ELECTRICITY
INTRO
•Intro battery/wire/light bulb activity
•Use multi meter to look at battery voltage.
•5.1.1 Define electric potential difference
•5.1.2 Determine the change in potential energy
when a charge moves between two points at
different potentials.
5.1.1 Define electric potential difference
•When two charges interact there is an electric
force between them.
•Potential energy from this is: Electrical
Potential Energy
•Similar to gravitational force
•Type of mechanical energy
•Metotal = KE + PEg + PEspring + PEelectric
Where does the “potential” come from
•With potential gravity, an object can
“potentially” fall from a height.
•With electrical potential, a charge can
“potentially” fall down an electric field.
•A positive charge is placed in a uniform electric
field and is displaced.
•The new position will have a ∆PE
Where does the “potential” come from
•With potential gravity, an object can
“potentially” fall from a height.
•With electrical potential, a charge can
“potentially” fall down an electric field.
•A positive charge is placed in a uniform electric
field and is displaced.
•The new position will have a ∆PE
•Electric Potential Energy Difference between
two points in an electric field is equal to the
work done in moving a charge from one
position to a position of higher electrical
potential energy in an electric field.
•See Video
•Make Drawing
Equation
•∆PEelectric = -q Ed
•q = charge
•E = electric field strength
•d = displacement from reference point
•(-) = inidcates that the electrical potential
energy will INCREASE if the charge is
NEGATIVE
•Unit is a Joule(J)
Potential Difference
•A measure of the difference in the electrical
potential energy between two positions in
space.
•AKA “voltage”
•Describes a change in energy per unit charge
•Unit is volt (V)
•Most batteries are 1.5V
•Similar idea to a water slide
***Draw diagram***
Equation
•∆V = ∆PEelectric / q
•∆V = voltage
•∆PEelectric = electric potential energy
•q = charge
Equation
•Combine the PEelectric equations with the ∆V
and you get….
•∆V = -Ed
***show substitution***
New Equation:
∆V = -Ed
Last one
•∆V = kc q/r
•kc = Coulomb constant (8.99 x 109Nm2/C2)
•q = charge
•r = distance separating the two charges
5.1.4 Solve problems involving electric potential
difference.
•Pg 599, Practice A
1) 6.4 x 10-19
2) -750V
3) (charge of electron = 1.6x10-19)
2.3 x 10-16J
•Section Review pg 601
CAPACITANCE!!!
•5.1.12 - Relate capacitance to the storage of
electrical potential energy in the form of
separated charges.
•5.1.13 - Calculate the capacitance of various
devices.
•5.1.14 - Calculate the energy stored in a
capacitor.
Capacitor
•Device that stores electrical potential energy.
•Uses
• tuning the frequency of radios
• Eliminating sparking in automobile ignition systems
• Storing energy in electronic flash units
Capacitor
•If, energized or charged has energy
•Energized by connecting the plates to the
terminals of a battery.
•Charge transfer between the plates stops when
the potential difference between the plates is
equal to the pd of the battery.
Capacitance
•Is the ability of a conductor to store energy in
the form of electrically separated charges.
•Is the ratio of charge to potential difference
•So…
•Capacitance = magnitude of charge/potential
difference
•C = Q/∆V
•SI unit is farad (F)
Capacitance
•Depends on the size and shape of the capacitor.
•Many capacitors have parallel plates with no
material between them
•The equation shows that as the area of
increases the capacitance increases
•C = ε A/d
•ε = permittivity of a vacuum (8.85 x10 -12 C2/Nm2)
•A = area of one of the plates
•d = distance between the plates
Dielectric
•The space between isn’t always empty
•Dielectric – insulating material that fills the
space between two capacitor’s plates
•Ex – air, rubber, glass, waxed paper
•Capacitance increases with use of dielectric.
•Molecules in a dielectric can align with the field
of the pates
•Reduces the charge on the capacitor plates.
•Q= C∆V
•The space between isn’t always empty
•Dielectric – insulating material that fills the
space between two capacitor’s plates
•Ex – air, rubber, glass, waxed paper
•Capacitance increases with use of dielectric.
•Molecules in a dielectric can align with the
field of the pates
•Reduces the charge on the capacitor plates.
Discharge
•Is the opposite of charging
•After the two plates are charged they will stay
charged until…
•Connected with a conductive material.
•Charges move back from one plate to the
other until both are balanced and uncharged.
•This is because this is the lowest potential
energy.
Discharge
•Examples:
• Flash in camera
• Keyboards
•Size of capacitors video
Stored Energy
•A charged capacitor stores electrical potential
energy.
•If each plate is neutral, then it takes almost no
work to move charges in the circuit.
•Once the plates start to build up a charge, a
potential difference develops
Stored Energy
•This requires more and more work to be done to
move the charge through the pd.
•This builds the electric potential energy.
•The work done on these charges is the transfer
of energy.
Stored Energy
•This energy can be calculated by:
•Electric potential energy = ½ (charge on
plate)(final potential difference)
•PEelectric = ½ Q ∆V
•By substitution this can become
•PEelectric = Q2/2C
•C= capacitance
Example
•A capacitor, connected to a 12V battery, holds
36 μC of charge on each plate. What is the
capacitance of the capacitor? How much
electrical potential energy is stored in the
capacitor?
•Identify your variables. Identify your equations.
Watch your units.
•Answer: C = 3.0 x 10 -6 μF, PE = 2.2 x 10-4 J
Practice
•Pg 607 Practice B #1-4
1) a) 4.8 x 10-5 C, b) 4.5 x 10 -6 J
2) a) 4.8 x 10-6 F, b) 5.4 x 10 -6 J
3) a) 9.00V
b) 5.0 x 10-12 C
4) 1.13 x 108 m2, ???
5.1.5 - Define electric current.
CURRENT
• So far static electricity (charges in electric
fields)
• Static electricity isn’t very useful
• Moving electricity IS!!
CURRENT
• The movement of electric charge
• Could be positive or negative charges
• Used everywhere
• Lights, radios, TV, air conditioners, Cars, computers
• Bodies, discovered in mid 1700s.
• Luigi Galvani experimented next to a dissected
frog
• Current transmit messages between muscles and
brain
CURRENT
•
•
•
•
More specifically….
Imagine a cross section of wire
Electrons are now moving through that wire
Current is the rate these charges move
CURRENT
• Current is the ratio of the amount of charge
to the time interval
• ∆q is amount of charge
• ∆t is the time it takes these charges to pass a
point
CURRENT
• Current is the ratio of the amount of charge to
the time interval
• ∆q is amount of charge
• ∆t is the time it takes these charges to pass a
point
• I = ∆q/∆t
• I = current
• Unit is Ampere, A
CURRENT
***Special note***
• Current is the OPPOSITE direction of the
movement of the negative charges.
Example
• The current in a light bulb is 0.835A. How long
does it take for a total charge of 1.67C to pass
through the filament of the bulb?
• Identify what you know. Identify what you
want to know. Identify your correct equation.
• Answer: 2.00s
Practice
• Pg 609 Practice C #2-5
• Answers:
1) 400s
2) 1.00s
3) 600s
4) 20C
5) a) 2.6 x 10-3A,
b) 1.6 x 1017 electrons
c) 5.1 x 10-3A
Drift Velocity
• When you flip the switch how fast do the
lights come on? Instantaneously?
• Does this mean the electrons are moving at
the speed of light?
• Not quite. 
Drift Velocity
• When you turn on the light the electron
motion near the switch changes the electric
field there.
• That change in the electric field then
propagates throughout the wire at nearly the
speed of light.
• The charges move much more slowly.
Drift Velocity
• When these charges do move do they move in
a straight line?
• Not quite. Sorry. 
• Remember that electrons in a metallic bond
jump from nucleus to nucleus.
• These jumps are random and sporadic.
Drift Velocity
• What causes these crazy movements?
• As they move the individual atoms are vibrating.
(see Brownian Movement)
• In these collisions energy is transferred, which
causes more vibration and an increase in
temperature. (think friction)
Drift Velocity
• Electrons want to accelerate in this electrical
field but can’t because of these collisions.
• Eventually things will balance out.
• Drift velocity is the net velocity of these
individual movements.
Drift Velocity
• So how fast does an electron really move???
• In a wire with current 10A, the drift speed
would be 2.46 x 10-4m/s.
• Meaning it would take about 68 minutes for it
to travel 1 meter.
Resistance
5.1.6 – Define resistance
5.1.7 – Apply the equation for resistance
5.1.8 – State and apply Ohm’s Law
Resistance
•
•
•
•
What is a conductor?
What is an insulator?
What is a resistor?
Is there a difference between an insulator and
a resistor?
• Is there a difference between a conductor and
a resistor?
Resistance
• A resistor is a conductor.
• It’s just not a good conductor.
• Opposition to the motion of charge through a
conductor is RESISTANCE.
• For many materials… resistance is constant
over a wide rang of applied potential
differences.
Resistance
• Earlier we learned that electrons do not move
in straight-lines through conductors.
• This is because of collisions with the metal
atoms.
• Anything that affect the number of collisions
will also affect the materials resistance.
• Primarily 4 factors.
•
•
•
•
Length
Cross-sectional Area
Material
Temperature
Resistance
• Two are purely geometrical factors.
• Length – the longer the wire the more
collisions which means more resistance.
• Cross-Sectional Area – a thinner wire physically
restricts the path an electron can take,
increasing the resistance.
Resistance
• Material – different material have different
structures of atoms. Some are more
conductive than others.
• Examples….
• Temperature – Temperature is a measure of
how much an object’s atoms are vibrating.
More vibrations means it’s harder for atoms to
travel, which means more resistance.
Resistance
• The relationship between resistance, voltage
and current can be expressed by saying
resistance is the ratio of potential difference
to current.
• OR
R = ∆V / I
• SI unit for resistance is ohm, Ω
Ohm’s Law
• Ohm’s law states that voltage will cause a
current to flow through a resistor.
• Commonly stated as V = IR
• Watch Video:
• https://www.youtube.com/watch?v=zYS9kdS5
6l8
Ohm’s Law
• Ohm’s law states that voltage will cause a
current to flow through a resistor.
• Commonly stated as V = IR
• Watch Video:
• https://www.youtube.com/watch?v=zYS9kdS5
6l8
Electric current and resistance
•Three different types of conductors
• Ohmic
• Filament Lamp
• Diode
Electric current and resistance
•Resistor is “ohmic” if the current flowing is
proportional to the voltage across its ends.
• Metals are ohmic if the temperature is constant.
• Gives a straight line on I-V graph
• Ohmic conductors obey “Ohm’s Law”
Electric current and resistance
•The filament lamp gets hot as there is an creasing
voltage. This means the resistance increases with
higher voltages.
Electric current and resistance
•Diodes
• Lets very little ore no current flow until voltage reaches a
threshold.
• 0.6V is very common
• Once the threshold is met, there is essentially zero
risistance and a large amount of current can flow.
• Act as a switch, only allowing current to flow in one
direction.
• Can turn AC in to DC
Practice
•The resistance of a steam iron is 19.0 Ω. What
is the current in the iron when it is connected
across a potential difference of 120V?
•Identify what you know. Identify what you are
looking for. Identify your equations. Solve.
•Answer: 6.32Ω
Practice
•A 1.5V battery is connected to a small light
bulb with a resistance of 3.5Ω. What is the
current in the bulb?
•Identify what you know. Identify what you are
looking for. Identify your equations. Solve.
•Answer: 0.43A
Resistance
•Pg 615 Practice D # 3-6
•5.2.4
•Draw circuit diagrams. Students should be able
to recognize and use the accepted circuits
symbols.
•Ch 18, Sec 1, SR pg 645
•Remember when we made the light bulb light
up at the beginning of this unit.
•Draw a picture of what we did.
•Volunteers to put their drawing on the board.
Schematic Diagrams
•A diagram that depicts the construction of an
electrical apparatus
•Uses symbols to represent components of a
circuit.
•Anybody from around the world can read
schematics
•Some basic symbols are…
Electric Circuits
•A path through which charges can flow.
•A schematic diagram is also called circuit
diagram.
•All circuits must have a source of potential
difference (ex… ), resistance, also called a
“load” (ex… ), and some thing connecting
them (ex… )
•It must be a continous/complete loop for the
charges to flow and work.
Short circuits
•With out a load, or resistance, the circuit has
very little resistance to the movement of
electrons.
•Happens when un-insulated wires connect
accidentally.
•They heat up, could possible melt/spark/catch
fire.
•Section Review Pg 645 #1, 3-5
•5.2.1
•Apply the equations for resistors in series.
•Ch 18, Sec 2, Practice A, SR pg656
Current in series
•Break up into groups of two.
•Each group gets a battery/wire/bulb
•Build a circuit.
Current in series
•Break up into groups of two.
•Each group gets a battery/wire/bulb
•Build a circuit.
•Combine and make both bulbs turn on.
•What happened to the second bulb?
Current
•Charge is conserved…. Meaning that the
charge that enters one bulb is the same
amount of charge that exits in a given time
interval
Current
•Only one path…. Meaning that charge that
exits the first bulb is the same amount of
charge that enters the second bulb in a given
time interval
Current
•Current is the amount of charge per time….
Meaning the current in bulb 1 is the same
current in bulb 2.
•CONCLUSION: When many resistors are
connected in series, the current in each
resistor is the same!
Current
•CONCLUSION: When many resistors are
connected in series, the current in each
resistor is the same!
•OR…. Current is constant in a series circuit
Resistance
•The total current in a series circuit depends on
how many resistors there are.
•To find the total current you must find the
total resistance,
•Total resistance is called equivalent resistance
•Equivalent resistance of a series combination
of resistors is always greater than any
individual resistance.
•Req = R1 + R2 + R3….
Voltage in a series
•The potential difference across the battery,
∆V, must equal the potential difference across
the load, ∆V1 + ∆V2…,
•So ∆V = ∆V1 + ∆V2….
CONCLUSTION: The total voltage is equal to the
sum of all the individual voltages.
Voltage in a series
•With a little substitution you get….
•∆V = IReq
•∆V = ∆V1 + ∆V2 + ∆V3….
•∆V = IR1 + IR2 + IR3….
•and
•∆V1 = IR1
and ∆V2 = IR2 and ∆V3 = IR3
Conclusions for a SERIES CIRCUIT
•Current is constant
•∆Vtotal = ∆V1 + ∆V2 + ∆V3 …..
•Req = R1 + R2 + R3 …..
•∆V1 = IR1
and ∆V2 = IR2 and ∆V3 = IR3
PS
•What happens when you unscrew one of the
light bulbs?
•They all go out. Why?
•Note: all elements MUST be connected for a
series circuit to work.
Sample Problem A
•A 9.0V battery is connected to four light bulbs.
They have 2.0Ω, 4.0Ω, 5.0Ω and 7.0Ω
resistance respectively. Find the equivalent
resistance for the circuit and the current in the
circuit.
•Draw a schematic. Label it. Identify what you
need. Identify what you know. Solve.
•Answer: Req = 18.0Ω, I = 0.50A
Practice as class
•A 12.0V storage battery is connected to three
resistors, 6.75Ω, 15.3Ω, and 21.6Ω respectively.
The resistors are joined in series.
a) Calculate the equivalent resistance.
b) What is the current in the circuit?
•Draw a schematic. Label it. Identify what you
need. Identify what you know. Solve.
•Answer: Req = 43.65Ω, I = 0.27A
Practice
•Pg 650, Practice A # 3-6
•5.2.2
•Apply the equations for resistors in parallel.
•Ch 18, Sec 2, Practice B, SR pg 656
•Get back into those groups.
•Now make both bulbs turn on in a parallel
circuit. Meaning one can be unscrewed and
the other still stay on.
•What happens to the brightness of the bulbs
as one is connected/disconnected?
•Resistors in parallel have the same potential
differences across them.
•Meaning….
•∆Vtotal = ∆V1 = ∆V2 = ∆V3….
Current in parallel circuits
•When the charges flow through the wire the
reach one bulb first. Some move through the
first bulb. Others move through the second
bulb.
•If one bulb has less resistance, more charges
move through that bulb. This is because that
bulb offers less opposition to the flow of
charges.
Current in parallel circuits
•The sum of currents in parallel resistors is
equal to the total current.
•I = I1 + I2 + I3 ….
Based upon that and ∆V = IR…. I = ∆V/R
So …. (∆V/Req) = (∆V/R1) + (∆V/R2) + (∆V/R3) …
Resistors in parallel
•Since
∆Vtotal = ∆V1 = ∆V2 = ∆V3….
•And
(∆V/Req) = (∆V/R1) + (∆V/R2) + (∆V/R3)
•We can then simplify to….
(1/Req) = (1/R1) + (1/R2) + (1/R3)
The total resistance must always be less than the
smallest resistor!?!?
Conclusions for a PARRALLES CIRCUIT
•Voltage is constant
•I = I1 + I2 + I3 ….
•(1/Req) = (1/R1) + (1/R2) + (1/R3)
Sample Problem B
•A 9.0V battery is connected to four resistors in
a parallel fashion. They include 7.0Ω, 5.0Ω,
4.0Ω, and 2.0Ω. Find the equivalent resistance
for the circuit and the total current in the
circuit.
•Draw a schematic. Label it. Identify what you
need. Identify what you know. Solve.
•Answer: Req = 0.917Ω, I = 9.8A
Class Practice
•The potential difference across the equivalent
resistance in sample problem B equals the
potential difference across each of the
individual parallel resistors. Calculate the value
for the current in each resistor.
•Draw a schematic. Label it. Identify what you
need. Identify what you know. Solve.
•Answer: 1.3A, 1.8A, 2.25A, 4.5A
Practice
•Pg 655, Practice B # 2-4