Understanding Current using nqvA

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Discussion Overview

The discussion revolves around the equation for electric current, expressed as Current = nqvA. Participants seek to clarify the relationships between the variables involved and how they collectively represent current, particularly in relation to time as current is defined as the rate of charge flow.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how the equation represents current, particularly regarding the role of time in the definition of current as the rate of charge carriers.
  • Another participant requests clarification on the variables in the equation and raises concerns about dimensional balancing based on their assumptions about the variables.
  • A participant provides definitions for the variables: n (density of charge carriers), q (charge of a charge carrier), v (average speed of charge carriers), and A (area of cross-section), asserting that the equation balances dimensionally as coulombs per second equals amperes.
  • Another participant confirms the relationship and explains the derivation involving a cylinder of charge carriers, emphasizing the calculation of charge passing a point over time.
  • A later reply mentions the use of current density (J) and provides an alternative expression for current density in relation to charge carriers and their velocity.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of the variables and the dimensional consistency of the equation. However, there remains some uncertainty regarding the conceptual understanding of how time factors into the equation and the overall clarity of the relationships among the variables.

Contextual Notes

Some participants express uncertainty about the context in which the equation is derived and how the variables are defined, indicating potential limitations in understanding the foundational assumptions behind the equation.

Who May Find This Useful

This discussion may be useful for students or individuals seeking to deepen their understanding of electric current, the relationships between charge carriers, and the mathematical representation of current in physics.

Googl
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Hi all,

I was wondering whether someone might help further understand the following equation for current. I mean get a good picture of how each valuable relates to each other and how they come to represent current.

Current = nqvA

Thanks. I know what each valuable represents but I can't seem to get a good picture how the equal represents current especially; Where time is taken into account since current is the rate of charge carriers/electrons.

I hope I have made my self clear.

Thanks.
 
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Oops. No need I understand it now.

...but I would appreciate any replies just to make sure.
 
Googl said:
Hi all,

I was wondering whether someone might help further understand the following equation for current. I mean get a good picture of how each valuable relates to each other and how they come to represent current.

Current = nqvA

Thanks. I know what each valuable represents but I can't seem to get a good picture how the equal represents current especially; Where time is taken into account since current is the rate of charge carriers/electrons.

I hope I have made my self clear.

Thanks.
Could you please say what the variables in that equation all stand for? Where did you find that equation and what was the context? I have a problem with balancing it, dimensionally if the variables are what I assume they are.
 
sophiecentaur said:
Could you please say what the variables in that equation all stand for? Where did you find that equation and what was the context? I have a problem with balancing it, dimensionally if the variables are what I assume they are.

n = density of charge carriers (1/m^3)
q = charge of a charge carrier. (coulomb)
v = average speed of charge carriers. (m/s)
A = area of cross-section of wire m^2.

If you multiply them together you get coulomb/s = Ampere
 
Just checking we're reading from the same hymn sheet..
This is the 'well known' relationship in which the dimensions are the same each side (that should be reassuring). The way it's derived is to consider a cylinder with of area A and unit length, moving at average speed v. The number of charges in the cylinder will be n times the volume, which is nA (times one, the unit length) so the charge in it will be nqA (per unit length) and the number of charges passing a point in one second will be nqAv. That's I, the current.
OK?
 
Googl,

I remember it as:

n or p (holes or electrons) = charge carriers
q = charge per carrier --±1.6E-19 C/(electron or hole)
v = velocity
A = area

So n*q*v*A or p*q*v*A equals current

Sometimes J (current density) is used. J = I/A = q*(p or n)*v.

EDIT: Sorry, I should have written in the first line, "concentration of n or p (holes or electrons) = charge carriers/m^3 .

Ratch
 
Last edited:

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