Online reading assignment: circuit basics

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.

The following questions were asked on reading textbook chapters and previewing presentations on circuit basics.

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.
"Electromotive force or emf is the voltage developed by a source of electrical energy. We can measure the current of this electrical energy by applying Ohm's law which states that the amperes of any circuit is equal to its voltage over its resistance. Simply stated, current is defined as the amount of charge (coloumbs) traveling per second through a circuit (referred to as amps)."

"Resistors use up voltage as current flows through them."

"What I was able to understand from tonight's reading was the direction of a basic circuit is from the positive terminal of a battery, through the circuit, and to the negative terminal of the battery; this direction is based off the way a positive charge will move, while electrons will flow in the opposite direction. I was also able to understand that a current is the same everywhere, and it will not get 'used up' by a bulb."

"I understand that the flow of electrons in a circuit flow opposite to the clockwise direction of how they are drawn. I understand how different chemical reactions have different amounts of potential energy to be used in volts."

"I understand this section pretty well. If we have a low resistance will have a higher current. If high resistance lower current."

"A battery is used to release EPE giving the charge that will move around the circuit. The circuit has a current that runs through it clockwise, but the electrons that are moving around are moving counterclockwise. The resistor that is in the circuit uses up voltage that is running through it so that if there is a large voltage applied, but not all of it is needed there will only be a small amount of current that ends up at the other end of the resistor that continues to flow through the circuit."

"I learned that 'stacking' batteries in a series results in the summation of the voltages of the individual sources, but putting them parallel will keep the voltage the same but increase the current supplied."

"I have a large background in electronics and electricity so I understood the chapter thoroughly."

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.
"Still not clear on how to to find total resistance, or break a circuit into parts yet."

"Terminology as well as the equations given in this sections readings were difficult to understand and follow. I'm confused as to how they each relate to each other as a whole."

"I don't understand resistors completely. Are they used to slow down current?"

"What I found a little confusing was the basic circuit. The current direction is confusing me a little."

"I found the relationship between current and voltage confusing. If the voltage is increased does the current increase or remain constant?"

"I do not understand why potential difference (voltage) ∆V is measured in volts and the resistance R is measured in ohms. I thought they should be the same, but when you put in values for ∆V/R, you get amperes?"

"The idea of 'stringing' (resistors in series) is not confusing, but I would like to know more about why we need to know about it. Also, could you explain electromotive source a little more. I tried to read about it in the blog and the book, but still didn't understand it. In Ohm's law will there always be multiple voltages and resistors in the numerator and denominator?"

"How all the concepts of a circuit connect (pun intended) is still unclear to me. The example of the stacked 9 V batteries makes no sense to me."

"I'm not sure what exactly causes a short circuit."

"I am kind of shaky on the basics of an actual circuit in regards to the flow of negatively charged electrons."

A wire is used to complete a circuit with a single 9.0 V battery. When a wire is used to complete a circuit with a system of 244 "stacked" 9.0 V batteries, there will be __________ voltage and __________ current, compared to the single 9.0 V battery circuit.
less; less.  [0]
less; more.  * [1]
more; less.  *** [3]
more; more.  ***************** [17]
(Unsure/guessing/lost/help!)  **** [4]

An emf source is connected to a container of water. When salt is dissolved in the water, there will be __________ resistance and __________ current, compared to the pure deionized water circuit.
less; less.  [0]
less; more.  ****************** [18]
more; less.  *** [3]
more; more.  ** [2]
(Unsure/guessing/lost/help!)  ** [2]

A metal screw completes a "short circuit" with a transformer emf source. This is dangerous due to the very __________ resistance of the metal screw, and the very _________ current flowing through it.
low; low.  [0]
low; high.  ****************** [18]
high; low.  ** [2]
high; high.  *** [3]
(Unsure/guessing/lost/help!)  ** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why does current flow opposite of the electrons in the circuit?" (Benjamin Franklin had a 50-50 chance of defining current flow in conductors right; he arbitrarily assigned the lack of electrons as being "one kind" of charge (positive), and the surplus of electrons as being the "other kind" of charge (negative). As it turned out, electrons are what easily flow through conductors, so the flow of electrons is a negative charge flow, which is the opposite of the definition of positive charge flow ("current").)

"So low resistance and high current is what causes short circuits?" (Well, all you need is low resistance. Whatever, the potential difference (voltage) is, a low enough voltage will cause a high current.)

"I thought it was interesting that higher voltage batteries use a series of stacked standard voltage batteries. I had no idea. Pretty cool!"

"How does the man in the 244-series batteries video not get shocked?" (Hopefully his hands are dry and have much higher resistance than the wire he is using to complete the circuit, so current will flow through the lower resistance wire than through his body.)

"How is the current relate to voltage? How does a resistor affect current and voltage?" (Let's say the potential difference (voltage) is fixed, because we're using an ideal 1.5 V battery. We can connect a low resistance resistor to the battery, and a lot of current will flow through it. Or we can connect a high resistance resistor to the battery, but in this case very little current will flow through it. If we don't connect anything to the battery, this is effectively an infinite resistance circuit, and zero current will flow. So the application of Ohm's law (I = ∆V/R) here is that resistance R will affect current I.)

"I don't understand the purpose of a resistor in a circuit. If the purpose of a circuit is to circulate charges, why would we add something that reduces circulation?" (While some devices need lots of current (electric heaters, stovetop ranges, blow dryers, etc.), some devices just need very specific amounts of small currents to carry out computational instructions in integrated circuit chips connected to a specific voltage source, so having resistors is how to control how much current flows through different parts of these circuits and devices.)

"How can we better prepare ourselves for the upcoming exam that is in about a week-and-a-half? Will there be a study guide for the midterm or should we just refer to the past midterms as a reference to what you are going to ask?" (The study guide on the topics covered on the midterm is now posted online, and there are now links to similar problems from past midterms. There will also be an in-class review session before the midterm.)

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