Voltage in RL Circuit Series: Why?

In summary, current flows through the inductor in the opposite direction of the natural current, and the higher voltage end of the inductor is where the current first enters.
  • #1
Jzhang27143
38
1
Suppose there is a circuit consisting of a battery, a resistor, and an inductor, and that there is initially no current. As current is increasing, the inductor creates an induced current in the opposite direction. I am confused as to which end of the inductor is at a higher voltage. Since the induced current is in the opposite direction as the natural current, shouldn't the natural current enter the inductor through the lower voltage end? I know this is not true because this would mean that there is an increase in voltage which would completely change the differential equation using Kirchoff. Why is the end of the inductor that the current first enters the higher voltage end?
 
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  • #2
Lenz' Law tells you that the induced voltage opposes the source. So the back emf (so called) will raise the voltage the L end of the resistor, to reduce the current that would flow if the L were just a piece of wire. If you have a very high Inductance, there could be no detectable current.
the Lenz law thing can be confusing at first but following the rule, 'verbally' can often tell you the result. It's not always at all obvious is you just try to follow intuition.
 
  • #3
Jzhang27143 said:
..., the inductor creates an induced current in the opposite direction. ...
For what it's worth, I don't believe the inductor creates an induced current: I believe it creates an induced emf. I believe there is only one current in the circuit and that it depends on the sum of all the emfs and resistances.
The only emf across the ideal inductance would be the induced emf and it would not generate its own current, but simply help determine the circuit current.

Jzhang27143 said:
Why is the end of the inductor that the current first enters the higher voltage end?
Because it is not a resistor (assuming we are talking ideal elements.)
Accepting your terminology:
The voltage across a resistor is always proportional to the current and the current enters at the higher voltage end.
The voltage across an inductance is proportional to the rate of change of current, not to the current itself. When the current is increasing, the higher voltage end will be where the current enters, when the current is decreasing, it will be the end where the current is leaving.
 
  • #4
We do have a lot of problems with signs, don't we? (Me too sir)
That's when drawing diagrams (or having them in your head - if you can) can be so important. If you actually say to yourself, the time worn phrases about these things and then add arrows and +- signs to the diagram, accordingly, then things can become a lot clearer. It's very easy to get locked into a nonsense conclusion if you don't follow things through from the basics and rely on that fickle friend intuition.
 

Related to Voltage in RL Circuit Series: Why?

1. What is the definition of voltage in an RL circuit series?

Voltage in an RL circuit series is the potential difference or electrical pressure between two points in the circuit. It is measured in volts (V) and represents the amount of energy required to move a unit of charge from one point to another.

2. Why does voltage change in an RL circuit series?

Voltage changes in an RL circuit series because of the presence of inductance. When current flows through an inductor, it creates a magnetic field that opposes the change in current. This opposition to the current flow creates a back EMF (electromotive force) that causes the voltage to change.

3. How does the inductance affect the voltage in an RL circuit series?

The inductance in an RL circuit series affects the voltage by creating a back EMF that opposes the current flow. This results in a voltage drop across the inductor, which can be calculated using the equation V = L di/dt, where V is the voltage, L is the inductance, and di/dt is the rate of change of current.

4. What is the relationship between voltage and current in an RL circuit series?

The relationship between voltage and current in an RL circuit series is that the voltage leads the current by 90 degrees in a purely inductive circuit. This means that when the voltage is at its maximum, the current is at its zero, and vice versa. This relationship can be visualized on a phasor diagram.

5. How does the value of resistance affect the voltage in an RL circuit series?

The value of resistance affects the voltage in an RL circuit series by limiting the current flow and thus reducing the amount of back EMF created by the inductor. This results in a decrease in the voltage drop across the inductor and an increase in the overall voltage in the circuit. Ultimately, a higher resistance value will result in a higher voltage across the inductor and a lower voltage in the circuit.

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