course Phy 202
Experiment 17: Capacitors and Current, Voltage, EnergyThe hand-cranked generator is connected to a large-capacity capacitor and the difficulty of cranking changes as time passes. This cranking difficulty vs. elapsed time is noted. The general nature of the current flow vs. time (i.e., increasing or decreasing) is inferred. The capacitor is connected in series and in parallel with a light bulb and the behavior of current vs. elapsed time inferred in each case; the effect of the light bulb is noted. The charged capacitor is allowed to discharge through the generator, then after recharging it is allowed to discharge through the light bulb; the nature of the capacitor is speculated upon.
Now connect the leads of the generator to the large capacitor, as shown on the video clip.
• Crank the handle of the generator at a constant rate of approximately two revolutions per second and keep cranking. After about a minute release the handle and see what happens.
Upon cranking the generator, the force to crank the generator was greater than the force required after cranking the generator for a period of time. When the handle was released, the generator handle continued to rotate some amount of time before stopping.
• What happened to the amount of force necessary to crank the handle? What do you think was therefore happening to the amount of current flowing in the circuit?
The required force decreased as the handle was cranked, and the capacitor charged. The reason for this change in force is due to the capacitor charging characteristics. As a capacitor charges less current flows, thus causing the overall circuit impedance to change, this in turn, results in the required force to crank the generator. Essentially, the charging current asymptotically approaches zero as the capacitor becomes charged
• What happened after the handle was released and how could you possibly explain this?
When the generator cranking ceases, the capacitor releases the stored energy and continues to keep the handle rotating. Once the stored energy is completely released, the handle stops rotating.
• What evidence do you have that the capacitor in some way stored at least part of the energy you produced when you turned the crank?
As stated above, it is evident that some type of energy has been stored and is released when the handle continues to rotate after the being released. This is the stored electrical in the capacitor created when the handle was cranked and the electrical energy stored in the capacitor.
Take one of the thin wire leads and clamp each end to a different post of the capacitor so that current can flow from one capacitor terminal to the other. After about 10 seconds remove the lead.
Now place a bulb in the holder and connect one of the tabs on the holder to one post of the capacitor using a thin wire lead. Connect one of the leads of the generator to the remaining tab of the bulb holder and the other to the remaining post of the capacitor, so that current must pass through the bulb to get to the capacitor.
This circuit is a series circuit consisting of the generator, the bulb and the capacitor.
• Crank the handle of the generator at a rate that causes the bulb to burn, but neither very brightly or very dimly. Continue cranking the handle at the same rate regardless of what happens. After about a minute, release the crank and see what happens.
• As you continue cranking, what do you notice about the force you have to exert, and what do you notice about the bulb?
The force required to crank the generator decreased over time, and the bulb became dimmer over time.
• After you stop cranking, what happens to the generator and what happens to the bulb?
When I stopped cranking the generator, the generator continued rotating without help and the bulb was not illuminated.
• What happens to the voltage produced by the generator as you continue cranking?
The voltage increases as the generator is cranked and the capacitor is charged.
If you crank at a constant rate you get pretty nearly constant voltage. The voltage across the capacitor increases and opposes the voltage of the generator.
It's clear from your subsequent answers that you understand this.
• Does the voltage increase, decrease, or remain the same? How can you tell?
As time progresses, the decreasing current causes progressively less voltage to be dropped across the resistor (R), resulting in the bulb becoming dimmer over time, and more voltage builds up across the capacitor (C). At time a certain period in time, the voltage felt across the capacitor is equal to the source voltage and the voltage dropped across the resistor (R) is equal to zero. This is the complete charge cycle of the capacitor. This is evident by both the bulb becoming dimmer and the required force to crank the generator becoming less over time, which indicates the resistance is becoming greater, which results in greater voltage.
• What happens to the current passing through the circuit as you continue cranking?
When the generator is initially cranked, the circuit current is greatest, due to the charging current and characteristics of the capacitor. Over time, continuing to crank the generator causes the circuit current to decrease.
• Does the current increase, decrease, or remain the same? How could you tell if you weren't looking at the light? How can you tell by looking at the light?
The circuit current decreases, which is evident by less force required to crank the generator, and the bulb becoming dimmer over time.
• Sketch an approximate graph showing how the current through a capacitor behaves at a constant voltage.
You have directly experienced the fact that the brightness of the light bulb depends on the voltage across the bulb (i.e., the faster you crank the generator when it is connected to a single bulb the brighter the bulb burns).
• What therefore do you conclude happens to the voltage across the bulb as you continue cranking the capacitor-and-bulb circuit?
The voltage, or potential difference, in the circuit becomes greater as the capacitor charges, eventually causing full voltage and no current across the resistor. Therefore, the voltage across the bulb increases and the bulb gets dimmer.
• Based on the force required to crank the generator, what happens to the current through the light bulb? Is this consistent with your answer to the preceding question?
Yes, based on the force required to crank the handle, it is evident that the current flow in the circuit decreases over time. Eventually, when the capacitor becomes fully charged, the current flow goes to zero and voltage becomes full supplied voltage. This is due to the capacitor acting like an open circuit when fully charged.
The total voltage across the capacitor and bulb remains constant as long as the generator is cranked a constant rate.
• Based on what you think happens to the voltage across the bulb as you continue cranking, what do you think happens to the voltage across the capacitor?
Eventually, when the capacitor becomes fully charged, the current flow goes to zero and voltage becomes full supplied voltage. This is due to the capacitor acting like an open circuit when fully charged.
"
Good work. See my notes and let me know if you have questions.