Which one of the following metals is used in the filaments of photo-electric cells that convert light energy into electric energy ?
- A.Tungsten
- B.Copper
- C.Rubidium
- D.Aluminium
Correct answer
Explanation
The correct answer is C, Rubidium. Rubidium, an alkali metal like caesium, gives off electrons easily when light falls on it, so it is used in photoelectric cells.
In the photoelectric effect, light striking a metal surface knocks electrons out of it, and these electrons make a current flow. Einstein explained this in 1905 and won the Nobel Prize for it. Alkali metals such as rubidium, caesium and potassium hold their outer electron loosely, so they have a low work function and respond even to visible light. That is why they coat the light-sensitive surface of photocells used in light meters, automatic doors and burglar alarms.
A is wrong because tungsten, with its very high melting point, is used in the filaments of electric bulbs, which turn electricity into light, the opposite job. B is wrong because copper is used in electric wires for its good conductivity. D is wrong because aluminium is used in power cables and utensils, and needs ultraviolet light to emit electrons.
Exam tip: photocells use alkali metals such as rubidium and caesium; bulb filaments use tungsten.
Practice Questions
View allWhich one of the following features is an indication for modification of stem of a plant?
- A.Presence of 'eye' on potato
- B.'Scale' found in onion
- C.'Tendril' found in pea
- D.'Hair' present in carrot
Show answer
The eyes of a potato are nodes with buds, and only a stem bears nodes and buds, so they prove that the potato is a modified stem.
Read the full explanation →Tooth enamel is made up of which one of the following calcium compounds ?
- A.Calcium carbonate
- B.Calcium sulphate
- C.Calcium hydroxide
- D.Calcium phosphate
Show answer
Tooth enamel is made mainly of calcium phosphate in the form of a hard crystal called hydroxyapatite. Enamel is the hardest substance in the human body, harder even than bone.
Read the full explanation →Which one of the following viruses used to be responsible for highly infectious disease Smallpox ?
- A.Adenovirus
- B.Variola virus
- C.Aichi virus
- D.Coxsackie virus
Show answer
Smallpox was caused by the variola virus, a large DNA virus of the pox family that spread from person to person through the air.
Read the full explanation →After fertilization, the fruit and the seed are produced by
- A.ovule and ovary, respectively
- B.ovary and ovule, respectively
- C.ovary, no ovule required
- D.ovule, no ovary required
Show answer
After fertilisation the ovary of the flower grows into the fruit, and each ovule inside it develops into a seed.
Read the full explanation →Which among the following statements with respect to carbon is/are correct?
1. Carbon forms the basis for all living organisms and many things we use
2. Carbon shows tetra-valency and the property of catenation
3. Carbon forms covalent bonds with itself and other elements
4. Carbon forms compounds containing triple and tetra bonds between carbon atoms
Select the correct answer using the code given below :
- A.1 only
- B.1 and 2 only
- C.1, 2 and 3
- D.2 and 4
Show answer
Statements 1, 2 and 3 are true, but carbon never forms a quadruple, or tetra, bond between two carbon atoms.
Read the full explanation →