LED (a semi-conductor device) is an abbreviation that stands for
- A.Licence for Energy Detector.
- B.Light Energy Device.
- C.Light Emitting Diode.
- D.Lost Energy Detector.
Correct answer
Explanation
The correct answer is C, Light Emitting Diode. An LED is a p-n junction diode that gives out light when current flows through it in the forward direction.
When an LED is forward-biased, electrons from the n-side and holes from the p-side meet at the junction and recombine, releasing energy as light. The colour depends on the semiconductor used, such as gallium arsenide, gallium phosphide or gallium nitride; blue and white LEDs use gallium nitride. LEDs need little power, produce little heat and last long, so they have replaced filament bulbs in homes, displays, traffic lights and vehicle lamps.
A is wrong: "Licence for Energy Detector" is not a real device name.
B is wrong: "Light Energy Device" is a made-up expansion; the D stands for diode.
D is wrong: "Lost Energy Detector" is also invented and has nothing to do with emitting light.
Exam tip: LED gives light from current, while a photodiode and a solar cell give current from light.
Practice Questions
View allWhich one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
Show answer
The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.
Read the full explanation →The frequency of ultrasound waves is
- A.less than 20 Hz
- B.between 20 Hz and 2 kHz
- C.between 2 kHz and 20 kHz
- D.greater than 20 kHz
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Ultrasound is sound with a frequency above 20,000 Hz, the upper limit of human hearing. Humans hear sound between about 20 Hz and 20 kHz.
Read the full explanation →Which one of the following is an example of a clean fuel ?
- A.Coke
- B.Propane
- C.Petrol
- D.Wax
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Propane is a light gaseous hydrocarbon that burns completely with a blue flame, leaving no smoke, soot or ash. A clean fuel burns completely, gives high heat and releases few pollutants.
Read the full explanation →A sample of 'soft soap' contains
- A.Caesium
- B.Potassium
- C.Calcium
- D.Magnesium
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Soft soaps are potassium salts of fatty acids, while ordinary hard bar soaps are sodium salts. Soap is made by saponification, heating an oil or fat with an alkali.
Read the full explanation →Which of the following sets of elements has the same valency?
- A.Na, Mg, Ca
- B.Na, Mg, Al
- C.Mg, Ca, K
- D.Mg, Ca, Ba
Show answer
Magnesium, calcium and barium are all alkaline earth metals of group 2, and each has a valency of 2. Elements in the same group of the periodic table have the same number of electrons in their outermost shell, and so the same valency.
Read the full explanation →