The bright red, green, and
yellow lights that you find on many electronic devices are light emitting
diodes or LEDs. Like any other diode, an LED carries current only in one
direction. But unlike a normal diode, an LED emits a photon of light whenever
an electron shifts from a conduction level in its n-type cathode to a valence
level in its p-type anode.
1. Explain why an LED’s brightness is proportional to the electric current flowing through it.
Answer: The LED's brightness is proportional to the number of photons it emits per second, which is proportional to the number of electrons shifting from conduction levels to valence levels, which is proportional to the number of electrons passing through the LED, which is proportional to the current flowing through the LED. Overall, the LED's brightness is proportional to the current flowing through the LED.
Why: Each photon of light is created by an electron passing through the LED. As more current flows through the LED, more electrons make the shift and emit photons of light.
2. The current passing through an LED experiences a voltage drop. Use conservation of energy to explain why this voltage drop must occur.
Answer: Light carries away energy. If the current passing through the LED experiences zero voltage drop, it doesn't leave any energy in the LED and the LED can't produce a steady output of light.
Why: Energy is a conserved quantity. If the LED is steadily emitting light and light carries energy, then the LED needs a steady source of energy. The electric current passing through the LED must leave energy in the LED and the result of such an energy delivery is a voltage drop for the current. After all, a drop in voltage indicates that the current has lost energy per charge.
3. A photon of green light has more energy than a photon of red light. Why must an LED that produces green photons have a larger voltage drop than an LED that produces red photons?
Answer: Since one electron produces one photon, an electron needs to give up more energy to produce a green photon than a red photon. The amount of energy an electron gives up as it flows through the LED is proportional to the voltage drop through that LED, so a green LED must have a larger voltage drop than a red LED.
Why: Each photon that you see coming from an LED is the result of a single electron dropping from a conduction level to a valence level. To produce a green photon, that electron must release more energy in completing the drop than it would if it were to produce a red photon. The current in a green LED is thus experiencing a larger voltage drop than the current in a red LED.
4. The power supply in a typical electronic device delivers current at too high a voltage for an LED. If that current were sent directly through the LED and then returned to the power supply, the LED would receive too much power, overheat, and burn out. To prevent such a disaster, the electronic device first sends the current through a resistor and then through the LED. How does the resistor protect the LED?
Answer: The resistor is lowering the voltage of the current by converting electrostatic energy into thermal energy. By the time the current reaches the LED, it has the appropriate voltage to power the LED.
Why: The LED alone isn't able to handle too large a voltage drop. If the current passing through the LED experiences too large a voltage drop, then the LED will overheat and burn up. The resistor's job is to throw away some of the current's energy and deliver that current to the LED at a more appropriate voltage.
You enjoy listening to your
little portable MP3 player while jogging, but it hasn’t been working properly
since it got wet in the rain last week. The problem is that it keeps turning
itself off. Because the player makes no “click” sound when you press the “on”
or “off” buttons and because it turns itself off automatically after an hour,
you know that the switch that controls the player's power is electronic. It’s
probably an n-channel MOSFET that’s connected in series with the player's
electronics so that current from the battery must pass through both the
electronics and the MOSFET before returning to the battery.
5. Why won’t any power reach the electronics when the MOSFET isn’t conducting current?
Answer: The circuit will be open and no electric current will flow through the electronics. With no current flowing, no power can move from the battery to the player.
Why: To maintain a flow of power between the battery and the player, charges must be able to flow out of the battery with extra energy through one wire and return to the battery with little energy through a second wire. Since the MOSFET is part of one of these wires and it isn't allowing current to flow, this circuit isn't active. Charges either can't flow out to the player or they can't return to the battery to pick up more energy.
6. (a) What must the “on” button do to make the n-channel MOSFET conduct current so that the MP3 player will operate? (b) What must the “off” button or the automatic shutdown do to stop the n-channel MOSFET from conducting current, so that the MP3 player will turn off?
Answer: (a) It must put positive charge on the gate of the MOSFET. (b) It must remove the positive charge from the gate of the MOSFET (or, equivalently, put negative charge on that gate).
Why: An n-channel MOSFET becomes a conductor when its central part or "channel" shifts from being p-type to being n-type. This shift can be caused by attracting negatively charged electrons into the channel by putting positive charge on the nearby gate. When there is no positive charge on the n-channel MOSFET's gate, it develops two back-to-back p-n junctions and acts like a pair of diodes arranged back-to-back. No current can flow through such a structure.
7. Water is a poor conductor of electricity, but with patience you can send charge through it. If water is slowly turning off the player, what is it probably doing?
Answer: The water is allowing positive charge to flow off the gate (most probably to the negative terminal of the battery).
Why: The water can't store charge, so it is simply acting as a conduit for that charge. Positive charge is leaving the gate through the water and going somewhere else. The most logical destination for this positive charge is the negative terminal of the battery.
8. You discover a small drop of water inside the “off” button, allowing positive charge to flow slowly from the gate of the n-channel MOSFET to the negative terminal of the battery. You remove the water and the MP3 player works perfectly. Why did the drop cause trouble and why did removing the drop fix the player?
Answer: The drop allowed positive charge to leak out of the MOSFET's gate and removing the drop trapped the positive charge on the gate (where it kept the MOSFET conducting current so that the radio could operating).
Why: Air is an insulator while water is not. Removing the water droplet prevented current from flowing away from the gate of the MOSFET and turning the MOSFET (and the player) off.