course Phy 202
Experiment 16: Current Flow and EnergyWhen the leads of a hand generator are connected to different objects the crank is sometimes easy to turn and sometimes difficult. The relationship between the force exerted and expected current flow, and therefore between energy and current flow, are examined. This examination is extended to series and parallel combinations of flashlight bulbs.
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In this experiment you will investigate the relationship between current flow and energy. Two other concepts, those a voltage and resistance, will be required to understand this relationship.
Your hand-cranked generator provides the 'push' necessary to create a flow of electrical current. The push is determined by the rate at which the handle is turned.
Clamp the ends of the leads coming from the generator to a piece of wood or plastic and turn the crank at about 2 complete turns per second. Then clamp the ends together and turn the crank again at the same rate.
• In which case was the crank easier to turn? In which case did you do more work per second (remember that work is the product of force and distance)?
More is done when the leads are connected to a conductor than an insulator. A conductor allows electron, while an insulator resists electron flow.
• If which case do you think more electrical current flowed through the wires attached to the generator?
More current flows through the wires when the leads are connected together. When the leads are connected together, resistance is low which allows more current flow, as opposed to having an insulator connected to the leads which has higher resistance and less, or no, current flow.
• How would you characterize the relationship between the current flowing and the difficulty of crank in the generator?
I would characterize the relationship between current flow and difficulty to turn the handle as dependent upon the resistance of the object. If the material in which the leads are connected across permits more current flow, the handle becomes harder to turn. On the contrary, if the material permits less, or no, current flow the difficulty to turn the handle decreases. Therefore, the difficulty to turn the handle is inversely proportional to the resistance of the object, and proportional to the current flow in the circuit.
• Does current flow more easily through the wires when they are attached to the wood or when they are clamped together?
Current flows more easily when the wires are clamped together than when the wood is attached to the leads.
• Would you say that the circuit resists the flow of electricity more with the wood between the clamps or when the clamps are directly attached to one another?
The wood resists the flow of electrons more than the leads being connected together.
• In which case is there more electrical resistance, the case when the generator is easy to crank or when it is more difficult crank?
More electrical resistance is indicated by the generator handle being easier to turn.
Go around testing different objects in your house to see which ones have high resistance and which ones have low resistance to the flow of electrical current. Try to find at least three different materials that have low resistance and it least three to have high resistance.
Low Resistance High Resistance
Car Key Domino
Paper-Clip Notebook Paper
Cooking Pot Handle Rubber-Band
Now insert a light bulb into a bulb holder and clamp the leads of the generator to the two tabs on the holder. Starting slowly at first, crank the generator faster and faster until the bulb glows, but not too bright so you don't burn it out.
• Count the number of times you crank the generator in 10 seconds while the bulb glows, and record this data. Note also the numbers marked on the bulb, and record them.
12 cranks / 10 seconds
Bulb #1 Information: 46 6.3 V 0.25 A
• Repeat for the other bulbs in your kit. Some bulbs may require faster cranking than others. Some may crank at the same rate or easier. Find a way to mark the bulbs and record which is which.
19 cranks / 10 seconds
Bulb #2 Information: 1487 14V 0.2 A
12 cranks / 10 seconds
Bulb #3 Information: 46 6.3 V 0.25 A
Now place two different types of bulbs in holders. Bulbs are of the same type if they require the same force and the same cranking rate. Connect a tab on the holder of one bulb to a tab on the holder of the other using a wire lead (one of the colored wires with alligator clips on the ends).
Connect the leads of the generator so that current will flow through the first bulb but not the second. Describe how you made the connection.
I connected the generator across the first bulb, so that there was not a completed circuit return path for the second bulb.
• Crank the generator to make the bulb burn, and note how much force is required to crank the generator and how fast it has to be cranked.
The resistance of the single bulb is evident, and requires some force to turn the handle and light the bulb.
Now connect the leads of the generator so that the current will flow first through the first bulb then through the wire lead connecting the two bulbs and finally through the second bulb and back to the generator. You will have a lead from the generator to the first bulb, another from the first bulb to the second and a third lead from the second bulb back to the generator.
• Crank the generator as before and note whether it requires more or less force to crank the generator, and whether the generator needs to be cranked faster or slower in order for the first bulb to burn as brightly as before.
Now, both bulbs are connected in series, and the handle is easier to crank since the resistance of the circuit has been increased, due to the added series resistance. The handle requires faster cranking to obtain the same illumination as the initial lamp across the generator, although there is not as much resistance to turning the handle.
• Do both bulbs burn with the same brightness? If not describe in terms of the previous observation of force and cranking rate the difference between the bulb that burns brighter and the bulb that burns more dimly.
No, both bulbs do not burn with the same brightness. If the same brightness is obtained as with the single bulb, the second bulb is just beginning to glow, but essentially not illuminated. If the second bulb is illuminated to a better degree of brightness, the handle must be turned significantly faster.
Finally connect the leads of the generator to the first bulb, as before, then complete a parallel circuit to the second in the following manner:
• Connect a lead from the tab of the first bulb to one tab of the second. It might not be possible to actually connect the second lead to the tab of the first bulb since there is already one lead connected to that tab; it can be connected to the first clip, which is already attached to the tab.
Connected both lights in a parallel circuit.
• Connect a second lead from the other tab of the first bulb to the remaining tab of the second.
Completed.
• The bulbs should be connected so that when the current flows into the out of the first generator lead it branches, with some current flowing into the first bulb and some branching off through the wire lead to the second bulb. The current passing through the second bulb will then travel through the second wire lead back to the second generator lead, where it will rejoin the current that has come through the first bulb.
Done.
• Crank the generator so that neither bulb burns too brightly and observe whether the generator requires more or less force to crank, compared to when the leads were attached to a single bulb. Note also whether the generator has to be cranked at the same rate, at a faster rate or at a slower rate in order for the first bulb to burn is brightly as before.
The generator requires more force to crank, thus indicating that the resistance has been lowered with this configuration. And, of course, a parallel circuit reduces the total resistance to less than the lowest resistance. Therefore, the lower resistance is expected.
You have experimented with bulbs connected in series and in parallel. The meaning of these terms is as follows:
• When the bulbs were connected so that current had to flow through the first bulb before flowing through the second, the bulbs were said to be connected in series.
• When the bulbs were connected so that the current branched, with one part going through the first bulb and the other through the second, the bulbs were said to be connected in parallel.
Answer the following questions:
• Which required more force to crank, the parallel or the series combination?
The parallel configuration requires more force to crank the generator handle.
• Which required greater cranking speed to achieve the same bulb brightness, the parallel or the series combination?
The series configuration requires greater cranking speed to obtain the same relative brightness, due to the greater resistance of the circuit.
• Did both bulbs have the same relative brightness when they were connected in parallel as when they were connected in series?
No the bulbs do not have the same relative brightness as when connected in parallel as when connected in series. This is due to the series voltage drop across the initial resistance, which permits less voltage to the second bulb. In the parallel circuit, the voltage is provided to both parallel branches, which allows the parallel circuit illumination to be brighter, in addition to overall lower resistance of the circuit.
In a series circuit the bulbs would, with the same cranking rate, burn the same if they were reversed. It doesn't matter which is first and which is second.
The reason is that the resistances add, giving the same result in either case, resulting in the same current. In a series circuit the same current flows through both bulbs, so they will glow identically if reversed.
• In which case do you think work was being done at the greater rate?
I think that work is being done at the greater rate in the parallel circuit, where the resistance is lower and the force to turn the handle is greater. Since power is the rate at which mechanical work is done, the power in the parallel circuit would be greater that the power in the series circuit. With Ohms Law, Power is define as Voltage times Current, and the difference between the series and parallel circuits is the overall circuit resistance. For the series circuit, let R represent the single load, and there are 2 loads in the circuit. Therefore, the total resistance = (R+R) = 2R. For the parallel circuit, the total resistance = (R*R)/(R+R) = R^2/2R = R/2. Because of this fact, the resistance in the parallel circuit is lower, and the current is higher, thus, the power in the parallel circuit is higher.
It turns out that the amount of force necessary to turn the crank is an indication of the amount of electrical current flowing in the circuit, while the rate at which the crank is turned, in revolutions/second, is an indication of the amount of electrical 'push', or voltage, in the circuit.
You haven't yet made the measurements necessary to draw this conclusion experimentally (though able to measure the speed of the cranking, we didn't measure the force, and it's hard just from the 'feel' of the circuit to tell in which case you had the greater force * distance per unit of time), but you have correctly worked out the comparison between the circuits if they are subject to the same voltage.
Note however that the greater cranking rate for the series circuit implies greater voltage. Power is V * I or V^2 / R. The parallel circuit has the lower R, but also the slower cranking rate and therefore the lower V.
If one of the bulbs is glowing the same in both circuits, then you know that it carries the same current in both. If both bulbs have identical resistance R, you conclude that the parallel circuit requires half the voltage of the series circuit but has resistance 1/2 R, while the series circuit would have resistance 2 R. The conclusion would be that V^2 / R is identical. The fact that both bulbs would glow at the same brightness in both circuits would confirm that the power consumption is identical.
The situation is more complicated if the bulbs are different, especially since the resistance of the bulb varies with the temperature of the filament.
More specifically:
• It is pretty much the case for this generator that the force F necessary to turn the crank is directly proportional to the current I flowing in the circuit: F = k1 * I, where k1 is a proportionality constant.
• It is also pretty much the case that the rate `omega at which the crank is turn is directly proportional to the voltage V pushing the current through the circuit: V = k2 * omega, where k2 is a proportionality constant.
In light of this information:
• Which circuit would you therefore say required the greater voltage, the series circuit or the parallel circuit?
Series circuits require more voltage due to voltage drops across the individual resistive devices. If enough voltage is not provided, then certain components may not work. Like the initial series circuit, if the handle was not cranked fast enough, then the second bulb would not illuminate. This is an indication that sufficient voltage is not provided to the circuit. In a series circuit, the current is constant and the voltage drops across the devices.
• Which circuit would you say required the greater current, the series circuit or the parallel circuit?
In a parallel circuit, the voltage is constant the total current is the sum of the individual branch currents. Therefore, the same devices implemented in a parallel circuit present less resistance and greater current.
Recall that power is the rate at which work is done: power = force * distance / `dt.
• As determined from the force necessary to crank the generator and from the rates at which the generator was cranked, which circuit seemed to require the greater power?
It seemed that the parallel circuit required more power than the series circuit, due to the handle being significantly harder to crank, but involved less revolutions to maintain the same relative, or better, illumination than the series circuit.
As determined from the brightness of the bulbs, which circuit seemed to require the greater power?
When trying to maintain the brightness of the bulbs, the series circuit appeared to require more power in order to maintain the brightness. Although the force to crank the handle was less, the number of revolutions was significantly greater than that required by the parallel circuit. This is due to the voltage drops across each bulb. In essence, the voltage required by the series circuit would be significantly more since the voltage drops across the series components, while parallel branches have constant voltage delivered to the branch loads. Therefore, the series circuit would require more revolutions with significantly less force required to crank the handle.
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Very good. See my notes and let me know if you have questions. Also, pollen allergies are starting to kick up, which can on some days muddle my thinking, so be sure to let me know if my notes don't make sense.