JEE MAIN - Physics (2023 - 6th April Evening Shift - No. 25)
A proton with a kinetic energy of $$2.0 ~\mathrm{eV}$$ moves into a region of uniform magnetic field of magnitude $$\frac{\pi}{2} \times 10^{-3} \mathrm{~T}$$. The angle between the direction of magnetic field and velocity of proton is $$60^{\circ}$$. The pitch of the helical path taken by the proton is __________ $$\mathrm{cm}$$. (Take, mass of proton $$=1.6 \times 10^{-27} \mathrm{~kg}$$ and Charge on proton $$=1.6 \times 10^{-19} \mathrm{C}$$ ).
Answer
40
Explanation
Given a proton with a kinetic energy of 2 eV, moving into a region of uniform magnetic field of magnitude $$\frac{\pi}{2} \times 10^{-3} T$$, and with an angle of $$60^{\circ}$$ between the direction of the magnetic field and the velocity of the proton, we want to determine the pitch of the helical path taken by the proton.
- First, calculate the proton's speed (v) using the kinetic energy (K.E) formula:
$$v = \sqrt{\frac{2 \times KE}{m}}$$
- Next, find the component of the velocity in the direction of the magnetic field (parallel component):
$$v_{\parallel} = v \cos \theta$$
In this case, θ is given as $$60^{\circ}$$, so $$\cos \theta = \frac{1}{2}$$.
- The pitch of a charged particle moving in a magnetic field with an angle θ to the direction of the magnetic field is given by the formula:
$$p = \frac{2 \pi m v_{\parallel}}{qB}$$
- Substitute the values for the mass of the proton (m), the kinetic energy (KE), the charge of the proton (q), and the magnetic field (B) into the formula:
$$p = \frac{2 \pi \times \sqrt{2mKE} \times \frac{1}{2} \times 2}{qB}$$
- After substituting the given values, the pitch of the helical path is found to be:
$$p = 0.4 \, m = 40 \, cm$$
In conclusion, the pitch of the helical path taken by the proton in the magnetic field is 40 cm.
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