the circuit.
FUSE – a ribbon of metal wire
that
meltsmuch current flows
through If current becomes too high,the fuse melts, and the circuit is open Can only be used oncethen must be replaced
it
Safety Precautions
Circuit Breaker opens a circuit
with a high current Uses an electromagnet that
responds to current overload by
opening the circuit Basically it is magnetic switch that
‗trips‘
The circuit breaker acts as a switch.
Can be used multiple times.reset once ―tripped.
Must be
Circuit
Diagrams
Uses symbols to
represent parts
of a circuit Shorthand way todescribe a realcircuit
Circuit
Symbols:
Howto Draw
On – Off SwitchElectrical Supply(Battery)
ResistorLight Bulb
Remember, the circuit
drawing needs to be a COMPLETE
path.
Circuit
Symbols:
Howto Draw
You try
to draw some
circuits:
A circuitswitch.
with
one
resistor, one battery,
and
one
1.
A circuit
with
two
resistors, three batteries, and
2.
one switch.
Circuits: 2 Diff Types
Can either
be series
or parallel.
Series:
1 path Parallel: 2+ Paths
Resistance (Ω)
Overview:
Series
v Parallel
Circuits
MUST CALCULATE
(Voltage drops to 0v after
Series
Parallel
Pathways for electrons
and current
Only 1 path. Multiple Pathways for current
Current (I) Always the same throughout
Different at each branch –
Voltage (V)VOLTAGE DROP after
everyresistor/bulb:
MUST CALCULATE each
separately
Each Branch starts
with same voltage
each branch)Add up all to get total Each branch different -
MUST CALCULATE each branch
Let‘s Practice: Identify WS…
Try some drawings!
1) Series: with 2 resistors (one is 4 ohm,
12V battery, and a 10 ohm light bulb.
one is 7ohm), a
2) Parallel: with a 1.5 volt battery and 3 light bulbs
(each on its own branch). Light bulbs have a resistanceof 2, 4, and 6
ohms.
3) Series: 6v battery, one switch, one 20ohm resistor.
4) Parallel: 12volt battery, 3 light bulbs.
2 light bulbs (23rdand 3 ohms) are on one branch, while
the(8ohms) is on another branch)
light bulb
Circuits: 2 Diff Types
Can either
be series
or parallel.
Series:
1 path Parallel: 2+ Paths
Series circuits
There is only 1 path forcurrent/electrons to travel If the circuit opens in
any way, the whole circuits stops working because current STOPS.
Series Circuits Current only takes one path for electronsIf you remove a light bulb or one
burnsout—the ENTIRE circuit stops working!
Current stays the sameas it flows through everypart of the
circuit Resistance (and therefore
voltage), will change at different points on a series circuit
Current in Series Current is the same at all
points Electric current always FLOWS
from positive (+) to negative (-) Which is OPPOSITE the flow
of electrons. Use Ohm‘s Law to find current
using total resistance andvoltage
Resistance in Series
Add up all resistors to get totalresistanceCurrent MUST
go through each resistor in
series because
there is only one path.
Voltage Series
Voltage “drops”after each resistorhas been passed.Calculate voltage drop by calculating voltage at each resistor
V = I*R V = (current *
resistor)
Sample Problem #1Draw a series circuit with a 3
different1.5
batteries (all together), 2 equal resistors, and a current of 0.5 A.
V
What is
3 * 1.5
What is
V = IR
What is
the total voltage of the circuit?
volts = 4.5 volts
1.
the
total resistance of the circuit?
2.
4.5v / 0.5A = 9ohmsth
eresistance of each resistor?
3.
9ohms / 2 equal resistors
= 4.5 ohms each
Sample Problem #2What is the total resistance of
the circuit?17ohms + 12ohms + 11ohms = 40 ohms
What is
V = IR
the
the
current for the circuit?60 volts / 40ohms = 1.5amps
What
is
voltage drop across
V1 = IR
V1 = 1.5a * 17ohm
V1 = 25.5v
each resistor?
V3 = IR
V3 = 1.5a * 12ohm
V3 = 18vV2 = IR
V2 = 1.5a * 11ohm
V2 = 16.5v
Parallel Circuits
Have more than one path for current to flow
Paths are also known asbranches
If you remove a resistor,
the other branches still
work
Voltage in Parallel
Voltage is the same
across each branchbecause
each branch is on the same wire
Voltage will drop to
after each branch.
0v
But we won‘t calculate
that…
Current in Parallel
Current depends on resistance in each branch
The current in each branch will be different if the resistors have different values It one branch has less resistance, more
charge will move through it because that bulb offers less
opposition to the movement of current.
The sum of the currents on each of the
MUST equal the total starting current
branches
EACH BRANCH iscalculated usingOhm‘s Law
Practice problem #3
Draw a parallel circuit with two resistors,
one 2 ohm and one 3 ohm (one on eachbranch) and a 12 V battery.What is the
12 volts
voltage through each resistor?
1.
What is
V = IR
What is
the
current flowing through each branch?
2.
12v/2ohms = 6v
total current?
12v/3ohm = 4v
the
3.
6v + 4v = 10v
Overview:
Series
v Parallel
Circuits
MUST CALCULATE
(Voltage drops to 0v after
Series
Parallel
Pathways for electrons
and current
Only 1 path. Multiple Pathways for current
Current (I) Always the same throughout
Different at each branch –
Voltage (V)VOLTAGE DROP after
everyresistor/bulb:
MUST CALCULATE each
separately
Each Branch starts
with same voltage
each branch)Add up all to get total
Each branch different - MUST CALCULATE each branch
Magnets A magnet is an object that produces a magnetic field.
Magnets can be natural or man made
The only natural metals that are naturally magnetized
(and can become magnetized) are Nickel, Iron, Cobalt,and Gadolinium.
If they are made, they are formed from things that are naturally magnetic .
Magnetic Poles All magnets have 2 poles:
North and South
Laws of attraction still applies Like poles repel, opposite poles attract
If a magnet is cut, each piece will still have two poles
Permanent Magnets Substances that are magnetic all the time
Can change anything into a magnet by rubbing a permanent magnet over it several times Permanent magnets can have magnetic
properties but not always be a magnet (ex: iron)
Permanent magnets can last for minutes, or forever
Magnetic Fields When magnets
repel or attract each other it’s because of their magnetic fields
Magnetic field – region where a magnetic force can be detected
Magnetic FieldsBelow: Iron filings
placed over a magnet align with the magnetic field of the magnet
Magnetism and Electric Currents Electric currents produce magnetic fields
Magnetism is produced by moving electriccharges
The magnetic field of a coil of wire resembles
that of a bar magnet
Electromagnets A strong magnet created when an ironcore is inserted into the
centercurrent-carrying coil of wire
Strength depends on: Thicker wire
Thicker wire = less resistance
the number of loops in the wire
More loops = stronger
the amount of current More current = stronger
of a
And remember, more voltage means more current
size of the iron core Bigger core = stronger
Electromagnets So why are
electromagnetsuseful?
Can turn them on / off
Cutting the current off turns off the magnet
Can control their strength Just like we talked about last
slide. More current and voltage Bigger Iron core More coils around the iron
core
Motors Electric motors change electrical energy to
mechanical energy Done by running an electric current
make an electromagnet.
through coils to
When the electric current runsthrough the armature, it becomesmagnetized
The armature spins because motors use other magnets to push and pull the armature and create motion.
Generators Change mechanical energy into electric energy
Done by moving a coil past a magnetic field. Use electromagnetic induction to produce an electric
current.
When a coil of wire moves through a magnetic field an
electric current can be produced. This is electromagnetic
induction. Produces AC current.
