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Solenoid

A solenoid is a long cylindrical coil of many turns of insulated wire.

In one line: Many turns, strong field.

Practice This Concept

Understand it

When current flows through a solenoid, it produces a magnetic field similar to that of a bar magnet.
One end behaves like a north pole and the other like a south pole.
The field inside a solenoid is strong and nearly uniform in the middle region.
This is why solenoids are used to make electromagnets and many electric devices.

Example

A tightly wound coil used in an electric bell or relay is a practical solenoid-based arrangement.

Formula and rule

A current-carrying solenoid behaves like a bar magnet, with a strong and nearly uniform field inside the coil.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1If a battery is connected to a coil of wire wound around a nail, the nail becomes magnetic while current flows.
2The coil acts as a solenoid and the nail becomes an electromagnet core.

Method you can reuse

1Identify the coil with many turns.
2State that current in the coil produces a magnetic field.
3Mention the north and south poles at opposite ends.
4Add that the field inside is strong and nearly uniform.

Sketch it in your answer

Draw a cylindrical coil with arrows showing current and field lines inside the coil as nearly parallel, crowded lines.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is a solenoid useful in making an electromagnet?

Think first, then show answer

A solenoid is useful because current through its many turns produces a strong magnetic field, and its ends behave like poles. This makes it easy to create a controllable electromagnet.

Common wrong answer

The solenoid always acts as a magnet without current.

Why it loses marks: Current is needed to produce the field.

Don't mix this up

Students often think a solenoid is a magnet by itself. In reality, it becomes magnetic only when current flows through it.

Exam tip

If asked about the field pattern, mention that the field inside a solenoid is strong and nearly uniform.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Define a solenoid, describe its field pattern, or explain why it behaves like a bar magnet when current flows.

1
Enough for 1 mark
A solenoid is a long cylindrical coil of many turns of insulated wire.
2
Add for 2 marks
Example: A tightly wound coil used in an electric bell or relay is a practical solenoid-based arrangement.
3
Add for 3 marks
When current flows through a solenoid, it produces a magnetic field similar to that of a bar magnet.

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