About polarity and resistance - Page 1
Forum section: electronics / beginners

author: iwkse
author_rank: Newbie
timestamp: November 20, 2014, 08:45:09 pm
content: Hi, I'm studying electronics since little time, so excuse me if my questions are very basic but I guess you can help me to clarify dubts.1. If I use a voltmeter and I measure a 4.5V battery, and I connect the COM cable to positive and Vohm cable to negative I get a negative number on the display. Is this normal? So it means there are n amount of electrons (negative sign) that still have to travel to the opposite side?2. I was reading that resistance reduce the voltage..here I had a doubt because I though it reduces the current. Could you clarify?Thanks!

author: IanB
author_rank: Super Contributor
timestamp: November 20, 2014, 09:00:34 pm
content: Quote from: iwkse on November 20, 2014, 08:45:09 pmVoltage is a measure of the difference in potential between two points. If you reverse the direction of a measurement you always should have a change in the sign of the measurement, so it is expected that +4.5 V in one direction becomes -4.5 V in the other direction.QuoteDepending on how it is used, a resistor may reduce voltage or current (or frequently both at the same time).A resistor causes a voltage difference to appear between its terminals when a current flows through it. This voltage difference has to come from somewhere else, so somewhere else you will measure less voltage once the resistor is in the circuit. But the loss of voltage across the resistor also reduces the driving force for current, so the current in the circuit will be reduced as well.

author: iwkse
author_rank: Newbie
timestamp: November 20, 2014, 09:13:32 pm
content: Quote from: IanB on November 20, 2014, 09:00:34 pmWell yes, I understood about the change of sign..just I was expecting a plus sign instead of a minus one.Quote from: IanB on November 20, 2014, 09:00:34 pmThanks about it, I understood!

author: IanB
author_rank: Super Contributor
timestamp: November 20, 2014, 09:30:42 pm
content: Quote from: iwkse on November 20, 2014, 09:13:32 pmYou understand this sentence doesn't make sense, right? If you understand about the change of sign, you must also expect a minus sign on the measurement when you put the red test lead on the battery negative terminal and the black lead on the positive terminal.

author: Sigmoid
author_rank: Frequent Contributor
timestamp: November 20, 2014, 09:46:16 pm
content: Quote from: iwkse on November 20, 2014, 08:45:09 pmUnderstanding electricity is a lot simpler if you don't bring electrons into it quite yet.Since batteries are electrochemical apparatuses, their internal physics involves electrolytes and ions. Let's talk in terms of abstract electric charge. The positive terminal of a battery emits positive charge, and the negative terminal sinks positive charge. Positive charge flows from higher potential to lower potential.The two terminals are at a potential, which is the voltage given on the battery's packaging, and which enables it to act as a power source. Say, you have a new 1.5V battery. In that battery, the positive terminal is at +1.5V in relation to the negative terminal. Which, conversely, means that the negative terminal is at -1.5V in relation to the positive terminal.Quote from: iwkse on November 20, 2014, 08:45:09 pmI = U/R- This is Ohm's law. The current across a resistor equals the voltage across the resistor, divided by the resistance.If you place a 1 kOhm resistor in series with your 1.5V battery, the full potential of the battery will appear over the resistor. Therefore, the current will be 1.5V/1kOhm = 1.5mA.Now, if you took ANOTHER 1kOhm resistor, and inserted it into the circuit in series (ie. battery - resistor - resistor - back to the battery), now the voltage of the battery appears across the two resistors. The current in the whole circuit will be 1.5V/(1kOhm+1kOhm) = 0.75mA. So yes, adding extra series resistance DID reduce the current flow.The voltage drop (ie. the potential difference) between the "hot" terminal of the first resistor (connected to the + terminal of the battery), and the "cold" terminal of the second resistor (connected to the - terminal of the battery) is still 1.5V, as the battery keeps it that way. But if you took a measurement across the two terminals of only the FIRST resistor, you'd see a voltage difference of 0.75V!Why? Because Ohm's law works in all combinations. U = R*I. So with a current of 0.75mA, the voltage across one resistor will be 1kOhm * 0.75mA = 0.75V! The voltage is divided across the two resistors, with each of themdropping0.75V, which adds up to the total 1.5V.Though if you connected the two resistors in parallel, things will look significantly different.BTW I don't know what kind of source you use to study some electronics, but I think you need better ones. The Electronics for Dummies book is pretty good for the basics, actually. There are also lots of good explanations on-line. And don't worry about electrons for now, they will just confuse you.

author: iwkse
author_rank: Newbie
timestamp: November 20, 2014, 10:59:43 pm
content: Quote from: IanB on November 20, 2014, 09:30:42 pmSo..this mean I have a negative sign because I measure electrons (with negative charge)? And when all the electrons will be attracted by the positive charge it will just show zero.I'm a enough near to the idea?Quote from: Sigmoid on November 20, 2014, 09:46:16 pmWell..to be sincere I prefer to understand at the root, I find it more difficult just to memorize something without knowing the reason of it.I'm using some italian book but I will also take a look at the one you mentioned, thanks. Anyway you helped to clear that though.

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 20, 2014, 11:24:29 pm
content: Quote from: iwkse on November 20, 2014, 08:45:09 pmWhen you say "reduce", you need to have a starting point for comparison. Depending on the circuit and how you're connecting things, adding a resistor can reduce or increase voltage, and it can reduce or increase current. When you add a resistor, are you inserting it into a conductor (starting from zero ohms), or are you wiring it across two terminals that had been disconnected (starting from infinite ohms)? Is the circuit powered by a constant voltage source, a constant current source, or something else? The answers to these questions will help determine what measurements are increasing and what are decreasing.What is true of a resistor is that the voltage across the resistor is proportional to the current through the resistor, and the proportionality is given by Ohm's law, V=IR.

author: Shock
author_rank: Super Contributor
timestamp: November 20, 2014, 11:39:00 pm
content: Quote from: iwkse on November 20, 2014, 09:13:32 pmWhen you place the black probe to the battery + terminal you are saying this my reference point to ground or 0v. When you attach the red probe to the - terminal the meter detects this as less positive than the + terminal. Therefore the reading is shown as 4.5v less positive/below the 0v reference, -4.5v.Normally to take this measurement you would connect the black probe to the - terminal so it displays correctly.

author: IanB
author_rank: Super Contributor
timestamp: November 21, 2014, 12:21:49 am
content: Quote from: iwkse on November 20, 2014, 10:59:43 pmNo, you are very far away from the idea. Thinking about electrons will only confuse you. Until you can see an electron and hold it in your hand, you should assume electrons do not exist. This is what people had to do around the year 1850.There is a thing called "voltage" which is a potential difference between two points. You can measure this potential difference with a voltmeter. The potential difference has a sign. If you measure it one way it will be positive, if you measure it the other way it will be negative.There is another thing called "current". Current will flow between two points with a potential difference or voltage between them if you connect these two points with something that conducts electricity. Current has a direction, and flows from the point with the higher potential to the point with the lower potential. Current can be measured with an ammeter if you cause the current to flow through the meter on its way from one point to another.The two paragraphs above represent an important concept in science and engineering calledabstraction. Abstraction means separating what you need to know from what you don't need to know. All problems are simplified if you only consider what you need to know and if you exclude unhelpful details.

author: Shock
author_rank: Super Contributor
timestamp: November 21, 2014, 12:23:28 am
content: We define current flow as traveling with the positive charge + to - (this is called "conventional current flow"). The electron flow is from - to +. You will see both referred to in electronics, it's easy to spot. The math we use is the same either way.

author: iwkse
author_rank: Newbie
timestamp: November 21, 2014, 01:19:49 am
content: Quote from: Shock on November 21, 2014, 12:23:28 amThanks man, this is what I needed to know and it was the cause of my confusion. Thanks to all others too for the precious comments.