Download Electricity Review

Physics
Name: __________________________
Date: _____________ Period: _______
Electricity
(Unit Review)
Key Vocabulary
Alternating Current
Anode
Attraction
Battery
Branch
Capacitor
Cathode
Charge
Circuit
Circuit Analysis
Conductor
Coulomb’s Law
Current
Diode
Direct Current
Electric Dipole
Electric Discharge
Electric Field
Electric Field Lines
Electrical Cell
Electrochemical Series
Electrode
Electrolyte
Electromagnetic Force
Electron Affinity
Friction
Ground Fault Circuit Interrupt (GFCI)
Induction Insulator
Kirchhoff’s Current Law
Kirchhoff’s Voltage Law
Light Emitted Diode (LED)
Lightning
Magnetic Levitation (Maglev)
Multimeter
Ohm’s Law
Parallel Circuit
Photocell
Potential Difference
Repulsion
Resistance
Resistor
Semiconductor
Series Circuit
Static Electricity
St. Elmo’s Fire
Switch
Thermocouple
Triboelectric Series
Voltage
Voltage Drop
1. List the four fundamental forces at work in the universe. Number them from strongest and weakest (1 =
strongest, 4 = weakest).
the strong force (1), the electromagnetic force (2), the weak force (3), and the gravitational force (4).
2. In our unit on matter and thermodynamics we talked about the relationship between the system and the
surroundings. How does that relate to electricity?
If charge is removed an object (the system), then it will increase somewhere else (the surroundings;
think about the net charge). If you want electricity to do work it is necessary to set up an electric
potential allowing for the conversion of electric potential into current. Remember that energy is never
created or destroyed, but converted from some other form of energy.
3. How is the definition of charge in physics different from that used in general terms?
Charge in physics is the technical term used to indicate that an object participates in electrical forces.
This needs to be distinguished from the common usage, in which the term is used indiscriminately for
anything electrical.
4. What force is exhibited between these charge pairs?
a. + and – attraction
b. + and + repulsion
c. – and – repulsion
5. Compare and contrast a conductor with an inductor.
Conduction involved the direct contact of objects, with electrons flowing through one object into
another. Objects that permit the free flow of electrical charges are called conductors. Most metals are
good conductors. (THIS IS WHERE I MADE A MISTAKE…I MEANT INSULATOR, BUT TYPED
INDUCTOR). An inductor is a conductor structured to create charge in a neutral object. Every
conductor has the ability to create inductance. It doesn't switch from being a conductor to being an
inductor, inductance just comes with conducting current. The better answer should be an insulator is an
object that resists the free flow of electrical charges.
6. _The flow of electrons_ is what carries energy in a circuit. This is often illustrated in an analogy with
water. Explain why this is not a perfect example.
While it might be easy to visualize the flow of electrons like the flow of water and the voltage as the
size of the opening for the faucet, unless you are using the water to perform some work (to turn a water
wheel???) it is hard to build in the relationship of voltage and current to Power (which may be better
illustrated with the roller coaster model).
7. Describe the relationship between voltage, current, and work.
It may have been better for me to ask the questions with respect to power instead of work. We know
the relationship between current, resistance, and voltage expressed by Ohm’s Law. Also, we have seen
that the voltage between two points can be represented as the amount of work needed to move charge
between the two points. If we reflect on this relationship, work can be defined as the voltage between
the two points times the charged that was moved between them.
From this relationship and the power equation, we can establish the following relationship:
8. Compare and contrast Alternating with Direct Current.
Electricity or "current" is nothing but the movement of electrons through a conductor, like a wire. The
difference between AC and DC lies in the direction in which the electrons flow. In DC, the electrons
flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes
going "forward" and then going "backward."
9. Explain how electron affinity relates to static electricity and the triboelectric series.
Electron affinity refers to the relative amount of “love” that a material has for electrons. The transfer of
electrons from one object to another without further movement is called static electricity. If one object
is rubbed against another, the object with the greatest electron affinity will end up negatively charged
and the object with the lower electron affinity will end up positively charged. If one were to record the
effect of doing this with many different materials, you could set up a triboelectric series (such as we
saw in the notes)
10. What are the three modes of creating static charge and how does each work?
a. Charging by Friction – The process of charging objects that results in a transfer of electrons
between the two objects that are rubbed together.
b. Charging by Conduction – When a charged object touches a neutral object, both objects acquire
the same type of charge. If a negative object is used to charge a neutral object, then both objects
become charged negatively (Sharing charge by touch)
c. Charging by Induction - In the induction process, a charged object is brought near but not
touched to a neutral conducting object. The presence of a charged object near a neutral
conductor will force (or induce) electrons within the conductor to move.
11. Two small equal sized and massed pith balls are 3 cm apart in air and carry charges of +3 x 10-9 and –
12 x 10-9 C respectively. Compute the force of attraction. If the balls are touched and then separated
by a distance of 3 cm, what will be the force between them?
9 ∙ 109 𝑁𝑚2
(
)(−4.5 ∙ 10−9 𝐶)(−4.5 ∙ 10−9 𝐶) 1.82 ∙ 10−7 𝑁
𝑘𝑞1 𝑞2
2
𝐶
𝐹=
=
=
= 2.025 ∙ 10−4 𝑁
𝑑2
0.03𝑚2
0.0009
12. Explain what an electric discharge is and how this relates to lightning and St. Elmo’s Fire.
An electric discharge is the release and transmission of electricity in an applied electric field through a
medium such as a gas. Lightning, which is thought to be caused by the frictional charging, creates a
separation of positive and negative charges in pockets. As these charge pockets build up, they seek
neutralization, which results in the often-spectacular discharge of electricity known as lightning. St.
Elmo’s Fire is a coronal discharge caused by the ionization of the surrounding atmosphere, generating
a luminous plasma. (So BOTH are forms of electric discharge seen on Earth)
13. Explain what an electric field is and how electric field lines work.
In its simplest form an electric field is a region around a charged particle or object within which a force
would be exerted on other charged particles or objects. Remember however that the electromagnetic
force is infinite and a field is actually a mathematical description of the relationship between two
particles expressed at a particular point in space.
14. Complete the following diagrams with field lines.
15. What is the electric field strength at a distance of 10 cm from a charge of 2 μC?
9 ∙ 109 𝑁𝑚2
(
)(2 ∙ 10−9 𝐶) 18000𝑁
𝑘𝑄
2
𝐶
𝐸= 2 =
=
= 1.8 ∙ 106 𝑁/𝐶
𝑑
0.1𝑚2
0.01𝐶
16. What is the relationship between current and work?
Current is the evidence of work being done. Electric current is defined as the rate at which charge flows
through a material (C/s). If we remember, work is force across a distance (W = Fd). So when current is
flowing we can use Coulomb’s Law to measure the force and thus the work. Power then is work per
time or current times volts (P = W/t = IV)
17. Compare and contrast a cathode and an anode. How are the two related to charge and “recharging” of a
battery?
The cathode is the positive terminal of a battery and has a higher potential. The anode is the negative
terminal of a battery and has a lower potential. The difference in electric potential between the
terminals is measured in volts and allows current to flow. The recharging process then is the process of
reestablishing the electric potential of the battery after it has been “used.” Remember that energy
cannot be created or destroyed, so we are converting electrical energy back into chemical energy in the
battery to allow for the resetting of the electrical potential.
18. An electric heater draws 3.5 A from a 110 V source. What is the resistance of the heating element?
V = IR
110 V = 3.5 A (R)
R = 31.4 Ω
19. Approximately how many milliamperes of current flow through a circuit with a 40 V source and 6.8
kΩ of resistance?
6.8 kΩ = 6800 Ω
V = IR
40 V = I (6800 Ω)
I = 5.9 mA
20. What are the benefits of using alternating current instead of direct current?
DC required clients to be within 2km of the power plant to receive electricity. While Edison didn’t like
it, Tesla had the answer to the P=I2R problem. With AC a transformer can be used to convert high
voltage and low current into low voltage and high current. So power could be sent through power lines
at low currents and high voltage and then stepped down to low voltage and high current when it got to
the house.
21. Compare and contrast the flow of electrons and the electric field in a circuit.
Ok, so a quick clarification. Due to some unusual history, the flow of current in a circuit is opposite the
flow of electrons. (Electrons flow from the negative terminal to the positive terminal, but the current
flows from the positive terminal to the negative terminal). Now, field lines point toward the negative
and away from the positive (since the field lines are based on a positive test charge), so current flows
with the field lines, but electrons flow against the field lines.
22. What is the frequency of the current in AC current in the United States?
60 Hz
23. Compare and contrast conductors, resistors, and superconductors.
A conductor is an object or type of material that allows the flow of electrical current (like a wire in a
circuit; low loss). Resistors are generally used to describe devices that pass electricity but impede the
flow of current to some extent, the more resistance, the harder for the current to flow. Essentially they
both have finite resistance and do become circuit elements that should be taken into account. Low
resistance means high conductance, and vice versa. In a superconductor resistance is decreased by
lowering the temperature to diminish vibrations within the material or object moving charge.
24. In an electrical shock, _voltage_ is the cause _current_ does the damage. Provide the number of amps
that allow you to feel the current, stop you from letting go of the source, and stop the heart.
You can typically feel electrical shock at 1 mA, at 10 mA one is unable to release the electrical source,
100 to 200 mA is enough current to stop the heart.
25. What is a Faraday cage and how does it work?
A Faraday cage (also known as a Faraday shield) is an enclosure formed by conductive material or by
a mesh of such material, used to block electric fields. A Faraday cage operates because an external
electrical field causes the electric charges within the cage's conducting material to be distributed such
that they cancel the field's effect in the cage's interior. The ultimate effect then is no electrical charge
within the cage. This is why the technology is used to protect those working with high voltage.
26. A Retina iPad mini charger draws 2.1 A from a 5-V socket, how much power does it require? How
much does it cost to charge the iPad if it takes about 3.5 hours for a single charge if the utility company
charges 8¢ per kWhr?
P = IV P = 2.1 A (5 V) = 10.5 W = 0.0105 kW ($0.08/hr) (3.5hr) = $0.00294
27. Explain how a superconductor works. What are some applications of superconductors?
Broadly speaking, the higher the temperature, the more thermal vibrations there are inside the
crystalline structure of the material and the harder electrons will find it to flow through. Conversely, if
you cool the material down, you reduce the vibrations and make it easier for electrons to flow.
Superconductors are used in maglev trains, Magnetic Resonance Imaging (MRI) and Nuclear magnetic
resonance (NMR) machines, magnetic confinement fusion reactors and in particle accelerators.
28. What difference will it make if the switch is located in either of
these two alternate locations in the circuit?
It won’t make a difference as the switch simply closes the
pathway by which current flows in a circuit.
29. What if we switched the orientation of the battery? Are the results
the same or different?
In a simple circuit there is no difference. However, there are some
electrical components that do work in only one direction and as
such the switching of the battery would either not work or burn
out the component.
30. Explain what the multimeter will read and why with respect to volts in the current set up (Assume a
battery of 6 volts).
The circuit is not going to read any voltage as there is no
differential in charge (with respect to the battery) between
one side of the battery and the air.
31. Draw arrows to show the different paths that electrons can take in this parallel circuit.
Branch #1
Branch #2
Electrons flow from (-) terminal (anode) to (+) terminal (cathode)
If a light bulb is missing or broken in a parallel circuit, will the other bulb light? Explain.
Yes the light bulb will light as the two are parallel and the current has 2 paths it can travel through.
32. Tell whether the bulbs will or will not light based on the circuit. Explain why. If it cannot light show
how you would fix it.
It would NOT light as the circuit is not
closed.
Is this a series or parallel circuit?
Series
Add a wire connecting
the two light bulbs to
complete the circuit.
33. Complete the following table for the values in this circuit.
V
I
R
P
R1
1.778 V
8.08 mA
220 Ω
14.36 mW
R2
1.778 V
13.67 mA
130 Ω
24.30 mW
R3
10.22 V
21.75 mA
470 Ω
222.3 mW
34. Draw a circuit diagram for a single circuit with the following conditions:
a. a light bulb (1) is in series with resistor (1) and (2)
b. resistors (1) and (2) are in parallel with each other
c. motor (1) is in series with light bulb (1)
d. a voltmeter is used to measure the voltage across resistor (2)
Total
12 V
21.75 mA
551.7 Ω
261.0 mW