Apparatus Competition

2007 AAPT Summer Meeting

Greensburo, NC

Visualizing Inductance

Steve Lindaas

Minnesota State University Moorhead, 1104 7^th Ave South, Moorhead MN 56563

(218) 477-4268

lindaas@mnstate.edu

Abstract

Can you turn on a light emitting diode (LED) with a AA battery? Normally the answer is no since LEDs typically have a turn on voltage greater than 1.5 volts. Adding an inductor in parallel to a battery and LED will allow you to flash the LED when the battery is disconnected. This simple circuit demonstrates the fundamental electromagnetic phenomena known as Faraday’s Law.

Construction of Apparatus:

There are two different but similar setups used.

Demo Setup:

Connect a battery (1.5 V), LED and inductor in parallel. Make sure the LED is back biased. You will want to disconnect the battery from the circuit quickly. A switch can be used or simply touching one end of the battery with an alligator clip is sufficient. Using a red LED with a coil with an inductance works well. A suitable inductor can be made by using 2.5 meters of 30 awg enameled wire that has been wound into a coil around your thumb.

Lab Activity Setup:

Connect a LED in series with a resistor which are then connected in parallel to a battery (1.5 V) and inductor. You will want to disconnect the battery from the circuit quickly. A switch can be used or simply touching one end of the battery with an alligator clip is sufficient. A digital oscilloscope will be required to record the induced voltage in the circuit. Measure the voltage across the LED and across the resistor. Using an inductor around 25 mH and a resistor around 500 W works well.

Use of Apparatus:

Demo Mode:

Complete the circuit by touching the loose alligator clip to the battery (or by closing the switch). Quickly disconnect the battery (open the switch). A flash of light will be observed from the LED. This flash occurs after the battery is disconnected from the circuit. Clearly the inductor is storing energy to power the LED.

The magnetic flux does not want to change in an inductor so it draws on the energy stored in the magnetic field to induce a voltage.

1) There is energy stored in an inductor.

2) The induced voltage is typically much larger than the 1.5V from the battery.

Reverse the polarity of the battery. Repeat and a weaker flash can sometimes be observed from the LED. This shows that the induced voltage can be quite large to force a current through the back biased LED. Note: This only works on some LEDs.

Lab Mode:

Connect a digital oscilloscope to the circuit to measure the voltage across the resistor (V_R) and across the resistor and diode (V_RD). Set the trigger to catch the transient voltage signal (eg positive slope greater than 0V). Complete the circuit by touching the loose alligator clip to the battery (or by closing the switch). Quickly disconnect the battery (open the switch). A flash of light will be observed from the LED. This flash occurs after the battery is disconnected from the circuit. Adjust the voltage and time step as necessary to capture a good signal.

The induced voltage can be seen to decay with a characteristic time constant ().

An example of a good signal is shown in Figure 1 where the exponential decay (). Figure 1 also shows that the voltage drop across the LED is constant at just under 2 V. The dependence of the time constant on the resistor is shown in Figure 2. The polarity is different from Figure 1 as the oscilloscope polarity was reduced for this setup. The voltage measured is the total voltage across the diode and resistor.

There are other possibilities for this lab since the area under the curve would be related to the change in magnetic flux, , since .

Parts List

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
L.E.D.	Radio Shack		3.95
Enameled Wire (1 m)	Radio Shack		2.50
AA Battery (1.5V)	Grocery Store		3.95
Alligator Clips (2x)	Radio Shack		5.50
Al Foil	Grocery Store		1.95
Duct Tape	Hardware Store		3.65
Total Cost			21.50

Note: Most of these parts can be found just laying about your room. The parts above are enough for quite a few setups.