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

AI Thread Summary
The discussion centers on the confusion regarding the relationship between the slope of a voltage vs. time graph and current. While many sources claim that the slope represents current, the mathematical analysis shows that the slope has units of V/s, not amperes. It is clarified that this relationship holds true only when considering capacitance, as the equation C * (dV/dt) = I indicates that the slope multiplied by capacitance yields current. Therefore, the slope alone does not equal current without including capacitance as a factor. This highlights the importance of context when interpreting voltage vs. time graphs in electrical circuits.
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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