Question

A positive charge enters a magnetic field and travels parallel to the magnetic field. The charge experiences

• A. an upward force
• B. a downward force
• C. a force in the direction of magnetic field.
• D. no force

Hint:

Force is \(qvB\sin\theta\) and \(\theta = 0\) here.

Answer 1

The Correct Option is D

Step 1: Approach:
Think about the direction rule. The cross product of two parallel vectors is the zero vector.

Step 2: Use the rule:
Since $\vec v \parallel \vec B$, we get $\vec v \times \vec B = \vec 0$. Then $\vec F = q\,\vec 0 = \vec 0$.

Step 3: Physical picture:
The magnetic force only turns the path sideways. When the motion is along the field lines there is no sideways component, so the charge keeps going straight at constant speed.

Step 4: Match:
No force is the fourth printed option.

Final Answer:
There is no magnetic force, option 4. \[ \boxed{\text{Option 4}} \]