Download AP 1 Quick Review on Electricity

AP Physics Quick Review – Electricity
1. Electrostatics
Formulas
F = kq1q2/r2
Like charges repel and unlike charges attract.
The total charge of a system will be conserved. When charged objects of identical
shapes and sizes are placed into contact, the objects will average out the total charge
so each ends up with the same charge.
The force between two charges is directly related to the product of the quantity of
charge on each object and just like the gravitational force, the force follows the
inverse square law with the distance.
Practice Questions
1. Sphere A is neutral and it touches an identical sphere B that has an excess of 100 electrons.
Then sphere A touches another identical sphere C that has an excess of 200 protons. What is
the charges of spheres A , B and C?
Sphere A
Sphere B
Sphere C
A. +100 e
+ 100 e
+ 100 e
B. -100 e
-50 e
+100 e
C. -75 e
-100 e
-75 e
D. +75 e
-50 e
+75 e
2. Two isolated charges, + q and - 2q, are 2 centimeters apart. If F is the magnitude of the force
acting on charge -2q, what are the magnitude force acting on charge + q when they are 4 cm
apart?
A. F
B. F/2
C. F/4
D. 2F
3. The diagram on the right shows 3 neutral metal spheres on insulating
stands touching each other. A negative rod is brought near sphere X
as shown in the diagram. The spheres are then separated and then the
negative rod is removed. What will be the charges on the spheres?
A. All three spheres will be positive.
B. Sphere X will be positive, Y will be neutral and Z will be negative.
C. Sphere X will be negative, Y will be neutral and Z will be positive.
D. Sphere X will be negative, sphere Y will be positive and Z will be
neutral.
4. A positively charged conductor attracts a second object. Which of the following statements
could be true? Select two answers
A. The second object is a conductor with positive net charge.
B. The second object is a conductor with zero net charge.
C. The second object is an insulator with zero net charge.
D. The second object is an insulator with positive net charge.
5. Two negatively charged objects are held together by a 0.020 m length of insulating string as
shown in the diagram above. The objects are initially at rest on a horizontal, non-conducting
frictionless surface. The effect of gravity on each object due to the other is negligible. The
charge of m1 is -4 nC and the charge of m2 is a -6 nC.
a. Calculate the tension in the string.
b. The string is now cut. On the axes below, qualitatively sketch a graph of the acceleration a of
the object of mass m2 versus the distance d between the objects after the string has been cut.
c. Describe qualitatively what happens to the speeds of the objects as time increases, assuming that
the objects remain on the horizontal, non-conducting frictionless surface.
2. Current Electricity
Formulas to know
The resistance in a wire depends on the type of material at a specific temperature which
relates to its resistivity (ρ) value, the length of wire (L) and the cross-sectional area of
the wire (A). The cross-sectional area of a wire is equivalent to r2. If the diameter of
a wire doubles, its cross-sectional area is quadrupled and that will lower its resistance
to ¼ of its value.
I = q/t
R = ρL/A
I = V/R
P = VI
Shorter, fatter, colder wires will have less resistance and be better choices for extension
cords than long thin wires. Temperature will be directly related to resistance so as
wires get hotter they have more resistance.
Series:
Resistance will remain constant if the material follows Ohm’s Law. Such materials are
called “ohmic”.
Rtotal= R1+R2+…
Parallel
1/Rtotal= 1/R1+1/R2+…
Ammeters measure current in amps. They are placed
into a circuit and have very low resistances.
Voltmeters measure potential difference (voltage)
in volts and are placed around a resistor.
They have very high resistance.
Name
Potential difference
Resistance
Current
Power
Energy
Charge
Variable
V
R
I
P
W
q
unit
Volt (V)
Ohm (
Ampere (A)
Watt (W)
Joule (J)
Coulomb (C)
Practice Questions
1. If the 2 ohm resistor is replaced with another one with a smaller resistance value,
then the
A. ammeter reading will decrease and voltmeter reading will increase.
B. ammeter reading will stay the same and the voltmeter reading will increase.
C. ammeter and voltmeter readings will both increase.
D. ammeter reading will increase and the voltmeter reading will stay the same.
2. The diagram below represents a lamp, a 10-volt battery,
and a length of nichrome wire connected in series. Which of
the following would increase the brightness of the lamp?
1.
2.
3.
4.
Decrease the radius of the nichrome wire.
Decrease the temperature of the room
Increase the length of the nichrome wire.
Put an ammeter into the circuit.
3. An air conditioner is designed to operate at 110 volts and is rated at 1800 watts. Is it possible to
use the air conditioner in a circuit supplied with 110 volts, but which can carry a maximum current of
15 amperes before the circuit breaker is set off?
[a] Yes, because the current needed is less than 15 amperes.
[b] No, because the voltage required is too high.
[c] Yes, because the voltage is lower than that needed.
[d] No, because the current needed is greater than the 15 amperes
4. A student constructs the
graph from data she collected
on the current and voltage
supplied to a lightbulb.
Discuss whether the bulb is
ohmic or not citing evidence
from the graph.
Series Circuit: Current is the same for all resistors and the total current is
determined by the total resistance in the circuit. Resistors add together to get
the total resistance.
Voltage is shared among all of the resistors with more voltage going to larger
resistors.
A break anywhere in this circuit stops the current flowing in the circuit.
Adding resistors in series increases the total resistance in the circuit. This will
reduce to the total current to all the resistors and each resistor will get less
voltage than before.
Less power is usually generated in series circuits. Larger resistors will get more
power than smaller resistors in this type of circuit.
Be aware that lower wattage light bulbs have more resistance so lower wattage
bulbs will be brighter than higher wattage bulbs in a series circuit.
Parallel Circuit: Voltage is the same for all resistors.
Each branch gets its own current and the total current is the sum of all of these
individual currents. The branch with the lowest resistance gets the most
current.
Adding resistors in parallel reduces the total resistance in the circuit and will
increase the total current, but the individual currents will not change.
The equivalent resistance of a parallel circuit will always be less than the value
of the smallest resistor in the circuit.
More power is usually generated in parallel circuits. Smaller resistors get more
power than larger resistors in this type of circuit so higher wattage bulbs will
be brighter than lower wattage bulbs in a parallel circuit.
The order of the components in the circuit does not matter as long as they are
connected in series or parallel.
Series-parallel combo: Determine which parts of the circuit are in parallel and
which are in series and apply the appropriate rules stated above to each part.
Redrawing the circuit and simplifying it can be helpful.
Remember that the brightness of a bulb is determined by its power which is a
product of its current and voltage.
A bulb that is burned out will not get current flowing through it, but it may still
have a potential difference across it.
Practice Questions
1. A radio is designed to be operated with a voltage of 6 volts and needs 10 mA of current to
operate properly. How will the radio be attached in a circuit with the 9 volt battery to function
properly?
A. The radio will be placed a series with a resistor of 300 ohms.
B. The radio will be placed in parallel a resistor of 300 ohms.
C. The radio will be placed in series with a 150 ohm resistor.
D. The radio will be placed in parallel with a 150 ohm resistor.
2. The diagram below shows a circuit with two resistors. If another resistor is
added in series to this circuit, what will happen to the ammeter reading?
A. The ammeter reading will be greater.
B. The ammeter reading will be lower.
C. The ammeter reading will stay the same.
D. It cannot be determined unless the value of the resistor is given.
3. The diagram represents an electric circuit
consisting of four resistors and a 12-volt battery.
If the 12 ohm resistor is removed, what happens to
the ammeter and voltmeter readings?
A. Ammeter and voltmeter increase.
B. Ammeter and voltmeter decrease.
C. Ammeter and voltmeter stay the same.
D. Ammeter increases and voltmeter stays the
same.
4. Two lamps are connected as shown. If lamp 1
burns out, then which of the following is true?
A. The second lamp will go out also.
B. The second lamp will stay lit, but will get
dimmer.
C. The second lamp will stay lit, but will get
brighter.
D. The second lamp will remain lit with the same brightness as before.
5. The circuit in the figure at the right contains two identical lightbulbs in series with a battery.
At first both bulbs glow with equal brightness. When switch S is closed, which of the
following occurs to the bulbs?
Bulb I
A. Goes out
B. Gets brighter
C. Gets brighter
D. Gets slightly dimmer
Bulb 2
Gets brighter
Goes out
Gets slightly dimmer
Gets brighter
V
The series-parallel combination circuit at the left shows that resistors 1 and 2 are
in series with a parallel combination of resistors 3 and 4. In this circuit, the
ammeter is placed to read the total current which is the same current that resistors
1 and 2 get. The current through resistors 3 and 4 will be less than the total
current but both currents will add to the total current in the circuit. The resistor
with the lower resistance will get the greater current in the parallel section
containing resistors 3 and 4.
Resistors 3 and 4 get the same voltage as measured by the voltmeter, but it will
not be the total voltage. The total voltage will be the sum of the voltages from the
parallel combination (the voltmeter reading) and the voltages of resistors 1 and 2.
Current is never used up in a circuit. The same current that leaves the battery
must return to the battery. (Current follows the junction rule.)
Voltage is used up by each resistor or parallel section so that the total volts left
must be zero by the time the current returns to the battery.(This is the loop rule.)
2. If the current in the 3 ohm resistor is 0.5 A, what is the emf
(voltage) of the power source?
A.
B.
C.
D.
1.5 V
3.0 V
4.5 V
6V
3. Five identical light bulbs -- A, B, C, D and E—are attached as
shown. Assume the bulbs are ohmic and there is negligible
resistance in the wires.
If bulb D blows out, rank the bulbs from dimmest to brightest,
placing equal brightness on the same line. Explain your ranking.
Dimmest _____ _____ _____ _____ _____ Brightest