Does the average resistance from each table (except flash bulb, which you should not average) match up with the resistance from therastance for the flash light bulb?
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- Can you describe your light bulb as an Ohmic device? Explain.
- Is it Ohmic for any range of values?
- Does the resistance change outside this range? Does the resistance go up or down?
- Can you explain this change in resistance at the molecular level?
Example of Data Table (for all 5 parts of the lab) :
Make separate tables for each part of the lab
| Current (Amp) | Voltage (Volt) | Resistance (Ohm) | ||
| Power supply meter | Ammeter (I) | Power supply meter | Voltmeter (V) | R= V/I |
Average resistance (from all the values in the resistance column) =
Error = (Max resistance – Min resistance)/2
Remember: Do not take the average of the resistance column or find error values for the flashlight bulb as the resistance changes with the increase in the voltage/current. You should never take average of any variable that changes in a systematic fashion.
For each of the five situations, you should have one table. Except for the flashlight bulb, calculate the average resistance and error for each table. Plot five voltage vs. current graphs for the five sets of data including the one for the flashlight bulb. From the slope of the best fit lines of the first four plots, find the resistances of the two individual resistors and also when they are connected in parallel and in series. Compare the two values you get for the same resistor in two different ways: One from the slope of the graph and the other from the average from the table. Using the two formulas for resistors in series and parallel, calculate the theoretical values of the combinations and compare that with what you got from the experiment. You cannot find resistance of a non-Ohmic resistor from the slope of a Voltage Vs. current plot. Question: Why do I say that? For such resistor, for certain voltage, you have to find the resistance from the ratio of V/I from the table.
Just for fun, (not necessary for the report) you may want to explore further by plotting the two additional graphs R vs. V and R vs. I for the flashlight. Study the nature of the graph and think if it makes sense. Is one of these graphs linear?
As an example, from the slope of the line in the following graph, the resistor has a resistance of 20.5 Ohm.
Figure 3, Typical Graph
Figure 4, Resistors combined in series.
When circuit components are connected in series the current traveling through one element is the same as the current traveling through the other. The charges moving through the first component have no where to go but through the second element.
Figure 5, Two resistors combined in parallel.
When circuit components are arranged in parallel the current arriving at one point becomes divided, some of the current goes through one element and the rest goes through the other element before recombining at the second junction.
Let us think of a set of tollbooths across a highway. The traffic traveling along the highway arrives at the plaza, then divides (different cars going through different booths) and then recombines as it leaves. Cars are like current and toll-booths are like resistors. Number of cars moving through each booth per unit time may not be same. However, total number of cars arriving to the plaza is same as the total number of cars leaving the plaza. This is an example of a parallel combination.
Figure 5 shows two resistors combined in parallel, the current divides, passes through the resistors A and B and recombines after passing through those. Note, the potential difference across the resistors A is same as that across the resistor B. Remember that you may not use resistor A and B and use resistor B and D instead.
Series Parallel
Figure 6: Connections for the lab for series and parallel connection with resistor A and B
Analysis
You should have 5 tables and 5 graphs by now. Of the 5 graphs, 4 should be linear and the fitted line should go through the origin (but do not force it).
From the first four graphs, find the values of resistance for the two individual resistors you used and the values of resistance for the series and parallel combinations.
Does the average resistance from each table (except flash bulb, which you should not average) match up with the resistance from therastance for the flash light bulb?How much is the deviation?
Qs. How much is the deviation?
Qs. Why do I ask you not to find the averag ge resiph?
When two resistors are combined in series the resistance of the combination, Rseries is the sum of the values of resistance of the two component resistors.
That is, Rseries = R1 + R2
Qs. Do your measured values support this?
When two resistors are combined in parallel the resistance of the combination is found as Rparallel
where: (Rparallel )-1 = (R1 )-1 + (R2 )-1
Qs. Do your measured values support this?
Qs. Does the voltage vs. current plot for flashlight bulb has a fixed slope?
Qs. If you try to fit the data points to a straight line (fixed slope), is the intercept zero?
Questions:
Why does the temperature increase with increase in current?
Ohm’s law states V=IR, or R = V/I. Thus it seems that resistance goes down as current increases. Same equation also suggests that resistance goes up as the voltage increases. However, we just found current increase as voltage increases. So what will happen to resistance when voltage goes up– will it go up or go down? Explain this conundrum.
For non-Ohmic material does the resistance always go up with increase in voltage? Explain.
Why did the resistance of the Flash Bulb go up even when you were doing the experiment at room temperature?
An incandescent lamp (bulb with filament) burns out mostly when you just turn it on, although it was fine when it was turned off last time. What changed to cause the bulb to fail?
Is a device always Ohmic?
What is a super conductor? How does it work? What advantage does it offer? Imagine you have discovered a room temperature superconductor. What would you like to do with it?
As current goes through an incandescent lamp, filament heats up and produces light. Will there be any advantage or disadvantage if you replace the filament of such a bulb with a filament made out of a hypothetical room temperature superconductor (resistance = zero)?
Will resistance of an object always go up with increase in voltage?
What is the scientific basis of people saying that incandescent light bulbs are energy inefficient?
Do you always need a filament inside a light bulb? Examples? How do they work differently?
What is the difference in mechanism between regular incandescent bulb and a halogen bulb? What is the advantage of one over the other?
