How Do You Determine the Direction of Potential Change in a Resistor?

In summary: I have a battery with plus and minus terminals and I pass through the terminals in the opposite order, ie from minus to plus then from plus to minus, will the voltage be the same? thanks,... so if I have a battery with plus and minus terminals and I pass through the terminals in the opposite order, ie from minus to plus then from plus to minus, will the voltage be the same?No, the voltage will be different.
  • #1
streetfightr4
8
0
hi, in the formula ChangeInPotential = Current X Resistance (formula for change in potential across a resistor) , if the resistor has two ends (point A and point B), how do I know wether it is the change in potential from point A to point B or from point B to point A
 
Physics news on Phys.org
  • #2
streetfightr4 said:
how do I know wether it is the change in potential from point A to point B or from point B to point A
You'll need to be told the orientation of the voltage source, if that matters. If all you need to find is the current, not its direction, you won't need to know.
 
  • #3
Doc Al said:
You'll need to be told the orientation of the voltage source, if that matters. If all you need to find is the current, not its direction, you won't need to know.

lets say my circuit is a direct circuit and I have a battery with plus and minus terminals connected to a load resistance. there are also wires between the load resistance and the battery terminals. And let's say

1) I go along the wire starting from the plus terminal all the way to the negative terminal and along the way I pass through points A and B , passing through A first. I know that there is a change in potential as I go from A to B. According to my textbook this change in potential is Current X Resistance.

2) I go along the wire but this time I start at the negative terminal of the battery and end at the positive terminal of the battery. Along the way I pass through points B and A. This time I pass through B first. IT SEEMS the change in potential is also = Current X Resistance which is the same as the change in potential in 1) (going from A first, then B), but how can this be because isn't it generally true that the change in potential from point1 to point2 is different from the change in potential from point2 to point1.
 
  • #4
(1) The change in potential from A to B is just the negative of the change in potential from B to A.
(2) The + terminal of the battery is the higher potential. Thus A is at a higher potential than B.
(3) The conventional current travels through the circuit from higher to lower potential. So going from A to B the potential drops by an amount equal to current X resistance.

When they say ΔV = current X resistance, they are often just talking about the magnitudes of those quantities. You put in the signs as explained above.
 
  • #5
Doc Al said:
(1) The change in potential from A to B is just the negative of the change in potential from B to A.
(2) The + terminal of the battery is the higher potential. Thus A is at a higher potential than B.
(3) The conventional current travels through the circuit from higher to lower potential. So going from A to B the potential drops by an amount equal to current X resistance.

When they say ΔV = current X resistance, they are often just talking about the magnitudes of those quantities. You put in the signs as explained above.

cool that was very helpful but how can one talk about the magnitude of ChangeInPotential when current in the formula can be negative?
 
  • #6
streetfightr4 said:
cool that was very helpful but how can one talk about the magnitude of ChangeInPotential when current in the formula can be negative?
You can talk about the magnitude of any quantity, if that's all you're interested in.

But try this. Imagine moving from A to B across the resistor. To find the voltage difference between A and B, including the sign, use: ΔV = -current X resistance. The sign of the current will be + if the current goes from A to B, otherwise it will be negative. In this case, the current is positive, so ΔV is negative.
 
  • #7
Doc Al said:
You can talk about the magnitude of any quantity, if that's all you're interested in.
So when my book says the "voltage drop as we go from higher potential A to lower potential B across the resistor is equal to Current X Resistance" , it is actually talking about the magnitude of the change in potential? Not the actual change in potential as the words "voltage drop" seems to imply?

DocAl said:
But try this. Imagine moving from A to B across the resistor. To find the voltage difference between A and B, including the sign, use: ΔV = -current X resistance. The sign of the current will be + if the current goes from A to B, otherwise it will be negative. In this case, the current is positive, so ΔV is negative.

thanks, this helps
 
  • #8
streetfightr4 said:
So when my book says the "voltage drop as we go from higher potential A to lower potential B across the resistor is equal to Current X Resistance" , it is actually talking about the magnitude of the change in potential? Not the actual change in potential as the words "voltage drop" seems to imply?
Since you are moving in the same direction as the current, the change in potential is negative. As your book says, it's a voltage drop across the resistor.
 
  • #9
thanks you were very helpful. I am going to now read everything you said and try to put it all together.
 
  • #10
As noted in the formula you'll need some information on orientation unless just the magnitude is asked.

In an actual circuit, if there is a ground symbol shown, the convention is to use that as the negative reference...but in some circuits it Is possible for positive to be the grounded...but that is very uncommon...

As a further confusion, conventional (positive) current flow gives one reference, actual current flow (negative electrons) gives another...

After a while you'll get used to the conventions and they become second nature...
 

Related to How Do You Determine the Direction of Potential Change in a Resistor?

1. What is electric potential?

Electric potential is a measure of the electric potential energy per unit charge at a given point in space. It is a scalar quantity and is measured in volts (V).

2. How is electric potential different from electric field?

Electric field is a vector quantity that describes the force a charge would experience at a given point in space. Electric potential, on the other hand, describes the energy of a charge at a given point. Electric field is related to electric potential by the equation E = -∇V, where E is the electric field, V is the electric potential, and ∇ is the gradient operator.

3. What is the relationship between electric potential and electric potential energy?

Electric potential energy is the potential energy a charge has due to its position in an electric field. It is directly proportional to the electric potential, with the equation U = qV, where U is the electric potential energy, q is the charge, and V is the electric potential.

4. How is electric potential measured?

Electric potential can be measured using a voltmeter, which measures the potential difference between two points in an electric field. The unit of measurement for electric potential is volts (V).

5. What is the difference between electric potential and electric potential difference?

Electric potential is a measure of the potential energy per unit charge at a single point, while electric potential difference is a measure of the difference in electric potential between two points. Electric potential difference is also known as voltage and is measured in volts (V).

Similar threads

Replies
21
Views
1K
  • Electromagnetism
Replies
11
Views
323
Replies
1
Views
865
Replies
7
Views
1K
Replies
4
Views
989
Replies
17
Views
1K
  • Electromagnetism
Replies
2
Views
441
Replies
4
Views
912
Replies
3
Views
621
Replies
3
Views
913
Back
Top