Magnetism Questions- Physics Review

What You Actually Need to Know About Magnetism Questions

Magnetism is one of those physics topics that trips up students constantly. Not because it's impossibly hard, but because most resources overcomplicate it. This guide cuts through the nonsense.

You'll get the core concepts, the formulas that actually matter, and real question types you'll face in exams. Nothing else.

The Basics: What Magnetism Actually Is

Magnetism comes from moving electric charges. That's it. Every magnetic effect you've ever seen—from fridge magnets to MRI machines—stems from electrons moving in some way.

In atoms, electrons spin and orbit the nucleus. These motions create tiny magnetic fields. In most materials, these fields cancel out. In magnetic materials, they don't.

Magnetic Poles

Every magnet has a north pole and a south pole. Cut a magnet in half and you get two smaller magnets, each with both poles. You cannot isolate a single magnetic pole—this is a fundamental property.

Like poles repel. Opposite poles attract. This is the foundation for solving half of all magnetism problems.

Key Formulas You Must Know

Don't memorize everything. Know these:

• Magnetic Force on a Moving Charge: F = qvB sin(θ)
• Magnetic Force on a Wire: F = BIL sin(θ)
• Magnetic Field from a Wire: B = (μ₀I)/(2πr)
• Magnetic Flux: Φ = BA cos(θ)
• Force on a Current-Carrying Wire: F = nIA B sin(θ)

Where:

• q = charge (Coulombs)
• v = velocity (m/s)
• B = magnetic field strength (Tesla)
• I = current (Amperes)
• L = length of wire (meters)
• θ = angle between field and motion/current

Right-Hand Rule: The Make-or-Break Skill

Most students fail magnetism questions because they can't apply the right-hand rule correctly. Here's how it actually works:

For a Straight Wire

Point your thumb in the direction of current flow. Your fingers curl in the direction of the magnetic field. Simple.

For a Solenoid

Curl your fingers in the direction of current flow. Your thumb points toward the north pole of the solenoid. This matters when you're figuring out which end is which.

For Force Direction

Point your fingers in the direction of velocity (for charges) or current (for wires). Point your palm in the direction of the magnetic field. Your thumb gives you the force direction. This one confuses people constantly.

Common Magnetism Question Types

1. Finding Force on a Moving Charge

Question format: A particle with charge q moving at velocity v enters a magnetic field B. Find the force.

How to solve:

• Identify q, v, and B from the problem
• Determine the angle θ between v and B
• Plug into F = qvB sin(θ)
• Check direction with right-hand rule

Critical point: If the charge moves parallel to the field, sin(0) = 0 and there's no force. If it moves perpendicular, sin(90°) = 1 and force is maximum. Students forget this constantly.

2. Circular Motion in Magnetic Fields

When a charged particle moves perpendicular to a uniform magnetic field, it travels in a circle. The magnetic force provides the centripetal force.

Set them equal: qvB = (mv²)/r

Solve for what the problem asks—radius, velocity, or magnetic field strength.

The radius depends on momentum. Faster particles trace bigger circles. This is how mass spectrometers work, and it's a favorite exam question.

3. Magnetic Field Around a Wire

Question format: Find the magnetic field at a point distance r from a current-carrying wire.

How to solve:

• Use B = (μ₀I)/(2πr)
• μ₀ = 4π × 10⁻⁷ T·m/A (this constant is always given)
• Direction comes from right-hand rule

Field strength drops off as 1/r. Double the distance, halve the field. This inverse relationship shows up constantly.

4. Force Between Two Parallel Wires

Two current-carrying wires exert forces on each other. Currents in the same direction attract. Currents in opposite directions repel.

Force per unit length: F/L = (μ₀I₁I₂)/(2πd)

This is how electric motors work. The principle shows up in many practical applications.

Comparison: Magnetic vs. Electric Forces

This table gets tested. Know it.

Electromagnetic Induction: Faraday's Law

When magnetic flux through a loop changes, an EMF is induced. This is Faraday's Law:

EMF = -N × (ΔΦ/Δt)

Where N is the number of loops and ΔΦ/Δt is the rate of change of magnetic flux.

The negative sign is Lenz's Law—it tells you the induced current flows to oppose the change that caused it. Don't ignore it. Exams expect you to explain this direction.

Flux changes when:

• Area changes
• Magnetic field strength changes
• Angle between field and area changes (cos θ factor)

How to Solve Any Magnetism Problem: Step-by-Step

Step 1: Identify What's Being Asked

Force? Field strength? Direction? Radius of path? Read the question twice. Students lose marks by solving for the wrong thing.

Step 2: Identify Given Values

List q, v, B, I, L, r, θ. Convert units if needed. Tesla is the unit for B—don't confuse it with other quantities.

Step 3: Pick the Right Formula

Match the situation to the formula. A moving charge in a field? F = qvB sin(θ). A wire in a field? F = BIL sin(θ). A changing flux? Faraday's Law.

Step 4: Plug In and Solve

Work through the algebra. Keep track of units. If your answer has wrong units, you messed up somewhere.

Step 5: Check Direction

Use the right-hand rule to verify the direction of any vector quantity. This catches mistakes before you submit.

Practice Problems You Should Master

Problem 1: An electron (q = -1.6 × 10⁻¹⁹ C) moves at 3 × 10⁶ m/s perpendicular to a 0.5 T magnetic field. Find the force magnitude and direction.

Solution:

• F = qvB sin(90°) = (1.6 × 10⁻¹⁹)(3 × 10⁶)(0.5)
• F = 2.4 × 10⁻¹³ N
• Direction: Use right-hand rule for positive charge, then reverse (electron has negative charge)

Problem 2: A wire carrying 10 A is placed in a 0.2 T field at a 30° angle. Wire length is 0.5 m. Find the force.

Solution:

• F = BIL sin(θ) = (0.2)(10)(0.5) sin(30°)
• F = 1 × 0.5 × 0.5 = 0.25 N

Problem 3: Magnetic flux through a 200-turn coil changes from 0.02 Wb to 0.08 Wb in 0.1 s. Find induced EMF.

Solution:

• EMF = -N × (ΔΦ/Δt) = -200 × (0.08 - 0.02)/0.1
• EMF = -200 × 0.6 = -120 V
• Magnitude is 120 V

Common Mistakes That Cost You Points

• Forgetting sin(θ): The angle matters. Always check whether motion is perpendicular to the field.
• Wrong direction on negative charges: Electrons curve opposite to what the right-hand rule predicts.
• Confusing Tesla with Gauss: 1 T = 10,000 G. Don't mix them up in calculations.
• Ignoring the negative sign in Faraday's Law: It's not optional. Lenz's Law is part of the equation.
• Using the wrong formula: Force on a charge vs. force on a wire are different. Know which applies.

What to Study Before Your Exam

Focus your review on:

• Right-hand rule applications until they're automatic
• The five core formulas and when to use each
• Circular motion in magnetic fields (setting centripetal = magnetic force)
• Faraday's Law and Lenz's Law together
• Force between parallel wires

Work through 10-15 practice problems from past exams. Magnetism is a skill—you learn it by doing, not by reading summaries.