Moving Charges and Magnetism for NEET UG
Moving charges create magnetic fields and feel magnetic forces. These two facts explain electric motors, MRI machines and particle accelerators.
PhysicsClass 1264 MCQs3 lessons13 flashcards3 memory tricks
Practise Moving Charges and Magnetism free
What you'll learn
- Magnetic force on moving charges
Find the force on a charge moving in a magnetic field and the radius of its circular path. - Magnetic fields from currents
Use the Biot–Savart law and Ampère's law for wires, loops and solenoids. - Force on wires and the moving-coil galvanometer
Find the force between current-carrying wires and the torque on a coil.
Key formulas
| Lorentz magnetic force | F = qvB sin θ (F = q v × B) |
|---|---|
| Radius in a uniform field (v ⊥ B) | r = mv/(qB) |
| Period of revolution | T = 2πm/(qB) |
| Cyclotron frequency | ν = qB/(2πm) |
| Long straight wire | B = μ₀I/(2πr) |
| Centre of a circular loop | B = μ₀I/(2R) |
| On the axis of a loop | B = μ₀IR²/[2(R² + x²)^(3/2)] |
| Inside a long solenoid | B = μ₀nI (n = turns per metre) |
Memory tricks
F = qv × B is always perpendicular to velocity, so it does no work and the kinetic energy stays the same. The charge moves in a circle of radius r = mv/qB.
Fleming's left-hand rule gives the force on a current in a field (motor). The right-hand rule gives the direction of induced current (generator). Thumb = motion/force, first finger = field, second finger = current.
f = qB/2πm does not depend on speed or radius, which is why a fixed-frequency oscillator can keep accelerating the particle.
Try 3 NEET-style questions
Q1. An ideal ammeter and an ideal voltmeter should have resistances, respectively, of
- infinite and zero
- both zero
- both infinite
- zero and infinite
Show answer
Answer: (4) zero and infinite
An ammeter is in series, so ideally it should not add resistance (zero). A voltmeter is in parallel, so ideally it should draw no current (infinite resistance).
Q2. According to the Biot–Savart law, the magnetic field produced by a small current element at a point lying along the line of the element (θ = 0°) is
- maximum
- zero
- infinite
- independent of the current
Show answer
Answer: (2) zero
dB ∝ I dl sin θ / r². On the line of the element, θ = 0, so sin θ = 0 and the field is zero.
Q3. Ampère's circuital law shows that the magnetic field inside a long ideal solenoid with n turns per unit length carrying current I is
- zero inside and strongest just outside
- strongest on the axis and zero at the walls
- B = μ₀I/2πr, falling with distance from the axis
- uniform, B = μ₀nI, and the same at all points inside
Show answer
Answer: (4) uniform, B = μ₀nI, and the same at all points inside
A rectangular Amperian loop gives B l = μ₀ n l I, so B = μ₀nI, uniform throughout the inside and almost zero outside.
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