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DC Circuits

  • I1-17: THERMOSTAT - MODEL

    I1-17
    Model of use of a bimetal strip in a thermostat.
    A bimetallic strip is set up to complete a circuit and turn on a bulb, as a model of how thermostats move with changing temperature to control furnaces or air conditioners. Heat the bimetal strip so that it curves toward the wire. When it touches the wire it completes the circuit, lighting the bulb. This is similar to the mechanism in a real bi-metal strip thermostat to turn on and off the power to the appliance.
    I1, I0, K6

    i1-17a

  • J4-03: PARALLEL PLATE CAPACITOR - SERIES CAPACITORS

    J4-03
    Demonstrate the effect of capacitors in series.
    The parallel plate capacitor is charged using a low-current DC power supply and separated as shown. A thin metal sheet is then inserted between the two capacitor plates, forming two capacitors in series. The voltage read by the electrometer remains virtually the same, indicating that the capacitance of the series capacitors is the same:

    C = C1 C2 / (C1 + C2),

    where either capacitance C is inversely proportional to the distance between the plates.
    J/K

    j4-03a

  • J4-42: CAPACITORS IN SERIES AND PARALLEL

    J4-42
    Demonstrate the voltage/charge/capacitance relationships in series and parallel capacitor circuits.
    A commercial bank of four identical capacitors can be connected in various series and/or parallel combinations and charged using an attached battery. The digital voltmeter is used to probe the voltage on any individual or group of capacitors and displayed for the class using a TV camera and video projector in the lecture halls (monitor in smaller classrooms).
    J4, ME2

    j4-42a

  • J4-43: CAPACITORS IN SERIES AND PARALLEL WITH PROJECTION METER

    J4-43
    Illustrate voltage relations in series and parallel capacitor circuits.
    Two capacitors can be connected in series or in parallel and charged with the battery, or they can be charged individually and then connected. The projection meter is used to read the voltage across either or both capacitors. BE CAREFUL: this uses large capacitors charged to 50 volts!
    J4

    j4-43a

  • K5-31 OHM'S LAW

    K5-31
    Demonstrates relationship between current, voltage, and resistance

    This simple circuit consists of a variable voltage power supply and a socket that can hold one of three modular resistor units, with a ammeter measuring the current through the resistor and a voltmeter measuring the voltage across the resistor. The whole circuit is mounted on a transparent plate that can be placed on an overhead transparency projector to show the wiring and the meters.

    The voltage can be varied to show how the voltage and current change together in a linear relationship to the resistance. Both two 1,000 Ohm resistors and one 2,000 Ohm resistor modules are available; the two 1,000 Ohm modules can be placed in parallel if desired.

    It can be valuable to ask students to make predictions about how the results will change when you change the resistance, then afterwards have them discuss their predictions and compare them to the results.
    K5

  • K5-42: TRANSISTOR

    K5-42
    Demonstrate a transistor circuit.
    A high-current transistor controlled by a flashlight battery operates an automobile headlamp by switching a 7.5volt heavy duty dry cell onto the light. The circuit for the system is shown above, and is included with the demonstration. By removing and replacing wires it can be shown that the flashlight battery alone cannot operate the light and that the heavy duty dry cell connected directly to the light turns the light on.

  • K5-43: NON-OHMIC DEVICE - V VS. I

    K5-43
    Demonstrates the non-linearity of the V vs I curve for a diode
    A zener diode is connected in series with a resistor across the output of a power supply. Observe the current vs. voltage as the voltage is varied. The voltage and current are read from projection meters when the device is placed on an overhead projector. The shunt with the zener diode has a series limiting resistor, so you will see a current in the milliamp range when the zener becomes conducting at about 7 volts.

    Note that this uses the same apparatus from K5-31: Ohm's Law, and can be taught in conjunction with that topic. The Zener diode can be used as an extension of the discussion of Ohm's Law, inviting students to hypothesize other non-ohmic devices.

  • K6-03 SERIES AND PARALLEL LIGHTS - BATTERY AND CLIP-ON WIRES

    K6-03
    Shows voltages and currents in series & parallel circuits
    Series and parallel combinations of light bulbs can be connected to the 7.5 volt battery source. Meters indicating current and voltage can be inserted in the circuit as required.

    Note that due to the aging of our large display galvanometers, this is better performed now with digital multimeters. A camera can be used to show them on the lecture hall screen if desired.
    K6, ME2

  • K6-11 CIRCUIT PARADOXES

    K6-11
    Series-parallel circuits to encourage dicussion about DC circuits
    Two circuits, with identical batteries and identical light bulbs, are connected with a switch through one branch of the circuit. (When using, be sure to check main power switch on underside.)
    K6

  • K6-12: SERIES/PARALLEL LIGHT CIRCUIT CONUNDRUM

    K6-12
    Test your ability to analyze series/parallel circuit.
    The circuit shown above uses three 1.5 volt batteries (a total of 4.5 volts) and five identical 1.5 volt light bulbs. In the photograph all of the switches are OFF, so none of the light bulbs is lit. The question is to rank the light bulbs in order of brightness when all of the switches are closed, putting all of the bulbs in the series/parallel circuit. The solution is shown above.
    K6

  • K6-32: POTENTIOMETER

    K6-32
    Find an unknown EMF using a voltage divider.
    A battery of voltage V is connected across a resistive wire of length L. A second, "unknown" lesser voltage battery is connected through a meter to the slide tap of the potentiometer. Find the distance L1 along the wire where the clip lead draws no current. The unknown voltage Vu is then: Vu=V L1/L External batteries can be used to provide a different battery configuration if desired.
    K6, ME2

  • K6-33: WHEATSTONE BRIDGE

    K6-33
    Demonstrate operation of a Wheatstone bridge.
    A Wheatstone bridge is connected as in the circuit below using two (approximately) 400 ohm resistors, a 0-90 ohm variable resistor and a 60 watt light bulb (16.7 ohms cold) with a 6 volt battery and a 0-5 mA ammeter (large lecture meter). The unknown resistance Rx (the light bulb) is: Rx = Rs (R2/R1), where Rs is the resistance of the active part of the variable resistor (equal to the fraction of its length used multiplied by 90 ohms).

  • K6-35: VOLTAGE DIVIDER

    K6-35
    Show how a voltage divider can be used to produce any voltage up to the maximum of the source.
    The circuit is assembled as shown in the diagram above: a 7.5 volt battery is connected across the entirety of an exposed resistive coil, with a voltmeter connected from the negative terminal of the battery to the adjustable slider that runs along the length of the resistor. This allows a variable voltage to be measured on the display meter.

  • K6-36: VISIBLE AMMETER

    K6-36
    See some of the important parts inside an ammeter.
    The shroud has been removed from a nonworking ammeter (11cm diameter) so that the working parts are visible. A digital camera can be made available upon request to show the device on the projection screens in the lecture halls, or the meter can be passed around in smaller classes.

  • K6-51: KIRCHHOFF'S LAWS

    K6-51
    Apply Kirchhoff's laws to a simple two-loop problem.
    The circuit is mounted on an overhead projector projectual to make it visible to a class. Analysis can be carried out using the two loops of the circuit to obtain the current in each part of the circuit. An analysis sheet can be used on another overhead projector. Finally, the voltmeter mounted on the circuit projectual can be used to measure the voltage drops across various parts of the circuit, checking the results of the calculations. Use a digital voltmeter to read small voltage drops more accurately.
    K6, PS1

  • K6-52: IMPEDANCE MATCHING - BATTERY AND JOULEMETER

    K6-52
    Demonstrate that the power transmission is maximized when the source impedance and the load impedance are equal.
    On the left of the picture above is a simple DC power supply consisting of a battery and a variable series resistor, which is set to about 70 ohms.

    Adjust the load resistor (at the right) until the power to the load resistor is maximized, as read by a joule meter (the box with all the connections in the center of the picture). The resistance of the load will also be about 70 ohms.

    This demonstration has been used as an introduction to impedance matching between the power amplifier and loudspeakers of an audio system.

    Note: a video camera can be made available upon request to aid in displaying this to large classes.

  • K7-05: SERIES MOTOR AND LIGHT BULB CONUNDRUM

    K7-05
    Generate thought about how a motor works.
    A 1.5 volt motor and 1.5 volt light bulb are wired in series such that a 1.5 volt battery can be connected across either of them individually, or across both in series. The first mpeg video linked below shows what happens when each is connected to the battery individually.
    Ask the students what will happen when both are connected in series: Will (a) only the light bulb operate, (b) only the motor operate, (c) both operate, or (d) neither operate?
    Again, only the motor spins, as seen in he second mpeg video below.

  • K7-11 RC CIRCUIT - RC TIME CONSTANT - OSCILLOSCOPE

    K7-11
    Observe the shape of the charging and discharging curves. Measures an RC time constant.
    An RC circuit with meters is mounted on an overhead projector projectual (the same setup as in K7-12). The voltage across the capacitor and the charging current are displayed. Either the voltage across the capacitor or the charging current (voltage across the series resistor) can also be displayed individually on the oscilloscope. Various combinations of resistance and capacitance can be plugged into the circuit.
    K7, ME2, PS1

  • K7-12: RC CIRCUIT - RC TIME CONSTANT - PROJECTION METERS

    K7-12
    Measure an RC time constant and to observe the shape of the charging and discharging curves.
    An RC circuit with meters is mounted on an overhead projector projectual (the same setup as in K7-11). The voltage across the capacitor and the charging current are displayed on analog meters. Various combinations of resistance and capacitance can be plugged into the circuit. See circuit diagram above.
    K7

  • K7-15: CURRENT IN RC CIRCUIT?

    K7-15
    Demonstrate charging of a capacitor in a possibly counterintuitive way.

    A switch is closed to charge a 1 farad (yes, one FARAD) capacitor with a 3 volt battery (actually two 1.5 volt batteries in series). The capacitor has 1.5 volt light bulbs on each side of it in the circuit, as shown in the circuit drawing above.

    Consider the following series of questions:

    Q: What will happen when the battery is connected (switch turned to left position):

    (a) both bulbs will light and stay lit,

    (b) both bulbs will go on momentarily,

    (c) only one bulb will light and stay lit (if so, which one?),

    (d) only one bulb will go on momentarily (if so, which one?),

    (e) neither bulb will go on at all,

    (f) something else will happen (if so, what?).

    A: (b), Both bulbs will go on momentarily as the capacitor charges, then they will fade out. Click below for video

    Q: What will happen when the switch is opened (center switch position)?

    A: Nothing: the capacitor remains charged and no current flows.

    Q: What will happen when the switch is closed to the right, completing the circuit including the capacitor and the two light bulbs?

    A: The capacitor will discharge through the bulbs, turning them on momentarily while current is flowing, with the intensity decreasing as the current falls to zero. Click below for video.
    K7