Experiment 5: Ohm’s Law; Electric Power.
Review of how to use meters:
Analog meters: Red connectors are for + and black for -, or they are just labeled + or - . There is
one of one sign, which you always connect to, and several of the other; which of these you use
determines which scale you read. Also, check that the meter is correctly zeroed.
Digital meters: Turn the dial to the function you want. For example, 20 in the area marked DCV
turns it into a DC voltmeter which reads from 0 to 20 V. DCA stands for DC amps. If the meter
has more than two connectors, the negative wire always goes to COM. Where the other wire goes
depends on whether you want to measure volts, amps, or milliamps. On the yellow meters, the dial
goes in the same place for the 10 A scale and for the 20 mA scale; which you are using depends on
which socket the + wire is in.
We usually don't get through this lab without someone blowing the fuse in a meter: Voltmeters go
in parallel, ammeters in series. Start with the least sensitive scale (such as 0 to 10 A), then change
scales (such as to milliamps) once you know the approximate reading.
Also, don't take circuits apart until the instructor approves your results. You may need to doublecheck something.
Part 1: Ohm’s Law.
In part a, a resistor is connected to a DC power supply and some meters. (This resistor has a coiled
up piece of fine wire inside; a more common type contains graphite.) Different voltages are applied
to it by adjusting the power supply, and the resulting current is observed. You then see if the results
are consistent with Ohm’s law. In part b, this is repeated using a light bulb as the resistor.
Part 1a, Resistor:
Select a resistor of 100 to 1000 Ω. Connect a circuit as shown.
(The power supply includes both a source of emf and a keyoperated on-off switch. R is the resistor, V is the voltmeter, and
A is the ammeter.) Start with the ammeter reading amps, not
milliamps; switch to milliamps if it turns out that the current is
small enough.
Each line on the diagram is a wire. Follow the wires like roads on a map. The current leaves the +
(red) terminal of the power supply, and travels to the + connector on the ammeter. So run a wire
between those points. After flowing through the ammeter, the current goes through another wire to
the resistor. And so on. Finally, run a wire from each end of the resistor to the voltmeter. (It
measures the difference in potential between the two points you connect it to.)
Check that the knob on the power supply is turned all the way down (counterclockwise). Keep an
eye on the meters as you turn the power on. IF ANY METER GOES OFF SCALE, TURN THE
POWER OFF IMMEDIATELY. (When a digital meter goes off scale, the display goes blank.) As
you slowly turn up the voltage, continue to be careful of this. Don't use a scale which barely moves
the needle either; that isn't very accurate. If the ammeter shows that the current is small enough, use
a milliammeter instead. However, do not use the 1.5 V or 3 V scales on an analog voltmeter; they
will not read accurately in a circuit with this much resistance.
Get several pairs of values for potential difference and current, covering as wide a range of values as
possible (0 to 23 V.). Graph the potential difference as a function of current. For error bars,
estimate how closely you can read an analog meter; with a digital meter, if you used the proper
scale, the error bars aren’t any larger than the dots you drew on the graph. Neatly draw the best
average line or smooth curve through the data.
Pick a point that lies right on your line, and use it to compute the resistance. The data points that lie
somewhat off the line presumably contain random errors due to your experimental uncertainty.
In your conclusion,
1. Compare the value you obtain to the value printed on the resistor. Do they agree within the
uncertainty printed on the resistor?
2. Comment on the shape of your graph. (The graph of V = IR, where R is a constant, is a
straight line. A linear voltage-current relationship is called ohmic behavior because it's what
Ohm's Law leads you to expect. Say whether the resistor is ohmic or nonohmic.)
Part 1b, Light Bulb:
Replace the resistor with a light bulb, leaving the circuit the same otherwise. Start again with an
ammeter, not a milliammeter, since the current might be larger now.
Get several pairs of values for V and I covering as wide a range as possible (all the way up to 23 V).
Include a few readings under 2 V. Include (0,0) as one of your data points, too. Nothing is wrong if
the bulb doesn't glow; it's just because of the low voltages you are using.
Graph the potential difference against the current as before. Calculate the resistance twice, using
points from near opposite ends of the graph.
In your conclusion,
1. State whether the bulb is ohmic or nonohmic. (The graph of V=IR, where R is a constant, is
a straight line.)
2. Ohm’s law implies that R is related to the slope of this graph. Comment on what the
resistance does as the bulb gets hotter, and how this relates to the graph’s shape.
Part Two: Power.
An electric heating element is immersed in water, and runs for a time t, producing a temperature
change ΔT. You will calculate its power from the formula covered in class, and see if it matches the
rate you observe energy being delivered as heat.
Procedure:
1. In the same circuit from part 1, replace the light bulb with the heating coil. The meters should be
able to handle around 3 A and 15 V.
2. Remove the inner aluminum cup, and measure its mass empty. Measure the mass again with at
least enough water to cover the heating coil. Subtract to obtain the water's mass.
3. Hang the inner cup through the ring in the top of the outer cup, and put on the lid which includes
the heating coil. (The stagnant air between the cups makes good insulation.)
4. Insert the thermometer through the hole in the lid, and record the initial temperature, Ti.
5. Start the stopwatch at the same time that you start about 3 amps through the heater. Run it until
the temperature changes 10°C or so. The water needs to be kept well stirred; the best way is to pick
the apparatus up and swirl it more or less the whole time. (Not so hard you spill it.)
6. Record the voltage and current.
7. Record the time, t, at which you turn off the power.
8. The temperature will probably continue to rise, for maybe half a minute, because the heat hasn't
finished flowing from the heater to the where the thermometer is. It's especially important to stir the
water well now. Record the highest temperature reached, Tf.
9. Determine the amount of temperature lost to the surroundings: With the water at some
temperature T1, restart the stopwatch from zero. Allow the water to cool off for the same amount of
time, t. Record T2, the temperature at the end of this time.
Calculations:
1. Correct Tf to compensate for heat leaks:
a. How much temperature was lost between T1 and T2?
b. Add this to your measured Tf to get (approximately) what it would have been if no heat
escaped from the apparatus. (For example, if T1 was 35.0 and T2 was 34.0 , 1.0 was lost. If
Tf was 35.2 , the corrected Tf would be 36.2 .)
2. Compute the heater’s power from the voltage and current.
3. Compute the rate heat energy was delivered to the cup:
a. Calculate how much heat was added to the water. Calculate how much heat was added to the
cup. Use the corrected final temperature. Add to obtain the total thermal energy produced. (If
necessary, review the material on heat and specific heat from PHY 131. The specific heat of
water is 4186 J/kg·°C; the specific heat of aluminum is 900 J/kg·°C.)
b. Power is energy delivered per unit time. (A watt is a joule per second.) So, divide the energy
by the time to deliver it.
Conclusion: Does the power calculated from I and V match the rate you observed energy being
delivered? (Expect a difference of 10% or so.)
After you're sure nothing needs to be rechecked, please dump out the water and leave the lid off so
everything will dry.
PHY 132
Report on Experiment 5: Ohm’s Law
Important: Never put an ammeter
in parallel:
Part 1a:
Value printed on resistor = ______________
V
I
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(attach graph.)
Compute R from data:
Part 1b:
V
I
__________
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(attach graph.)
Compute R from small V and I:
Compute R from large V and I:
Part Two:
mcup =
mcup+water =
V=
I=
Ti =
Tf =
T1 =
T2 =
mwater =
t=
Calculations: (Please show how you calculated each number, not just the answer.)
Find corrected Tf:
Electric power of heater:
Heat produced,
in water =
in cup =
total =
Rate heat was delivered to cup: