General Physics PHY 2140

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General Physics (PHY 2140)
Lecture 9

• Electrodynamics
• Electric current
• temperature variation of resistance
• electrical energy and power

http//www.physics.wayne.edu/apetrov/PHY2140/
Chapter 17-18
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Department of Physics and Astronomy announces the
Fall 2003 opening of The Physics Resource
Center on Monday, September 22 in Room 172 of
Physics Research Building.
Hours of operation Mondays, Tuesdays,
Wednesdays 11 AM to 6
PM Thursdays and Fridays 11 AM to 3
PM Undergraduate students taking PHY2130-2140
will be able to get assistance in this Center
with their homework, labwork and other issues
related to their physics course. The Center
will be open Monday, September 22 to Wednesday,
December 10, 2003.
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Lightning Review

• Last lecture
• Current and resistance
• Current and drift speed
• Resistance and Ohms law
• I is proportional to V
• Resistivity
• material property
• Review Problem Consider two resistors wired one
  after another. If there is an electric current
  moving through the combination, the current in
  the second resistor is
• a. equal to
• b. half
• c. smaller, but not necessarily half
• 
  the current through the first resistor.

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17.4 Resistivity - Example
(a) Calculate the resistance per unit length of a
22-gauge nichrome wire of radius 0.321 m.
Cross section
Resistivity (Table) 1.5 x 10-6 Wm.
Resistance/unit length
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17.4 Resistivity - Example
(b) If a potential difference of 10.0 V is
maintained across a 1.0-m length of the
nichrome wire, what is the current?
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17.4 Temperature Variation of Resistance - Intro

• The resistivity of a metal depends on many
  (environmental) factors.
• The most important factor is the temperature.
• For most metals, the resistivity increases with
  increasing temperature.
• The increased resistivity arises because of
  larger friction caused by the more violent motion
  of the atoms of the metal.

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• For most metals, resistivity increases approx.
  linearly with temperature.

r
T Metallic Conductor

• r is the resistivity at temperature T (measured
  in Celsius).
• ro is the reference resistivity at the reference
  temperature To (usually taken to be 20 oC).
• a is a parameter called temperature coefficient
  of resistivity.

• For a conductor with fixed cross section.

r
T Superconductor
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17.5 Temperature Variation of Resistance - Example

• Platinum Resistance Thermometer
• A resistance thermometer, which measures
  temperature by measuring the change in the
  resistance of a conductor, is made of platinum
  and has a resistance of 50.0 W at 20oC. When the
  device is immersed in a vessel containing melting
  indium, its resistance increases to 76.8 W. Find
  the melting point of Indium.

Solution Using a3.92x10-3(oC)-1 from table
17.1.
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Platinum Resistance ThermometerA resistance
thermometer, which measures temperature by
measuring the change in the resistance of a
conductor, is made of platinum and has a
resistance of 50.0 W at 20oC. When the device is
immersed in a vessel containing melting indium,
its resistance increases to 76.8 W. Find the
melting point of Indium.

• Solution
• Using a3.92x10-3(oC)-1 from table 17.1.
• Ro50.0 W.
• To20oC.
• R76.8 W.

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Appendix Superconductivity

• 1911 H. K. Onnes, who had figured out how to
  make liquid helium, used it to cool mercury to
  4.2 K and looked at its resistance

• At low temperatures the resistance of some
  metals?0, measured to be less than
  10-16?conductor (i.e., ?lt10-24 Om)!

• Current can flow, even if E0.
• Current in superconducting rings can flow for
  years with no decrease!

• 1957 Bardeen (UIUC!), Cooper, and Schrieffer
  (BCS) publish theoretical explanation, for
  which they get the Nobel prize in 1972.
• It was Bardeens second Nobel prize (1956
  transistor)

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17.7 Electrical energy and power

• In any circuit, battery is used to induce
  electrical current
• chemical energy of the battery is transformed
  into kinetic energy of mobile charge carriers
  (electrical energy gain)
• Any device that possesses resistance (resistor)
  present in the circuit will transform electrical
  energy into heat
• kinetic energy of charge carriers is transformed
  into heat via collisions with atoms in a
  conductor (electrical energy loss)

D
C
A
B
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Electrical energy

• Consider circuit on the right in detail
• AB charge gains electrical energy form the
  battery
• (battery looses chemical energy)
• CD electrical energy lost (transferred into
  heat)
• Back to A same potential energy (zero) as before
• Gained electrical energy lost electrical energy
  on the resistor

C
B
A
D
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Power

• Compute rate of energy loss (power dissipated on
  the resistor)
• Use Ohms law
• Units of power SI watt
• delivered energy
  kilowatt-hours

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Example

• Power Transmission line
• A high-voltage transmission line with resistance
  of 0.31 W/km carries 1000A , starting at 700 kV,
  for a distance of 160 km. What is the power loss
  due to resistance in the wire?

Given V700000 V r0.31 W/km L160 km I1000
A Find P?

• Observations
• Given resistance/length, compute total resistance
• Given resistance and current, compute power loss

Now compute power
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Mini-quiz

• Why do the old light bulbs usually fail just
  after you turn them on?

When the light bulb is off, its filament is cold,
so its resistance is large. Once the switch it
thrown, current passes through the filament
heating it up, thus increasing the
resistance, This leads to decreased amount of
power delivered to the light bulb, as Thus,
there is a power spike just after the switch is
thrown, which burns the light bulb. Resume
electrical devices are better be turned off if
there is a power loss
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Direct Current Circuits
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18.1 Sources of EMF

• Steady current (constant in magnitude and
  direction)
• requires a complete circuit
• path cannot be only resistance
• cannot be only potential drops in direction of
  current flow
• Electromotive Force (EMF)
• provides increase in potential E
• converts some external form of energy into
  electrical energy
• Single emf and a single resistor emf can be
  thought of as a charge pump

V IR E
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EMF

• Each real battery has some internal resistance
• AB potential increases by E on the source of
  EMF, then decreases by Ir (because of the
  internal resistance)
• Thus, terminal voltage on the battery DV is
• Note E is the same as the terminal voltage when
  the current is zero (open circuit)

B
C
r
R
E
A
D
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EMF (continued)

• Now add a load resistance R
• Since it is connected by a conducting wire to the
  battery ? terminal voltage is the same as the
  potential difference across the load resistance
• Thus, the current in the circuit is

B
C
r
R
E
A
D
Power output
Note well assume r negligible unless otherwise
is stated
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Measurements Voltmeters measure Potential
Difference (or voltage) across a device by being
placed in parallel with the device.
Ammeters measure current through a device by
being placed in series with the device.
A
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Direct Current Circuits
Two Basic Principles Conservation of
Charge Conservation of Energy Resistance
Networks
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Resistors in series Conservation of Charge I
I1 I2 I3 Conservation of Energy Vab V1
V2 V3
Voltage Divider
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I
Resistors in parallel Conservation of Charge I
I1 I2 I3 Conservation of Energy Vab
V1 V2 V3
a
R1 V1I1R1
R2 V2I2R2
R3 V3I3R3
b
Current Divider
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Example Determine the equivalent resistance of
the circuit as shown. Determine the voltage
across and current through each
resistor. Determine the power dissipated in each
resistor Determine the power delivered by the
battery
R14W
E18V
R36W
R23W