AP Physics 1 Circuit Lab – Ohm’s Law

The purpose of this lab is to verify Ohm’s Law and determine the properties of various electrical devices.  It will also serve as practice analyzing and understanding circuits and correctly using voltmeters and ammeters.

All circuits will be constructed on the Vernier Circuit Board.  The board is powered by two D-cells, in series that provide 3 volts to terminals 1 (+) and 35 (–) when switch SW1 is in the “3V” position.  It is good practice to leave this switch off except when actually making observations and measurements of the circuit (the “External” position is equivalent to OFF unless an external power supply is connected to terminals J1 and J2).  Jumper cables with alligator clips are used to connect the various parts of the circuit.  Note that the circuit elements on the board have numbered binding posts to which two alligator clips may easily be connected – one atop the other. 

Use jumper cables to create a simple circuit consisting of a battery and two resistors in series.  Connect a voltmeter and an ammeter to measure the voltage across resistor R₂ and the current passing through R₂.  The resistor R₂ will be the bulb in the first experiment (posts 26 and 27) and it will be a 68 ohm resistor (posts 12 and 13) in the second experiment.  R₁ will be either zero or some other number of ohms depending on what other resistor or combinations of resistors (if any) are connected in series.  Vary the value of R₁ in order to produce a set of data that shows a nice range of different values that can be graphed.  For best results the range of voltages recorded for R₂ should be from zero to about 3 volts with regularly spaced values in between to show the relationship of voltage and current.

Collect data to complete the table and create one graph showing the two different sets of data.  Use different symbols, colors, etc. and include a key or legend.  Determine appropriate curve fits for each device.

Optional:  If time permits and you so desire try collecting a third set of data for the LED/resistor combination between posts 24 and 25.  Create your own data table.  Do not try to bypass the 220 ohm resistor connected to the LED – it prevents excessive current from damaging the LED.  Also, you will find that only one direction of current through the LED will cause it to light up!

Bulb		68 ohm Resistor
Voltage (V)	Current (mA)	Voltage (V)	Current (mA)

Questions

1. Discuss whether or not each device is ohmic and refer to the graph to explain and support your response.

2. Determine the resistance of the resistor based on the curve fit equation (should equal one of the coefficients in your equation).  Determine the percent difference with the “nominal value” of 68 ohms.

A complete report (30 pts): 

q Data Table  (10)

q Voltage vs. Current graph, with two sets of data and curve fits.  (16)

q Responses to the questions.  (4)