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NEET 2010 (AIPMT, Prelims) · Cathode Rays, Thomson's Experiment & Gas Discharge

Cathode Rays, Thomson's Experiment & Gas Discharge question from NEET 2010 (AIPMT, Prelims)

Q1NEET 2010 · AIPMT · PRELIMSEasy

Cathode Rays, Thomson's Experiment & Gas Discharge

A beam of cathode rays is subjected to crossed electric (E) and magnetic fields (B). The fields are adjusted such that the beam is not deflected. The specific charge of the cathode rays is given by (Where V is the potential difference between cathode and anode)

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Answer: (d)

No deflection: $eE = evB \Rightarrow v = \frac{E}{B}$ .
Accelerated through $V$ : $eV = \frac{1}{2}mv^2 \Rightarrow \frac{e}{m} = \frac{v^2}{2V}$ .
Substitute $v$ : $\frac{e}{m} = \frac{E^2}{2VB^2}$ .
So the answer is (d) $\frac{E^2}{2VB^2}$ .

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