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Physics 2 Exam 2
Terms in this set (55)
Driven by AC power, the same electrons are moving back and forth across a circuit element like a lightbulb over and over again. How is it that these electrons haven't dissipated all their energy in the lightbulb after the first pass?
Each time the AC voltage changes, there is a new potential difference. The electrons might dissipate energy into the lightbulb as they pass through the first time, but on the return trip, they have a new potential difference and thus a new potential energy change across the bulb for each electron. This decrease in potential energy as the electron moves is what is translated into the energy dissipated by the bulb so you are always giving the circuit more energy which can then be dissipated over and over as well.
You have three capacitors and a battery. Would you store more energy in the capacitors if they were connected in parallel or in series with the battery? Explain.
You would store more energy if the capacitors were connected in parallel. In parallel, each capacitor would have the same potential drop across it (say 1.5 V) while in series each capacitor would only have a portion of the total potential difference from the battery across it (say, 0.5 V, 0.5 V, and 0.5 V). Since the capacitor's energy storage is proportional to C*V^2, having a larger V across each capacitor would add up to a considerably larger total energy stored.
You have a capacitor that is connected to a battery. If you insert a dielectric between the plates while the battery is still connected, what happens to the 1) charge on the capacitor plates, 2) the electric field between the plates, 3) the capacitance, 4) the potential difference across the capacitor and 5) the energy stored in the capacitor?
Inserting the dielectric will: 1) increase the charge on the plates, 2) keep the electric field the same, 3) increase the capacitance, 4) keep the potential difference the same, and 5) increase the energy stored in the capacitor.
For what purpose(s) would you connect batteries in series? For what purpose(s) would you connect batteries in parallel? Does it matter whether the batteries are similar or not in either case?
You would connect batteries in series if you wanted a larger voltage. Each battery in series would increase the voltage along the circuit. You would connect batteries in parallel if you wanted a smaller voltage but required a larger current or a longer lifetime for the device. For series, the batteries do not need to be similar except that you'd want them to have a similar lifetime so that they tended to be completely discharged at the same time. For parallel, you'd probably want the batteries to have similar voltage as well to maintain a consistent voltage to whatever device they are powering. This would also limit the potential current/charging loops over the batteries to a minimum.
In an RC circuit, current flows from the battery until the capacitor is fully charged. How does the energy depleted in the battery compare to the energy stored in the capacitor? If they are equal, explain why. If they are different, where does the extra energy go or come from?
The energy stored in the capacitor is less than the energy depleted from the battery. Since current flows through the circuit to charge the capacitor, it flows through the resistor in the circuit as well. When current flows through a resistor, it dissipates power, which means that it uses energy as well. Thus some energy is lost to the capacitor and some energy is lost to the resistor.
Does a magnet have a "charge" where all the magnetic field lines go out (like a positive charge) or all the lines go in (like a negative charge) ? For example, if you cut a bar magnet in half between the North and South ends, would part cut off at the North end act only like a North, and the part cut off at the South end only act like a South?
If you cut a magnet in half, the North and South ends would both act like a new bar magnet each with a North and South end of their own. So there isn't really a magnetic "charge". Experimentally, there have not been found to be any magnetic monopoles, so in most situations, there is no magnetic "charge" that can be separated from the basic dipole quality of a magnet. Although it might be possible to find a magnetic monopole in the future, none have been found so far meaning that for everyday purposes there is no magnetic "charge".
Which is easier to make: an electric motor using DC power or an electric motor using AC current? Explain.
A electric motor is easier to create with AC current. Typically, a motor is created with a current loop in a magnetic field that causes it to spin. If the current in the loop is DC, the loop will turn until it reaches a particular orientation and might oscillate around that point but it would not continue to turn around and around. Using AC, the force on the loop will alternate with the current which will allow the loop to continue to turn after it reaches the same stopping point that the DC version would have reached. Thus the loop can continue to turn with the AC power. This is one example of an advantage of AC power over DC power.
You have two wires running parallel to each other up and down. They have current running in opposite directions, one with current flowing up and one with down flowing down. What is the direction of the force on the wire (attractive, repulsive, other)? Explain this in terms of the magnetic field created by the wires and the force produced by the magnetic field.
The wires will be repelled from each other. Starting with the upward going current wire, this creates a magnetic field curling around the wire in the counterclockwise direction (viewed from above). If the downward current is to the left of the upward current wire, the magnetic field at the downward going wire from the upward going wire is pointed towards me. Using the right-hand rule, the downward going current crossed with the magnetic field pointed toward me means that the force on the downward wire is to the left, which is away from the upward going wire. The same basic argument can be made for the other wire. But in either case, the wire is repelled from the other wire.
What type of current distribution would create a magnetic field that is uniform in space with no dependence on distance from the current? Explain.
An uniform infinite plane of current would produce a magnetic field that is constant with no dependence on the distance from the current source. If one imagines a rectangular Ampere's Law loop around some subsection of an infinite plane of current, the line integral will increase with the length of the plane it encircles but so will the amount of current it encloses thus these two effects cancel each other out and the net result is to find that the magnetic field is constant and does not depend on the distance from the current. You can also make the same argument about magnetic fields as you could with an infinite plane charge and electric fields; with an infinite plane of current, there is no preferred distance scale so being far away "looks" the same as being close to the plane. Since the magnetic field falls off as a 1/r^2 law from a point of current (from the Biot-Savart Law) just like the electric field does from a point, this means that the net effect will be similar for similar types of distributions and since a uniform plane charge produces a uniform electric field, a uniform plane current should produce a uniform magnetic field. The main difference is that the uniform plane current will produce a magnetic field that is not perpendicular to the plane of current but rather points parallel to it but still at a 90 degree angle from the direction of the current.
How do you create a permanent bar magnet like the ones we've seen in class or the ones you see on your fridge?
To create a permanent magnet out of a non-magnetized material, you must apply a strong enough magnetic field to the material that is magnetically "hard" or in other words, which has a hysteresis curve that leaves a strong internal magnetic field even after a strong magnetic field that initially aligned the magnetic domains in the material has been turned off or is removed. This point is often reached upon approaching saturation and the material is most often a ferromagnetic material like iron.
In terms of potential what direction do positive and negative charges move?
positive charges move toward lower potential and negative charges move toward higher potential
What is the definition of current?
the flow of positive charges or the motion of the gaps between electrons
What is the definition of resistance?
the ability of material to resist or impede charge flow
Units of resistance
units of current
Amperes or Coulombs/sec
What is current proportional to?
voltage and 1/resistance
Does a larger resistor have a smaller or larger resistance?
smaller because the larger area gives the particles more options in terms of paths
What does a high current say about the power?
What does a high current say about the resistance?
What is a fuse? What is disadvantageous about it?
a safety device consisting of a strip of wire that melts and breaks an electric circuit if the current exceeds a safe level. you can't reset it which isn't ideal
What is a circuit breaker and how does it work? Draw.
A circuit breaker is kind of like a reset-able fuse. There is a bimetallic strip connected to the circuit that, if it heats up too much, bends and breaks the circuit. Once it cools down, you can bend it back to its original spot.
Draw the voltage and current graphs for AC.
What is Vrms?
root mean squared voltage
What is the average voltage and current of an AC circuit?
0 and 0
Draw Vrms curve
If you change the frequency of the AC voltage in the wall outlet what will happen to the average power?
It will stay the same because the average power isn't dependent on frequency
If you change the RMS voltage in an AC outlet from 120 V to 60v what happens to the average power?
decreases by a factor of 4
What is a capacitor?
two plates of conducting material that store energy and charge when a voltage is applied
Show what happens to a capacitor when voltage is applied
What is capacitance?
the ability to store energy as separated charges
units of capacitance
Farad (F) or Coulombs/volts
What does doubling the distance between capacitor plates do to the charge that can be stored in them?
decreases charge by 1/2
What does doubling the separation between two capacitor plates after disconnecting them from the circuit do to the electric potential?
increases it by a factor of two
What is the effect of putting a dielectric in a capacitor?
What is the dielectric constant?
K is always greater than one
What is the difference between resistance in a parallel circuit and a series circuit?
Less resistance in a parallel circuit because there are more paths for the current to travel
What are Kirchoff's rules?
1) junction rule- if you have some current I0 coming in and the circuit branches off, the two branches of current (I1 and I2) must equal I0. in other words, the sum of the current flowing into a junction must equal the sum of the currents flowing out of the junction
2) loop rule- in any closed loop in a circuit, the voltage changes across all the components must sum to zero
Why is the energy in a capacitor only have of the battery's energy?
the resistor depletes 1/2 of the battery's energy by charging it up
What do you do with the battery voltages when they are connected in parallel vs in series
in series you add the voltages of the batteries together
How do you not want batteries oriented in parallel
in opposite- you want both positive sides to be connected and both negative sides to be connected
What way does magnetic force go?
from north to south
Do monopoles exist for magnets?
not as far as we know
draw an electric and magnetic quadripole
draw two magnets interacting n to s and n to n
What is the earth's dipole from?
motion of molten core
What direction is the force on a negative charge traveling upward when the magnetic field points toward the right?
going toward you because it is a negative charge which flips the direction of the force
What direction is the force on a negative charge traveling in the direction of the magnetic field?
no force because the velocity is the same direction as the magnetic field
What direction is the force on a charged particle at rest in a magnetic field?
no force bc no velocity- has to be moving for there to be a force
right hand rule for a magnetic field produced by current
point thumb in the direction of the current and wrap fingers around the wire, which gives you your fingers pointing in the direction of the magnetic field
right hand rule for force on electric current due to magnetic field
fingers point straight along current then bend along magnetic field to give you thumb pointing in the direction of the force
right hand rule for force on electric charge q+ due to magnetic field
fingers point along particle's velocity v then along magnetic field which gives thumb pointing in the direction of force F
You put this capacitor with the dielectric inserted into a circuit with a 10 Volt battery. Then you remove the quartz sheet. Did the potential energy stored in the capacitor change when you removed the quartz? If not, why not? If so, by how much, did it increase or decrease, and where did the energy come from or go to?
The energy change is due to 2 effects. First, you did work by pulling the dielectric quartz out of the capacitor. Initially, one might think that this would increase the stored energy since you are adding energy by doing positive work. This would be true if the capacitor was already removed from the circuit, but in this case it is still hooked up to the battery. Since it is still hooked up to the battery, the voltage change across the capacitor is forced to be the same as the terminal voltage from the battery which means that current actually flows from one side of the capacitor to the other. So essentially, the dielectric was "holding" some of the charge in place on the capacitor and the removal of the dielectric allowed some charge to "fall" on back to the other side of the capacitor, releasing stored potential energy in the process.
How do the rules for connecting resistors in series and parallel compare to the rules for connecting capacitors in series and parallel?
The rules are basically switched. Adding resistors in series increases the equivalent resistance (R_eq = R_1+R_2+...) while adding capacitors in series decreases the equivalent resistance (1/C_eq = 1/C_1 + 1/C_2+ ...). Adding resistors in parallel decreases the equivalent resistance (1/R_eq = 1/R_1 + 1/R_2+ ...) while adding capacitors in parallel increases the equivalent capacitance.
You have a wire with current running through it that runs left to right. You place the wire in a magnetic field that runs up and down. What direction is the magnetic force on the wire? Explain.
The force on the wire will be towards you. The current I and magnetic field B are multiplied using a cross product thus you use the right hand rule to figure out the direction of the force (thumb toward the right, index finger up means middle finger toward you).
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