Electric Charges
A little chemistry


All matter is made up of atoms
Atoms are made of protons,
neutrons, and electrons




Protons: (+)
Neutrons ( no charge )
Electrons: (-)
Atoms can have a positive charge, a
negative charge, or no charge at all.
Charges

Electrical charge – caused by an
imbalance of protons and electrons



Positive (more protons than electrons)
Negative (more electrons than protons)
Neutral (same number of protons and electrons)


(Electric Fields of each shown above…)
Opposites attract
Likes repel
More Chemistry

A charge can never be created or destroyed.

It is transferred from one object to another.

This happens when electrons move from one object
to another
Opposite Charge
Like Charge
Transferring Charge

Electric energy is never created or destroyed,
only transferred

Objects (generally those that are conductors)
can become charged by one of 3 ways:
1. Induction
2. Conduction
3. Friction
Transferring Charge
– Transferring a charge by bringing a
charged object NEAR a neutral object without
touching it.
Induction
The
total charge on the door knob will still be neutral,
but the opposite sides will have opposite charges
Transferring Charges
Conduction -
Electrons are transferred from one
object 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 Charge
Friction - two objects rub up against each other,
leaving electrons built up on one side. Both
objects become charged.

i.e. rubbing a balloon on your head, walking across
carpet, 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 Charge
Electrical Conductor:
the metal inside the cord presents a
path of little resistance for the
charge to flow
Electrical Insulator:
the rubber/plastic has a high
resistance, to keep the charge from
leaving the metal path of wire
(and to ensure you don’t shock yourself
when you touch the cord!)
CURRENTS AND OHM’S LAW
What?
Electricity – flow of electric current
 Electric current – the movement of an
electrical charge.

In most cases, we
think of an electric
current running
through a circuit.
 In comes Ohm’s Law.

Ohm’s Law
V=IxR
V = Voltage (units are volts, v)
I = Current (units are ampres, A)
R = Resistance (units are ohms, Ω)
Voltage

Voltage is the difference in electrical potential
between two places where electrons are flowing.
A
negative charge (electron) wants to move away
from other negative charges (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 push
different 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 potential electrical
difference that sets a charge in
motion
Voltage is the PUSH and is measured
in 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 in
pressure on either side of an object
 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 electrons make up the
flow of charge.
 Electrons
travel
negative to positive
 Current travels in
the OPPOSITE direction
as electrons do.
Types of Current
THERE ARE TWO TYPES OF CURRENT:
In a direct current (DC) the current always moves from one
terminal to the other in the same direction.
- example: battery
In an alternating current (AC) the current will
always alternate directions at regular intervals.
- example: appliances at home
Remember, 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 circuit


Example – you could say that…
 Amps measure how much water comes out of a hose.
 Volts measure how hard the water comes out of a
hose.
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 of
current 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 into
thermal energy and light.
 Resistance
is caused by internal friction, which slows the
movement of 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 will
move through the material in the presence of an electric
field


ie: metal wires
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…


2 ways that we can increase resistance in a wire:
 make
the wire longer (ie: the loops)
 make wire thinner
 Thicker
wire = less resistance
 Thin wire = more resistance
Ohm’s Law
V=IxR
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 12V
battery. What is the resistance of the headlights if
they draw 3.0 A of current when turned on?
Use your Ohm’s Law Triangle
- Insert your values
- Use units
- Solve!
SERIES AND PARALLEL
CIRCUITS
Circuits

Provides a path for electricity to travel
Similar
to water pipes in your house
 Because of the voltage of an outlet, electrons
will travel through the circuit
Electrons NEED to travel
around the ENTIRE path
for anything to work

Open vs. Closed Circuits



When there is a complete path, the circuit is considered a
closed circuit.
When there is NOT a complete path, the circuit is
considered an open circuit.
A switch allows you to open and close a circuit
Conductive Ink!!!


The ink on this page conducts electricity (yes, it is special
ink).
What happens to the
when the page is folded
over?
 … to the circuit?
 … to the lightbulb?
 … to the current?
Safety Precautions
If too many devices (tv, radio, hair dryer, etc) are
connected to an outlet, the overall resistance of the
circuit is lowered
 This increases the current traveling through the
circuit, possibly more than a safe level of current.

 This
is called an overloaded circuit.
Too much current traveling though
a wire can cause fires.

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 melts when too
much current flows through it

 If
current becomes too high,
the fuse melts, and the
circuit is open
 Can only be used once
then must be replaced
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. Must be
reset once ―tripped.
Circuit Diagrams


Uses symbols to
represent parts
of a circuit
Shorthand way
to describe a
real circuit
Circuit Symbols: How to Draw
Electrical Supply
(Battery)
Resistor
On – Off Switch
Light Bulb
Remember, the circuit drawing needs to be a COMPLETE path.
Circuit Symbols: How to Draw
You try to draw some circuits:
1.
2.
A circuit with one resistor, one battery, and one
switch.
A circuit with two resistors, three batteries, and
one switch.
Circuits: 2 Diff Types
Can either be series or parallel.

Series: 1 path
Parallel: 2+ Paths
Overview: Series v Parallel Circuits
Series
Parallel
Only 1 path.
Multiple Pathways for current
Always the same throughout
Different at each branch –
MUST CALCULATE
Voltage (V)
VOLTAGE DROP after every
resistor/bulb: MUST
CALCULATE each
separately
Each Branch starts
with same voltage
(Voltage drops to 0v
after each branch)
Resistance (Ω)
Add up all to get total
Pathways for electrons
and current
Current (I)
Each branch different - MUST
CALCULATE each branch
Let‘s Practice: Identify
WS…
Try some drawings!




1) Series: with 2 resistors (one is 4 ohm, one is 7ohm), a
12V battery, and a 10 ohm light bulb.
2) Parallel: with a 1.5 volt battery and 3 light bulbs
(each on its own branch). Light bulbs have a resistance
of 2, 4, and 6 ohms.
3) Series: 6v battery, one switch, one 20ohm resistor.
4) Parallel: 12volt battery, 3 light bulbs. 2 light bulbs (2
and 3 ohms) are on one branch, while the 3rd light bulb
(8ohms) is on another branch)
Circuits: 2 Diff Types
Can either be series or parallel.

Series: 1 path
Parallel: 2+ Paths
Series circuits

There is only 1 path for
current/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 electrons
If you remove a light bulb or one burns
out—the ENTIRE circuit stops working!
Current stays the same
as it flows through every
part 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 and
voltage

Resistance in Series

Add up all
resistors to get
total resistance
Current MUST
go through
each resistor in
series because
there is only
one path.
Voltage Series

Voltage “drops”
after each resistor
has been passed.
Calculate
voltage
drop by calculating
voltage at each
resistor
V
= I*R
 V = (current * resistor)
Sample Problem #1

1.
Draw a series circuit with a 3 different1.5 V
batteries (all together), 2 equal resistors,
and a current of 0.5 A.
What is the total voltage of the circuit?
3 * 1.5 volts = 4.5 volts
2.
What is the total resistance of the circuit?
V = IR
3.
4.5v / 0.5A
=
9ohms
What is the resistance of each resistor?
9ohms / 2 equal resistors
= 4.5 ohms each
Sample Problem #2
What is the total resistance of the circuit?


What is the current for the circuit?



17ohms + 12ohms + 11ohms = 40 ohms
V = IR 
60 volts / 40ohms = 1.5amps
What is the voltage drop across each resistor?
V1 = IR
V1 = 1.5a * 17ohm
V1 = 25.5v
V3 = IR
V3 = 1.5a * 12ohm
V2 = IR
V2 = 1.5a * 11ohm
V3 = 18v
V2 = 16.5v
Parallel Circuits

Have more than one path
for current to flow
 Paths
are also known as
branches

If you remove a resistor,
the other branches still
work
Voltage in Parallel

Voltage is the same
across each branch
 because
each
branch is on the
same wire
 Voltage
will drop to 0v
after each branch.
 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 branches
MUST equal the total starting current
EACH BRANCH is
calculated using
Ohm‘s Law

Practice problem #3

1.
Draw a parallel circuit with two resistors,
one 2 ohm and one 3 ohm (one on each
branch) and a 12 V battery.
What is the voltage through each resistor?
12 volts
2.
What is the current flowing through each branch?
V = IR
3.
12v/2ohms = 6v
What is the total current?
6v + 4v = 10v
12v/3ohm = 4v
Overview: Series v Parallel Circuits
Pathways for electrons
and current
Current (I)
Voltage (V)
Series
Parallel
Only 1 path.
Multiple Pathways for current
Always the same throughout
Different at each branch –
MUST CALCULATE
VOLTAGE DROP after every
resistor/bulb: MUST
CALCULATE each
separately
Each Branch starts
with same voltage
(Voltage drops to 0v after
each branch)
Add up all to get total
Each branch different - MUST
Magnets
 A magnet is an object that produces a
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 Fields
Below: 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 electric
charges
 The magnetic field of a coil of wire resembles
that of a bar magnet
Electromagnets
 A strong magnet created when an iron
core is inserted into the center of a
current-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
 And remember, more voltage means more current
 size of the iron core

Biggercore = stronger
Electromagnets
 So why are electromagnets
useful?
 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 through coils to
make an electromagnet.
 When the electric current runs
through the armature, it becomes
magnetized
 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.