# Solving Models: Deriving Expressions for Transient Capacitor Voltage

• MattRSK
I am unsure of how to make v(t) the subject of the last one?I think you might want to brush up on your algebra skills before trying to tackle this problem. The steps you have taken are incorrect and you are not understanding the explanations given to you. Until you have a
MattRSK
1. Homework Statement
The transient behaviour of a capacitor has been studied by measuring the voltage from across the device as a function of time. The following data has been obtained.

Time (s)/Voltage (v)
0 / 10
1 / 6.1
2 / 3.7
3 / 2.2
4 / 1.4
5 / 0.8
6 / 0.5
7 / 0.3
8 / 0.2
9 / 0.1
10 /0.07
12 /0.03

2. Homework Equations

The following relationship for the change in voltage, V, with time, t, has been proposed:

DV/DT = -k fcn(v)

Where k is a constant and fcn(v) could have one of the following forms:

1. fcn(v) = c (where c is a constant)
2. fcn(v) = V
3. fcn(v) = V2

(fcn is an abbreviation of the term "a function of")

I am to derive three corresponding expressions for V as a function of time.

3. The Attempt at a Solution

I am having trouble with where to start. I believe the aim is to find which equation, using the data, creates a linear relationship. Any help on how to derive the equations would be much appreciated.

MattRSK said:
1. Homework Statement
The transient behaviour of a capacitor has been studied by measuring the voltage from across the device as a function of time. The following data has been obtained.

Time (s)/Voltage (v)
0 / 10
1 / 6.1
2 / 3.7
3 / 2.2
4 / 1.4
5 / 0.8
6 / 0.5
7 / 0.3
8 / 0.2
9 / 0.1
10 /0.07
12 /0.03

2. Homework Equations

The following relationship for the change in voltage, V, with time, t, has been proposed:

DV/DT = -k fcn(v)

Where k is a constant and fcn(v) could have one of the following forms:

1. fcn(v) = c (where c is a constant)
2. fcn(v) = V
3. fcn(v) = V2

(fcn is an abbreviation of the term "a function of")

I am to derive three corresponding expressions for V as a function of time.

3. The Attempt at a Solution

I am having trouble with where to start. I believe the aim is to find which equation, using the data, creates a linear relationship. Any help on how to derive the equations would be much appreciated.
Surely the problem is NOT to "find which equation, using the data, creates a linear relationship" because that's trivial- (1) is the only constant derivative and so the only linear function. The problem simply asks you to "derive three corresponding expressions for V"

In (1), since fcn(V)= c, a constant, your equation becomes dV/dt= kc. What do you get when you integrate that? Use three of the given values to determine k, c, and the constant of integration.

In (2), since fcn(V)= v, your equation becomes dV/dt= kV. That can be written as dV/V= kdt and integrated.

In (3), since fcn(V)= V2 (I assume that's what you mean), your equation becomes dV/dT= kV2. That can be written as dV/V2= kdt and integrated.

For the equation

dv/dt= kc

With intergration I get
V(t2)-V(t1)=kc[t2-t1]

I am unsure of how to use the 3 values to determine k c and the constant could you give me some idea please?

MattRSK said:
For the equation

dv/dt= kc

With intergration I get
V(t2)-V(t1)=kc[t2-t1]

I am unsure of how to use the 3 values to determine k c and the constant could you give me some idea please?

Firstly, if k is a constant and c is a constant then k x c = a constant - lets' call it m.

So you could re-write your equation as

V(t2) - V(t1)=m x (t2-t1)

Suppose t2=4 sec and t1=3 sec - could you not refer to the time-voltage values list and find the corresponding voltages at those times?
This would lead you to a value of the unknown m over the interval from t=3 to 4 sec.

You could also find the value of m for other time spans - choose whatever you wish. The point is that if the (first) proposed function is to fit the given data, then m would be constant over the whole range of the data. If it doesn't turn out to be so, you would discard this solution.

This would seem to be the point of the exercise. Which proposed function best fits the data?

Ok thanks for all that. I understand getting the model which fits best, however I am struggling with the integration parts, here is my attempts at the other 2.

dv/v=kdt

becomes

ln (vt2)/(vt1) = k[t2-t1]

and for

dv/v2=kdt

becomes
k[t2-t1]= [(v(t2)-1)/-1] - [(v(t1)-1)/-1]

Is that correct, my next step would be to graph them

Looks correct, except that in the first post you had "-k".

Also, the expression from integrating dv/v2 could be simplified.

By simplify do you mean saying,

t2 would be anytime t
and t1 would be the initial voltage ie t1=0

So simplifying the expressions would give:

for ln (vt2)/(vt1) = -k[t2-t1]

Becomes ln V(t)/V=Kt
making that ln V(t)=ln V(-kt)

and for k[t2-t1]= [(v(t2)-1)/-1] - [(v(t1)-1)/-1]

becomes -kt= [(v(t)-1)/-1] - [(v-1)/-1]

I am unsure of how to make v(t) the subject of the last one?

Could it be

V(t)-1= v-1 -kt

MattRSK said:
Could it be

V(t)-1= v-1 -kt

Perhaps,

$$V(t)^{-1}$$=$$V_{0}^{-1}-{kt}$$

Where $$V_{0}$$ is the value at time t=0

You might also write the equation in the form V(t) = ... etc.

MattRSK said:
By simplify do you mean saying,

t2 would be anytime t
and t1 would be the initial voltage ie t1=0
Yes, for starters. Though I think you meant t1 would be the initial time, and v1 is the initial voltage at t=0.

So simplifying the expressions would give:

for ln (vt2)/(vt1) = -k[t2-t1]

Becomes ln V(t)/V=Kt
Okay so far.
(You do mean ln(V(t) / V), right?)

making that ln V(t)=ln V(-kt)
Not quite. Note that

ln(V2/V1) = ln(V2) - ln(V1), not ln(V2)/ln(V1)​

and for k[t2-t1]= [(v(t2)-1)/-1] - [(v(t1)-1)/-1]

becomes -kt= [(v(t)-1)/-1] - [(v-1)/-1]

I am unsure of how to make v(t) the subject of the last one?
Use another simplification I had in mind, namely

x-1 / (-1) = -1/x

Firstly thanks for all your replies you are very generous with your time.

ln(vt2) - ln(vt1) = -k(t2-t1)

Becomes

ln v(t) - ln v0 = -kt

which then becomes

ln v(t) = -kt + ln v0

and I plot the ln v against time.

and the last equation becomes
1/v(t) = -kt + 1/v0

and I plot the inverse of voltage against time!

Yup, that's the idea.

p.s. You're welcome.

Thank you to everyone who replied to this thread. This is my first experience with the Physics Forum and its just great! I will endeavor to be helpful to others on the site. You guys have given me a huge helping hand so thanks very much!

Cheers
Matthew

Just an update!

I got an A for my assignment, thanks again for your help.

Regards
Matthew

## 1. How do I derive an expression for transient capacitor voltage?

To derive an expression for transient capacitor voltage, you need to use Kirchhoff's laws and the equations for capacitor charging and discharging. You can also use differential equations to solve for the voltage over time.

## 2. What is the purpose of solving models for transient capacitor voltage?

The purpose of solving models for transient capacitor voltage is to understand and predict the behavior of capacitors in circuits during charging and discharging processes. This information is crucial for designing and analyzing electronic circuits.

## 3. What factors affect the transient capacitor voltage?

The transient capacitor voltage is affected by the capacitance value of the capacitor, the voltage applied to the circuit, and the resistance in the circuit. The initial voltage and time constant also play a role in the transient voltage.

## 4. Can I use the same method to derive expressions for different types of capacitors?

Yes, the same method can be used to derive expressions for different types of capacitors. However, the equations and values may vary depending on the type of capacitor (e.g. electrolytic, ceramic, etc.) and the circuit configuration.

## 5. How can I test the accuracy of my derived expression for transient capacitor voltage?

You can test the accuracy of your derived expression by comparing it to experimental data or simulations. If the results match closely, then your expression is likely accurate. You can also check for consistency with known relationships and principles, such as Kirchhoff's laws and Ohm's law.

• Engineering and Comp Sci Homework Help
Replies
2
Views
2K
• Calculus and Beyond Homework Help
Replies
2
Views
1K
• Engineering and Comp Sci Homework Help
Replies
2
Views
811
• General Engineering
Replies
0
Views
677
• Introductory Physics Homework Help
Replies
3
Views
227
• Calculus and Beyond Homework Help
Replies
7
Views
1K
• Engineering and Comp Sci Homework Help
Replies
0
Views
485
• Calculus and Beyond Homework Help
Replies
1
Views
614
• Engineering and Comp Sci Homework Help
Replies
4
Views
1K
• Introductory Physics Homework Help
Replies
2
Views
960