
The circuit shown in Figure P28.78 is set up in the laboratory to measure an unknown capacitance C in series with a resistance R = 10.0 MΩ powered by a battery whose emf is 6.19 V. The data given in the table are the measured voltages across the capacitor as a function of lime, where t = 0 represents the instant at which the switch is thrown to position b. (a) Construct a graph of In (ε/Δv) versus I and perform a linear least-squares fit to the data, (b) From the slope of your graph, obtain a value for the time constant of the circuit and a value for the capacitance.
Δv(V) | t(s) | In (ε/Δv) |
6.19 | 0 | |
5.56 | 4.87 | |
4.93 | 11.1 | |
4.34 | 19.4 | |
3.72 | 30.8 | |
3.09 | 46.6 | |
2.47 | 67.3 | |
1.83 | 102.2 |
(a)

To draw: The graph of ln(ε/Δv) versus time t and perform a linear least squares fit to the data.
Answer to Problem 28.78AP
The graph of ln(ε/Δv) versus time t, where ln(ε/Δv) on y axis and time t on x axis is,
And the equation of best fit is ln(ε/Δv)=(0.01178)t+0.08798.
Explanation of Solution
Introduction:
The linear least square is a technique of fitting a statistical model to a set of variables. This shows the data points linearly in terms of unknown parameters.
Given: The capacitance is C and the resistance R is 10.0 MΩ, the emf ε of a battery is 6.19 V, the time t at which the switch is thrown to position b is 0 and the data given in the table is,
Δv (V) | t (s) |
6.19 | 0 |
5.55 | 4.87 |
4.93 | 11.1 |
4.34 | 19.4 |
3.72 | 30.8 |
3.09 | 46.6 |
2.47 | 67.3 |
1.83 | 102.2 |
Write the equation of line which gives a linear relationship.
y=mx+c
Here,
y is the ordinate.
x is the abscissa.
m is the slope of the line.
c is the intercept on y axis.
Take ln(ε/Δv) on y axis and time on x axis due to which y=ln(ε/Δv) and x=t
Substitute ln(ε/Δv) for y and t for x in above equation.
ln(ε/Δv)=mt+c (1)
Calculate the values of ln(ε/Δv) for each values of Δv.
Δv (V) | x=t (s) | y=ln(ε/Δv) |
6.19 | 0 | 0 |
5.55 | 4.87 | 0.109 |
4.93 | 11.1 | 0.227 |
4.34 | 19.4 | 0.355 |
3.72 | 30.8 | 0.509 |
3.09 | 46.6 | 0.694 |
2.47 | 67.3 | 0.918 |
1.83 | 102.2 | 1.218 |
Write the formula of slope.
m=N(∑xiyi)−(∑xi)(∑yi)N(∑x2i)−(∑xi)2
Here,
N is the number of points.
The number of data points N is 8.
Calculate (∑xiyi), (∑xi), (∑yi) and (∑x2i) from the table.
x=t (s) | y=ln(ε/Δv) | x2 | xy |
0 | 0 | 0 | 0 |
4.87 | 0.109 | 23.7169 | 0.53083 |
11.1 | 0.227 | 123.21 | 2.5197 |
19.4 | 0.355 | 376.36 | 6.887 |
30.8 | 0.509 | 948.64 | 15.6772 |
46.6 | 0.694 | 2171.56 | 32.3404 |
67.3 | 0.918 | 4529.29 | 61.7814 |
102.2 | 1.218 | 10444.84 | 124.4796 |
∑xi=282.27 | ∑yi=4.03 | ∑x2=18617.617 | ∑xiyi=244.216 |
Substitute 8 for N, 244.216 for (∑xiyi), 282.27 for (∑xi), 4.03 for (∑yi) and 18617.617 for (∑x2i) in above equation to find m.
m=(8)(244.216)−(282.27)(4.03)(8)(18617.617)−(282.27)2=0.01178
Thus, the slope of line is 0.01178.
Write the formula of intercept.
c=(∑x2i)(∑yi)−(∑xi)(∑xiyi)N(∑x2i)−(∑xi)2
Substitute 18617.617 for (∑x2i), 4.03 for (∑yi), 282.27 for (∑xi), 244.216 for (∑xiyi) and 8 for N in above equation to find c.
c=(18617.617)(4.03)−(282.27)(244.216)(8)(18617.617)−(282.27)2=0.08798
Thus, the value of intercept on y axis is 0.08798.
Substitute 0.01178 for m and 0.08798 for c in equation (1).
ln(ε/Δv)=(0.01178)t+0.08798
Thus, the equation of best fit is ln(ε/Δv)=(0.01178)t+0.08798.
The graph of ln(ε/Δv) versus time t, where ln(ε/Δv) on y axis and time t on x axis is,
Figure 1
(b)

The value of time constant of the circuit and the value of the capacitance.
Answer to Problem 28.78AP
The value of time constant of the circuit is 84.8 s and the value of the capacitance is 8.48 μF.
Explanation of Solution
Given: The capacitance is C and the resistance R is 10.0 MΩ, the emf ε of a battery is 6.19 V and the time t at which the switch is thrown to position b is 0.
Write the equation of time constant.
τ=RC (2)
Here,
τ is the time constant.
R is the resistance.
C is the capacitance.
The time constant from the slope is,
τ=1m
Substitute 0.01178 for m in above equation to find τ.
τ=10.01178=84.8 s
Thus, the time constant is 84.8 s.
Substitute 84.8 s for τ and 10.0 MΩ for R in equation (2) to find C.
(84.8 s)={10.0 MΩ(106 Ω1 MΩ)}CC=8.48×10−6 F(1 μF10−6 F)C=8.48 μF
The capacitance is 8.48 μF.
Conclusion:
Therefore, the value of time constant of the circuit is 84.8 s and the value of the capacitance is 8.48 μF.
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Chapter 28 Solutions
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