# How to get Voltage vs. Time graph from having Current and Voltage values.

## Homework Statement

Make a Voltage vs. Time graph. We know the Resistance, Voltage values, and corresponding Current Values.

## Homework Equations

We have data points for the voltage, which ranges from 0-10 volts, and we have the corresponding current values.

V(t) = V(0)*e^-(t/rc)

## The Attempt at a Solution

I was looking at equations that might help me solve this problem.

I know that V(t) = V(0)*e^-(t/rc)

rc is the time constant. But isn't RC the Resistance x the Capacitance. We had three light bulbs connected in series, so there was no capacitance. And I already solved for the resistance.

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If you're confused with the setup, here is the whole unedited problem.

!!In the following: BECAUSE OF THE POTENTIAL FOR SHOCK OR DAMAGE TO THE
ELECTRONIC INSTRUMENTS, YOU MUST ASSEMBLE YOUR CIRCUIT WITH ALL
MULTIMETERS AND POWER SUPPLY TURNED OFF.
!! THE TA WILL INSPECT YOUR CIRCUIT, AND THE TA WILL THEN TURN ON THE
POWER SUPPLY AND MULTIMETERS.
B. Keeping in mind the important
caveats above, assemble your
components as indicated in the
figure. This arrangement
should allow you to measure
the current-voltage curve for
three light bulbs in series. You
should take data points starting
from about 10 Volts down to 0
Volts. Do not exceed 10 Volts
output from the power
supply. Since the voltage
readout on the power supply is very crude it is important to record the voltage as
measured by Multimeter #1 rather than using the power supply readout. Multimeter #2
is placed between the two light bulbs and will be set to measure current; you should use
the 200 mA maximum setting for current measurement. Record a total of 20 – 30 data
points, about 2 for each unit of voltage, and be sure to take at least four points in the
range 0.10 – 1.00 Volts.
Questions
1. Use Logger Pro or some other curve plotting program to plot the current-voltage
behavior for the three light bulbs. Note any nonlinearities in your plot. What is the
source of this nonlinearity? Is it really a breakdown of Ohm’s law?
2. Fit a smooth curve to your current-voltage data using the Logger Pro fitting function or
Matlab. From this data extract the slope of the current-voltage curve when voltage is
near zero. Compare this “differential” resistance with the result of your resistance
measurement from part A for the three light bulbs. Are the two values similar?
3. Include your plot of voltage versus time for the RC circuit.

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