Which one of the following metals is most commonly used for making filament of incandescent electric bulbs?
- A.Aluminium
- B.Silver
- C.Copper
- D.Tungsten
Correct answer
Explanation
The correct answer is D, Tungsten. The filament of an incandescent bulb must glow white-hot without melting, and tungsten has the highest melting point of all metals, about 3,400 °C.
An incandescent bulb works by the heating effect of electric current: the thin coiled filament gets so hot, around 2,500 °C or more, that it gives out light. Tungsten suits this job because it does not melt at such temperatures, has a high resistivity and can be drawn into very fine wire. To stop the hot filament from burning in oxygen, the bulb is filled with an inert gas such as argon or nitrogen. Even so, only a small part of the energy becomes light; most is lost as heat, which is why LEDs have replaced these bulbs.
A is wrong: aluminium melts at about 660 °C and would melt at once. B is wrong: silver is the best conductor of electricity but melts at about 960 °C. C is wrong: copper, used for wiring because of its low resistance, melts at about 1,085 °C.
Exam tip: bulb filament = tungsten, filled with argon or nitrogen; heating elements use nichrome; fuse wire is a low-melting alloy.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
Show answer
In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
Show answer
Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
Show answer
Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
Show answer
It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
Show answer
They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
Read the full explanation →