Need help with this challenge problem for my physics lab. complete answers will
ID: 2110261 • Letter: N
Question
Need help with this challenge problem for my physics lab. complete answers will recieve full points.
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Plug two extra long connecting wires into the decade box and place the box it at the front of your lab
bench. Make sure your power supply is turned off. Then hook up the following circuit: power supply in
series with a single bulb in series with the parallel combination of a 50 Ω resistor and the decade box.
Make sure to leave your decade box at the front of your lab table. Don’t turn on your power supply yet!
Call over an instructor. She will set your decade box to 1 Ω, 10 Ω, 100 Ω or 1000 Ω. (Don’t look at the
setting!) Now turn on your power supply and set it to 5.00 volts.
Your challenge:
Using only a single measurement with the DMM, determine the setting on the decade box. You may
use the DMM as either a voltmeter or an ammeter. You may also refer to the I-V data that you
generated for the light bulb in Activity II. Once you have a prediction, check the decade box to see if
you were correct. If you were not correct, review your analysis to try and spot any mistakes. Make
notes in the space below so that you can write your mini-report when doing the lab HW.
Explanation / Answer
Using the I-V curve for the light bulb, you can find its resistance since R = V/I.
Now imagine the bulb to be behave just like any ordinary resistance. Use the DMM as a voltmeter and measure the voltage across the bulb. Since the total voltage is 5V, the remaining voltage drops across the parallel combination of the 50â„ ¦ and the decade box. Assume the value of the decade box to be R. The parallel combination will lead to an equivalent resistance of 50R/(50+R) = Req. The current through the bulb and the parallel combination is the same.
Hence, V(bulb, found using DMM) / R(bulb, from I-V curve) = (5 - V(bulb))/Req.
Only R is unknown and can be found.