# How do we know if a resistor is added or subtracted?

• k_squared
In summary, The book says that adding resistors causes a current heading north, while subtracting resistors causes a current heading south. However, the rule is that if you walk through a resistor in the same direction as the current flow, then a potential drop occurs. If you walk through the resistor against the flow of the current, then a potential rise occurs.
k_squared

## Homework Statement

I was reading in my physics book about loop currents. However, some resistors are added to the result (R*I) whereas some resistors are subtracted from the result -(R*I). The book uses the phrase "uphill" and "downhill" to describe this, and, indeed, the so called "uphill" resistors where the values are added instead of subtracted the chosen current was headed north as it passed through them, but I doubt that can be the rule.

My answer, however, is WHY does this work? Is it not possible to construct a circuit where the resistor could be on either side of the loop (like a circuit with only one loop, a battery and two resistors?)

So how do we know whether to add or subtract resistors?

(N/A)

## The Attempt at a Solution

(see above.)[/B]

k_squared said:
So how do we know whether to add or subtract resistors?
You don't add or subtract resistors. It's the potential changes that are added or subtracted. Have you studied Kirchhoff's voltage law? The term "uphill" indicates rise in potential and "downhill" indicates drop in potential. In any circuit loop, the sum of all the potential changes is ZERO.

KVL I understand (or at least I think I do. However, I can't seem to identify whether the voltage drops or rises across a resistor. (Sorry that I may have worded that poorly, I've been up for 22 hours.)

When you are analyzing a circuit using KVL the first thing you do is assign currents through all the components, usually indicated by arrows showing their assumed directions (whether those assumed directions turn out to be correct or not doesn't matter, the math will take care of sorting that out). When you do your "KVL walk" around a loop, the rule is that if you walk though a component in the same direction as its designated current, then a potential drop occurs. If you walk through the component against the flow of the current, then a potential rise occurs. To extend your book's analogy, it's like following a stream where the running water represents the current flow. If you walk against the flow you are moving uphill. If you walk with the flow, you are moving downhill.

CWatters
k_squared... Perhaps try posting an example circuit, apply the method gneill describes and we can see where you are going wrong if at all.

## 1. How do we physically determine if a resistor is added or subtracted?

To physically determine if a resistor is added or subtracted, you can use a multimeter to measure the resistance before and after the addition or subtraction. If the value increases, a resistor was added. If the value decreases, a resistor was subtracted.

## 2. Can we determine if a resistor is added or subtracted by just looking at the circuit diagram?

No, it is not possible to determine if a resistor is added or subtracted just by looking at the circuit diagram. The circuit diagram only shows the components and their connections, but not the actual values of the resistors.

## 3. How does the overall resistance change when a resistor is added or subtracted?

The overall resistance of a circuit will change when a resistor is added or subtracted. The amount of change depends on the value of the resistor and its position in the circuit. Adding a resistor in series will increase the overall resistance, while adding a resistor in parallel will decrease the overall resistance.

## 4. Is there a way to calculate the change in resistance when a resistor is added or subtracted?

Yes, you can use Ohm's Law (R = V/I) to calculate the change in resistance when a resistor is added or subtracted. This equation relates the resistance (R) to the voltage (V) and current (I) in the circuit. By measuring the voltage and current before and after the addition or subtraction, you can calculate the change in resistance.

## 5. Why is it important to know if a resistor is added or subtracted in a circuit?

Knowing if a resistor is added or subtracted in a circuit is important because it affects the overall resistance and can impact the performance of the circuit. It also helps in troubleshooting and understanding the function of the circuit. Additionally, keeping track of changes in the circuit helps in making accurate measurements and calculations for future modifications.

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