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Electricity
Electric Charge
Electrons have negative charge
Protons have positive charge
Just like magnets where like poles repel, and unlike attract
In Electricity:
•Like charges repel (negative to negative)
•Unlike charges attract (negative to positive
Electric charges are different than magnets
Remember that magnetic poles can not exist alone,
But, Electric charges CAN exist by themselves.
Electric Force
Electric force is another force just like gravity or
magnetism
Electric Force is the attraction or repulsion
between electric charges
An electric field extends around any electric
charge.
•Invisible field just like a magnetic field
•Field lines can be drawn to show the direction of
the field
•Electric field lines point outward from a positive
charge
•Electric field lines point inward towards a
negative charge
Electric Fields
The strength of an electric field is related to
distance
• This is just like gravity and magnetism!!
• As the distance between the charges
increases, the force between the charges
decreases
• Distance is inversely proportional to strength of
the field
•Where field lines are close together, the
strength of the field is strong
•Strongest force is felt near the charge
•Farther from the charge the weaker the force
Electricity
Electricity occurs in 2 different forms
• Static
• Is stationary
E.g. Brush your hair
Wool socks in
tumble drier
• Current
• Flows around
circuit
E.g. turn on light
Static Electricity
In static electricity, charges build
up on an object, but they do not
flow…they are stationary!
•
•
•
•
Occurs with materials which are insulators
Rubbing adds or removes electrons
Object becomes charged
Like objects repel, unlike attract
Transferring Charge
Law of Conservation of Charge:
Charges are never created or
destroyed, they can be transferred
from one object to another
3 Ways to transfer charge:
A. Charging by Friction
B. Charging by Conduction
C. Charging by Induction
Transferring Charge
Charging by Friction:
Transfer of electrons from one uncharged object
to another by rubbing
•Caused by two objects rubbing against each
other
•Object that gains electrons becomes negatively
charged
•Object that loses electrons becomes positively
charged
Example:
Walking on a carpet with socks, the socks pick
up electrons and become negatively charged
The carpet loses electrons and becomes
positively charged
Transferring Charge
Charging by Conduction:
Transfer of electrons from a charged object to an
uncharged (or neutrally charged) object by direct
contact
•Electrons are transferred from the object with
greater negative charge to the one with more
positive charge
Example:
When a negatively charged sock touches your
skin the electrons are transferred by direct
contact.
Transferring Charge
Charging by Induction:
The movement of electrons to one part of an object
that is caused by the electric field of a second
object
•The electric field around a charged object attracts
or repels electrons in the second object
Example:
•The negative charges in the your fingertip produce
an electric field that repels the electrons on the
surface of the doorknob.
•The electrons on the doorknob move away from
your finger
•This movement of charge produces an induced
positive charge on the doorknob nearest your finger
Static Discharge
These stationary charges don’t stay there
forever as you’ve experienced by touching a
doorknob!
ZAP!!!
When a negatively charged object and a
positively charged object are brought together,
electrons transfer until both objects have the
same charge
This is called a static discharge
•Can produce a spark
•Pet your cat at night
•Pull a blanket away from another one
•Lightning
Detecting Charge
Electric charge is invisible
Can be detected with an instrument called an
electroscope
•Metal rod
•Knob on top
•Two metal leaves
When a charged object is brought close to the
knob, the leaves repel each other and spread
apart
Current Electricity
To understand electricity and
circuits you need to understand
Current
Voltage
Resistance
Conductors and Insulators
And how they are all related!
Current Electricity
• Current is electrons flowing
continuously through a material
• Flow is from negative to positive
• Circuit = continuous loop for electrons
to flow
• A circuit needs an energy supply to
allow electrons to flow
Current Electricity
•Electric charges flow because of a difference in
electric potential energy (EPE)
•EPE is related to the amount of energy in the
source (battery or generator)
•Voltage causes a current in an electric
circuit…it’s a force pushing the electric current
through the circuit
•Voltage (V) – Difference in electrical potential
energy between two places in a circuit (can be a
gain or loss of energy)
Current Electricity
In other words,
Voltage is the short name for the
electrical force need to drive an
electric current between two points!
Current Electricity
Resistance: The measure of how
difficult it is for charges to flow
through a material
•The greater the resistance, the
less current there is for given
voltage
•Resistance is measured in Ohms
(Ω)
Current Electricity
4 Factors that determine resistance
•What the resistance material is made of
•Conductors have less resistance than
insulators
•Length of the resistor
•Greater lengths give more resistance
•Diameter of the material
•Small diameters have more resistance
•Temperature of the resistance
•Higher temperatures cause greater
Current Electricity
Path of least Resistance….
Electric charges act just like
you…they take the easy way!
This is why a bird is not harmed
when sitting on a electric fence
or wire.
The bird’s body has more
resistance than the wire, so the
electricity picks the “path of least
resistance”….the wire, not the
bird!
Resistance (R)
The amount that a component
slows the current
•As the electrons are slowed by a resistor, energy is
lost in the form of heat.
•This means that current, resistance and voltage must
be linked.
•This is Ohms law
•The unit of resistance is the ohm, symbol 
V
I × R
Current Electricity
In the 1800’s Georg Ohm developed a law that
demonstrated how current, voltage, and
resistance are related…
Called…..Ohm’s Law (how original!)
Ohm says…
Resistance in a circuit is equal to the voltage
in the circuit divided by the current
Resistance = Voltage ÷ Current
in units this is:
Resistance (Ω) = Voltage (V) ÷ Amps (A)
Current Electricity
Another way to look at this law:
Voltage = Current X Resistance
Or
Current = Voltage ÷ Resistance
Current Electricity
So looking at the basic equation
Resistance (Ω) = Voltage (V) ÷ Current (I)
How does changing the 3 factors effect each other?
What happens to the current, if voltage is doubled?
….If
nothing
with
the
What
happens
tochanges
the current
, ifthe
theresistance,
resistancethen
is increased?
current
is doubled
Remember….C
= Vtoo
÷R
So, the current will decrease!
What happens to the Voltage when the resistance is
increased?
Remember ----V = C ÷ R
So, Voltage also decreases!
Electrical Circuits
Features of a circuit
1.Circuits have devices that are run by
electricity
2.A circuit has a source of electrical
energy
3.Electric circuits are connected by
conducting wires
Electrical components
…for electric circuits!
• Wire
• Ammeter
• Battery
• Fuse
• Connected
wires
• Bulb
• Switch
• Voltmeter
• diode
• Resistor
• Rheostat
Electric circuits
Electric Circuits
Conductors: Material that allows electrons
to flow easily
•Metals (copper, silver, aluminum, iron)
•Water (both fresh and salt)
•Humans (yikes!!!)
Insulators: Material that DOES NOT allow
electrons to flow easily
•Rubber
•Plastic
•Glass
•Wood
•Resistors can be made of conductors or
insulators!
CIRCUIT TYPES
The simplest type of circuit involves electricity going around with
no “choices” (electrons don’t really choose).
This is called a Series circuit.
Draw the path the electrons travel.
The other main type of circuit has two
or more branches.
This is called a Parallel circuit.
Draw on the electron flow.
What sort of circuit is this?
A parallel… but, more
importantly…
A short circuit.
(we’ll talk more of this later!)
Series circuit
• Has a single loop for electrons to
travel round
• Components are connected one after
another...in series
• Current has to travel through all
components in turn
• Current is the same at all points
• Voltage is shared between
components
Series circuit
Disadvantages to Series
Circuits
•If you have two or more
lights hooked up in a series
circuit, when one goes out,
they ALL go out
•As you add lights the lights
become dimmer as more
lights are added
Parallel circuit
• Has two or more paths for electrons
to flow down
• Current is shared between the
branches
• Sum of the current in each branch =
total current
• Voltage loss is the same across all
components
Parallel circuit
Advantages of Parallel Circuits
•Unlike in a series circuit, when on light burns
out in a parallel circuit, the other lights
connected in circuit stay on.
•Switches can be added so that components
can be turned on and off without affecting the
other components
•Resistance decreases in the circuit when
adding more branches or loops to the circuit
•Remember Ohm’s law, if resistance decreases
then current increases!
Current (I)
Current is the flow of electrons
around a circuit
Two types of current:
• DC = direct current like battery
• Electrons flow in one direction
• AC = Alternating current like mains
• Electron flow changes direction 50x per
second
Ammeter
•
•
•
•
Measures CURRENT(I)
Unit = Amp (A)
Current is flow of electrons
Connect in series at the point
you wish to measure
• RED to RED and BLACK to
BLACK
Voltage (V)
Gain or loss of energy as it
passes through a component
• Voltage lost = voltage gained
• In series voltage loss is shared
between components
• In parallel voltage loss is the
same across all components
Voltmeter
• Measures voltage
• Unit = Volt (V)
• Voltage increases as it passes
through energy suppliers
• Voltage is decreased as it
passes through users
• Connect in parallel around a
component
SUMMARY
In Series
Current
Voltage
Always the same
Voltage from source =
voltage used
Voltage is shared
between power users
In Parallel
The branches share
electrons and add to the total
Voltage is the same in all
branches
Electric Power
The rate transformation of electrical
energy into another form is power
Electric Power = Voltage X Current
P=VI
The unit of Power is watts
Power
Energy used by component per
second
• Unit of power is the Watt, symbol is W
• One watt means that 1 joule of electrical
energy is being used up per second.
• Current, voltage and power
are linked
P
I × V
PROBLEMS
1. Fill in the gaps in the table.
a
b
c
d
e
g
i
k
j
f
h
l
Electricity Generation
Fossil Fuels
Hydro Dam
Hydroelectric power
Nuclear power
Wind Power
Tidal Power
Geothermal
Solar
• Solar water heating
• Photovoltaic cells
• Solar furnace
Electrical Safety
Electricity in the wrong place can
cause damage and injury!
Potential Dangers
•Short Circuit
•Electric Shock
•Ungrounded Wires
Electrical Safety
Short Circuit
A connection that allows current to take
the path of least resistance…usually a
unintended path.
Example:
A downed electric wire on the ground
and someone touching it, causes a short
circuit.
Their body becomes the path of least
resistance and the electricity flows
through them instead of the wire!
Ouch!
Electrical Safety
Electric Shocks
The severity of an electric shock
depends on the current
•A current of less than .001 A is almost
unnoticeable
•A current greater than .2 A can be
dangerous, causing burns, or even
stopping the heart
•Household voltage: 120 V, if only 1Ω
resistance, then the current would be
120 A!!! Yikes!