Physics II PHY 202222 Electricity

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Physics II PHY 202/222 Electricity

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• Rock Hill, SC 29730
• www.yorktech.com

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Electricity Test 4
Beiser Chapters 23-26 Multiple Choice
Odd Supplementary Problems Every Other Odd
(1,not3,5,not7) Browne Chapter 20-25 for PHY
222 20 3,13 21 8 22 5 24 7 25 9
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Chapter 23 Electricity
Beiser p.266
4
Electric Charge

• Positive charge from protons
• Negative charge from electrons
• Measured in Coulombs (C)
• e 1.6 x 10-19 C

Like charges repel. Unlike charges attract.
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_
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Coulombs Law
Beiser p.266
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Coulombs Law Example
_

Find the force of attraction between a ball with
a charge of 0.2 C and a ball with a charge of
-0.3 C if they are separated by .5 m.
Beiser p.266,7
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Superposition of Electric Forces

• Find the electric force on Q3 from the other
  charges.

Fnet
350
Beiser p.269
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Electric Fields
If you had a small positive test charge and
placed it near other charges, it would experience
a force at every point in space. Mapping these
lines of force show the electric field. Measured
in N/C or V/m.
Beiser p.269,270
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Potential Difference

• The amount of work needed to move a charge of 1C
  from one point to another.
• Measured in volts (V)
• 1V 1J/C

Beiser p.266
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Chap 23 - Summery
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236,8
11
23.10
12
23.12
q2
q
x
40 - x
40 cm
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23.14
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23.16
100 V
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23.18
- - - -

F
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23.20
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Chapter 24 Electric Current
Beiser p.277
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Current

• The flow of charge
• Measured in Amperes (or Amps), A
• 1 A 1 C/s

Beiser p.277,8
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Direction of Current

• A complete circuit is needed for electrons to
  travel.
• Electrons actually travel from negative terminal
  of battery to positive.
• Current is said to go from positive to negative.

Direction of current is positive to negative

V 12 V
Mr. Electron sez Im going this way!
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Conductors Insulators

• Conductors a material through which current
  flows easily
• Insulator - a material through which current will
  not (generally) flow
• Resistor a material through which current flows
  with some difficulty
• Semiconductor - a material that is sometimes a
  conductor sometimes a resistor
• Superconductor a material that carries current
  effortlessly with no loss

Beiser p.277
21
Resistance

• A measure of the opposition to current in a given
  material
• Measured in Ohms (O)
• 1 O 1 V/A
• A resistor is a device with resistance

Resistor color band example red yellow blue is 2
4 6 so 24x106O
R 24x106 O
Beiser p.278
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Ohms Law

• Relates current, voltage and resistance.
• It takes more voltage to push current through a
  high resistance material

Beiser p.278
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Power

• The rate at which work is done to maintain
  current.
• or
• The rate at which a current at a voltage can do
  work
• Measured in Watts (W)
• 1 W 1 J/s

Beiser p.282
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24.2
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24.4
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24.6
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24.8
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24.10
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24.12
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24.14
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24.16
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24.18
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24.20
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Chapter 25 Direct Current Circuits
Beiser p.288
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Resistors in Series

• Current has to fight its way through R1 then
  R2 and then R3
• Add resistors in series.
• RTotal R1, R2 R3 100 O 300 O 500 O
  900 O

Beiser p.288
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Resistors in Parallel

• Current can choose to go through R1 or R2, so
  total resistance is less that either individual
  resistor.
• Use formula to get total resistance

TI-83 keystrokes
Mr. Electron sez Whee! I can go either way.
That makes it easy for me/hard for you!
Beiser p.290
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Combinations of Resistors
Mr. Electron sez looks like fun!
1) Add the parallel resistors
2) Add the series resistors
RTotal 191 O
3) Add the parallel resistors
Beiser p.293
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EMF Internal Resistance

• Batteries have a small internal resistance so
  that
• V Ve Ir or
• Terminal Voltage emf potential drop due to
  internal resistance
• The total internal resistance of batteries in
  series is the sum of the individual internal
  resistances.
• The total voltage of batteries in series is the
  sum of the individual batteries.

Beiser p.294
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Batteries in Series

• The total voltage of batteries in series is the
  sum of the individual batteries.
• The total internal resistance is the sum of the
  individual internal resistances

Beiser p.295
40
Batteries in Parallel

• Batteries in parallel should always have the same
  voltage, so that back currents dont flow through
  the weaker batteries and waste power.
• The total voltage of batteries in parallel is the
  voltage of any of the batteries.
• The total internal resistance is added like
  resistors in parallel.

Beiser p.295
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Kirchhoffs Rules

• The sum of the currents into any point is equal
  to the sum of the current from that point,
• The sum of the voltage around a loop is zero.

Beiser p.298
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Kirchhoff Example 1

• Step 1 pick a point where all the legs of the
  circuit come together.
• Step 2 pick a direction that you think current
  will flow in each leg and label each leg as I1,
  I2
• Step 3 The current into point A the current
  out of point A I1 I2 I3
• Step 4 Trace a complete circuit and add the
  voltages batteries increase and resistors
  decrease see 4 12V I1R1 I2R2 0 (If
  moving against the current reverse the signs)
• Step 5 Trace another path see 5 12V
  I1R1 I3R3 0
• Solve the three simultaneous equations

Beiser p.299-302
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Solving by Substitution Method
Substitute both into eq.1
I1 I2 I3 12 I1R1 I2R2 0 12 I1R1 I3R3 0
Solve eq.2 for I2, and eq.3 for I3.
Substitute into eq.2 and eq.3
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Solving by matrices
On TI-83
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25.2
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25.4
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25.6
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25.8
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25.10
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25.12
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25.14
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25.16
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25.18
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Example 25.21
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25.22
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25.24
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Chapter 26 Capacitance
Beiser p.308
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Capacitors Capacitance

• A capacitor is a device that stores charge.

A voltage can push electrons around to store
charge on a capacitor. Capacitance is the ratio
of charge to voltage.
Capacitance is measured in farads.
Beiser p.308
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Capacitors in Parallel
Beiser p.310
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Capacitors in Series
If C1 100 F and C2 300 F TI-83 keystrokes
Beiser p.310
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Energy of a Capacitor
When charge is stored in a capacitor, the amount
of stored or potential energy is given by any of
the following
Beiser p.312
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Capacitor Charging

• For a capacitor that initially has no charge,
  these three formulas govern capacitor charging
  when the switch is closed
• The first gives current or rate of charging at
  any time
• The second is the TIME CONSTANT a which tells
  how long it takes to charge a capacitor to 63 of
  its capacity.
• The last gives the amount of charge at any time.

Beiser p.313
63
Capacitor Discharging

• For a capacitor that initially is fully charged,
  when the switch is closed
• The first is the TIME CONSTANT a which tells
  how long it takes to discharge 63 of the full
  charge of the capacitor (or how long it takes to
  fall to 37 of capacity)
• The last gives the amount of charge at any time.

Beiser p.314
Beiser p.313
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26.2
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26.4
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26.6
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26.8
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26.10
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26.12
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26.14
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26.16
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26.18