Download Topic - Advanced Physics Lab

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Ohm’s Law Lab
6/5/2014
168
Seed Question: Suppose you were to apply different voltages across a light-bulb and
record the corresponding currents through the bulb. What do you think the graph of
Voltage vs Current would look like? Sketch it!
V vs I
I
V
V
R
0
I
Exploration: Let’s investigate the relationship between voltage across a light bulb and
current through it, by testing a bulb to destruction. Hook up the light bulb to the power
supply, being careful not to short circuit the power supply by touching the ends of the leads
together. Before you turn on the power supply make sure the voltage knob is rotated fully
counterclockwise: 0 V. Keep the current knob rotated fully clockwise for this lab. Read the
voltage (in volts V) and current (in amps A) right off the power supply panel, recording it by
hand here. Start from 0 and increase go up by 0.5 V, recording your data rapidly until the bulb
burns out. You only get 1 shot at this. Do a large, hand plotted graph of voltage vs current:
Make sure V is on the y-axis. Make sure your data stretches over at least the top half of page
169. Plot a best-fit line of the linear portion of your graph and find its slope. Show your
calculation directly on the graph: draw a large triangle, indicate the rise and run and don’t
forget the units!
V (V)
I (A)
Why did your burn out? Look at your light bulb carefully and
0
0
see how it is different. What changed?
0.5
1.0
etc
What can you conclude about the way voltage depends on
current? State it mathematically.
What does the slope of your graph represent?
Big Idea: The light bulb is not quite an ideal resistor. Ideal resistors have voltages that depend
linearly on current: they have a constant resistance and obey Ohm’s Law:
Ohm’s Law: V = I R
Units:
volts (V) amps (A) ohm’s Ω
1 V = 1 A∙Ω
The resistance R is the slope of the graph of V vs I, and has units of Ω:
1 Ω = 1 V/A
For Further Contemplation: ?