Relationship of voltage and resistance

[Googl]

There is supposed to be a voltmeter. This is a better image.

 

[sophiecentaur]

So, after pages and pages of discussion, we have arrived at the Potential Divider (/potentiometer). Halleluya. Isn't this in every elementary textbook with all the sums laid out for you?

 

[Googl]

sophiecentaur,

Isn't a variable resistor also known as a Potential Divider? I remember using the keywords voltage divider/variable resistor/potentiometer somewhere. These are all similar terms, am I not correct?

Anyway to not change subject and over complicate things, how would you describe voltage across the resistor as the knob is adjusted along the potentiometer? This is a good analysis of the relationship/proportionality of voltage against resistance as opposed to having a fixed resistor. I have given my idea above somewhere would you say this is true?

Logically it would seem that as the knob is adjusted to the full 20kΩ of the potentiometer, the voltage measured across would be 0V but this isn't true. So with a 20V battery when the knob is moved to the full end of the potentiometer at 20KΩ the voltage measured would be 20V. In fact here we could use the current formula to find the current at this total, keep it constant then work out what is the voltage as the knob is moved across (from 20KΩ, 18KΩ, 16KΩ, 14KΩ, 12KΩ, 10KΩ, 8KΩ, etc). I hope I have been clear there. I understand this is not current which is what you might think I am assuming here.

The idea that at a 20KΩ resistor will provide a 20V when connected to a 20V battery is what is hard to grasp. This would take me back to my original post;

"Would you say voltage drop across a resistor is directly proportional to the resistance.

I am trying to understand the relationship between voltage and resistance."

I think I have been constant throughout with my question...

 

[dlgoff]

I hate the water analogy but let's give it a try

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html#c1"

just for the heck of it.

 

[Andrew Mason]

...In fact here we could use the current formula to find the current at this total, keep it constant then work out what is the voltage as the knob is moved across (from 20KΩ, 18KΩ, 16KΩ, 14KΩ, 12KΩ, 10KΩ, 8KΩ, etc). I hope I have been clear there. I understand this is not current which is what you might think I am assuming here.

Stop right there. A potentiometer has three terminals: one on one end of the resistor, one on the other end, and a sliding contact that slides along the resistor. If you do not connect a load between the sliding contact and one of the other contacts, the voltage measured from the sliding contact to one end will be proportional to the distance the sliding contact is from that end to the total length of the resistor.

The idea that at a 20KΩ resistor will provide a 20V

Stop right there. A resistor does not provide voltage. A battery provides voltage, which is energy per unit charge. A resistor uses energy. The energy it uses is the potential drop (change in potential energy/unit charge) x charge passing through the resistor. The rate of energy use (power consumption) is the potential drop x current (charge/unit time).

AM

 

[Googl]

Stop right there. A resistor does not provide voltage. A battery provides voltage, which is energy per unit charge. A resistor uses energy. The energy it uses is the potential drop (change in potential energy/unit charge) x charge passing through the resistor. The rate of energy use (power consumption) is the potential drop x current (charge/unit time).

AM

I understand that I just did not find the right words at the time.

So having DROP at the end of voltage is very significant (voltage drop). A circuit will have 20V across it without a resistor. When a resistor is added a voltage drop will occur across the resistor which will be directly proportional to the size of resistance. So the greater the resistor resistance the greater the voltage drop across the resistor that occurs.

I think I did not have enough confidence in what the quantity of voltage actually is, which is the difference of two points electrical potentials. So when a voltmeter is set at two terminals it will measure the potential difference between the two terminals, so the electrical potential at one terminal could be 5V while the other 2V and the voltage would be 5V-2V which is equal to 3V.

 

[Andrew Mason]

I understand that I just did not find the right words at the time.

So having DROP at the end of voltage is very significant (voltage drop). A circuit will have 20V across it without a resistor. When a resistor is added a voltage drop will occur across the resistor which will be directly proportional to the size of resistance. So the greater the resistor resistance the greater the voltage drop across the resistor that occurs.

This is not correct. An electric potential s applied to the resistor by the voltage source (battery). The amount of resistance placed in the electric field created by the voltage source (ie. the load resistance to which the voltage is applied) does not affect the magnitude of this potential difference. The resistor material provides charges that move in response to the electric field created by the electric potential. The greater the resistance, the fewer charges that flow in response to the electric field. The potential energy of charges present in that field (ie. in the resistor having uniform resistance per unit length) decreases in a linear fashion as one goes from one end of the resistor to the other, from 20 Joules/coulomb at one end down to 0 at the other.

I think I did not have enough confidence in what the quantity of voltage actually is, which is the difference of two points electrical potentials. So when a voltmeter is set at two terminals it will measure the potential difference between the two terminals, so the electrical potential at one terminal could be 5V while the other 2V and the voltage would be 5V-2V which is equal to 3V.

This is correct but somewhat misleading. Voltage (potential difference) is potential energy per unit charge at one point relative to another. Potential difference at a point is always measured relative to some other point. So we can only talk about 5V or 2V in relation to TWO points, not one.

Voltage does NOT depend on the size of the load resistance to which the voltage is applied (unless the load resistance is in the same order of magnitude as the internal resistance of the voltage source).

Think of Niagara Falls. The "potential difference" is determined only by the height of the falls. Whether I run a garden hose from above the falls to a point at the bottom to run a lawn sprinkler, or a 5 metre diameter channel to a turbine generator, the potential drop for both loads is the same.

AM

 

[Dale]

I am trying to understand the relationship between voltage and resistance.

Ohm's law: V=IR

They are proportional to each other given a fixed current.

 

[sophiecentaur]

@Googl
I'm afraid that you started this thread with a question that used the 'wrong' words with not enough context for people to understand what you were really getting at. You then drew a diagram which (strictly) was not of the circuit you were discussing - which further confused everyone.

This sort of thing would not occur if you were prepared to get information from a standard text, rather than trying to follow your own line of enquiry. If you had read around a bit then you would have known that a variable resistor is not a potential divider and that a 'potentiometer' is a three terminal device that can be connected in various ways. You would also have found the (very elementary) formula for the way a potential divider works and your question would have been answered without all this confusion.
I don't want just to be grumpy about this but there are better ways of getting to grips with electronics - which involve taking information as it is normally taught. It is presented as it is because that method works.
But don't stop asking relevant questions!

 

[semih93]

we use wires

I think that we can not say r is equal to 20ohm volt will be 20 volt ,because we use wires to connect resistance and battery and these wires's resistance are not zero hence they take voltage,theoretical we say v=I.r and if I has constant value then volt and ohm are proportional but in there we don't condider wires's resistance and you guess that in real this is not like that

 

[sophiecentaur]

I think that we can not say r is equal to 20ohm volt will be 20 volt ,because we use wires to connect resistance and battery and these wires's resistance are not zero hence they take voltage,theoretical we say v=I.r and if I has constant value then volt and ohm are proportional but in there we don't condider wires's resistance and you guess that in real this is not like that

There are remarkably few 'electronics' circuits in which you actually need to take into account the resistances of connecting wires. For a long mains supply cable or a short lead from battery to car starter motor, the wire resistance is very relevant but, in most cases, you really don't need to bother. Even when you do, you just need to treat the connecting wires as additional resistances in the circuit and follow the formula through.

I do not understand why people are trying to describe what happens in a circuit in words when the tried and tested formulae work so well. At this level, electrical theory is really not suited to 'verbal' descriptions. Just learn and use the formulae guys your alternative, 'chatty', analysis is not revealing anything significant about the Physics of what's happening.

 

[Benny Jacob]

As I understand,Current is just the magnitude of the amount of electrons and voltage is a pressure force (potential difference) which causes for the flow of electrons.. Without this pressure(voltage) there is no current/electron flow...And resistance is something (like a property of a material depending upon temperature) which opposes the flow of electrons.
Actually resistance is not getting varied unless there is change in temperature or length or the cross sectional area...
Mathatically resistance is just the proportionality 'constant' to derive the Ohm's law. and it is an independent from voltage or current.