ELECTRICITY
What do you already know about it?
Voltage?
Resistance?
Current?
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 Do Smarty Demo 
Electric Circuit
A path over which electrons travel, out through the negative
terminal, through the conductor, back in through the positive
terminal.
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http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/Conductivity.htm
http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/Conductivity.htm
Electric Current
http://www.youtube.com/watch?v=0oTmAP_EESI&feature=player_embedded
Alternating Current and Direct Current
If a single electron has a charge of 1.60 × 10-19
Coulombs, then
1.60 1019 Coulombs
1Electron
Now flip to get the number of electrons in 1
coulomb of charge
1

 Electron

19 
 1.60 10  Coulombs
Electron
6.24  10
Coulombs
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A collection of 6.24 × 1018 electrons has a
charge of one Coulomb
Electric current is the flow of electrons
through a wire. Measured in Amperes (A).
1 Amp is 1 coulomb of charge per second.
That means that 6.24 × 1018 electrons flow past
any single point in the circuit.
Current is the amount of charge flowing
through a circuit in a set amount of time then
the formula for current would be
q
I
t
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Example:
A battery can deliver 14.7 coulombs of charge in
30 s. Find the current.
q 14.7C
I

 0.49 A
t
30s
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Voltage can be thought of as, “The driving
force which causes the current to flow”
Current flows from high electric potential
to low electric potential.
Just like a ball will roll down a hill in an
attempt to shed off its gravitational potential
energy, an electron will flow towards a proton
to shed off its electrical potential energy.
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Electric Potential - work needed or energy
acquired when a positive charge is moved from
infinity to a certain point in an electric field.
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Potential Difference – The change in the
potential energy of a charge as it is moved
between 2 points within an electric field.
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The difference in this electric potential energy
is what is known as voltage
Current does not flow unless there’s a potential
difference (voltage) present.
1 Volt is when a battery cell provides 1 Joule of
electrical potential energy to each coulomb of
charge as it moves through the cell.
1Joule
1Volt 
1Coulomb
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In a battery the potential difference (voltage)
between the positive and negative terminals sets
up an electric field.
This electric field
causes electrons to
accelerate in the
direction of the
positive terminal thus
causing current to
flow.
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As a charge moves through an electric field it
gains either Potential Energy or Kinetic Energy
If "Opposites" move closer they gain Kinetic
Energy They speed up
If 'Opposites' separate they gain Potential Energy
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If "Likes" separate they gain Kinetic Energy
If "Likes" move closer they gain Potential Energy
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If an amount of Energy is supplied to charge as
it moves between 2 points there is an electrical
potential difference between the points.
E
V
q
V = Potential Difference (V)
E = Energy (Joules)
q = Charge (Coulombs)
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Example:
If a battery cell provides 3.0 J of electrical
potential energy in moving 2 coulombs of
charge through the cell, what is the potential
difference (or voltage) of the cell?
3.0  Joules 
E
V 
 1.5 Volts 
q 2.0  Coulombs 
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Example:
It takes 6 Joules of work to move 2 coulombs of
charge between 2 points in an electric field. What
is the potential energy difference (voltage)
between these 2 points?
W  E
6  Joules 
E W
V


 3 Volts 
q
q 2  Coulombs 
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Resistance
As electrons move through a material they
experience collisions with other atoms.
These collisions cause energy to be given off
in the form of heat and can be thought of as a
type of resistance to the movement of the
electrons.
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Electrical resistance is when a material
resists or does not allow current to flow
through the material.
Electrical resistance is measured in ohms ()
Electrical resistance is the
“friction”
of the electrical world*
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Resistance Depends on 4 Factors:
1. Temperature
Increasing the temperature causes an increase in
the collision of particles, which then in turn
increases the resistance.
2. Length of Material
Increasing the length of the material causes an
increase in the number of collisions, which in
turn increases the resistance .
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3. Cross Sectional Area of Wire
Increasing the space, decreases the number of
collisions, which then decreases the resistance.
4.
Nature of the Material
Metals differ in both the number of the free
electrons and the space available for electron
movement.
The number that indicates the resistance of
materials per meter in length of 1m2 crosssectional area, at a given temperature, is called
resistivity of the material.
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Insulators have very high values of resistivity
whereas good conductors have very low
resistivities.
Resistors are materials with a mid-range
resistivity.
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Resistance
http://www.youtube.com/watch?v=WzUTOeoxREM&feature=related
http://www.stmary.ws/highschool/physics/home/notes/electricity/intro/wiresResistanceCurrent.swf
Ohms Law
The potential difference across a load
equals the product of the current through the
load and the resistance of the load.
V = IR
R = Resistance (Ohms )
V = Potential Difference (Volts V)
I = Current (Amperes A)
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Example:
A 120 Volt potential difference applied to the
terminals of a light bulb produces a 2.2 A
current through the bulb. Calculate the
resistance of the light bulb.
V 120
R 
 55
I 2.2
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Electrical Power:
The rate at which electric energy is
converted to other forms of energy is called
the power output in the circuit.
E
P
t
P = Power in J/s (Watts)
E = Energy in J
t = time in seconds
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We can use unit analysis to put the equation for
power in to a different form as well.
Since Volts are J/C and Amps are C/s.
J C J
 
C s s
Therefore: (P=VI) must also be true.
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Example:
Find the power consumed by the light bulb in
the following circuit.
P = VI = 1.5  0.1 = 0.15 W
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Electricity
http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/ResistanceWire.htm
Homework
Pg. 692 #’s 1, 2
Pg. 696 #’s 4, 6
Pg. 714 #’s 21, 23
Pg. 737 #’s 40, 41, 42
RRHS Handout
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