A light bulb with a large curly filament is connected alternately to 110 V AC and DC sources. A magnet is brought near the bulb. The filament under goes a steady deflection in the case of DC, but vibrates impressively in the case of AC.
The difference can be further illustrated by hooking a large inductance or capacitance in series with the bulb. The inductance "passes" DC but "blocks" AC, whereas the capacitance "blocks" DC but "passes" AC. See Capacitors and Inductors [1] for details.
Here is another demonstration of AC: A bicolor LED is connected directly to the 110V AC line. When the instructor swings the LED around her/his head, the light flashes green then red, showing that the LED is lit for only one half of the AC cycle. The same LED can be connected to a DC source, then the LED has one polarity, either red or green.
- This is a circuit that looks very much like the canonical series circuit, but the switches and lights don't do what's expected. There has to be a deeper level of knowledge and understanding to figure it out.
Here's the apparent circuit.
The puzzle is, what components can be added to this circuit to give the observed behaviour? One could think about the effects of common components such as resistors, inductors, capacitors, diodes, transistors, etc. I can tell you it doesn't involve magnets or radio frequency transmitters. If you figured it out, check the solution. If you haven't, I can give you three hints:
Diodes provide the coding and decoding by phase of the AC current. A diode is put in parallel with the each switch and light.
A diode is a uni-directional valve. It will let current pass in one direction but not the other. A diode in this direction will let positive current pass from left to right but will block negative current. From the other direction negative current will pass but positive current will be blocked. Diodes are marked with the vertical bar to indicate the direction.
At the switches:
When the switch is open, all current is forced to flow through the diode. One switch diode blocks positive current and one blocks negative current. If both switches are open, all current is blocked and no lights light up.
When a switch is closed, the diode is bypassed, shorted out, and both polarities can pass through the switch. Otherwise, each diode blocks one polarity.
At the lights:
The lights act as resistors and will pass both polarities. Here, each diode acts as a short circuit for one polarity, and acts to bypass the light. The other polarity, blocked by the diode, is forced through the lamp and causes it to glow.
With two switches there are 4 possibilities:
open open 0 current
open closed + current
closed open - current
closed closed + and - current
One light turns on from the + current and one from the - current.
Here's a circuit where the lights are replaced with led's. See if you can figure out what should happen.
Various low-pass and high-pass RC and RL filters can be constructed to your taste. The Pasco Waveform Analyzer has tunable low, high, and band-pass filters built into its circuitry so the harmonics of a square, sawtooth, or other complex waveform as from the Fourier Synthesizer, can be analyzed.
A simple RC circuit will integrate or differentiate waveforms:
(Of course, the derivative and integral of a sine wave is the leading and lagging cosine wave; these are just the normal 90° phase shifts.) The circuit below integrates.
The resistance R is made large and the capacitative reactance Xc is made small by using a large C and/or a large W. Then the current into the circuit is set by R and proportional to Vin. The capacitor stores and integrates the charge.
To differentiate the circuit is wired as below:
We arrange for nearly all the input voltage to drop across the capacitor (Vc >> Vout ) by making R small and Xc large using a small C and/or w Thus the voltage drop across R measures i without disturbing Vc.
The circuit looks essentially like a capacitor to the input. The current is set by C and the small R is placed in series to sense it. RL circuits will perform the same operations.
A relaxation oscillator circuit as seen in RC Time Constant [3] demonstrates the RC time constant by a flashing neon bulb.
With a square wave input the voltage across the capacitor shows the exponential decay on an oscilloscope.
You could roughly measure the time constant t c= RC from the oscilloscope trace. With a similar circuit for RL you could estimate the time constant tL = L/R. These circuits are analyzed and their oscilloscope traces depicted in Halliday and Resnick, Part 2; for RC, Section 32-8, pp 705 -709; and for RL in Section 36-3, pp 798 - 801. As discussed in E.7.5, the same circuits will integrate and differentiate waveforms.
A simple, graphic demonstration of a series RLC circuit is to use a small light bulb for the R. Then you can tune through resonance and see the bulb's brightness reach a maximum, or you can set the frequency a little below resonance and insert an iron core into the inductor and see the bulb's brightness go through a maximum.
A parallel RLC circuit is set up so it can be driven with a signal generator and its resonance observed on an oscilloscope. The R (and thus Q), L and C are all variable. Alternatively, the circuit can be pulsed and the exponentially damped oscillations observed on a scope.
(The small input and output capacitors serve to isolate the RLC circuit.)
If the signal generator is replaced with a sweep frequency generator, the oscilloscope can be caused to actually draw the resonance curve.
The sweep generator repeatedly sweeps across a band of frequencies including the resonance. The output of the RLC circuit is then amplitude modulated by the resonance curve. Its average is still zero, but after passing through the diode which acts as a detector and being smoothed by the capacitor the final result is the response curve of the RLC circuit as a function of frequency. You can vary the position of the peak of the curve by changing C or L, or you can vary the width of the curve (Q) by varying R.
You may become confused when you try to use the dual trace feature of the scope, for example, to demonstrate the 90° phase shift of a capacitor, unless you understand how the leads are grounded.
One side of both scope leads is grounded, and one side of the signal generator is grounded. This prevents you from hooking up the naive circuit below to show the 90° phase shift.
You can "fake" the situation by using a small resistor (1000W) as shown below.
The voltage across the resistor alone shows the phase of the current through the capacitor. The voltage across both is the voltage across the capacitor -- mostly, if R<< Xc. Then these two voltages are almost
90° out of phase. For a capacitor, then, you want to use a low frequency so Xc is large.- In the similar circuit with an inductor you would use an high frequency so XL is large.
But the simplest way of demonstrating the same phase shift is use a two prong adaptor on the plug of the signal generator. Then the signal generator ground is floating, and the circuit can be hooked up as below.
You are reading the voltages across the resistor and capacitor in the opposite directions, so press the invert button on channel 2 of the scope to show properly that the voltage lags the current in a capacitor.
Adjust the two signals to have the same amplitude, and then turn the sweep rate knob counter-clockwise to the X-Y position to display the 90° phase shift in the form of a perfect circle.
The similar situation with an inductor is well shown with L = 50 mH and R = l0K.
Links:
[1] https://demoweb.physics.ucla.edu/node/167
[2] https://demoweb.physics.ucla.edu/node/169
[3] https://demoweb.physics.ucla.edu/node/185