Unit 5A
Electric Circuits
Light the Bulb!
 What needs to happen to get the bulb to
light???
Two Requirements for electricity to “flow”
1. A potential difference - Voltage
2. A closed loop - Circuit
Potential Difference
AKA: VOLTAGE
Remember when we studied Electric Fields,
and compared to Gravitational Fields
Using a mass:
Using a + test charge:
Potential in Circuits
• Potential lines point away from
positive terminal of battery
• Going with the lines:
Decreasing in potential
• Going against the lines:
Increasing in potential
Potential Difference
Falling objects have more potential energy
when they are “HIGHER” above the ground
Water stored high in a tower will want to
flow downhill
The direction of movement of the water
naturally follows the gravitational field lines
Work must be done to lift it
Potential Difference
Charge also naturally “flows” in the
direction of the field lines
Like the water pressure, this electrical
“pressure” is due to the potential difference
So how do we “pump” the charge?
Galvani vs. Volta
Luigi Galvani
(1791)
Alessandro Volta
(1800)
Biology vs. Chemistry
Galvani’s frog dissection
Using two different metals, touching one to
spinal cord other to muscle:
Frog leg jerked!
Animal Electricity
Biology vs. Chemistry
Volta found, however that
animals weren’t necessary…
It was the two different metals
that were important
He made a pile of copper and
zinc plates separated by thin
paper soaked in an electrolyte
Voltaic Pile = Battery
Voltage
The chemical reaction Volta created
allowed electrons to move from one metal
plate to the next
Because the unit of Potential Difference is
named after Volta (the Volt), it is often
called VOLTAGE
To measure: Use a Voltmeter
2 probes (1 for reference point)
Why is a bird on a wire safe?
Circuit
The 2nd requirement for charges to flow:
There must be a closed loop from + to –
In order for there to be a potential
difference, the charges have to have the
electric field set up with a high potential
(+ anode) and low potential (- cathode)
Circuit
The 2nd requirement for charges to flow:
There must be a closed loop from + to –
In order for there to be a potential
difference, the charges have to have the
electric field set up with a high potential
(+ anode) and low potential (- cathode)
Circuit
Which charges are moving?
Positive or negative?
And how do they move?
Fast or slow?
Instantaneously, or is there a delay?
Is a battery a source of charges that move all
the way through the wires to the other side?
Do the charges get “used up” when the
batteries die?
Electron Drift Velocity
When the switch is open:
Free electrons (conducting electrons) are always
moving randomly, colliding with atoms.
The random speeds are at an order of 106 m/s.
BUT… there is no net movement of charge
across a cross section of a wire.
Electron Drift Velocity
But when the switch is closed:
Electrons start to “drift” gradually in the
direction of the electric field
Average Velocity = about 0.01 cm/s
So how do lights turn on instantly when you
flip a switch?
Electron Drift Velocity
An electric field is established almost
instantly (at the speed of light, 3x108 m/s).
Free electrons, while still randomly moving,
immediately begin drifting due to the
electric field, resulting in a net flow of
charge.
Electron Drift Velocity
When the electric field is established, all
the charges in the circuit move
Charges don’t come out of the battery and
go all they way around the circuit…
Charges in the wires are set in motion
They don’t get “used up”
Conservation of charge!
What does get “used up” when batteries
die? Does it really get used up? What’s a
better way to explain this?
Conventional Current
Even though we know the smaller electrons
are the ones doing the moving…..
“By convention” we say that current flows
from the + terminal of the battery to the –
The flow of + charge is called current (I) and
is measured in Amperes (A) using an
Ammeter
1 Amp = 1 C/s
Q
I = ---t
AC/DC
Alternating current vs. Direct current
Batteries provide Direct current
DC – Charges travel in ONE direction
Generators (electrical plants) provide AC
Charges oscillate back and forth
Compare to a longitudinal wave
In US – AC frequency = 60 Hz, in Europe =50 Hz
Household circuits (plugs in walls)
Battery-operated equipment needs a
transformer to plug into the wall
Factors Affecting Current
Potential Difference
Electrical Resistance
Resistance
Resistance is a measure
of opposition to the flow
of charge…
Causing the electrical
energy to be converted to
thermal energy or light.
Studied by Georg Ohm
Unit = Ohm ()
Measured with an
Ohmmeter
Resistance
Electrical current acts like water currents:
Factors Affecting Resistance
Resistivity
Conductor vs. Insulator
Temperature – Metals: Resist more when HOT,
can be superconductors when very cool;
Semiconductors: Opposite
Length of wire
Long – more collisions, Short – fewer collisions
Cross-sectional Area
Wide vs. Thin
Number of Paths
Resistivity of Materials
Ohm’s Law
Voltage is directly related to the current
Resistance is inversely related to the
current
V = I R
Most resistors follow Ohm’s Law under
normal circumstances
Some resistors are “non-Ohmic” due to
conductivity or temperature
Ohm’s Law
As current flows through a circuit, the
energy is dissipated in the resistors and the
potential drops
Ohm’s Law
A hair dryer operates on 110 V and
draws 1100 mA. What is the resistance
of the hair dryer?
V = I R
Don’t forget to convert milliAmps!
110 V = (1.1 A) R
R = 100 Ω
Schematic Diagrams
Circuits typically contain a voltage source,
wire conductors, and one or more devices
which use the electrical energy.
Schematic Diagrams
How would you draw the following circuit?
Series Circuits
Resistors are said to be connected in Series
when there is only ONE pathway for
charges.
If the circuit breaks, current will stop and all
devices will go off
Current in Series Circuits
Because there is only ONE path, all the
current coming out of the battery goes
through each resistor
I1 = I2 = I3 … Same in all!
Each resistor causes the voltage to drop,
the amount of current depends on the
Equivalent Resistance of the whole circuit
Ibattery= I1 = I2 = I3 = Vbattery/ Req
Equivalent Resistance in Series
The amount of resistance that a single
resistor would need in order to equal the
overall affect of the collection of resistors
that are present in the circuit.
For a series circuit: Req = R1+ R2 + R3 ….
Voltage Drop in Series
Total Voltage drop around a circuit (+ to –
terminals of battery) = Voltage of battery
Sum of voltage drops across each resistor must
add up to the Voltage of the battery
ΔVbattery = ΔV1+ ΔV2 + ΔV3 ….
ΔV1 = I • R1
ΔV2 = I • R2
ΔV3 = I • R3
Series Circuit Problem
 Calculate the following quantities:
•
•
•
•
•
Req = R1 + R2 + R3 = 17Ω + 12Ω + 11Ω = 40Ω
Itot = Vbattery / Req = (60V) / (40Ω) = 1.5 A
V1 = I1R1
V2 = I2R2
V3 = I3R3
V1 =(1.5A)(17Ω) V2 =(1.5A)(12Ω)
V3 =(1.5A)(11Ω)
V1 = 25.5 V
V2 = 18 V
V3 = 16.5 V
Parallel Circuits
Have multiple pathways for the current to
flow.
If the circuit is broken the current may pass
through other pathways and other devices
will continue to work.
Voltage Drops in Parallel Circuits
Each resistor has a direct path, connected
to both terminals of battery
Vbattery = V1 = V2 = V3 = ... Same in all!
So the amount of current passing through
each can be calculated using Ohm’s Law
I1 = V1 / R1
I2 = V2 / R2
I3 = V3 / R3
Eq. Resistance in Parallel
Multiple pathways = Less Resistance
Eq. Resistance in Parallel
Multiple pathways = Less Resistance
1/Req = 1/R1 + 1/R2 + 1/R3 + ...
Current in Parallel Circuits
Multiple pathways = More current can flow
Itot = I1 + I2 + I3 … = ΔV / Req
Current in Parallel Circuits
Multiple pathways = More current can flow
Itot = I1 + I2 + I3 … = ΔV / Req
More current will take the path of least
resistance
So the amount of current passing through
each can be calculated using Ohm’s Law
I1 = V1 / R1 I2 = V2 / R2
I3 = V3 / R3
Parallel Circuit Problem
 Calculate the following quantities:
•
•
•
•
•
1/Req = 1/R1 + 1/R2 + 1/R3 = 4.29 Ω (rounded)
Itot = Vbattery / Req = (60V) / (4.29 Ω) = 14 A (rounded)
I1= V1/R1
I2 = V2/R2
I3 = V3/R3
I1 =(60V)/(17Ω) I2 =(60V)/(12Ω)
I3 =(60V)/(11Ω)
I1 = 3.53 A
I2 = 5 A
I3 = 5.45 A
Series vs. Parallel Circuits
SERIES
PARALLEL
SAME IN ALL: Current SAME IN ALL: Voltage
Add more, Resistance Add more resistors,
goes up, so Current
Resistance goes down
goes down (bulbs
so Current goes up
dim)
(brighter)
Req = R1+ R2 + R3
1/Req= 1/R1+ 1/R2 + 1/R3
Itot = ΔV / Req
Itot = ΔV / Req
ΔV1 = IR1, ΔV2 = IR2
I1 = V1 /R1, I2 = V2/R2
ΔVtot = ΔV1+ ΔV2 + ΔV3 Itot = I1 + I2 + I3 …
Combination Circuits
When circuits contain Series and Parallel
segments
FIRST: Find Req for Parallel
THEN: Find Req for all in Series
Household Circuits
Which circuit type would work best in your
home?
Measuring Voltage
VOLTMETER
To find a potential
difference, you need 2
probes: black is reference
point ( -#, switch!)
Connect in PARALLEL
Voltmeters must have
HIGH Resistance so as not
to affect the current
Measuring Current
AMMETER
To measure current, you
want all current to flow
through but not be
reduced
Connect in SERIES
Ammeters must have
VERY LOW Resistance so
as not to affect the
current
Measuring Resistance
OHMMETER
To measure resistance,
you want the meter to
send current through at a
calibrated voltage
Connect in PARALLEL
Make sure to check the
scale and where the
probes are plugged in
every time you measure!
Short Circuit
A short circuit is a parallel path in a circuit with
zero or very low resistance.
Short circuits can be made accidentally by
connecting a wire between two other wires at
different voltages.
Short circuits are dangerous because they can
draw huge amounts of current.
Electrical Power
Remember: Power is the rate at which
Energy is transferred
P = E / t
P = (Electrial PE) /t
P = (qV) / t
Also remember…. q / t = I
P = I V
(Pigs In Venus!)
Unit: Watt (W) = J/s = AV = CV/s
Watt’s Law
P = I V is called Watt’s Law
Can be written other ways, depending on
what variables are known, along with
Ohm’s Law
P = I V . . . And I = V/R so…
P = (V/R)V = V2/R
OR… Using V = IR..
P = I (IR) = I2R
P = I V = V2/R = I2R
Electrical Power Problems
How much power is dissipated by a 100-Ω
resistor if 50 mA of current flow through it?
P = I V = V2/R = I2R
P = I2R
P = (0.050 A)2 (100 Ω) = 0.25 W
If the circuit above is left close for 10 s, how
many Joules of energy are used?
P = E / t
E = P t = (0.25 W) (10 s)= 2.5 J
Electrical Power
Electric companies charge for the number of
kilowatt-hours used during a set period of
time, often a month.
One kilowatt-hour (kWh) means that a
kilowatt of power has been used for one hour.
Since power multiplied by time is energy, a
kilowatt-hour is a unit of energy.
One kilowatt-hour is 3.6 x 106 J.
Typical Power Ratings
Appliance
Stove
Clothes Dryer
Heater
Dishwasher
Photocopier
Iron
Vacuum Cleaner
Coffee Maker
Refrigerator
Portable Sander
Fan
Personal Computer
TV Receiver
Fax Transmitter/Receiver
Charger for Electric Toothbrush
Power (W)
12,000
5,000
2,000
1,600
1,400
1,000
750
700
400
200
150
150
120
65
1
Electrical Power - Transmission
Electric companies need to send energy
long distances
Wires have resistance and dissipate energy
as heat – rate of Ediss = Power
Which has best results in reducing Power?
More or less Voltage?
More or less Current?
Answer: Reducing Current
Why? How?
Electrical Power - Transmission
P = I V can be rewritten: I = P / V
For a given amount of Power needed,
increasing the Voltage will decrease the
Current (and therefore the # of collisions)
P = I2R
So if V increases 10x, I decreases 10x, and
Power lost decreases 100x
Power lines typically transmit at very high
voltages to reduce heat losses
Human Body and Electricity
Resistance: Dry, clean skin – up to 500,000
Ω; Wet, broken, burned skin – 500-1000 Ω
Which is more lethal: High Voltage or
Current?
Answer: CURRENT - 100-200 mA AC
Most dangerous if through the heart
VERY high voltages and currents not as
lethal, heart/lungs clamp but don’t fibrillate
AC more lethal than DC
Human Body and Electricity
If even low currents happen for a prolonged
time, skin will burn
Burnt skin reduces resistance, allows more
current!
Summary
Electricity is
wonderful….
But must be treated
with respect!
Good luck finishing
the packet!