A metallic wire having resistance of 20 Ω is cut into two equal parts in length. These parts are then connected in parallel. The resistance of this parallel combination is equal to
- A.20 Ω
- B.10 Ω
- C.5 Ω
- D.15 Ω
Correct answer
Explanation
The correct answer is C, 5 Ω. Each half of the wire has a resistance of 10 Ω, and two 10 Ω resistors in parallel give 5 Ω.
The resistance of a wire is R = ρL/A, so it is directly proportional to its length when the material and thickness stay the same. Cutting the 20 Ω wire into two equal lengths gives two pieces of 10 Ω each. For resistors in parallel, 1/R = 1/R₁ + 1/R₂ = 1/10 + 1/10 = 2/10, so R = 5 Ω. A quick rule: n equal pieces joined in parallel give the original resistance divided by n squared, here 20 divided by 4.
A is wrong because 20 Ω is the resistance of the whole wire before cutting, or of the two halves joined in series. B is wrong because 10 Ω is the resistance of only one half. D is wrong because 15 Ω does not follow from any arrangement of two 10 Ω pieces.
Exam tip: cut into n equal parts and join in parallel, the resistance becomes R divided by n squared.
Practice Questions
View allWho among the following built a model steam engine in 1698 called "Miner's Friend" to drain mines ?
- A.Thomas Savery
- B.Thomas Newcomen
- C.James Watt
- D.Richard Arkwright
Show answer
The English engineer Thomas Savery patented a steam pump in 1698 and called it the Miner's Friend, as it was meant to pump water out of flooded mines.
Read the full explanation →Myopia is a defect in human vision where an image of a
- A.nearby object is focused beyond the retina.
- B.nearby object is focused before the retina.
- C.distant object is focused before the retina.
- D.distant object is focused beyond the retina.
Show answer
In myopia, or short-sightedness, a person sees near objects clearly but distant objects look blurred because their image forms in front of the retina.
Read the full explanation →A homogeneous mixture contains two liquids. How are they separated ?
- A.By filtration
- B.By evaporation
- C.By distillation
- D.By condensation
Show answer
Two miscible liquids that form a homogeneous mixture are separated by distillation, which uses the difference in their boiling points.
Read the full explanation →Cartilage is NOT found in
- A.larynx
- B.nose
- C.ear
- D.urinary bladder
Show answer
The urinary bladder is a hollow muscular sac made of smooth muscle and a stretchable lining; it has no cartilage.
Read the full explanation →The device used to produce electric current is known as
- A.motor
- B.generator
- C.ammeter
- D.galvanometer
Show answer
An electric generator converts mechanical energy into electrical energy and so produces electric current. A generator works on electromagnetic induction, discovered by Michael Faraday in 1831.
Read the full explanation →