Light Emitting Diodes (LED’s) for grades 9-12
Part 1: Circuits and Efficiency (9, 11, 12C)
1) Attach the indicated load to two D cells. Describe what you notice in each case.
a) A resistor and an LED in series: What happens if you reverse the connections on the battery?
The LED only works one way. Note: The resistor is needed to protect the LED from too much current. It is not needed for the blue or uv LED.
b) A small incandescent light bulb: What happens if you reverse the connections on the battery?
The bulb lights up and there are no changes when the directions are switched.
2) Observe a string of small Christmas lights swung in a circle. What does the demonstration tell you about the type of lights in the string and about the electricity coming from the outlet? Explain.
They will see a series of dashed lines. This shows that the electricity is AC and that the lights must be LEDs. How else can you show that the current from an outlet is AC? One other demonstration that I use involves some old power supplies. When I attach the AC to a small speaker, you can hear a 60 Hz hum. When you attach the speaker to the DC, you hear a hum that is one octave higher – 120 Hz. The 60 Hz AC is converted to a poor version of DC using a rectifier. A better power supply would also smooth this with a capacitor.
3) Attach the indicated load to a hand-cranked generator. Describe what you notice
a) A resistor and an LED in series: What happens if you crank it at different speeds?
The LED is tested before the bulb because it is easier to light. At really slow speeds it won’t light up. At higher speeds it gets brighter but the colour doesn’t change..
b) A small incandescent light bulb: What happens if you crank it at different speeds?
At slow speeds the bulb does not light up. As you crank it faster, it goes from red to orange to white. It also gets brighter. It takes more force o turn than the LED.
4) Use the solar panels to get the largest voltage possible. Measure this with a voltmeter.
a) What voltage did you get? What did you have to do?
The small solar panels will produce up to 0.45 V in bright light. The larger ones can produce 1.5 V.
b) Predict what you will see if an LED is attached to the solar panels. Explain. Test this.
The LED is tested first because the students will be able to predict the results based on their previous experiences. The LEDs need around 3V (especially the blue) and so they won’t work unless two large solar panels are connected in series. The LED won’t be very bright unless you go outside or bring the panel near a large light bulb or point an LED flashlight at it or add two more solar panels in parallel. This shows how solar panels provide reasonable voltage but small current. This is just like what happens if you have the class make electrical cells from fruit or potatoes (my favorite). You need to connect a few in series to make a battery that can light an LED. If you want to light an incandescent bulb, you will need many more cells in parallel.
c) Predict what you will see if an incandescent bulb is attached to the panels. Explain. Test this.
The bulbs need around 3V and so they won’t work unless two large solar panels are connected in series. This still won’t light up the incandescent bulb. The students may predict this because of their experience with the hand-cranked generators. The panels have sufficient voltage but can’t push sufficient current. It is like trying to use a watch battery when a D cell is needed. Solar panels can light up incandescent bulbs but you will need many of them in parallel to produce the current. This illustrates the inefficiency of the bulbs again and how current is different from voltage.
5) Charge up a capacitor to 3 V. Predict what you will see if an LED or an incandescent bulb is attached the capacitor. Explain. Test this.
The capacitor is an ultra-capacitor of 1 F. These are great tools to show electrical energy storage because they charge much faster than rechargeable batteries. They are also becoming very important in electrical energy storage because they are lighter, have a much longer lifetime and they can support more current.
The capacitor can be charged in less than a minute by attaching it to the batteries or by using a hand-cranked generator. Students should be able to predict that the capacitor will last longer with the LED’s. This is true and the contrast is quite dramatic. The incandescent bulb dims and then dies within seconds. The LED lasts for more than 15 minutes. This illustrates again, the inefficiency of incandescent bulbs and the difference between voltage and current.
6) Many areas of the world do not have easy access to electricity. The people often need to work during the day and can only get an education at night. How can the materials that you have examined be able to provide light at night? What are all the components needed?
You need to combine solar panels (to generate electricity), capacitors (or rechargeable batteries to store energy) and LED’s (to produce light efficiently). You can show how these three are combined in cheap garden lights. Using hand-cranked lights are another possibility. You can show this with hand-cranked flashlights. They also contain capacitors so that you don’t have to crank while using them.
7) A 100 W incandescent bulb, a 26 W compact fluorescent light (CFL) and a 20 W LED (light emitting diode) light each produce about the same amount of light.
a) How do the temperatures of these compare?
The incandescent is burning hot (be careful!), the CFL is warm and the LED is at room temperature.
b) How do the different temperatures explain the differences that you noticed in the circuits above?
The incandescent was much harder to light and used up energy faster because most of the energy goes to generating heat and only some of it is transformed into light.
c) Calculate the cost of lighting a bulb for 25 kh (about 23 years) for each type.
bulb cost / rebate / life (kh) / E cost/25 kh / Number Bulbs / Total costIncand / $1 / $ 0 / 1.2 / $180 / 25/1.2 = 20.8 / 20.8*$1 + $180 = $201
CFL / $10 / $ 3 / 8 / $42 / 25/8 = 3.1 / 3.1*$(10-3) 2 + $42 = $64
LED / $12 / $ 3 / 25 / $30 / 25/25 = 1 / $(12-3) + $30 = $37
d) When will LED bulbs save you the most money, in the winter or summer? Explain.
You will save the most money in the summer because you won’t need air conditioning to get rid of the extra heat from the incandescent bulbs. In the winter this extra heat can reduce the need to heat the room.
8) Attach the long legs of the dimmer switch to two D cells. Attach the short legs to a bulb.
a) What happens as you turn the knob from one extreme to the other?
At one extreme, the light does not glow. As the knob is turned to the other extreme the light gets brighter and if you look very carefully, you can see that it goes from red to orange to white.
b) Measure the voltage and current through the light at five very different positions of the switch and graph your results with V vertical.
V (V)I (mA)
c) How does this graph differ from that of a resistor?
The resistance of a resistor is constant and the graph is a straight line. However, the graph of the bulb is a curve with resistance increasing with current because of increased temperature.
9) Attach the long legs of the dimmer switch to two D cells. Attach the short legs to a blue LED.
a) What happens as you turn the knob from one extreme to the other?
This experiment gives similar qualitative information as the hand cranked generator. However, it also allows the students to see the changes more clearly and to associate them with data. It also shows that Ohm’s Law does not apply to all loads. The light does not glow for quite a range. Once turned on it gets brighter but the colour stays the same. Note: Use a blue LED so that current from zero resistance does not fry the LED. If you use the red LED, you will need to include a resistor.
c) How is this different from the incandescent bulb?
The incandescent bulb changes in colour and intensity and has a range where it is not quite off but not quite on. The LED is either on or off and only changes in brightness not colour.
d) Measure the voltage and current through the light at five very different positions of the switch and graph your results.
c) How does this graph differ from that of a resistor?
The behaviour is like an on-off switch. There is no current until there is sufficient voltage. Afterwards the current increases, but the voltage doesn’t change much.
Part 2: Light and Colour (10, 12U)
10) Use the diffraction gratings to examine the colours produced by the three light sources.
a) Show the spread of the spectrum by shading the boxes below. Mark where the light is brightest.
Make sure that the lights are off. Tell the students to look off to the side to see the spectrum. The photo below shows the amber LED on the right and the spectrum on the left at an angle of almost 40o. The finer the spacing of the diffraction gratin, the better the spread of the spectrum and the greater the angle at which you will see the spectrum.
Bulb Type red orange yellow green blue violet
Incandescent
Red LED
______LED
b) How do the three spectra differ?
The incandescent bulb shows a full spectrum - from red to violet. All of the colours are strong but there may be a broad peak noticed in the middle. A red LED shows a narrow spectrum; reds and possibly some orange and ir. The other LED’s have very wide spectra. The image below shows the yellow LED. The uv LED shows quite a bit of violet light, so it is easy to see when it is working. The ir LED shows a small amount of red light but you need to look directly at the LED to see it. You probably should not stare directly at the ir LED because your eye will not blink if it is too intense. You can safely view it through the camera on your phone. You can also tell if the LED is on, because the voltage across it will be around 1V. If the ir LED is damaged it will be a short circuit and will read the 3 V of the batteries.
11) How is colour related to voltage in an LED?
a) Make a circuit with two D cells, a resistor and the uv LED. Measure and record the voltage across the LED and repeat the process for each of the other LED’s.
The resistor is needed to use up the extra energy. Without it, the low frequency LED’s will get fried. Something from 10 to 300 W will work well. The resistor that PI provides works fine.
.
V (V) 20 ohm / E (x 10-19 J) / V (V) PI 300 ohm / E (x 10-19 J) / l (x 10-7 m) / f (x 1015 Hz)2.99 / 4.79 / 2.86 / 4.58 / 4.54 / 0.66
2.96 / 4.74 / 2.63 / 4.21 / 4.70 / 0.64
2.18 / 3.49 / 1.89 / 3.03 / 5.65 / 0.53
2.18 / 3.49 / 1.83 / 2.93 / 5.91 / 0.51
2.01 / 3.22 / 1.86 / 2.98 / 6.00 / 0.50
1.93 / 3.09 / 1.67 / 2.68 / 6.61 / 0.45
1.46 / 2.34 / 1.20 / 1.92 / 9.40 / 0.32
b) What does the data tell you about how colour and energy are related?
The energy increases as you go toward the violet end of the spectrum and beyond.
c) Why is uv light harmful and why are x-rays even more dangerous?
These colours have more energy and can burn your skin and can cause changes in your DNA. This connection of light and colour is also useful for grade 9 Astronomy.
d) The frequencies of the LED’s have been given. However, the LEDs produce a range of frequencies. What does this given frequency represent?
This is the peak frequency under normal use.
e) Graph the energy (vertical) vs. frequency using Excel and sketch it above, What is the slope of the line?
A typical value with the PI resistor and LEDs plus a uv and ir LED is E = 7.7 x 10-34 f - 0.80. When the experiment was repeated with a 20 resistor the equation was E = 7.5 x 10-34 f - 0.30. The main effect of the resistor is on the intercept – not the slope. Each experiment had R2 values of 0.95, and so were equally good fits to the data.
f) The slope is Planck’s constant, h = 6.626176 x 10-34 Js. Calculate your percentage error.
You can usually get within 20 %. The above results are +16% and +14%. Note: If you do not include the data for a uv and an ir LED, the results are not as good. The equations are E = 8.6 x 10-34 f – 1.4 (300 ohm) and E = 9.3 x 10-34 f – 1.3 (20 ohm). The errors in Planck’s constant are now +30% and 41 %. You can get good results without the ir and uv if you use the PI version of the experiment that measures the voltage when the LED just barely turns on. This experiment takes longer to do but is closer to the photoelectric effect experiments.