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
Correct answer
Explanation
The correct answer is A, tungsten. 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. It also has a fairly high resistivity and a low rate of evaporation at working temperature, which is why the filament lasts for hundreds of hours inside a bulb filled with an inert gas.
Option B, silver, is the best conductor of electricity of all metals, but for that very reason it would produce hardly any heat, and it melts at a temperature far too low for a filament. Option C, nichrome, is the right choice for the element of a heater or a toaster, where a red heat is wanted, but it cannot reach the white heat needed to give useful light. Option D, copper, is used for connecting wires precisely because it has a low resistance and stays cool while carrying current.
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 →The direction of the magnetic field produced around a straight current-carrying conductor is given by
- A.Fleming's left hand rule
- B.the right hand thumb rule
- C.Fleming's right hand rule
- D.Lenz's law
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Hold the conductor in your right hand with the thumb stretched along the direction of the conventional current; the way your fingers curl round the wire is the direction of the concentric magnetic field lines.
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