The resistance of a uniform metallic wire is
- A.directly proportional to its area of cross section
- B.inversely proportional to its length
- C.directly proportional to its length and inversely proportional to its area of cross section
- D.independent of both length and area of cross section
Correct answer
Explanation
The correct answer is C. The resistance of a wire equals the resistivity of the material multiplied by the length and divided by the area of cross section. A longer wire gives the electrons a longer path and more collisions, so the resistance rises with length; a thicker wire gives them more room, so the resistance falls as the area grows.
Option A reverses the dependence on thickness, which is why a thin wire heats up more than a thick one carrying the same current. Option B reverses the dependence on length. Option D is wrong because only the resistivity, not the resistance, is independent of the size of the specimen; resistivity is a property of the material and changes only with the substance and its temperature. A useful check: doubling the length doubles the resistance, while doubling the diameter makes the area four times larger and cuts the resistance to a quarter.
Read the full article: Electricity and Magnetism: Laws, Rules and PYQs
Practice Questions
View allIf the potential difference across the ends of a conductor is halved, what happens to the current flowing through it?
- A.It gets increased
- B.It gets doubled
- C.It gets decreased
- D.It gets halved
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By Ohm's law, at a constant temperature the current through a conductor is directly proportional to the potential difference across its ends, since the current equals the potential difference divided by the resistance.
Read the full explanation →What is the work done to move a unit charge from one point to another in an electric circuit carrying some current, between those two points?
- A.Pressure distance
- B.Potential difference
- C.Potential distance
- D.Pressure difference
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The potential difference between two points in a circuit is defined as the work done in moving a unit positive charge from one point to the other.
Read the full explanation →Which of the following is NOT based on the heating effect of current?
- A.Electric heater
- B.Electric bulb (with filament)
- C.Electric iron
- D.Microwave
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A microwave oven does not cook by passing a current through a resistance. It generates microwaves in a magnetron, and these waves make the water molecules in the food rotate rapidly, so the food heats from within by dielectric heating.
Read the full explanation →A 12 V battery is connected across an unknown resistor. If a current of 2 mA is flowing through the circuit, the value of the resistance is _________ .
- A.600 ohm
- B.6 ohm
- C.6000 ohm
- D.60 ohm
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By Ohm's law the resistance equals the potential difference divided by the current. Here the current must first be converted into the base unit: two milliampere is 0.002 ampere.
Read the full explanation →Thin wires of which of the following metal is used for making the filament of electric bulbs?
- A.Tungsten
- B.Silver
- C.Nichrome
- D.Copper
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Tungsten has the highest melting point of all metals, well above three thousand degrees Celsius, so a thin tungsten wire can be heated until it glows white without melting.
Read the full explanation →