20170331

Physics 205B, spring semester 2017
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"Kirchhoff's junction rule states that the sum of all the currents flowing into a system must be equivalent of the the current flowing out, Kirchhoff's loop rule states that there are rises and drops in potential but the net result in a complete loop stays the same. When an ammeter is used to break a circuit it essentially acts as a wire with zero resistance, in contrast a voltemeter has almost infinite resistance and will likely block any current if used to break a circuit."

"Kirchhoff's rules--for the junction rule the sum of all currents flowing into a junction must equal the sum of all the currents flowing out of the same junction. For the loop rule, if there is a complete loop in a circuit, where we end up at the same spot as we did when we started, all of the electric potential rises added together must equal all the electric potential drops added together."

"A change in electric potential represents the potential energy used by a charge. For each charge that uses a certain amount of electric potential, the amount of electric potential energy used is equal to charge times electric potential."

"Resistance increases in series circuits and decreases in parallel. Voltmeters measure potential and have high resistance, Ammeters measure current and have negligible resistance."

"I now understand how a voltmeter takes its reading of the change in voltage across a circuit by connecting in parallel to the system. Finding the current of the circuit and multiplying by each of the resistors between the connections of the voltmeter show the appropriate reading."

"As resistance goes down in a shorted battery circuit, there is more danger associated. For example, the gum wrapper being set on fire as it completed the circuit. Had the gum wrapper had a higher R value, there would be less risk of fire."

"Nothing, honestly."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"Not much, just need to do a few more circuit-solving problems."

"When you have to redraw the circuits because of parallel or not parallel. I just need to practice redrawing them."

"I think I need to see more examples of how to use the equations when using either an ammeter or a voltmeter."

"The different types of switches than can either be open or closed and the effect that has on resistance."

"The stuff that we covered on Monday in class with the voltmeter and ammeter stuff. I feel like there isn't enough understanding of the concept to go off of when we did the worksheet. So the worksheet was more so guessing my way through it than really understanding things. I left class with a bunch of question marks in my head."

"Using given equations to make substitutions for power dissipation."

"The power equations. I do not know how power can be expressed three different ways."

"A lot of stuff, as usual."

What are the resistances of these (ideal) devices?
(Only correct responses shown.)
Ideal light bulb: some finite value between 0 and ∞ [67%]
Burnt-out light bulb: ∞ [48%]
Ideal wire: 0 [52%]
Real (non-dead) battery: some finite value between 0 and ∞ [76%]
Ideal switch, when open: ∞ [24%]
Ideal switch, when closed: 0 [33%]

Two light bulbs with different resistances r and R, where r < R, are connected in series with each other to an ideal emf source. Select the light bulb with the greater quantity.
(Only correct responses shown.)
More current flowing through it: (there is a tie) [14%]
Larger potential potential difference: light bulb R [52%]
More power used: light bulb R [57%]

Two light bulbs with different resistances r and R, where r < R, are connected in parallel with each other to an ideal emf source. Select the light bulb with the greater quantity.
(Only correct responses shown.)
More current flowing through it: light bulb r [57%]
Larger potential potential difference: (there is a tie) [38%]
More power used: light bulb r [38%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Could we go over power dissipation examples?"

"Doing Monday's lab really helped me distinguish the difference between an ammeter and a voltmeter and what each one measures."

"What makes the resistivity of materials strongly dependent on temperature?" (Since current is the flow of electrons, there are two things that either help or do not help these electrons move. The speed of these electrons depends on the temperature--cold temperatures mean that they can move slowly, and hot temperatures mean that they can move quickly, so raising the temperature tends to increase the speed of electrons and decrease the resistivity of the material. However, electrons need to pass through all the atoms in the material, and the vibrational motion of these stationary atoms also depends on the temperature--cold temperatures mean that they randomly vibrate very little, and hot temperatures mean that they randomly vibrate a lot, so raising the temperature tends to increase the random vibrations of the atoms, which can "block" to motion of electrons, and increase the resistivity of the material. These are two competing effects, and for different materials one effect will typically dominate over the other, and even this might change at different temperature ranges for the same material.)

"I don't understand how adding plugs will become dangerous when the ideal wires have low resistance." (If you start to add more resistors in parallel, the equivalent resistance will begin to decrease. E.g., five 100 Ω resistors in parallel have an equivalent resistance of 20 Ω, ten Ω resistors in parallel have an equivalent resistance of 10 Ω. Since all the outlets in a single household circuit are wired in parallel, plugging in more appliances (with a given resistance) means more are connected in parallel with each other; and then the equivalent resistance goes down, and the amount of current that will flow through the wire supplying that part of the house will increase. Power dissipated in the wire is I2·R, so even if the resistance of the wire is small, the current-squared will be very high, and the wire can get dangerously hot enough to melt its insulation wrapping, and start a fire behind the walls!)

"Why can we disregard the positive versus negative flow when dealing with an ideal battery? "For an emf source (such as an ideal battery) with a potential difference of ε, we do not pay attention to the direction of current flowing through the battery, but instead watch how we move through the battery with respect to the positive (+) and negative (–) terminals, which respectively represent the higher and lower electrical potential ends of the battery." (If this sounds non-physical, it kind of is--this is done when applying the loop rule (looking for potential drops and rises), so "walking around" a loop may or may not correspond to the actual flow direction of current. When applying the loop rule, if you travel into the (–) terminal and out of the (+) terminal of a battery, then you increase in potential (this is the conventional way a battery would be placed in a circuit); but if you happen to travel into the (+) terminal and out of the (–) terminal of a battery (which means that it is put in "backwards"), then you decrease in potential.)

"Brooo, it's April already." (We're still in March. #toosoon)