Download Electric Current Notes PPT

Electrical Energy and Current
Preview
Section 1 Electric Potential
Section 2 Capacitance
Section 3 Current and Resistance
Section 4 Electric Power
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Section 1
Electrical Energy and Current
TEKS
The student is expected to:
6B investigate examples of kinetic and
potential energy and their transformations
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Section 1
Electrical Energy and Current
Section 1
What do you think?
• You may have purchased batteries for radios,
watches, CD players, and other electronic
devices. Batteries come in a variety of different
sizes and voltages. You probably have 1.5 volt,
3 volt, and 12 volt batteries in your home.
• What do volts measure?
• Is the number of volts related to the size of the
battery?
• How is a 3 volt battery different from a 1.5 volt
battery?
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Electrical Energy and Current
Section 1
Electrical Potential Energy
• A uniform electric field exerts a
force on a charged particle
moving it from A to B.
• Will the particle shown gain or
lose PEelectric as it moves to the
right?
– Lose energy (because it is moving
with the force, not against it)
– Similar to a falling object losing PEg
– PEelectric = Wdone = Fd = -qED
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Electrical Energy and Current
Section 1
Electrical Potential Energy
• PEelectric is positive if the charge is negative and moves
with the field.
• PEelectric is positive if the charge is positive and moves
against the field.
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Electrical Energy and Current
Section 1
Classroom Practice Problem
• A uniform electric field strength of 1.0 x 106 N/C
exists between a cloud at a height of 1.5 km and
the ground. A lightning bolt transfers 25 C of
charge to the ground. What is the change in
PEelectric for this lightning bolt?
• Answer: -3.75 x 1010 J of energy
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Electrical Energy and Current
Section 1
Gravitational Potential Difference
• Suppose a mass of 2.00 kg is moved from point A
straight up to point B a distance of 3.00 m. Find the
PEg for the mass if g = 9.81 m/s2. Repeat for a mass
of 5.00 kg.
– Answer: 58.9 J and 147 J
• What is the PEg per kg for each?
– Answer: 29.4 J/kg for both
• The change per kg does not depend on the mass. It
depends only on points A and B and the field strength.
• There is an analogous concept for electrical potential
energy, as shown on the next slide.
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Electrical Energy and Current
Section 1
Potential Difference
• Potential difference (V) is the change in electrical
potential energy per coulomb of charge between two
points.
– Depends on the electric field and on the initial and final positions
– Does not depend on the amount of charge
– SI unit: joules/coulomb (J/C) or Volts (V)
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Electrical Energy and Current
Section 1
Potential Difference
• The potential difference is
calculated between two points, A
and B.
– The field must be uniform.
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Electrical Energy and Current
Electrical Potential Energy
Click below to watch the Visual Concept.
Visual Concept
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Section 1
Electrical Energy and Current
Section 1
Potential Difference Near a Point Charge
• The above V determines the potential energy per
coulomb at a point compared to a very distant point
where V would equal zero.
• Potentials are scalars (+ or -) so the total potential at a
point is the sum of the potentials from each charge.
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Electrical Energy and Current
Section 1
Superposition Principle and Electric Potential
Click below to watch the Visual Concept.
Visual Concept
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Electrical Energy and Current
Section 1
Batteries
• A battery maintains a constant potential
difference between the terminals.
– 1.5 V (AAA, AA, C and D cell) or 9.0 V or 12 V (car)
• In 1.5 V batteries, the electrons use chemical
energy to move from the positive to the negative
terminal.
– They gain 1.5 joules of energy per coulomb of charge
• When connected to a flashlight, the electrons
move through the bulb and lose 1.5 joules of
energy per coulomb of charge.
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Electrical Energy and Current
Section 1
Now what do you think?
• You may have purchased batteries for radios,
watches, CD players, and other electronic
devices. Batteries come in a variety of different
sizes and voltages. You probably have 1.5 volt,
3 volt, and 12 volt batteries in your home.
– What do volts measure?
– Is the number of volts related to the size of the
battery?
– How is a 3 volt battery different from a 1.5 volt
battery?
© Houghton Mifflin Harcourt Publishing Company
Electrical Energy and Current
TEKS
The student is expected to:
6B investigate examples of kinetic and
potential energy and their transformations
© Houghton Mifflin Harcourt Publishing Company
Section 2
Electrical Energy and Current
Section 2
What do you think?
• The battery shown has a potential
difference of 6.0 volts. It has just
been connected to two metal
plates separated by an air gap.
There is no electrical connection
between the two plates and air is
a very poor conductor.
• If a light bulb replaced the two metal plates and the battery
was connected, electrons would flow out of the negative and
into the positive terminal. Will this also occur with the two
metal plates?
• If not, why not?
• If so, is the flow similar or different from that with the light bulb?
Explain.
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Electrical Energy and Current
Section 2
Capacitors
• The two metal plates are
electrically neutral before the
switch is closed. What will
happen when the switch is
closed if the left plate is
connected to the negative
terminal of the battery?
– Electrons will flow toward lower
PE.
• From the battery to the left plate
• From the right plate to the
battery
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Electrical Energy and Current
Section 2
Parallel Plate Capacitors
• Electrons build up on the left plate, giving it a net
negative charge. The right plate has a net positive
charge.
– Capacitors can store charge or electrical PE.
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Electrical Energy and Current
Capacitance
Click below to watch the Visual Concept.
Visual Concept
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Section 2
Electrical Energy and Current
Section 2
Capacitance
• Capacitance measures the ability to store charge.
• SI unit: coulombs/volt (C/V) or farads (F)
• In what way(s) is a capacitor like a battery?
• In what way(s) is it different?
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Electrical Energy and Current
Section 2
Capacitance
• How would capacitance change if the metal plates had
more surface area?
– Capacitance would increase.
• How would it change if they were closer together?
– Capacitance would increase.
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Electrical Energy and Current
Section 2
Capacitance
•  is a constant that is determined by the material
between the plates (0 refers to a vacuum).
• Combining the two equations for C yields:
Q 
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0 A
d
V
Electrical Energy and Current
Parallel-Plate Capacitor
Click below to watch the Visual Concept.
Visual Concept
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Section 2
Electrical Energy and Current
Dielectrics
• The space between the
plates is filled with a
dielectric.
– Rubber, waxed paper, air
• The dielectric increases
the capacitance.
– The induced charge on
the dielectric allows more
charge to build up on the
plates.
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Section 2
Electrical Energy and Current
Capacitor Applications
• Connecting the two plates of a
charged capacitor will discharge it.
– Flash attachments on cameras use a
charged capacitor to produce a rapid
flow of charge.
• Some computer keyboards use capacitors
under the keys to sense the pressure.
– Pushing down on the key changes the
capacitance, and circuits sense the
change.
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Section 2
Electrical Energy and Current
Section 2
Energy and Capacitors
• As the charge builds, it requires more and more work to
add electrons to the plate due to the electrical repulsion.
– The average work or PE stored in the capacitor is (1/2)QV.
– Derive equivalent equations for PEelectric by substituting:
Q = CV and
V = Q/C
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Electrical Energy and Current
Section 2
Classroom Practice Problem
• A 225 F is capacitor connected to a 6.00 V
battery and charged. How much charge is stored
on the capacitor? How much electrical potential
energy is stored on the capacitor?
– Answers: 1.35 x 10-3 C , 4.05 x 10-3 J
© Houghton Mifflin Harcourt Publishing Company
Electrical Energy and Current
Now what do you think?
• If a light bulb replaced the
two metal plates and the
battery was connected,
electrons would flow out of
the negative and into the
positive terminal. Will this
also occur with the two
metal plates?
• If not, why not?
• If so, is the flow similar or
different from that with the
light bulb? Explain.
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Section 2
Electrical Energy and Current
TEKS
Section 3
The student is expected to:
5E characterize materials as conductors or
insulators based on their electrical properties
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Electrical Energy and Current
What do you think?
• The term resistance is often used
when describing components of
electric circuits.
• What behavior of the components
does this term describe?
• Do conductors have resistance?
• If so, are all conductors the same?
Explain.
• What effect would increasing or
decreasing the resistance in a
circuit have on the circuit?
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Section 3
Electrical Energy and Current
Electric Current
• Electric current (I) is rate at which
charges flow through an area.
• SI unit: coulombs/second (C/s) or
amperes (A)
– 1 A = 6.25  1018 electrons/second
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Section 3
Electrical Energy and Current
Conventional Current
• Conventional current (I) is
defined as the flow of
positive charge.
– The flow of negative charge
as shown would be
equivalent to an equal
amount of positive charge in
the opposite direction.
• In conducting wires, I is
opposite the direction of
electron flow.
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Section 3
Electrical Energy and Current
Conventional Current
Click below to watch the Visual Concept.
Visual Concept
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Section 3
Electrical Energy and Current
Section 3
Velocity of Electrons Through Wires
• When you turn on a wall switch for a light,
electrons flow through the bulb. Which speed
below do you believe most closely approximates
that of the electrons?
–
–
–
–
The speed of light (300 000 000 m/s)
1 000 m/s
10 m/s
0.0001 m/s
• Why do you think so?
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Electrical Energy and Current
Drift Velocity
• Electrons undergo
collisions with atoms in the
metal.
– They “drift” through the wire.
– Drift velocity for a copper
wire with a current of 10 A is
0.000246 m/s.
• The E field moves through
the wire near the speed of
light, causing all electrons
in the wire to move nearly
instantly.
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Section 3
Electrical Energy and Current
Drift Velocity
Click below to watch the Visual Concept.
Visual Concept
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Section 3
Electrical Energy and Current
Section 3
Resistance to Current
• Resistance is opposition to the flow of charge.
– SI unit: volts/ampere (V/A) or ohms ()
• Ohm’s Law : V = IR
– Valid only for certain materials whose resistance is constant
over a wide range of potential differences
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Electrical Energy and Current
Section 3
Classroom Practice Problems
• A typical 100 W light bulb has a current of 0.83
A. How much charge flows through the bulb
filament in 1.0 h? How many electrons would
flow through in the same time period?
– Answers: 3.0  103 C, 1.9  1022 electrons
• This same 100 watt bulb is connected across a
120 V potential difference. Find the resistance of
the bulb.
– Answer: 1.4  102 
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Electrical Energy and Current
Section 3
Resistance of a Wire
• On the next slide, predict the change necessary
to increase the resistance of a piece of wire with
respect to:
–
–
–
–
Length of wire
Cross sectional area or thickness of the wire
Type of wire
Temperature of the wire
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Electrical Energy and Current
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Section 3
Electrical Energy and Current
Factors that Affect Resistance
Click below to watch the Visual Concept.
Visual Concept
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Section 3
Electrical Energy and Current
Section 3
Applications
• Resistors in a circuit can change the current.
– Variable resistors (potentiometers) are used in
dimmer switches and volume controls.
– Resistors on circuit boards control the current to
components.
• The human body’s resistance ranges from
500 000  (dry) to 100  (soaked with salt
water).
– Currents under 0.01 A cause tingling.
– Currents greater than 0.15 A disrupt the heart’s
electrical activity.
© Houghton Mifflin Harcourt Publishing Company
Electrical Energy and Current
Now what do you think?
• The term resistance is often used
when describing components of
electric circuits.
• What behavior of the components
does this term describe?
• Do conductors have resistance?
• If so, are all conductors the same?
Explain.
• What effect would increasing or
decreasing the resistance in a
circuit have on the circuit?
© Houghton Mifflin Harcourt Publishing Company
Section 3
Electrical Energy and Current
TEKS
The student is expected to:
6B investigate examples of kinetic and
potential energy and their transformations
© Houghton Mifflin Harcourt Publishing Company
Section 4
Electrical Energy and Current
Section 4
What do you think?
• Hair dryers, microwaves, stereos, and other
appliances use electric power when plugged into
your outlets.
• What is electric power?
• Is electric power the same as the power discussed in the
chapter “Work and Energy?”
• Do the utility companies bill your household for power,
current, potential difference, energy, or something
else?
• What do you think is meant by the terms alternating
current (AC) and direct current (DC)?
• Which do you have in your home?
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Electrical Energy and Current
Types of Current - Direct
• Batteries use chemical energy to give electrons
potential energy.
– Chemical energy is eventually depleted.
• Electrons always flow in one direction.
– Called direct current (DC)
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Section 4
Electrical Energy and Current
Types of Current - Alternating
• Generators change
mechanical energy into
electrical energy.
– Falling water or moving steam
• Electrons vibrate back and
forth.
– Terminals switch signs 60 times
per second (60 Hz).
– Called alternating current (AC)
– AC is better for transferring
electrical energy to your home.
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Section 4
Electrical Energy and Current
Section 4
Energy Transfer
• Is the electrical potential
energy gained, lost, or
unchanged as the
electrons flow through the
following portions of the
circuit shown:
–
–
–
–
A to B
B to C
C to D
D to A
• Explain your answers.
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Electrical Energy and Current
Energy Transfer
–
–
–
–
A to B (unchanged)
B to C (lost in bulb)
C to D (unchanged)
D to A (gained in battery)
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Section 4
Electrical Energy and Current
Electric Power
Click below to watch the Visual Concept.
Visual Concept
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Section 4
Electrical Energy and Current
Section 4
Electric Power
• Power is the rate of energy consumption (PE/t ). For electric
power, this is equivalent to the equation shown below.
– SI unit: joules/second (J/S) or watts (W)
– Current (I) is measured in amperes (C/s).
– Potential difference (V) is measured in volts (J/C).
• Substitute using Ohm’s law (V = IR) to write two other equations
for electric power.
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Electrical Energy and Current
Section 4
Classroom Practice Problems
• A toaster is connected across a 120 V kitchen
outlet. The power rating of the toaster is 925 W.
– What current flows through the toaster?
– What is the resistance of the toaster?
– How much energy is consumed in 75.0 s?
• Answers: 7.7 A, 16 , 6.94  104 J
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Electrical Energy and Current
Section 4
Household Energy Consumption
• Power companies charge for energy, not power.
– Energy consumption is measured in kilowatt•hours
( kw•h).
• The joule is too small.
– A kw•h is one kilowatt of power for one hour.
• Examples of 1 kw•h:
– 10 light bulbs of 100 W each on for 1 h
– 1 light bulb of 100 W on for 10 h
• 1 kw•hr = 3 600 000 J or 3.6 x 106 J
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Electrical Energy and Current
Relating Kilowatt-Hours to Joules
Click below to watch the Visual Concept.
Visual Concept
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Section 4
Electrical Energy and Current
Section 4
Electrical Energy Transfer
• Transfer of energy from power plants to your
neighborhood must be done at high voltage and
low current.
– Power lost in electrical lines is significant.
• P = I2R
• Power lines are good conductors but they are very long.
• Since power companies can’t control the resistance (R), they
control the current (I) by transferring at high voltage.
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Electrical Energy and Current
Section 4
Now what do you think?
• Hair dryers, microwaves, stereos, and other
appliances use electric power when plugged
into your outlets.
– What is electric power?
• Is electric power the same as the power discussed in the
chapter “Work and Energy?”
– Do the utility companies bill your household for
power, current, potential difference, energy, or
something else?
– What do you think is meant by the terms alternating
current (AC) and direct current (DC)?
• Which do you have in your home?
© Houghton Mifflin Harcourt Publishing Company