How is the slope of a voltage vs time graph the current?

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Discussion Overview

The discussion revolves around the relationship between the slope of a voltage vs time graph and current, particularly in the context of capacitors. Participants explore the mathematical implications and units involved, questioning the validity of common assertions found on various websites.

Discussion Character

  • Debate/contested
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • Some participants assert that the slope of a voltage vs time graph does not yield current, as the units derived from the slope are V/s, which does not equate to amperes.
  • Others challenge this by suggesting that the relationship holds true when considering capacitance as a constant factor, specifically in the context of capacitors.
  • A participant points out the need for proper variable usage, introducing the concept of capacitance and its role in the relationship between voltage and current.
  • One participant requests clarification on which websites claim that the slope represents current, indicating a desire for specificity in the sources of information.
  • Another participant agrees that the slope of voltage multiplied by capacitance can yield current through a capacitor, suggesting a conditional relationship based on the context of capacitive circuits.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of the slope of a voltage vs time graph as current. While some argue against it, others propose that it can be valid under specific conditions involving capacitance. The discussion remains unresolved with competing views presented.

Contextual Notes

Limitations include potential misunderstandings of the relationships between voltage, current, and capacitance, as well as the need for clarity in the definitions and contexts being discussed.

x86
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A lot of websites say that if you take the slope of a voltage vs time graph, you get the current. However, the math tells a different story.

where V = voltage, J = joules, C = coulombs, A = amperes, s = seconds

A = C / s
C = A * s
V = J / C
J = C / V

if we take the slope of Voltage vs Time, our unit is:

V/s = J/(C * s) = J / (A * s^2) = (C * V) / (A * s^2) = (C * V) / (C /s * s^2) = V/s

No matter what I do, I can never get the unit ampere.

How is it mathematically possible that the slope of a voltage vs time graph has the unit of the current? I don't get it.
 
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x86 said:
A lot of websites say that if you take the slope of a voltage vs time graph, you get the current. However, the math tells a different story.

where V = voltage, J = joules, C = coulombs, A = amperes, s = seconds

A = C / s
C = A * s
V = J / C
J = C / V

if we take the slope of Voltage vs Time, our unit is:

V/s = J/(C * s) = J / (A * s^2) = (C * V) / (A * s^2) = (C * V) / (C /s * s^2) = V/s

No matter what I do, I can never get the unit ampere.

How is it mathematically possible that the slope of a voltage vs time graph has the unit of the current? I don't get it.
The slope does not have the units of current. The units are V/s (which can be expressed in different ways) as you have shown. Perhaps you misread the information on the websites or you looked at websites giving wrong information.
 
Last edited:
x86 said:
A lot of websites say that if you take the slope of a voltage vs time graph, you get the current.
Which websites?

However, the math tells a different story.

where V = voltage, J = joules, C = coulombs, A = amperes, s = seconds

A = C / s
C = A * s
V = J / C
J = C / V
If V = J / C then J ≠ C / V

if we take the slope of Voltage vs Time, our unit is:

V/s = J/(C * s) = J / (A * s^2) = (C * V) / (A * s^2) = (C * V) / (C /s * s^2) = V/s

No matter what I do, I can never get the unit ampere.

How is it mathematically possible that the slope of a voltage vs time graph has the unit of the current? I don't get it.
You forgot to consider capacitance.

First the variables need to be used properly.
Type______Symbol______Unit
Voltage...E...V
Current...I...A
Charge...Q...C
Capacitance...C...F
time...t...s

Q = C * E
∴ E = Q / C
E / t = Q / (C * t)
E / t = I / C
 
x86 said:
A lot of websites say that if you take the slope of a voltage vs time graph, you get the current.
That is true for capacitors if you include capacitance as constant factor.
 
mfb said:
That is true for capacitors if you include capacitance as constant factor.

I agree with that if you are referring to:

C*dv/d(t)=i(t)

Then yes, the slope of the voltage multiplied by the capacitance will give the current at any given time thru the capacitor.
 

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