Transcript Electric Current and Circuits

Electric Current and Circuits
Potential Energy
• Movement of a positive test charge within an electric field is accompanied
by changes in Potential Energy:
 Moving the charge AGAINST the direction of an electric field is like moving
a mass upward within Earth’s gravitational field. (requires work by an outside force)
 Would increase the PE of the object
 Moving the charge in the SAME direction of an electric field is like a mass falling
downward within Earth’s gravitational field. (occurs without an outside force)
 Would decrease the PE of the object
Electric Potential
• Electric Potential = the amount of Potential
energy per charge
Electric Potential = Potential Energy
charge
• Electric current is the continuous flow of
electrical charge
• Current flows when there is potential difference
(voltage) between two points
• To create this difference, work must be done on
the charge by a charge pump
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Charge Pumps (sources of pumping electrons):
1. Photocell - converts light energy into electricity
Examples: solar calculator, solar cell
2. Battery - converts chemical energy into
electrical energy
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3. Generator – converts mechanical energy into
electrical energy
Examples: steam turbine, nuclear electric plant
Batteries
• The first battery was created by physicist
Alessandro Volto by stacking alternating
layers of zinc, brine-soaked cloth, and
silver.
• It was the first device to have a lasting
current (flow of charge)
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Anatomy of a battery
• Each battery has 2 terminals – positive
and negative
• A cathode connects to the positive
terminal
• An anode connects to the negative
terminal
Collectively, these are called electrodes and
are where chemical reactions take place
Electric Circuit:
• A complete conducting path through which
electrons can flow
• For a current to flow it must include a
charge pump and be closed
This is an open circuit. No
current would flow while open
A load is a device using electrical energy. In a load, work is
done by the electrons, and the PE of the electrons decreases.
Since it opposes the flow of current, a load is a type of resistor
Open Switch
Conductor
+
-
Battery
Closed Switch
Load
A light is an
example of a
load
Fuse
Basic
Circuit
Resistor
• Electric energy (from charge pumps) can
be converted into:
• Heat
• Light
• Sound
Activity 1
• Draw a simple circuit diagram with a
battery (don’t forget the electrode signs), a
load, and a closed switch.
Load
Battery
Closed Switch
Fact or fiction?
• When you turn on a light switch, electrons
move rapidly from the socket to the lamp
to the bulb.
What electrons really do
• Electrons move slowly bouncing around in
many directions as they move in one
direction overall.
Overall Direction
Electron Drift
• Heat is always a side effect of electric
current (because of electron drift).
Overall Direction
Electron Drift
• A potential difference (V) between two
points creates an electric field
• An electric field moves through a circuit at
the speed of light, setting electrons in
motion
The electrical field caused by the voltage makes
all electrons start to move simultaneously
Voltage (V) – energy per unit charge
• Unit:
Volt (V)
• 1V = 1J/C
Current (I) – rate of flow of charge
• Unit:
Amp (A)
• 1A = 1C/s
Resistance (R) – opposition to current
• Unit:
Ohm (Ω)
Factors affecting resistance of a wire
1. Thickness: the thicker the wire the lower the
resistance
2. Length: The shorter the wire the lower the
resistance
3. Type of metal: gold, silver, and copper have
the lowest resistance
4. Temperature: The lower the temperature, the
lower the resistance
Physics Challenge: When do most
bulbs blow and why?
• When first turned on (cooler).
Less resistance = more
current.
• A large surge of current can
cause a weak filament to
break.
In a circuit, current is directly proportional to
voltage (I α V) and inversely proportional
to resistance (I α 1/R) .
Ohm’s Law: V = I R
Problem Set 1
1. What is the resistance of an electric frying pan
that draws 12 amps of current when connected
to a 110v circuit?
2. How much current is drawn by a 23Ω lamp
when a voltage of 12v is applied?
3. What is the voltage of a battery if it produces a
current of 0.75 amps in a 12Ω resistor?
Problem Set 1
1. What is the resistance of an electric frying
pan that draws 12 amps of current when
connected to a 110v circuit?
Problem Set 1
2. How much current is drawn by a 23Ω
lamp when a voltage of 12v is applied?
Problem Set 1
3. What is the voltage of a battery if it
produces a current of 0.75 amps in a 12Ω
resistor?
Effects of current on the body
• 0.001a (1 ma) – barely felt
• 0.005a (5 ma) – painful
• 0.010a (10 ma) – muscles contract
• 0.015a (15 ma) – loss of muscle
control
• 0.100a (100 ma) – can be fatal if the
current goes through the heart
Safety Notes
• Electricians often put their hand in their
pocket to avoid current going through
heart (just in case)
• Electricians touch a wire they think is not
live with the back of their hand so they are
not stuck clinching the wire if they were
mistakenly wrong
Problem set 2
For each problem, calculate the current running through the
body and describe its effect:
1. Your hands are wet (R= 2400Ω) and you touch the
terminals of a 12v battery.
2. R = 7000Ω and a paper clip is inserted into an 110v
electrical socket.
3. A 9.0v portable CD player falls into a hot tub where your
R = 100Ω
Problem Set 2
1. Your hands are wet (R= 2400Ω) and you
touch the terminals of a 12v battery.
Problem Set 2
2. R = 7000Ω and a paper clip is inserted
into an 110v electrical socket.
Problem Set 2
3. A 9.0v portable CD player falls into a hot
tub where your R = 100Ω
Problem Set 3
If your pet Peety the parakeet perches on an electric wire.
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1. Why isn’t the bird electrocuted?
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2. If you fell from a tree onto the same wire, would you
be electrocuted?
•
3. Should you reach up and touch the bird to rescue
him?
Problem Set 3
If your pet Peety the parakeet perches on an electric wire.
1. Why isn’t the bird electrocuted?
No potential difference between feet = no current
2. If you fell from a tree onto the same wire, would you be electrocuted?
No potential difference between hands = no current
3. Should you reach up and touch the bird to rescue him?
No, you would be the path from the bird to the ground and both of
you would be toast. There would be potential difference in this
case.
Electric Power• Rate of converting electric energy into other energy forms
• P=W
t
power = work
time
• The unit for power is the watt 1w = 1J/s
• Another power equation
P=IV
Problem Set 4
1. An appliance draws 13 A when
connected to a 110 v circuit.
a) What is the power of the appliance
b) What is its resistance
2. What is the power of a light bulb that has
a resistance of 190 ohms in a 120 v
circuit?
Problem Set 4
1. An appliance draws 13a when connected
to a 110 v circuit.
a) what is the power of the appliance
b) What is its resistance
Problem Set 4
2. What is the power of a light bulb that has
a resistance of 190 ohms in a 120 v
circuit?
“Power” companies sell us energy. The unit they
use for energy is the kilowatt hour.
Physics challenge:
What is the smallest denomination coin you could use to pay
for the energy used by a 60w bulb burning for 8 hours? (at
a cost 16.0412 cents per kilowatt hour)
“Power” companies sell us energy. The unit they use
for energy is the kilowatt hour.
Physics challenge:
What is the smallest denomination coin you could use to pay
for the energy used by a 60w bulb burning for 8 hours? (at
a cost 16.0412 cents per kilowatt hour)
The multimeter
• Used to measure current,
voltage, resistance, etc.
• The black lead is plugged into the
common port
• The red lead is switched
depending on what you are trying
to record
Measuring Voltage
–
Set up the multimeter to detect voltage.
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The black cord should be connected in
the middle slot of the multimeter (this will
stay here for all readings)
The red cord will be placed in the slot to
the right of the black that has V or Voltage
as one of its symbols
Turn the knob to read Voltage for a direct
current. Look for V and the symbol for
direct current (The symbol for direct
current is like an equal sign with the
bottom line dashed)
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The voltmeter should be placed outside
the circuit
Measuring Voltage
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•
You are measuring the
potential difference from one
side of the resistor to the other
The volt meter should not be
part of the circuit
Touch each probe to opposite
sides of the resistor
Set up the multimeter to detect current
•
•
Set up the multimeter to
detect current.
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The black cord should be
connected in the middle slot of
the multimeter (this will stay
here for all readings)
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The red cord will be placed in
the slot to the left of the black
that has A for a reading of
Amperes.
Turn the knob to read
Amperes for a direct current.
Look for A and the symbol
for direct current.
To read current
•
The current must flow through the
multimeter and it should be part of
the circuit
•
Make it part of the circuit so that
electrons must flow through it.
•
Open the circuit and attach one
probe to each open end
•
This will once again make it a
closed circuit with the multimeter
acting as an ammeter in series.
Direct Current (dc)
• Electrons move in one direction
– Electrons follow a gradient to where there are
less electrons (but we don’t draw it this way)
Alternating Current (ac)
• Electrons change direction
• In USA, direction changes 60 times per
second
• Conventional way to draw current
– Show current moving from the negative to
positive
Load
Battery
Closed Switch
Series circuits
• Series circuit:
• Resistors are connected in a single path
• Current must go through every resistor in order
Circuit diagram:
In a series circuit:
• Current – stays the same in all parts of the circuit
Equation: IT = I1 = I2 = I3 = …
• Voltage – drops across resistors but adds up to total
voltage provided by the battery. (energy is conserved)
Equation: VT = V1 + V2 + V3 + …
• Resistance – Total resistance is found by adding all the
resistors
Equation: RT = R1 + R2 + R3 + …
Current Explanation
• Think of three fuel tankers going to fuel
stations along a single path. There is only
one way to travel so they must all travel
T stands for total
past each gas station (resistor) 1 for first
2 for second
I1 = 3A
3 for third
1
I2 = 3A
IT =3A
Equation: IT = I1 = I2 = I3 = …
T
I3 = 3A
2
3
IT, total current that you start with must go to every resistor
Voltage Explanation
• Voltage: every time a new station is visited,
some energy is lost. Hence voltage drops.
• All the voltage is lost at the end, so total voltage
is the sum of its parts
• A fuel pump is needed for a recharge
1
VT
V1
Equation: VT = V1 + V2 + V3 + …
2
V3
3
V2
Resistance Explanation
• Resistance: The more resistors the more
total resistance since all the current must
pass all the resistors.
R1
1
RT
Equation: RT = R1 + R2 + R3 + …
2
3
R3
R2
• Ohms law (V=IR) can be used to calculate
an unknown
V1=I1R1
1
V2=I2R2
VT=ITRT
2
3
V3=I3R3
R2
• Use the facts about circuits to discover
givens in other parts of the circuit so you
can solve for variables there.
V1=I1R1
1
VT=ITRT
Equation: IT = I1 = I2 = I3 = …
Equation: VT = V1 + V2 + V3 + …
Equation: RT = R1 + R2 + R3 + …
3
V3=I3R3
V2=I2R2
2
R2
When you start a circuit problem:
Include all of the missing parts to ohms law
V1 =
I1 =
V2 =
I2 =
VT =
IT =
RT =
V3 =
I3 =
Example Problem
If you find current anywhere you
know it’s the same everywhere.
You don’t have enough
information off the bat but can
figure out RT and use that to
solve for current
Example Problem
If you find current anywhere you
know it’s the same everywhere.
You don’t have enough
information off the bat but can
figure out RT and use that to
solve for current
Example Problem
Next use VT and RT to solve for IT
which is the same throughout the
circuit
Example Problem
Now we can solve individually for
everything
Example Problem
Now we can solve individually for
everything
Example Problem
Example Problem
Example Problem
Example Problem
Example Problem
Example Problem
Parallel Circuits
• Resisters are connected in separate
branches
• There is more than one path for current
Parallel Circuits
• Resisters are connected in separate
branches
• There is more than one path for current
Parallel Circuits
• Resisters are connected in separate
branches
• There is more than one path for current
Parallel Circuit Rules:
Why resistors in parallel have less
resistance than the least one.
• Think of resistors as a speed bump that
slows down traffic.
Why resistors in parallel have less
resistance than the least one.
• Think of resistors as a speed bump that
slows down traffic.
• 1 resistor slows down the flow or current
Why resistors in parallel have less
resistance than the least one.
• Think of resistors as a speed bump that
slows down traffic.
• 2 resistors in series slows down the flow
even more since all cars must go over 2
speed bumps.
Why resistors in parallel have less
resistance than the least one.
• Adding another resistor in parallel also
adds another lane. The additional lane
speeds up traffic and has a greater effect
than the resistor added.
Problem Set #5
Problem Set #5
• Find the resistance of the following resistors in
parallel
1. 12 ohms, 12 ohms, 12 ohms
2. 36 ohms, 12 ohms, 9 ohms, 36 ohms
3. 60 ohms, 60 ohms, 30 ohms
Problem Set #5
• Find the resistance of the following resistors in
parallel
1. 12 ohms, 12 ohms, 12 ohms
2. 36 ohms, 12 ohms, 9 ohms, 36 ohms
3. 60 ohms, 60 ohms, 30 ohms
Problem Set #6
24V
24V
12V
R2
12
6
12V
R2
12
6
12V
R2
12
6
12V
R2
12
6
3a
30V
30
3a
30V
30
3a
30V
30
3a
30V
30
Complex Circuits
• Are a mixture of parallel and series circuits
• Suggestion: Work your way to one big series
circuit where all the resistors are in series.
• If the current has to go through the resistor and
has no other option, it is in series.
Problem Set #7
Determine which of the following are in
series, parallel, or both.
A. Series
B. Parallel
C. Parallel
D. Series
E. Series
F. Series
G. Series w/ H & parallel
H. Series w/ G & parallel
I. Parallel
B-CEq
• Start by finding the resistors connected in
parallel
• B and C are in parallel
• The equivalent of B and C act like it is in parallel
with A and D
• Suggestion: Work your way to one big series
circuit where all the resistors are in series.
• If the current has to go through the resistor and
has no other option in order to get back to the
power source, it is in series.
Try this one
• G and H are in series
• The combo of series G-H and I are in parallel
• That whole combo of G-H series in parallel with I
act as a single resistor in series with F and E
Try this one
Complex Circuit (Example G)
• Try This
Complex circuit
Complex circuit
Complex circuit
Complex circuit
Like a 2 ohm
Resistor in series
Problem Set #8 (Find the RT for each)
Solving complex circuit problems
Example H:
• Start by seeing if you are given all the
values of the resistors
• If you are calculate the total resistance
Solving complex circuit problems
Example H:
Solving complex circuit problems
Example H:
• Now we have 2 of three totals and can do
ohms law
Solving complex circuit problems
Example H:
• I1 is in series and therefore all the current
from the battery flows here.
Solving complex circuit problems
Example H:
• V1 and V2 both have the same voltage since they
are in series.
• 30 V are left over since RT = 40V and I1 = 10V
Solving complex circuit problems
Example H:
Problem Set #9 (1)
30V
24
2
12
Problem Set #9 (1)
30V
24
2
12
Problem Set #9 (1)
30V
24
2
12
Problem Set #9 (1)
30V
24
2
12
Problem Set #9 (1)
30V
24
2
12
Problem Set #9 (1)
30V
24
2
12
Problem Set #9 (2)
12v
12
6a
12
12v
12
6a
12
12v
12
6a
12
12v
12
6a
12
Problem Set #9 (3)
30V
6
12
24V
30V
6
12
24V
Honors Book Fun!!
I can do this with Physics!!
#39
• Three resisters, 25, 45, and 75 ohms are
connected in series and a 0.51-A current passes
through them. What is (a) the total resistance
(b) the potential difference across the resistors?
#39
• Three resisters, 25, 45, and 75 ohms are
connected in series and a 0.51-A current passes
through them. What is (a) the total resistance
(b) the potential difference across the resistors?
#43
#43
#54
#54
#59
#59
#60
#60
#61
#61
Notes on short circuits
• Can be caused by an overload
• Usually occurs when wires become fray
and touch one another
• The current bypasses the devices plugged
in and there is a large decrease in
resistance.
• The current increases as a result causing
the wire to heat up
Two main short circuit dangers
1. Electric shock
2. Fire – the byproduct of current is heat
Safety devices to stop a short
circuit
• Fuses- a short thin piece of metal wired
into a circuit that will melt at a certain
temperature caused by a certain
current. (must be replaced when they
blow)
• Circuit breakers- composed of a
bimetallic strip. One metal expands
faster than the other causing it to curl.
The curling causes a break in contact
that opens the circuit. The circuit would
have to be reset to be closed again.
How a circuit breaker works
Problem set #6
1. An ammeter is connected in __________
because current ___________________
2. An ammeter has ________ resistance so
that it will not greatly alter the current in
the circuit.
Problem set #6
1. An ammeter is connected in __________
because current ___________________
2. An ammeter has ________ resistance so
that it will not greatly alter the current in
the circuit.