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F3.1 · Describe fields of permanent magnets and electromagnets

Learn to describe fields of permanent magnets and electromagnets through clear examples and targeted practice.

Ontario Grade 11 Physics

Electricity and Magnetism

Permanent magnets and electromagnets — Ontario Grade 11 Physics, F3.1

A magnet can affect other magnets or magnetic materials without touching them. The region where this effect can be detected is called a magnetic field. In this lesson, the system is the magnet or electromagnet and the space around it. We use a fixed viewpoint and describe directions on the page: left, right, up, and down. A magnetic field has both strength and direction, so it is a vector. Before comparing field patterns, recall that arrows show direction and that closer field lines represent a stronger field. The lesson focuses on describing fields, not calculating forces or measuring them.

What you will learn

Describe the shape and direction of the field around a permanent magnet.
Explain how field-line spacing represents relative field strength.
Describe how an electric current creates the field of an electromagnet.
Compare the field patterns of permanent magnets and electromagnets.

1. Permanent magnets and field-line diagrams

A permanent magnet produces a magnetic field even when it is not connected to a power source. A bar magnet has two ends, called poles. One end is labelled north and the other south. The labels identify the magnet’s poles; they are not geographic directions.

A field diagram is a model that helps us describe a field. Its lines are not physical threads in space. The arrows on the lines show the direction of the magnetic field. By convention, outside a magnet, the arrows point from its north pole toward its south pole. Inside the magnet, the field continues from south toward north, so the lines form closed paths.

The field direction at a point is the direction a small north-seeking compass end would point there. A compass needle is a small magnet, and its north-seeking end aligns with the local field direction. The field-line pattern around a bar magnet curves from one pole to the other. Lines are drawn closer together near the poles because the field is stronger there than in regions where the lines are farther apart.

A field line gives a direction at each point along it. Field lines do not cross in a field diagram. If they crossed, the crossing point would have two different field directions, which would make the diagram unclear.

Outside a bar magnet, field arrows point from north to south.
Closer field lines indicate a stronger field relative to areas with wider spacing.
A magnetic field is a vector: it has strength and direction.
Field lines are a representation, not material objects.

2. Describing an electromagnet’s field

An electromagnet is a magnet whose field is produced by electric current. A current is the movement of electric charge through a conductor. When current flows through a wire, a magnetic field forms around the wire. Winding the wire into a coil makes the fields from its turns combine into a pattern like the field of a bar magnet. The coil has a north end and a south end while current flows.

For a simple coil, use the right-hand grip rule to identify the field direction. Curl the fingers of your right hand in the direction of conventional current around the coil. Your thumb points toward the coil’s north end. Conventional current is the direction positive charge would flow in a circuit; it is the direction used in circuit diagrams. Do not confuse it with the direction electrons move.

An iron core placed inside the coil can make the electromagnet stronger. The core becomes magnetized while the current flows and adds to the field produced by the coil. The electromagnet’s field can be changed by changing the current or the coil. If the current is switched off, the field from the current stops; an iron core may retain some magnetism, but the coil is no longer being energized.

The field pattern of an electromagnet resembles that of a bar magnet: it has two ends and curved field lines outside the coil, directed from north to south. Its important difference is that the field depends on current. Reversing the current reverses the field direction and exchanges which end acts as north and south.

Current in a wire produces a magnetic field around the wire.
A coil’s field has a north end and a south end, like a bar magnet’s field.
The right-hand grip rule links conventional current direction to the coil’s field direction.
Reversing the current reverses the electromagnet’s poles.

3. Comparing and reading field patterns

To describe any field diagram, first identify the object and its poles or current direction. Next, follow the arrows to state the field direction. Then compare line spacing to describe where the field is relatively stronger or weaker. This method describes the pattern without claiming an exact field strength.

Magnetic field strength is measured in teslas, symbol

\mathrm{T}

. A field diagram usually shows relative strength through line spacing rather than giving a numerical value. Do not infer a particular number of teslas just by counting drawn lines; the number and spacing depend on how the diagram was made.

A permanent magnet has a field without an electric current in a coil. An electromagnet’s field is produced by current and can be controlled by changing or reversing that current. Both can be represented by field lines, and both have field direction at every point.

A useful description is precise but modest: name the field source, state the direction shown by the arrows, and compare the spacing. For an electromagnet, also state the conventional current direction or whether it has been reversed. Do not say that field lines are visible or that they physically flow.

Describe source, direction, and relative strength from the diagram.
The SI unit of magnetic field strength is the tesla, $\mathrm{T}$ .
Field-line spacing supports comparisons, not an exact numerical measurement.
Permanent magnets and electromagnets have similar field patterns but different sources.

Field-description guide

Source	How to identify direction	What spacing tells you
Permanent bar magnet	Outside: north toward south	Closer lines mean relatively stronger field
Current-carrying coil	Use the right-hand grip rule to find north; outside: north toward south	Closer lines mean relatively stronger field
Coil with reversed current	Poles and field direction reverse	Spacing indicates relative strength, not an exact value

Worked example

Reading a bar-magnet diagram

A field diagram shows a bar magnet with its north pole on the left and south pole on the right. Arrows outside the magnet curve from left to right. The lines are most closely spaced near the poles. Describe the field.

Identify the source
The source is a permanent bar magnet. Its field exists without a powered coil.
Read the arrows
Outside the magnet, the arrows point away from the north pole and toward the south pole. Here, that direction is from left to right.
Compare spacing
The closest spacing is near the poles, so the field is relatively stronger there than where the lines are farther apart. This is a comparison, not a numerical field measurement.

Answer: The permanent magnet’s external field points from left to right, from north toward south. It is relatively strongest near the poles, where the field lines are closest together.

Check: The stated direction agrees with the arrows, and the strength comparison agrees with the line spacing.

Worked example

Finding an electromagnet’s north end

A coil is viewed end-on. Conventional current travels counterclockwise around the visible end. Use the right-hand grip rule to identify that end’s pole.

Set the viewpoint
Look straight at the specified end of the coil. The current direction is counterclockwise from this viewpoint.
Apply the grip rule
Curl the fingers of your right hand in the direction of conventional current. Your thumb points out toward the viewer, so the visible end is the north end.
Describe the external field
Outside the coil, the field points away from this north end and toward the opposite south end.

Answer: The visible end is north. The external field points away from it and curves toward the coil’s south end.

Check: The thumb points out of the visible end when the fingers curl counterclockwise, consistent with the stated pole.

Worked example

Predicting the effect of reversing current

An electromagnet has a north pole at its upper end and a south pole at its lower end. The current in the coil is reversed. Describe the resulting field pattern.

Keep the source in view
The source remains an electromagnet: current still flows in the coil, but its direction has changed.
Reverse the pole labels
Reversing current reverses the magnetic field direction. The upper end becomes south and the lower end becomes north.
State the outside direction
Outside the coil, field arrows go from the new north end toward the new south end. They therefore point generally upward, from the lower end toward the upper end.

Answer: The lower end becomes north and the upper end becomes south. Outside the coil, the field points from the lower end toward the upper end.

Check: The poles and external field direction both reverse, while the coil remains an electromagnet.

Common mistakes and how to avoid them

Saying magnetic field arrows point from south to north outside a magnet.

Correction: Outside a magnet, the conventional field direction is from north to south. The complete field path continues through the magnet.

Treating field lines as real strands or saying they move.

Correction: Field lines are a diagramming tool. Their arrows show direction, and their spacing compares relative strength.

Using the right-hand grip rule with electron flow instead of conventional current.

Correction: The rule here uses conventional current direction. Identify that direction before curling your fingers.

Assuming an electromagnet’s poles cannot change.

Correction: Reversing current reverses the field direction and swaps the north and south ends.

Reading exact field strength from a sketch’s line spacing.

Correction: A sketch supports a relative comparison. A numerical value requires a measurement or a specified field value.

Lesson summary

A magnetic field describes the region around a magnet where magnetic effects can be detected.
Field arrows show direction. Outside a magnet, they point from north to south.
Closer field lines indicate a relatively stronger field.
A current-carrying coil produces an electromagnet’s field. The right-hand grip rule identifies its north end.
Reversing the current reverses the electromagnet’s field and swaps its poles.

Check your understanding

Question 1

Outside a bar magnet, which way do the field arrows point?

From north to south
From south to north
From the centre of the magnet outward in every direction
correctIndex

Show answer and explanation

From north to south

By convention, the external field direction is from the north pole toward the south pole.

Question 2

What does closer spacing of field lines show in a field diagram?

The field is relatively stronger in that region
The field direction has reversed
The magnet is moving faster
correctIndex

Show answer and explanation

The field is relatively stronger in that region

Closer lines represent greater field strength relative to regions with wider spacing.

Question 3

What happens to an electromagnet’s poles when the current direction is reversed?

The north and south ends switch
The poles stay in the same places
The field becomes a permanent-magnet field
correctIndex

Show answer and explanation

The north and south ends switch

Reversing current reverses the coil’s magnetic field, so its north and south ends exchange.

Key terms

Magnetic field: The region around a magnet or current-carrying wire where magnetic effects can be detected.
Field line: A drawn line used to represent a field’s direction and relative strength.
Vector: A quantity that has both magnitude, or size, and direction.
Permanent magnet: A magnet that produces a magnetic field without current flowing through a coil.
Electromagnet: A magnet whose field is produced by electric current, usually in a coil.
Conventional current: The direction positive charge would flow in a circuit; it is the direction used for circuit diagrams and the coil rule in this lesson.
Right-hand grip rule: A rule for a coil: curl the fingers of the right hand with conventional current; the thumb points toward the coil’s north end.
Tesla: The SI unit of magnetic field strength, with symbol $\mathrm{T}$ .

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Physics (SPH3U), expectation F3.1. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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