Lab 1 TF EIM

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Kirchoff's Law (50 pnts)

Smoky Circuits

When doing these labs it is important to determine the limitations of your electrical components in order to avoid damaging them

This lab uses resistors. Resistors are vulnerable to melting if you push too much current through them.

Resistors have power ratings ranging from 1/8 Watt up to several Watts. The common resistors are rated at 1/4 Watt. Let's assume this rating for the resistors in is lab.

Determine the maximum voltage of the DC power supply you will use. (ie, 30 Volts)

Determine the range of currents which the voltmeter can measure. (ie 1 mA)

You can now determine the maximum resistance which you should use in the Lab.

[math] P =I^2R \Rightarrow R = \frac{P}{I^2} \lt \frac{1/4 \mbox{Watt}}{ \left (1 \mbox{mA}\right)^2} \lt 250 k\Omega[/math]

Construct the circuit below.

TF EIM Lab1.png

Enter the values of the DC voltage and Resisters that you used.

Use a voltmeter to measure the potential difference and resistances.

Variable Measured Value
[math]V_A [/math]
[math]V_B [/math]

Enter the measured and predicted quantities in the table below

Use Kirchoff's laws to predict the Voltage ([math]V_C[/math]) and current values and compare them to your measurements by filling in the table below.

Variable Measured Value Predicted Value % Difference

Internal resistance (30 pnts)

Measure the internal resistance of your power source by graphing the potential difference on the x-axis and the current on the y-axis for several values of the resistance [math]R_L[/math] shown in the circuit below. Begin with [math]R_L = 1k\Omega[/math] and then decrease it by a factor of 5 for each subsequent measurement. You can use a volt meter to measure the current and potential difference.

Questions (20 pnts)

  1. What conservation law is involved in Kirchoff's Loop Theorem?
  2. What does the slope in the internal resistance plot above represent?