An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of th

NCERT Exemplar · Q4

Q. An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true?

(a) The electron will be accelerated along the axis.

(b) The electron path will be circular about the axis.

(d) The electron will continue to move with uniform velocity along the axis of the solenoid.

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Concept understanding — Charged Particle in Magnetic Field

Charged Particle in a Magnetic Field

When a charged particle moves through a magnetic field, the field grabs it sideways. Unlike an electric field, which can speed a charge up or slow it down, a magnetic field only bends the path — it never changes the particle's speed. Understanding why leads directly to circular and helical motion, the basis of cyclotrons, mass spectrometers and the aurora.

The force: always sideways

A particle of charge $q$ moving with velocity $\vec{v}$ in a magnetic field $\vec{B}$ feels the magnetic (Lorentz) force:

\vec{F} = q\,(\vec{v} \times \vec{B})

Because of the cross product, $\vec{F}$ is perpendicular to both $\vec{v}$ and $\vec{B}$ . Its magnitude is

F = |q|\,v\,B\sin\theta

where $\theta$ is the angle between $\vec{v}$ and $\vec{B}$ .

Important

Since $\vec{F} \perp \vec{v}$ , the force does no work: $\vec{F}\cdot\vec{v} = 0$ . Therefore the kinetic energy and the speed stay constant — the field only changes the direction of motion, never the magnitude.

Case 1: velocity perpendicular to the field → a circle

If $\vec{v} \perp \vec{B}$ ( $\theta = 90^\circ$ ), the force $F = qvB$ stays constant in size and always points toward one central point. That is exactly the condition for uniform circular motion, with the magnetic force acting as the centripetal force:

qvB = \frac{mv^2}{r}

Solving for the radius:

r = \frac{mv}{qB}

The time period of one revolution is

T = \frac{2\pi r}{v} = \frac{2\pi m}{qB}

Note

The period $T$ (and the frequency $f = qB/2\pi m$ , the cyclotron frequency) does not depend on the speed or the radius. A faster particle traces a bigger circle but takes exactly the same time per loop. This speed-independence is what makes the cyclotron work.

Case 2: velocity at an angle → a helix

If $\vec{v}$ makes an angle $\theta$ with $\vec{B}$ , split it into two parts:

Perpendicular component $v_\perp = v\sin\theta$ — feels the magnetic force and drives circular motion of radius $r = \dfrac{mv_\perp}{qB}$ .
Parallel component $v_\parallel = v\cos\theta$ — feels no force (since $\vec{v_\parallel}\times\vec{B}=0$ ) and carries the particle steadily along the field line.

Combining a circle with a steady drift gives a helix. The distance advanced along the field in one full turn is the pitch:

p = v_\parallel\,T = v\cos\theta \cdot \frac{2\pi m}{qB}

A quick example

An electron ( $m = 9.1\times10^{-31}\ \text{kg}$ , $q = 1.6\times10^{-19}\ \text{C}$ ) enters a $0.02\ \text{T}$ field at $10^{6}\ \text{m/s}$ , perpendicular to $\vec{B}$ :

$r = \frac{mv}{qB} = \frac{(9.1\times10^{-31})(10^{6})}{(1.6\times10^{-19})(0.02)} \approx 2.8\times10^{-4}\ \text{m}$ …

Why this formula?

Charged Particle in a Magnetic Field — Why the Key Formulas Hold

Let's build this from first principles. The core idea is that a magnetic field exerts a force only on a moving charge, and that force is always perpendicular to both the velocity and the field.

1. The Fundamental Force Law: Lorentz Force

The starting point is the Lorentz force for a charge $q$ moving with velocity $\vec{v}$ in a magnetic field $\vec{B}$ :

\vec{F}_m = q (\vec{v} \times \vec{B})

Why this form?

Cross product $\vec{v} \times \vec{B}$ means the force is perpendicular to both $\vec{v}$ and $\vec{B}$ .
Magnitude: $F_m = |q| v B \sin\theta$ , where $\theta$ is the angle between $\vec{v}$ and $\vec{B}$ .
Direction: given by the right-hand rule (for positive $q$ ).

Key insight: Because $\vec{F}_m \perp \vec{v}$ , the magnetic force does no work — it changes only the direction of velocity, not its speed.

2. Circular Motion in a Uniform Magnetic Field

Consider a charge $q$ moving with speed $v$ perpendicular to a uniform $\vec{B}$ (so $\theta = 90^\circ$ , $\sin\theta = 1$ ).

Step 1: Force provides centripetal acceleration

The magnetic force is the only radial force:

F_m = q v B

This must equal the centripetal force required for circular motion:

F_c = \frac{m v^2}{r}

Step 2: Equate and solve for $r$

q v B = \frac{m v^2}{r}

Cancel one $v$ (assuming $v \neq 0$ ):

q B = \frac{m v}{r}

Thus:

\boxed{r = \frac{m v}{q B}}

This is the radius of the circular path (cyclotron radius).

Why this makes sense:

Larger mass $m$ → harder to turn → larger $r$
Larger charge $q$ or stronger $B$ → stronger force → tighter turn → smaller $r$
Faster speed $v$ → more momentum → larger $r$

3. Angular Frequency (Cyclotron Frequency)

From the circular motion relation:

\omega = \frac{v}{r}

Substitute $r = \frac{m v}{q B}$ :

\omega = \frac{v}{\frac{m v}{q B}} = \frac{q B}{m}

Thus:

\boxed{\omega_c = \frac{q B}{m}}

Why this is remarkable:

$\omega_c$ is independent of speed $v$ — all particles with same $q/m$ have the same angular frequency, regardless of how fast they move.
This is the principle behind cyclotrons (particle accelerators).

4. General Motion: Helical Path …

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Inside the solenoid the electron's velocity is parallel to $\vec{B}$ , so $\vec{v}\times\vec{B}=0$ and the magnetic force vanishes - the electron keeps moving with uniform velocity along the axis, matching stem option (d).

Setting up the field and velocity

A long current-carrying solenoid produces a magnetic field that is (very nearly) uniform inside and directed along its axis. The electron is projected with velocity $\vec{v} = v_0\hat{i}$ also along the axis, so $\vec{v} \parallel \vec{B}$ .

Applying the Lorentz force law

\vec{F} = q(\vec{v}\times\vec{B})

When two vectors are parallel (or antiparallel), their cross product is zero:

\vec{v} \times \vec{B} = 0 \quad\Rightarrow\quad \vec{F} = 0.

Consequence

With no magnetic force (and no other force stated - gravity is negligible for an electron), Newton's first law applies: the electron continues at constant velocity, unchanged in magnitude and direction, straight along the axis.

Watch out

This is a common trap: a magnetic field does not automatically deflect every charge that enters it. It deflects only the component of velocity perpendicular to $\vec{B}$ . Motion purely along the field line feels no force at all.

Checking against the stem …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

KEAM 2025Set pha-2025-0424F4 marksMCQ
$Q. When a proton moves in a uniform magnetic field such that its velocity has a component along the direction of magnetic field, its trajectory will be a (A) circle (B) straight line (C) helix (D) parabola (E) ellipse$
›Reveal solutionSolution

The parallel velocity component moves the proton uniformly along $\vec{B}$ while the perpendicular component makes it circle; together these produce a helix.

The magnetic force $\vec{F}=q\vec{v}\times\vec{B}$ acts only on the velocity component perpendicular to $\vec{B}$ , causing circular motion in that plane. The component of velocity along $\vec{B}$ experiences no force and stays constant, giving uniform …
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KEAM 2025Set pha-2025-0429F4 marksMCQ
$Q. If an electron moves with a velocity v in a magnetic field B, the magnetic force on the electron is maximum when the angle between v and B is (A) 30º (B) 180º (C) 60º (D) 90º (E) 0º$
›Reveal solutionSolution

The Lorentz magnetic force magnitude is $F = qvB\sin\theta$ , which is greatest when the velocity is perpendicular to the field, i.e. $\theta = 90^\circ$ .

The force on a charge moving in a magnetic field is

$\vec F = q\,\vec v \times \vec B, \qquad |\vec F| = qvB\sin\theta.$ …
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KEAM 2024Set eng-2024-06054 marksMCQ
$B$
›Reveal solutionSolution
The perpendicular velocity component makes a circle while the parallel component moves uniformly along $B$ — the combination is a helix.

Resolve $v$ into components parallel and perpendicular to $B$ :

The perpendicular component gives uniform circular motion (magnetic force $qv_\perp B$ ). …
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KEAM 2024Set pha-2024-06104 marksMCQ
$3 \mu C$
›Reveal solutionSolution

Compute the Lorentz force $q\vec v\times\vec B$ .

Given $q = 3\,\mu C = 3\times10^{-6}$ C, $\vec v = (\hat i + \hat j)\times10^5$ m/s, $\vec B = 5\hat j$ T.

$\vec v\times\vec B = 10^5(\hat i+\hat j)\times 5\hat j = 5\times10^5\,(\hat i\times\hat j + \hat j\times\hat j) = 5\times10^5\,\hat k.$ …
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KEAM 2023Set eng-2023-P1-A14 marksMCQ
$\left(10^{-4}\hat{k}\right)\mathrm{T}$
›Reveal solutionSolution
With F perpendicular to B, v = F/(qB) = (5 x 10^-12)/(10^-9 x 10^-4) = 50 m/s.

Concept and Intuition

The magnetic force is F = q v x B. Here B is along z and the force lies in the x-y plane, so the velocity component producing the force is perpendicular to B; the magnitude relation reduces to F = q v B.

Step-by-Step Solution

Magnitude of force: |F| = sqrt(4^2 + 3^2) x 10^-12 = 5 x 10^-12 N.

B = 10^-4 T, q = 10^-9 C, and v is perpendicular to B. …
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KEAM 2022Set eng-2022-P1-A14 marksMCQ
$v$
›Reveal solutionSolution
Both the electron and proton move perpendicular to $\vec{B}$ , so each follows a circular path.

Concept and Intuition

A charged particle entering a uniform magnetic field with velocity perpendicular to the field feels a force $\vec{F} = q\vec{v}\times\vec{B}$ that is always perpendicular to the velocity and constant in magnitude. This is a centripetal force, producing uniform circular motion. Both the electron and proton are charged and moving perpendicular to $\vec{B}$ .

Step-by-Step Solution

Force magnitude $F = qvB$ (since $v \perp B$ ), always perpendicular to $v$ .

A constant perpendicular force gives circular motion of radius $r = mv/qB$ . …
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Q.An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true?

Concept understanding — Charged Particle in Magnetic Field

Charged Particle in a Magnetic Field

The force: always sideways

Case 1: velocity perpendicular to the field → a circle

Case 2: velocity at an angle → a helix

A quick example

Why this formula?

Charged Particle in a Magnetic Field — Why the Key Formulas Hold

1. The Fundamental Force Law: Lorentz Force

Why this form?

2. Circular Motion in a Uniform Magnetic Field

Step 1: Force provides centripetal acceleration

Step 2: Equate and solve for $r$

Why this makes sense:

3. Angular Frequency (Cyclotron Frequency)

Why this is remarkable:

4. General Motion: Helical Path …

Setting up the field and velocity

Applying the Lorentz force law

Consequence

Checking against the stem …

Method: Classifying Charged-Particle Motion by the Angle Between v and B

Steps

Concept and Intuition

Step-by-Step Solution

Concept and Intuition

Step-by-Step Solution

Q.An electron is projected with uniform velocity along the axis of a current carrying long solenoid. Which of the following is true?

Setting up the field and velocity

Applying the Lorentz force law

Consequence

Checking against the stem …

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