Transcript AP Physics Electricity Notesx

Chapters 16-1,2,3,5; 17-1; 18-2,3,4,5
Coulomb’s Law, Ohm’s Law, Power and
Resistivity
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Electric Charge
A charged system or object can
attract a neutral one by
changing the distribution of
charge in the neutral object
Units of Charge: Coulombs (C)
Charges produced by rubbing
are typically around a
microcoulomb:
1 µC = 10−6 C
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Smallest unit of charge is the electron at 1.6x10-19C (on the equation sheet)
and this is the elementary charge (e).
Charge can only be lost or gained by this amount. You cannot lose or gain a
fraction of an electron. So objects lose or gain charges in amounts of 1e, 2e,
3e, -500e, etc.
Electron has a charge of –e and proton as having a charge of +e.
The charges of electrons, neutrons and protons
result from their quark compositions
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Law of Conservation of Electric Charge
Electric charge is conserved—the total charge of a system
cannot change in any interaction (electrons and protons are
conserved)
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Coulomb’s Law
The electric force between two point charges has to do with size of
the chargesand distance between
them.
Different on
equation
sheet
k = 9.0 × 109 N∙m2/C2
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Direction of Electrical Force – along the line between the charges
• All contact forces (friction, normal, tension, etc) result
from interatomic electric forces
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Practice Problem
16-1. What is the magnitude and direction of the electric force on the
electron of a hydrogen atom exerted by a single proton (Q2=+e) that
is in the atom’s nucleus. The distance between them is 0.53x10-10m.
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Practice Problem
16-2. Two positive point charges Q1=50µC and Q2=1µC are separated
by a distance r. Which is larger in magnitude, the force exerted by Q1
on Q2, or the force of Q2 on Q1? How is the direction related?
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Electric Potential Energy and
Potential Difference
PE in an electric field
Electrostatic force is a conservative force,
so there is electric potential energy, work
can be done and electric PE can convert
into KE
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Analogy between gravitational and electrical potential energy. Just as the
more massive rock has more potential energy, so does the larger charge:
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Electric potential (V) is the electric potential energy per unit charge.
V=PE/q Not on equation sheet
Just like with Ug = mgΔy, only a difference in electric potential is measureable.
So we look at ∆V– this is the potential difference. Power sources maintain a
nearly constant potential difference.
Electric potential and potential difference are measured in Joules/Coulomb
which is equal to Volts
Voltage and potential difference are the same thing
Batteries and power plants transform other types of energy into a potential
difference
Electric Current
Electric current is the rate of flow of charge through a
conductor. It is
Different on
equation
sheet
Unit of electric current: the ampere, A.
1 A = 1 C/s
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In order for current to flow, there must be a path from one
terminal, through a closed circuit, and back to the other battery
terminal.
Circuit
symbols:
Current is not a vector, but it has direction.
By convention, current is defined as
flowing from + to – (flow of positive
charge). Electrons actually flow in the
opposite direction. – blame the guys 200
years ago that didn’t know about electrons!
Current and charge do not get used up (if it
goes in one part of the circuit, it comes out
the other) – law of conservation
Ohm’s Law
Ohm’s Law:
Different on
equation
sheet
Resistance is a property for metal conductors.
Ohm’s law doesn’t hold true under all conditions. (not
really a true law – not sure why it hasn’t been demoted
like Pluto)
We use resistors to
control the amount of
current.
In many conductors, the resistance is independent of the
voltage (death to Ohm’s Law). Materials that do not
follow Ohm’s law are called nonohmic.
Practice Problem
18-1. A steady current of 2.5 A exists in a wire for 4.0 min. (a) How much
total charge passes by a given point in the circuit during those 4.0 min? (b)
How many electrons would this be?
18-2. Identify what, if anything, is wrong with each circuit below.
Practice Problem
18-3. A small flashlight bulb draws 300mA from a 1.5-volt battery. (a)
What is the resistance of the bulb? (b) If the battery becomes weak and
the voltage drops to 1.2-volts, how would the current change? Assume
the bulb is approximately ohmic.
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Practice Problem
18-4. Current enters a resistor as shown in this figure. A)Is the
potential higher at point A or at point B? B)Is the current greater at
Point A or Point B?
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Resistivity
The resistance of a wire is found using its length, cross
sectional area and a property called resistivity (ρ ):
The constant ρ, the resistivity, is characteristic of the
material.
Resistivity increases with temperature (perhaps atom
movement is interfering with electron movement) –
exception is in some semiconductors
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18-4 Resistivity
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Practice Problem
18-5. Suppose you want to connect your stereo to speakers that will be
placed far away from the stereo. A)If each wire must be 20m long,
what diameter wire do you need to keep the resistance less than 0.10Ω
if the resistivity of copper is 1.68x10-8 Ωm? B) If the current to each
speaker is 4.0A, what is the voltage drop across each speaker?
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Practice Problem
18-6. Suppose a wire could be stretched uniformly until it is twice its
original length. What would happen to the resistance? Assume the
amount of material, and therefore the volume, doesn’t change.
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Electric Power – or Energy Dissipated over Time
Power is the energy transformed by a device per unit
time:
Not on
equation
sheet
Different on
equation
sheet
For ohmic devices, we can derive these equations:
Not on
equation
sheet
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Direct and Alternating Current
Current from a battery
flows steadily in one
direction (direct current,
DC). Current from a power
plant varies sinusoidally
(alternating current, AC).
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Practice Problem
18-8. Calculate the resistance of a 40W car headlight designed for a
12V battery.
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Chapter 19-1,2,3
DC Circuits
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Resistors in Series
A series connection has a single path.
The current through each resistor is the same, but the voltage drop is
different for each resistor. The sum of the voltage drops across the resistors
equals the battery voltage.
Not on
equation
sheet
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Equivalent resistance (total resistance) –
Different on equation
sheet
Resistors in Parallel
A parallel connection splits the current. Each resistor has a different
current, but the voltage across each resistor is the same.
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The total current is the sum of the currents across each resistor:
Not on
equation
sheet
Not on
equation
sheet
The Req is less than the R of any individual resistor.
Different on equation
sheet
Rules about Circuits
Series Circuits:
All resistors have the same I which
is equal to the total I
Parallel Circuits:
Each resistor has a different I that
adds to the total I
V is different for each resistor and
they add to the total V
All resistors have the same V which
is equal to the total V
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Kirchhoff’s Rules
Some circuits are much more complicated and need to be broken down or
collapsed into different series and parallel parts.
Junction rule: The sum of currents entering a junction equals the sum of the
currents leaving it (conservation of charge).
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Loop rule: The sum of the changes in potential around a closed loop is zero
(conservation of energy).
Solving Circuit Problems
50Ω
50Ω
50Ω
50Ω
Follow up Questions
19-2 A) the lightbulbs in this figure are identical. Which configuration produces more light? B)Which way
do you think the headlights of car are wired?
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Solving Circuit Problems – A bit harder now
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Practice Problem
[19-3] Two 100 Ω resistors are connected in parallel and in series to a 24.0
volt battery. What is the total resistance and the current in each circuit?
Practice Problem
[19-4] How much current is drawn from the battery in the circuit?
Practice Problem
[19-5] What is the current through the 500 Ω resistor?
Practice Problem
19-6. The circuit here has 3 identical bulbs. A)When the switch is
closed, how will the brightness of bulbs A and B compare with that of
C? B)What happens when the switch is opened? Answer in terms of
brightness compared to before.
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Practice Problem
Exercise C. A 100W light bulb and 60W bulb are connected to a 120V
power source in two different ways. In each case, which bulb glows
more brightly? Why?
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