Typical Observations and Answers to Questions
Questions:
1. Which bulbs (in the circuits you constructed) have the greatest voltage? Which have the lowest voltage?
Since P = V2/R, and brightness is related to power, the bulbs with the greatest brightness would have to have the greatest voltage as long as the resistance of each bulb is the same. These would include the single bulb in circuit #1 and BOTH bulbs in Circuit #2. You will soon learn that each of these bulbs have a voltage of 3 volts.
Oppositely, the least bright bulbs would have the lowest voltage. These include the two bulbs in Circuit #5 that don't light. Again, as you will see how to determine in a future lesson, the voltage across each of these is about 1 volt. That is lower than the voltage needed to light up so that a human eye could see them. Of course, these answers assume that the resistance of all the light bulbs are the same. If we knew the current in each bulb we could also show that brightness (power) was related to current through each bulb by I2R (again, as long as the resistance of each bulb was the same).
2. How do the voltages of the bulbs in circuit #3 compare to the voltages in circuit #2?
Based on the brightness of the bulbs, the voltages of the bulbs in circuit #3 (each about 1.5 v) are lower than those in circuit #2 (each about 3 v)
3. How do the voltages of the bulbs in circuit #5 compare to the voltages of the bulbs in circuit #6?
The bulbs in circuit #6 all light up, but they are not as bright as the one bulb that lights up in circuit #5. Thus, the bulbs in circuit #6 have lower voltage than the lit bulb in circuit #5. As you will see, the bulbs in circuit #6 all have 1.5 v across them, while the bulb in circuit #5 that does light has 2v across.
4. How do the brightnesses of the bulbs in circuit #3 compare to those in circuit #6? Give a reason for your answer based on how the circuits are constructed.
The bulbs in both circuit #3 and circuit #6 all have the same brightness and the same voltage. You can see from the connections that circuit #6 has two identical branches (one outer and one inner) that are connected to the same two points in the circuit as the the single branch in circuit #3.
5. In your own words, give a reason why the bulbs in circuit #3 do not have the same brightness as the bulbs in circuit #2.
The bulbs in circuit #3 have to share the same current which is lowered by the increased resistance of the two bulbs in the same branch. The bulbs in circuit #2 have their own currents, which are higher than the single current mentioned in circuit #3 (because the resistance in any one branch of circuit #3 is lower than the resistance of the single branch in circuit #2). You will see in an upcoming presentation that the bulbs in circuit #2 are said to be in parallel while the bulbs in circuit #3 are said to be in series.
6. In the previous practice problem set, you learned that resistivity is affected by temperature. How do you think the observations in this lab could be affected by that fact?
For all of the resonses above it was assumed that the resistance of each bulb was the same. But when a bulb is lit up it also warms up, increasing its temperature. Therefore, the brightest bulbs are at the highest temperatures making their resistivities (and resistances) greater. This heating effect would make the currents in the various branches (and the power for each individual bulb) slightly different than what we are predicting. Since a measurement of "brightness" is very subjective determination, it is not the best way to describe the workings of these circuits. Instead, we will use devices called ammeters and voltmeters that can accurately measure the current through and the voltage across each bulb.