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Magnetic Field Due To Current In A Solenoid

Key idea: O Level electromagnetism: the magnetic field pattern due to a current in a solenoid, how to find its direction, and how to strengthen the field.

  • SEC G3 Physics 2027
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Learning objectives

  • State the properties of magnets
  • Describe induced magnetism
  • Distinguish temporary and permanent magnets
  • Determine magnetic-field direction with a compass or bar magnet
  • Interpret bar-magnet field patterns
  • Draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets
  • Interpret the field pattern around a straight current-carrying wire
  • Draw the magnetic field pattern around a straight current-carrying wire
  • Interpret the field pattern around a current-carrying solenoid
  • Draw the magnetic field pattern around a current-carrying solenoid
  • Relate current magnitude and direction to magnetic field
  • Describe electromagnet applications
  • Describe experiments showing the force on a current-carrying conductor in a magnetic field
  • Describe magnetic force on a charged-particle beam
  • Predict force reversal when current or field reverses
  • Use Fleming’s left-hand rule
  • Explain the turning effect on a current-carrying coil
  • Explain how current and turns increase the turning effect
  • Describe split-ring commutator action
  • Describe the effect of winding a motor coil on a soft-iron cylinder
  • Deduce that a changing magnetic field can induce an e.m.f.
  • Deduce that induced e.m.f. opposes the change producing it
  • Deduce factors affecting induced e.m.f. magnitude
  • Describe a simple a.c. generator and slip rings
  • Sketch a simple a.c. generator voltage–time graph
  • Describe a simple iron-cored transformer
  • Apply ideal-transformer equations
  • Explain cable loss and high-voltage transmission

1. Definition

A solenoid is a long coil of insulated wire. A current-carrying air-core solenoid produces a magnetic field like a bar magnet, with a north and a south pole. Adding a soft iron core makes it a much stronger temporary magnet—an electromagnet.

What you need for this course

You should be able to describe a solenoid’s magnetic field pattern, use the right-hand grip rule and explain how to increase field strength.

2. Key Ideas

  • A current in a solenoid produces a field like a bar magnet.
  • Inside the solenoid: field lines are nearly parallel → field is more uniform.
  • Ways to make the solenoid field stronger:
    • increase the current
    • increase the turns per unit length (wind more turns into the same coil length)
    • insert a soft iron core to make an electromagnet
  • Reverse the current → reverse the solenoid poles and field direction.

3. Detailed Explanations

A. Field pattern (like a bar magnet)

Outside the solenoid, field lines curve from N to S like those of a bar magnet. Inside, they run from S to N and are close, nearly parallel and evenly spaced, representing a strong, approximately uniform field in a long solenoid.

B. Right-hand grip rule for a solenoid (direction and poles)

Wrap your right hand around the solenoid so your fingers point in the direction of conventional current around the coil. Your thumb points to the north pole end of the solenoid.

For a quick end-view check: current anticlockwise around an end makes that end north; current clockwise makes that end south.

Current-produced fields and the motor effectThree panels show an anticlockwise field around current out of the page; a longitudinal section through a solenoid, with current out of the page in the upper parts of every turn and into the page in the lower parts, giving internal field to the right and a north pole at the right end; and the motor-effect directions for field right, current out of the page and force up.Straight wireSolenoidMotor effectcurrent out of pagethumb points out; fingers curlanticlockwise around the wireLarger current → stronger field.Further away → weaker field.Section through the coil axisupper parts: I out of pageSNlower parts: I into pageinside field: S → NEach upper/lower pair belongsto the same coil turn.Curl fingers with current;thumb points right, to N.Return field outside not shown.B: N → SBIout of pageFupFleming’s left handfirst finger → field Bsecond finger → current Ithumb → force FReverse B or I → F reverses.Reverse both → F is unchanged.
Scroll diagram horizontally to read all labels.
Use the right-hand grip rule for fields made by currents; use Fleming’s left-hand rule for the force on a current in an external magnetic field.

C. How to strengthen the solenoid field

  • More current → stronger field.
  • More turns per unit length → stronger field. Merely making the coil longer while keeping the turns equally spaced does not increase this quantity.
  • Soft iron core → a much stronger electromagnet because soft iron is easily magnetised and demagnetised.
Simulation checkpoint

In the Magnetism & Induction Lab, select the electromagnet view. Predict the north pole, reverse the current to check the pole swap, then compare the field-strength result when you change current and coil turns.

4. Common Mistakes

  • Using electron flow instead of conventional current in the right-hand grip rule.
  • Forgetting that reversing current reverses the poles.
  • Saying the field is uniform everywhere (it is most uniform inside the solenoid).

5. Exam Tips

  1. If asked “how to strengthen the electromagnet”, list: more turns per unit length, larger current, soft iron core.
  2. If asked for direction/poles, state: “use right-hand grip rule (thumb points to N)”.
  3. Mention “inside: nearly parallel lines” if asked about uniform field.

6. Worked Examples

Modelled example 1

Reversing current

Core

Problem

Describe what happens to the direction of the magnetic field inside a solenoid when the current direction is reversed.
Study the worked solution

  1. Track the cause

    Method

    Reverse the conventional current around every turn.

    Reason

    The current direction determines the solenoid’s magnetic-field direction.

    Working

    The right-hand grip rule now points the thumb towards the opposite end.

  2. State both consequences

    Method

    Reverse the field inside the solenoid and interchange its poles.

    Reason

    The thumb identifies the north-pole end.

    Working

    The former north end becomes south, and the former south end becomes north.

Guided practice 2

Increasing field strength (no numbers)

About 4 min

Problem

State two methods to increase the magnetic field strength of a solenoid.

Name and explain two controls

Hints

Hint 1: one electrical, one geometrical
Consider the current in each turn and how many turns contribute over a given length.
Hint 2: a material option also exists
An easily magnetised core can reinforce the coil’s field.
View solution step by step

  1. Increase the coil contribution

    Method

    Increase the current or increase the turns per unit length.

    Reason

    Either change strengthens the combined magnetic effect of the current-carrying turns.

    Working

    Any two valid methods are sufficient.

  2. Use a suitable core

    Method

    Alternatively, insert a soft iron core.

    Reason

    The core becomes strongly magnetised and reinforces the solenoid field.

    Working

    Valid set: larger current, more turns per unit length, soft iron core.

Common misconception 3

Choosing a core

Find and correct the mistake

Learner choice

An electromagnet must turn on and off quickly. A learner chooses steel because it “stays strongly magnetic”. Locate the mismatch and choose the correct core.

Repair the choice

Core material

View solution step by step

  1. Match both switching states

    Method

    Require easy magnetisation when current flows and little retained magnetism when it stops.

    Reason

    A retained field would delay or prevent the off-state.

    Working

    “Stays magnetic” conflicts with rapid switching.

  2. Choose soft iron

    Method

    Use a soft iron core.

    Reason

    It magnetises readily and loses most of its magnetism readily when the current stops.

    Working

    Soft iron makes the magnetic effect follow the coil current.

Examiner practice 4

Finding the north pole end

2 marks

Examination question

Viewed from one end of a solenoid, the conventional current around the turns is anticlockwise. Identify that pole and explain the rule used. [2 marks]

Write the two-mark response

View solution step by step

  1. Apply the grip rule

    1 mark

    Method

    Curl the right-hand fingers in the anticlockwise conventional-current direction.

    Reason

    The thumb then gives the solenoid’s internal field direction and north-pole end.

    Working

    The thumb points outwards from the viewed end.

  2. Identify the pole

    1 mark

    Method

    State that the viewed end is north.

    Reason

    Magnetic field lines emerge from a north pole.

    Working

    Viewed end: north pole.

Challenge 5

“Uniform field” meaning

Minimal support

Diagram-to-claim transfer

Inside a long solenoid, the field lines are drawn nearly parallel and evenly spaced. What does this tell you about the magnetic field inside?

Interpret both visual features

Hints

Hint 1: read direction and spacing separately
Parallel lines indicate a common direction; similar spacing indicates similar strength.
View solution step by step

  1. Interpret the line direction

    Method

    Use the nearly parallel lines to infer nearly the same field direction throughout the region.

    Reason

    Each field-line tangent represents the local direction.

    Working

    Direction is approximately constant.

  2. Interpret the line spacing

    Method

    Use the nearly even spacing to infer similar field strength.

    Reason

    Comparable field-line density represents comparable strength in the diagram.

    Working

    The field inside is approximately uniform.

7. Mind Stretchers

Mind stretcher 1: Uniform field useExtension

Why are solenoids useful when you want a region of nearly uniform magnetic field?

Show Answer

Inside a solenoid, the field lines are nearly parallel and evenly spaced, so the field is approximately uniform over a region.

Mind stretcher 2: Why iron core helps so muchExtension

Why does adding a soft iron core make a solenoid’s magnetic field much stronger than increasing turns a little?

Show Answer

Soft iron is easily magnetised. The core becomes strongly magnetised in the solenoid’s field, so it adds to the magnetic effect and concentrates the field inside the solenoid.

8. Practice and next step

Test current, turns and polarity in the Magnetism & Induction Lab, then see the solenoid applied in an Electric Bell.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027