A few questions about Resistance and DC Circuits

In summary: There is no real answer to this question. It is just something that is included in the definition of a resistor.
  • #1
Feldoh
1,342
3
I'm just sort of looking for some clarification on some of this...

1) Say we have an ideal emf (battery, I guess...) connected in a circuit to a resistor and nothing else, why is the current the same on either side of the resistor? My textbook doesn't really explain this that well, but I think it's because charge is conserved, and if we pick any two points on the circuit the "flow" of electrons has to be the same for charge to be conserved.

2) So because of this, can we think of resistor as applying a resistance over the entire loop as opposed to one part of it? This seems weird to me because when we draw a circuit diagram representing this situation the resistor is included in only one part of the circuit as opposed over the entire loop, is there a reason for this?

3) Because of this resistors essentially have the effect of decreasing the current at any point in the loop by decreasing the the charge flow over a time interval, correct?

4) When talking about DC circuits we can consider the ideal emf as the work done per charge. When we talk about an emf in terms of work, we're talking about the ability of the emf to (literally?) pump electrons through the actual emf device, correct?

I have a few more questions, but if these are wrong there would be no point in asking them lol, so I'll save the rest for later...
 
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  • #2
Feldoh said:
1) Say we have an ideal emf (battery, I guess...) connected in a circuit to a resistor and nothing else, why is the current the same on either side of the resistor? My textbook doesn't really explain this that well, but I think it's because charge is conserved, and if we pick any two points on the circuit the "flow" of electrons has to be the same for charge to be conserved.
Yes.

Feldoh said:
2) So because of this, can we think of resistor as applying a resistance over the entire loop as opposed to one part of it? This seems weird to me because when we draw a circuit diagram representing this situation the resistor is included in only one part of the circuit as opposed over the entire loop, is there a reason for this?
I wouldn't look at it this way at all. It may work for a simple circuit like this, but it certainly would not work for typical circuits, and it is not realistic even for this simple circuit.

Feldoh said:
3) Because of this resistors essentially have the effect of decreasing the current at any point in the loop by decreasing the the charge flow over a time interval, correct?
Well, a resistor is defined as a device where the current is proportional to the applied voltage, so I would say the resistor makes the current proportional to the voltage rather than saying that it "decreases" it. It then affects the current in the rest of the loop by conservation of charge as you mentioned above.

Feldoh said:
4) When talking about DC circuits we can consider the ideal emf as the work done per charge. When we talk about an emf in terms of work, we're talking about the ability of the emf to (literally?) pump electrons through the actual emf device, correct?
Your wording is a little confusing, can you try again?
 
  • #3
feldoh said:
When we talk about an emf in terms of work, we're talking about the ability of the emf to (literally?) pump electrons through the actual emf device, correct?

The ability of the electric field to force a bunch of charged particles to go in the direction that is contrary to their spontaneous direction, to make them go in the direction that, all by themselves, those charged particles would naturally be repelled from.

I've been an EE for over thirty years and I still have to visualize circuit behavior with an analogy to gravity. Current is like someone telling you to carry a bucket filled with so many rocks. Voltage is like someone telling you that the place you have to carry those rock to is somewhere at the top or the bottom of a mountain.
 
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  • #4
Feldoh said:
So because of this, can we think of resistor as applying a resistance over the entire loop as opposed to one part of it? This seems weird to me because when we draw a circuit diagram representing this situation the resistor is included in only one part of the circuit as opposed over the entire loop, is there a reason for this?

That resistor symbol tells you, in effect: although there is an infinitely large number of infinitely small resistors throughout the length of this path, this one resistor conveniently represents the sum of all of them.
 
  • #5
Feldoh said:
why is the current the same on either side of the resistor? My textbook doesn't really explain this that well, but I think it's because charge is conserved, and if we pick any two points on the circuit the "flow" of electrons has to be the same for charge to be conserved.

The same reason that, if you have a garden hose, and water is going into one end of it at a rate of three gallons per minute, then water must also be coming out of the other end at a rate of three gallons per minute. It doesn't have a hiding place for it to store the stuff.
 

What is resistance?

Resistance is the measure of how much a material or device impedes the flow of electric current. It is represented by the symbol R and is measured in ohms (Ω).

What factors affect resistance?

The resistance of a material or device is affected by three main factors: its length, its cross-sectional area, and its material's resistivity. The longer the object, the higher the resistance. The wider the cross-sectional area, the lower the resistance. And materials with higher resistivity have higher resistance.

How is resistance calculated?

Resistance is calculated using Ohm's law, which states that resistance (R) is equal to the voltage (V) divided by the current (I). This can also be written as R = V/I. We can also use the formula R = ρL/A, where ρ is the resistivity, L is the length, and A is the cross-sectional area.

What is a DC circuit?

A DC (direct current) circuit is a closed loop through which current flows in only one direction. This is in contrast to an AC (alternating current) circuit, where the current periodically reverses direction. Examples of DC circuits include batteries, solar cells, and DC power supplies.

How do you calculate the total resistance in a series circuit?

In a series circuit, the total resistance is equal to the sum of all the individual resistances. This can be written as Rtotal = R1 + R2 + R3 + ... In other words, the more resistors that are added in series, the higher the total resistance will be.

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