Physics - Study Mode

[#251] A copper wire of radius r and length l has a resistance of R. A second copper wire with radius 2r and length l is taken and the two wires are joined in a parallel combination. The resultant resistance of the parallel combination of the two wires will be
Correct Answer

(D) R/5

Explanation

Solution: 1st copper wire: R = pl/A = pl/ πr2 2ndcopper wire: R’ = p’l’/A’ = pl/(4 πr2) = R/4 If joined in parallel, (1/Req) = (1/R) + (1/R’) = 1/R + 4/R = 5/R Req = R/5

[#252] Which of the following is a good conductor of heat but a bad conductor of electricity ?
Correct Answer

(D) Mica

Explanation

Solution: Mica is a good conductor of heat but bad conductor of electricity.

[#253] When a number of resistors are connected in series in a circuit, the value of current:
Correct Answer

(D) Remains the same

Explanation

Solution: The same current flows through each resistor in series. Individual resistors in series do not get the total source voltage, but divide it. The total resistance in a series circuit is equal to the sum of the individual resistances: RN(series)=R1+R2+R3+… +RN.

[#254] A good conductor while carrying current is
Correct Answer

(B) Negatively charged

Explanation

Solution: Electrons carry current in a good conductor of electricity and they are negatively charged. This makes a good electrical conductor negatively charged.

[#255] Sound waves in air are
Correct Answer

(B) longitudinal

Explanation

Solution: Sound waves in air (and any fluid medium) are longitudinal waves because particles of the medium through which the sound is transported vibrate parallel to the direction that the sound wave moves. A vibrating string can create longitudinal waves as depicted in the animation below.