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When 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.
Note video clip(s) associated with this experiments on the CD entitled
'Experiments'. The link is
Experiment 16: Current Flow and Energy . The link will not work within this document; go to the CD, run the html file in the root folder which contains
'experiments' in the filename, and click on the link.
Note that on the video file the bulb holders are mounted on a block of wood. The
wood block is no longer included in the kit; the holders can simply lie on the
tabletop.
In this experiment you will investigate the relationship between current flow and energy. Two other concepts, voltage and resistance, will be developed.
The following is a useful model of what you will experience in this experiment. This model doesn't apply in all its details to conduction in wires, but can provide a framework in which a more detailed understanding can be built.
- Your hand-cranked generator creates an electrical field that accelerates free charges, increasing their kinetic energies.
- In a typical circuit these charges frequently collide with other free charges and/or with fixed atoms, so that kinetic energy gains are quickly converted to randomly directed kinetic energies, i.e., thermal energy, which is typically dissipated to the surrounding environment.
- As a result, a single charge moving the length of the circuit will dissipate a certain amount of energy. The amount of energy dissipated per unit of charge is called the voltage drop. Voltage is a measure of energy per unit of charge.
- The faster you turn the crank the more energy a given charge will dissipate as it passes through the circuit--i.e., the faster you turn the crank, the higher the voltage drop.
- You provide the energy with the work you do as you turn the crank.
- At a given cranking rate, the more free charges are available, the more work it takes and the more force you therefore have to exert.
The above isn't a complete description of conduction, but it is sufficient to understand the observations you will make in this experiment. At the quantum level, in fact, the electrons behave within as waves, and experience various forms of interference as they move through the material.
The two things you do want to keep in mind are:
- The faster you crank the greater more work is done per charge carrier, i.e., the higher the voltage.
-
The more free charges are available, the more force you're going to need to
exert and the more charge will flow.
- The rate of flow of charge is in fact directly proportional to the force
required, and the voltage is directly proportional to the rate at which the
generator is cranked.
The rate of flow of charge is called current. The greater the number of charges that flow
past a given point per unit of time, the greater the current.
You also need to recall that work is the product of the force you exert and distance in the direction of the force (abbreviated, we say that work = force * distance, or `dW = F * `ds).
Answer the following:
If you are cranking at twice the rate with the same force:
- how many times as much voltage are you producing?
- how many
times as much current?
- how many times as much work is therefore done per minute?
****
#$&*
If you are cranking at twice the rate with double the force:
- how many times as much voltage are you producing?
- how many
times as much current?
- how many times as much work is therefore done per minute?
****
#$&*
If you are cranking at twice the rate with half the force:
- how many times as much voltage are you producing?
- how many
times as much current?
- how many times as much work is therefore done per minute?
****
#$&*
------>>>>> voltage current work/min: 2*rat, 2*force, 2*rat half the
force
Explain your reasoning for the preceding three sets of questions and discuss whether your answers are consistent with one another.
****
#$&*
To begin, if the wire leads are not inserted into the 'jack' in the back of the generator, insert them now.
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, and note the force you have to exert.
Then clamp the ends to one another 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)?
- In which case do you believe more charges were available?
- In which case do you think more electrical current flowed through the wires attached to the generator?
Answer these questions in the box below, and explain the basis for each answer in terms of what you observe:
#$&*
------>>>>> crank easier, more work per sec, more charges available, more
current thru wires
Now consider the following questions:
- Is it harder or easier to turn the crank when current is flowing?
- Does current flow more easily through the wires when they are attached to the wood or when they are clamped together?
- Would you say that the circuit resists the flow of electricity more with the wood between the clamps or when the clamps are directly attached to one another?
Answer these questions below:
****
#$&*
------>>>>> harder/easier with current, more easily thru wood or clamped
together, resistance to current greater when
There are two types of resistance in this situation. There is the mechanical resistance of the crank, which is what you feel when you turn it. And there is the resistance of the material to the flow of current. When little or no current flows in response to the field you create, the electrical resistance is high. When current flows easily in response to the field, the electrical resistance is said to be low.
Answer the following below:
- When current flows, do you have to exert more or less mechanical force on the crank?
- When current flows, is electrical resistance high or low?
- When the mechanical force you have to exert is high, does this indicate a high or a low electrical resistance?
- Is high mechanical resistance therefore associated with high electrical resistance or low electrical resistance?
****
#$&*
------>>>>>more/less mech force with current, with current is electrical
res high or los, high mech force -> hig/low elec res, high mech res -> high or
low elect res
Go around testing different objects in your house to see which ones have high resistance and which ones have low resistance to the flow of electrical current. Try to find at least three different materials that have low resistance and it least three that have high resistance.
Indicate three materials with low resistance in the first line, three materials with high resistance in the second, and in the third line explain how you were able to tell which had high and which had low resistance:
#$&*
------>>>>> 3 materials low, 3 high res
You kit contains three flashlight bulbs and holders into which you can screw the bulbs. Each holder has two tabs. You can clamp one lead from your generator into each tab.
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 just barely glows. If you crank
fast enough the bulb will shine brightly, and if you crank too fast you can easily burn the bulb out. It is suggested that you don't crank too
fast.
- Note the numbers marked on the bulb, and record them. The numbers are small and can be
difficult to read; either of the convex lenses in your kit can be used as a magnifying glass.
- Count the number of times you crank the generator in 10 seconds while the bulb glows, and record this data.
- Repeat for the other bulbs in your kit. Some bulbs may require faster cranking than others, some will require more force than others. Determine the cranking speed needed to get each bulb to barely burn, and note which bulb takes the least force and which takes the most.
You will list the following below: in the first comma-delimited line the lowest
cranking rate, in turns per second, then the marking on the bulb, and finally
the phrase 'most force, least force, in-between force' to describe the amount of
force necessary. In the second line list the cranking rate in the middle,
the marking on that bulb and the appropriate phrase describing the force.
In the third line list the same information for the remaining bulb.
****
#$&*
------>>>>>lowest crank rate to glow _ marking _ mostLeastInbetween force
each bulb
Now place the bulb requiring the lowest cranking rate in one holder, and the bulb requiring the greatest cranking rate in the other. Connect a tab on the holder of one bulb to a tab on the holder of the other using a wire lead (the wire leads are the colored wires with alligator clips on the ends).
Connect the leads of the generator so that current will flow through the first bulb (the one that required the slower cranking rate) but not the second. Describe how you made the connection.
#$&*
------>>>>> current thru 1st (slower cranking) not 2d 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.
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. Only these leads will be connected. Don't crank the handle until you make the following predictions:
If you crank at the same rate as before:
- Do you expect that one bulb will glow, that both will glow or that neither will glow?
- Do you expect that you will need to exert more force, less force or the same force to achieve the same cranking rate?
Answer in the
space below, and include your reasoning:
#$&*
------>>>>> thru st then 2d then back which will glow, predict comparison
of force to preceding
Now crank the generator and answer the preceding question based on your experience, noting whether your predictions were true or not.
Give the best explanation you can of why the circuit behaves as it does:
#$&*
------>>>>> compare actual with predictions
Crank the generator so that both bulbs glow, but at least one of the bulbs just barely glows.
- Does this require a faster, a slower or the same rate as before?
- Did one bulb glow more brightly than the other?
- If so, which?
- Does it seem to require more force, less force or the same force as before?
Give your best explanation of why the system behaves as it does:
#$&*
------>>>>> both glow one barely: faster or slower, same or different
brigntness, which, compare forces
Finally disconnect everything, then connect the leads of the generator to the first bulb only. Then complete a parallel circuit to the second in the following manner:
- Connect a lead from the tab of the first bulb to one tab of the second. It might not be possible to actually connect the second lead to the tab of the first bulb since there is already one lead connected to that tab; it can be connected to the first clip, which is already attached to the tab.
- Connect a second lead from the other tab of the first bulb to the remaining tab of the second.
- The bulbs should be connected so that when the current flows into the out of the first generator lead it branches, with some current flowing into the first bulb and some branching off through the wire lead to the second bulb. The current passing through the second bulb will then travel through the second wire lead back to the second generator lead, where it will rejoin the current that has come through the first bulb.
What cranking rate do you now predict will cause one bulb to just barely glow, and which bulb do you think it will be (or will both just barely glow)?
Explain your reasoning.
#$&*
------>>>>> now with branching
Crank the generator so that one bulb just barely glows. The other bulb might barely glow, or it might not glow at all. Measure the cranking rate necessary to accomplish this, and in the first line below give the cranking rate. In the second line identify which bulb or bulbs glowed.
#$&*
------>>>>> one barely glows: rate, which glowed
Compared to the process of cranking the glowing bulb by itself:
- Does this require more force, less force or the same force?
- Does this require a greater, a lesser or the same cranking rate?
You can easily make a direct comparison as follows:
- Be sure that the leads from the generator are clipped the tabs of the bulb that glowed (if both glowed then this won't make any difference).
- Disconnect the leads connecting this bulb to the other.
- See if the force and/or the cranking rate changes as a result of disconnecting the second bulb.
You can alternately connect and disconnect the second bulb to see if there is any difference.
In the box below report your observations:
#$&*
------>>>>> compared to glowing bulb alone: moreLessSame force,
greaterLesser rate
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.
You just compared the behavior of series and parallel combinations of the 'slowest-cranking' and 'fastest-cranking' bulbs.
Now use repeat using the two bulbs which required the least cranking rate, setting aside the bulb that required the fastest crank. Make all the comparisons requested above.
Between the series and parallel circuit, which (if either) required the greater cranking rate, which (if either) required the greater force in order to get one bulb to barely glow? Answer below with succinct but complete statements and your best explanations:
#$&*
------>>>>> compare series parallel: which greater rate, which greater
force for one bulb barely glowing
In which case do you think work was being done at the greater rate? Give the best possible support for your reasoning:
#$&*
Returning to the idea that the cranking rate dictates the amount of work done per charge, and for a given cranking rate the force required is an indication of the current:
- For two given bulbs, which circuit, the parallel or the series circuit, requires the greater cranking rate to get one bulb glowing?
- Which circuit requires the greater force?
- Which circuit requires more work per minute?
- Could your last answer vary depending on which bulbs are used to build the two circuits?
Give your answers in the
space below,
and be sure to include your reasoning:
#$&*
------>>>>> compare: which greater rate, which greater force, which more
work per min, does this depend on which bulbs
In a circuit in which one of the bulbs is not glowing, do you think the non-glowing bulb dissipates the greater energy in the series or in the parallel combination?
Explain the reasons for your conclusion.
#$&*
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.
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?
- Which circuit would you say required the greater current, the series circuit or the parallel circuit?
Be sure to explain your
reasoning.
#$&*
------>>>>> which greater voltage, which greater current
Finally, using the two 'slow-crank' bulbs, set up a series then a parallel circuit. In each, crank until both bulbs glow, with the dimmer bulb just barely glowing. Compare the force and the cranking rate required for the series with the force and cranking rate necessary for the parallel combination.
Recall that power is the rate at which work is done: power = force * distance /`dt.
- As determined from the force necessary to crank the generator and from the rates at which the generator was cranked, did the series or the parallel circuit seem to require the greater power?
- As determined from the brightness of the bulbs, which circuit seemed to require the greater power?
Answer below and give your best explanation of each answer.
#$&*
Your instructor is trying to gauge the typical time spent by students on these
experiments. Please answer the following question as accurately as you
can, understanding that your answer will be used only for the stated purpose and
has no bearing on your grades: