Magnetism is one of the highest-scoring and most predictable chapters in NDA Physics. Almost every paper asks about Oersted's experiment, the right-hand rule, electromagnets, or the working of a motor. At The Cavalier, we teach you to picture the invisible magnetic field first, then the formulas follow naturally. Master this page and you bank easy marks.
Why This Topic Matters for NDA
Magnetism links two ideas that look separate — electricity and magnets — into one neat package called electromagnetism. The NDA exam loves this chapter because it can test memory (facts and rules) and reasoning (direction of force, poles) at the same time.
You will typically see 2 to 4 questions from this area in the General Ability Test. They are usually direct, so a clear concept gives you near-guaranteed marks.
Every electric current is surrounded by a magnetic field, and every changing magnetic field can push a current. These two sentences are the heart of the whole chapter.
Magnets and Magnetic Poles
A magnet is a material that attracts iron, cobalt, and nickel and points north-south when hung freely. Every magnet has two poles: a north pole (N) and a south pole (S).
The basic law of poles
- Like poles repel (N–N or S–S push apart).
- Unlike poles attract (N–S pull together).
An important fact: magnetic poles always exist in pairs. If you cut a bar magnet in half, you do not get one N and one S piece — you get two smaller magnets, each with its own N and S. A single isolated pole (a magnetic monopole) has never been found.
Magnetic and non-magnetic materials
Materials that are strongly attracted by a magnet — iron, cobalt, and nickel — are called ferromagnetic. Materials like wood, plastic, copper, and rubber are non-magnetic and are not attracted. A piece of iron can itself become a temporary magnet when brought near a strong magnet; this is called magnetic induction in materials.
Magnetic field lines flow from N to S outside the magnet and from S to N inside the magnet, forming closed loops. They never cross each other.
The Magnetic Field and Field Lines
The space around a magnet where its force can be felt is called the magnetic field. We draw it using magnetic field lines (also called lines of force).
Properties of field lines
- They start at the north pole and end at the south pole outside the magnet.
- They are closed continuous loops.
- They never intersect — if they did, the compass would point in two directions at once, which is impossible.
- Where lines are crowded, the field is strong; where they are spread out, the field is weak.
The strength of the magnetic field is measured in tesla (T) in the SI system. The symbol for magnetic field strength (flux density) is B. One tesla is a fairly large unit, so weaker fields are often given in gauss, where 1 T = 104 gauss. The Earth's magnetic field at the surface is only about 0.5 gauss, which is why a compass needle has to be light and freely pivoted to respond to it.
You can map a magnetic field easily with a small plotting compass or with iron filings sprinkled on a card over a magnet. The filings line up along the field lines and reveal the pattern, with the densest clusters near the poles where the field is strongest.
A common NDA question asks which property is true of field lines. The safe answers: they form closed loops and never cross. Memorise these two.
Earth as a Giant Magnet
The Earth behaves like a huge bar magnet buried at its centre. This is why a freely suspended magnetic needle (compass) always settles in the north-south direction.
An important twist
The geographic North of Earth actually behaves like a magnetic south pole. That is why the north pole of your compass needle (which is attracted to a south pole) points towards geographic north.
- Magnetic declination: the angle between geographic north and magnetic north.
- Magnetic inclination (dip): the angle the needle makes with the horizontal.
A compass works because Earth is a magnet. Sailors and soldiers have used it for navigation for centuries — a fact the NDA paper sometimes frames in a defence context.
Oersted's Experiment: Current Makes Magnetism
In 1820, Danish scientist Hans Christian Oersted noticed that a compass needle placed near a wire deflected the moment current flowed through the wire. When the current stopped, the needle returned. When the current direction reversed, the needle deflected the opposite way.
This was a landmark discovery: an electric current produces a magnetic field around it. Electricity and magnetism are linked.
A current-carrying conductor creates circular magnetic field lines around it. More current → stronger field. The field gets weaker as you move farther from the wire.
Right-hand thumb rule
To find the direction of the field around a straight wire: point the thumb of your right hand in the direction of the current; the curl of your fingers shows the direction of the magnetic field lines.
Solenoids and Electromagnets
A solenoid is a long coil of insulated wire wound in many turns. When current flows, it behaves exactly like a bar magnet — one end acts as N, the other as S. The field inside a solenoid is strong and uniform.
Making it stronger: the electromagnet
If you place a soft iron core inside the solenoid, you get an electromagnet — a magnet you can switch on and off with current.
- More turns in the coil → stronger magnet.
- More current → stronger magnet.
- A soft iron core greatly boosts the strength.
Soft iron is used in electromagnets because it loses magnetism quickly when current stops. Steel is used for permanent magnets because it retains magnetism.
Force on a Current-Carrying Conductor
If a wire carrying current is placed in a magnetic field, the field exerts a force on the wire. This is the reverse side of Oersted's idea, and it is the principle behind every electric motor.
Force on a conductor: F = B I L sinθ, where B = magnetic field, I = current, L = length of conductor in the field, and θ = angle between current and field. Force is maximum when θ = 90° and zero when the wire is parallel to the field (θ = 0°).
Fleming's left-hand rule
Stretch the thumb, first finger, and middle finger of your left hand at right angles:
- First finger → direction of Field (B)
- Middle finger → direction of Current (I)
- Thumb → direction of Motion/force (thrust)
Use left hand for motors (current and field given, find motion) and right hand (Fleming's right-hand rule) for generators (motion and field given, find current).
How an Electric Motor Works
An electric motor converts electrical energy into mechanical (rotational) energy. It uses the force on a current-carrying coil placed in a magnetic field.
Working in simple steps
- Current passes through a rectangular coil placed between the poles of a magnet.
- By Fleming's left-hand rule, one side of the coil is pushed up and the other down — this makes the coil rotate.
- A device called the split-ring commutator reverses the current every half turn, so the coil keeps spinning in the same direction.
The commutator reverses current direction every half rotation. Without it, the coil would just oscillate back and forth instead of spinning continuously.
Motors run fans, water pumps, mixers, electric trains, and countless defence machines — a practical link the NDA exam appreciates.
Electromagnetic Induction and the Generator
In 1831, Michael Faraday discovered the opposite effect: a changing magnetic field produces a current. This is called electromagnetic induction, and the current is an induced current.
Faraday's law (in words)
Whenever the magnetic field (or magnetic flux) through a coil changes, an EMF (voltage) is induced in it. The faster the change, the larger the induced EMF.
An electric generator converts mechanical energy into electrical energy — exactly the reverse of a motor. Rotating a coil in a magnetic field induces a current.
- AC generator uses slip rings and produces alternating current.
- DC generator uses a commutator (split rings) to give direct current.
Do not swap them: Motor = electrical → mechanical; Generator = mechanical → electrical. Mixing these up is the single most common NDA error in this chapter.
Worked Example and Common Mistakes
A straight wire of length 0.5 m carries a current of 4 A. It is placed at right angles (90°) to a uniform magnetic field of 0.3 T. Find the force on the wire.
So the wire feels a force of 0.6 newton. If the wire were placed parallel to the field (θ = 0°), sin 0° = 0, so the force would be zero.
Watch the angle. Many NDA numericals are easy once you remember that force is greatest at 90° and zero when the wire is along the field.
Common mistakes to avoid
These slips cost marks in almost every batch. Fix them now.
- Using the wrong hand — left hand is for motors, right hand for generators.
- Thinking field lines can cross — they never do.
- Believing a single magnetic pole exists — poles always come in pairs.
- Saying soft iron makes the best permanent magnet — it is the opposite; steel holds magnetism, soft iron for electromagnets.
- Confusing Oersted (current → magnetism) with Faraday (changing magnetism → current).
Geographic north of Earth acts as a magnetic south pole. Students often write it the wrong way and lose an easy mark.
Previous-Year Style Question
Q. The device that converts electrical energy into mechanical energy is called a:
Answer: An electric motor. It uses the force experienced by a current-carrying coil in a magnetic field (Fleming's left-hand rule) and a split-ring commutator to produce continuous rotation. A generator does the reverse — mechanical to electrical.
Q. Oersted's experiment demonstrated that:
Answer: An electric current produces a magnetic field around it. A compass needle near a current-carrying wire deflects, proving electricity and magnetism are connected.
Quick Revision
- Like poles repel, unlike poles attract; poles always exist in pairs.
- Field lines: N to S outside, closed loops, never cross.
- Earth is a giant magnet; compass points north-south.
- Oersted: current → magnetic field (right-hand thumb rule).
- Force on conductor: F = B I L sinθ; maximum at 90°, zero at 0°.
- Fleming's left hand = motor; right hand = generator.
- Motor: electrical → mechanical, uses a commutator.
- Faraday: changing field → induced current; generator: mechanical → electrical.
- Soft iron for electromagnets; steel for permanent magnets.
Revise this recap the night before the exam and you will recognise most magnetism questions instantly. At The Cavalier, repetition of these core rules is how toppers lock in easy marks.
Frequently asked questions
What is the difference between a motor and a generator?
A motor converts electrical energy into mechanical (rotational) energy, while a generator converts mechanical energy into electrical energy. They are exact opposites of each other.
Why can't magnetic field lines cross each other?
If two field lines crossed, the compass at that point would have to point in two different directions at once, which is impossible. So field lines never intersect.
Why is soft iron used in electromagnets instead of steel?
Soft iron gains and loses magnetism very quickly, so it makes a strong temporary magnet that switches off when the current stops. Steel retains magnetism, so it is used for permanent magnets.
What did Oersted's experiment prove?
It proved that an electric current produces a magnetic field around the conductor. A compass needle near a current-carrying wire deflects, showing electricity and magnetism are linked.
Which rule gives the direction of force on a current-carrying conductor?
Fleming's left-hand rule. The first finger shows the field, the middle finger the current, and the thumb shows the direction of force or motion.
Is the geographic North Pole of Earth a magnetic north or south pole?
It behaves like a magnetic south pole. That is why the north pole of a compass needle, which is attracted to a south pole, points towards geographic north.
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