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UNIT 23: DIRECT CURRENT CIRCUITS
current is passing through. How does the potential difference across a resistor
depend on the current through it? In order to explore this, you will need:
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4 D-cell batteries, 1.5 V, alkaline
4 D-cell holders
1 multimeter
1 ammeter, 0.25 A
1 ammeter, 1 A
1 resistor, approximately 68Ω
1 SPST switch
Recommended Group Size:
2
Interactive Demo OK?:
N
23.6.1. Activity: Experimental Relationship of i and ΔV
a. What do you predict will happen to the voltage drop, ΔV, across the
resistor as the current, i, through it increases? Sketch a graph of your
predicted relationship.
Fig. 23.8. Circuit resistor
symbol
b. Set up a circuit to test your prediction by placing the resistor in series
with one, two, three, and then four batteries. Set up the voltmeter and
ammeter to measure the voltage across the resistor and the current
through it. Carefully describe your procedures and results, and sketch
your circuit diagram. Note: The circuit symbol for a resistor is slightly
different from that for a bulb. It is shown in Figure 23.8.
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UNIT 23: DIRECT CURRENT CIRCUITS
If potential difference is measured in volts and current is measured in
amperes, then the unit of resistance is the ohm, which is usually represented by
the Greek letter Ω, “omega.”
23.6.2. Activity: Statement of Ohm’s Law
a. State the mathematical relationship found in Activity 23.6.1 between
potential difference and current for a resistor in terms of ΔV, i, and R.
b. Based on your graph, what can you say about the value of R for a
resistor—is it constant or does it change as the current through the
resistor changes? Explain.
c. From the slope of your graph, what is the experimentally determined
value of the resistance of your resistor in ohms? How does your slope
agree with the rated value of the resistor? If you use a resistor rated at
68Ω, the slope should be close to 68Ω but is unlikely to be exactly 68Ω.
d. Complete the famous pre-exam rhyme used by countless introductory
physics students throughout the English-speaking world:
Twinkle, twinkle little star, ΔV equals
________________
times
________________
Fig. 23.9.
(1) If a circuit element (such as the carbon resistor you have been using) has
the same resistance over a wide range of conditions, it is called ohmic. This is
because it obeys Ohm’s Law with ΔV and i being proportional to each other.
(2) A circuit element like a light bulb that changes resistance with the amount
of current in it is called non-ohmic.
Note: The resistance for a non-ohmic resistor is still determined by R
= ΔV/i but R is not constant. Instead, it depends on ΔV.
23.7. USING A MULTIMETER
A digital multimeter is a device that can be used to measure either current,
voltage, or resistance depending on how it is set up. The following activity
will introduce you to the digital multimeter and give you some practice in
using it. You will need: