Physics · Moving Charges And Magnetism · NEET
No. A magnetic field only pushes a charge that is MOVING (and moving across the field, not along it). A charge sitting still feels zero magnetic force. This is the biggest difference from an electric field, which pushes any charge whether it moves or not. Remember: magnetic force needs motion.
They show the direction of B when it is perpendicular to the paper. A dot (a circle with a point) means the field comes OUT of the page toward your eyes. A cross (a circle with an X) means the field goes INTO the page, away from you. We use these symbols because you cannot draw a straight arrow pointing at or away from a flat page.
The tesla is a very LARGE unit. The Earth's magnetic field is only about 3 to 6 x 10^-5 T. A strong lab magnet is about 1 T. So in problems you often see small numbers like 10^-4 T. A smaller unit called the gauss is also used, where 1 T = 10^4 gauss.
The magnetic field B is a property of space around a magnet or current; it just exists there. The magnetic force is what a moving charge or a current-carrying wire actually FEELS when it enters that field. B is the cause, force is the effect. In NEET, F = qvB sin(theta) links them.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The SI unit of magnetic field B is the tesla (T). One tesla is a strong field; the Earth's field is only about 10^-5 T. The older unit gauss is also used, with 1 T = 10^4 gauss.
A dot means the magnetic field points straight OUT of the page, toward you. It is like seeing the tip of an arrow coming at your eye.
A cross (X) means the magnetic field points straight INTO the page, away from you. It is like seeing the tail feathers of an arrow moving away from you.
Magnetic field B is a vector. It has both a magnitude (in tesla) and a direction. That is why we need dot and cross symbols to show its direction when it is perpendicular to the paper.
By convention in physics, B is the symbol for the magnetic field (also called magnetic flux density). The letter M is usually saved for magnetisation. NEET and NCERT both use B for the magnetic field.
