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Chapter 21 Electric Current and Direct- Current...
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Chapter 21 Electric Current and Direct- Current Circuits
Transcript of Chapter 21 Electric Current and Direct- Current...
Chapter 21
Electric Current and Direct-
Current Circuits
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• Electric Current
• Resistance and Ohm’s Law
• Energy and Power in Electric Circuits
• Resistors in Series and Parallel
•
HW # 5
Pg. 754 – 759:
# 7, 8, 11, 19, 22, 28, 32, 44, 49, 73, 78
PHYS 1402.01
Due on Monday, Oct. 7
PHYS 1402.02
Due on Tuesday, Oct. 8
Electric Current
Electric current is the flow of electric charge from
one place to another.
A closed path through which charge can flow,
returning to its starting point, is called an
electric circuit.
Electrical current
(A) Amperesecond
Coulomb :units
dt
dQI
Up to now, we have been considering equilibrium
configurations of charges -- electrostatics. Now we
will consider steady-state motions of charges.
By convention +I denotes the direction of
positive charge flow (or the opposite
direction of negative charge flow).
AnqvI d
# charges/volume
drift velocity
Electric Current
A battery uses chemical reactions to produce a
potential difference between its terminals. It
causes current to flow through the flashlight
bulb similar to the way the person lifting the
water causes the water to flow through the
paddle wheel.
Example # 1
The cell membrane separates the interior
of a living cell from its surroundings. So-
called ion channels penetrate the
membrane, allowing passage of materials
into an out of the cell. A particular
channel opens for 1.0 ms and allows
passage of 1.1 X 10 singly ionized
potassium ions during this time. What’s
the current in the channel?
Example # 2
How many coulombs of charge are in one
ampere-hour?
Example # 3
A steady current of 2.5 A exists in a wire
for 4.0 min. (a) How much total charge
passed by a given point in the circuit
during those 4.0 min? ( b ) How many
electrons would this be?
Electric Current
A battery that is disconnected from any circuit
has an electric potential difference between its
terminals that is called the electromotive force or
emf:
Remember – despite its name, the emf is an
electric potential, not a force.
The amount of work it takes to move a charge
ΔQ from one terminal to the other is:
Electric Current
The direction of current flow – from the positive
terminal to the negative one – was decided
before it was realized that electrons are
negatively charged. Therefore, current flows
around a circuit in the direction a positive charge would move;
electrons move
the other way.
However, this
does not matter
in most circuits.
Resistance and Ohm’s Law
Under normal circumstances, wires present
some resistance to the motion of electrons.
Ohm’s law relates the voltage to the current:
Be careful – Ohm’s law is not a universal law
and is only useful for certain materials
(which include most metallic conductors).
Resistance and Ohm’s Law
Solving for the resistance, we find
The units of resistance, volts per ampere,
are called ohms:
Resistor symbol ??
Practical resistors:
Example # 4
When a potential difference of 18 V is
applied to a given wire, it conducts 0.35 A
of current. What is the resistance of the
wire?
Resistance and Ohm’s Law
Two wires of the same length and diameter will
have different resistances if they are made of
different materials. This property of a material is
called the resistivity.
Resistance and Ohm’s Law
The difference between
insulators,
semiconductors, and
conductors can be clearly
seen in their resistivities:
Resistance and Ohm’s Law
In general, the resistance of materials goes up
as the temperature goes up, due to thermal
effects. This property can be used in
thermometers.
Resistivity decreases as the temperature
decreases, but there is a certain class of
materials called superconductors in which the
resistivity drops suddenly to zero at a finite
temperature, called the critical temperature TC.
Example # 5
Nichrome is a nickel –chromium alloy
used in heating applications like electric
toasters, because it has a relatively high
resistivity and heats up when current
passes through it. Suppose you have a
nichrome wire 0.20 mm in diameter and
75 cm long. ( a ) What’s its resistance?
( b) Find the current when a potential
difference of 120 V is connected across
the wire’s ends.
Energy and Power in Electric Circuits
When a charge moves across a potential
difference, its potential energy changes:
Therefore, the power it takes to do this is
Energy and Power in Electric Circuits
In materials for which Ohm’s law holds, the
power can also be written:
This power mostly becomes heat inside the
resistive material.
Consider a 60 W light bulb, connected to
a 120 V voltage source.
What is the current passing
through the wire in the bulb?
(A) 0.5 A (B) 1.0 A (C) 2.0 A
(D) 240 A
What is the resistance of the
wire in the bulb?
(A) 0.5 W (B) 1.0 W (C) 2.0 W (D)
240 W
Example # 6 (Your turn)
1. 0.5 A
2. 1.0 A
3. 2.0 A
4. 240 A
0%
0%
0%
0%
What is the current passing
through the wire in the bulb?
1. 0.5 A
2. 1.0 A
3. 2.0 A
4. 240 A
0%
0%
0%
0%
What is the resistant of the wire in
the bulb?
1. 5 -W
2. 10-W
0%
0%
A battery that produces a potential difference V is
connected to a 5-W lightbulb. Later the 5-W
lightbult is replaced with a 10- W lightbulb. (a) In
which case does the battery supply more current?
Conceptual Checkpoint 21 - 2
1. 5 -W
2. 10-W
0%
0%
A battery that produces a potential difference V is
connected to a 5-W lightbulb. Later the 5-W
lightbult is replaced with a 10- W lightbulb. (b)
Which lightbulb has the greater resistance?
Conceptual Checkpoint 21 - 2
1. 0.60 kW
2. 1.0 kW
3. 0.58 kW
4. 2.6 kW
0%
0%
0%
0%
Example # 7 pb. # 29 a) Find the power dissipated in a
25-Ω electric heater connected to a 120- V
outlet.
Energy Use: Energy and Power in
Electric Circuits When the electric company sends you a bill,
your usage is quoted in kilowatt-hours (kWh).
They are charging you for energy use, and kWh
are a measure of energy.
1. 2.1 kW
2. 1.0 kW
3. 1.1 kW
4. 2.6 kW
0%
0%
0%
0%
Example # 8 Electric utilities measure energy in kilowatt-hours (kWh),
where 1 kWh is the energy consumed if you use energy
at the rate of 1 kW for 1 hour. If your monthly electric bill
(30 days) is $100 and you pay 12.5c/kWh, what’s your
home’s average power consumption and average current,
assuming a 240-V potential difference between the wires
supplying your home? Response for first question
1. 4.0 A
2. 4.6 A
3. 3.0 A
4. 2.6 A
0%
0%
0%
0%
Example # 8 Electric utilities measure energy in kilowatt-hours (kWh),
where 1 kWh is the energy consumed if you use energy
at the rate of 1 kW for 1 hour. If your monthly electric bill
(30 days) is $100 and you pay 12.5c/kWh, what’s your
home’s average power consumption and average current,
assuming a 240-V potential difference between the wires
supplying your home? Response for 2nd question
1. 9.0 A
2. 8.6 A
3. 9.03 A
4. 8.33 A
0%
0%
0%
0%
Example # 9 Several male students in the same dorm room want to
dry their hair. Having taken PHYS 1402 at UTPA, they
have set their hair dryers to the “low, “ 1000-W settings.
Assuming a standard 120-V how many hair dryers can
they operate simultaneously without tripping the 20-A
circuit breaker?
Resistors in Series
Resistors connected end to end are said to be in
series. They can be replaced by a single
equivalent resistance without changing the
current in the circuit.
1. A
2. B
3. C
4. D
0%
0%
0%
0%
Example # 10 Two resistors, one having half the resistance of the
other, are connected to a battery as shown on the board.
What is the voltage across the bigger resistor?
2/bV
3/bV
2/3 bV
3/2 bV
1. A
2. B
3. C
4. D
0%
0%
0%
0%
Example # 11 Two resistors, one having half the resistance of the
other, are connected to a battery as shown on the board.
What is the voltage across the bigger resistor?
2/bV
3/bV
2/3 bV
3/2 bV
Resistors in Series
Since the current through the series resistors
must be the same in each, and the total potential
difference is the sum of the potential differences
across each resistor, we find that the equivalent
resistance is:
Resistors in Series
Since the current through the series resistors
must be the same in each
Total potential difference from point A to point B
must be the emf of the battery ε
ε = V1 + V2 + V3 …….
Resistors in Series and Parallel
Resistors are in parallel
when they are across the
same potential
difference; they can
again be replaced by a
single equivalent
resistance:
Resistors in Series and Parallel
Using the fact that the potential difference
across each resistor is the same, and the total
current is the sum of the currents in each
resistor, we find:
Note that this equation gives you the inverse of
the resistance, not the resistance itself!
Resistors in Series and Parallel
If a circuit is more complex, start with
combinations of resistors that are either purely
in series or in parallel. Replace these with their
equivalent resistances; as you go on you will be
able to replace more and more of them.
