What is a magnetic field?

A magnetic field is a vector field near a magnet, electric current, or changing electric field where magnetic forces can be observed. Earth's magnetic fields cause magnets to align with the field's direction, and they force moving, charged particles into circular or helical paths. This force on electric currents in wires within a magnetic field is the basis for electric motors. Magnetic fields can be visualized using continuous lines of force, also known as magnetic flux, that emerge from north-seeking magnetic poles and enter south-seeking magnetic poles. The density of these lines indicates the strength of the magnetic field. Magnetic fields are measured in teslas (T), and they vary widely in strength, from the fields inside atomic nuclei to those in interstellar space.

How can you detect the presence of a magnetic field?

You can detect a magnetic field in several ways: * Compass: A compass needle aligns with the magnetic field vector, indicating the field's direction. * Dip Meter: A dip meter measures the inclination angle by aligning with the total field vector after the device aligns with the magnetic meridian as defined by a compass. * Oscillation Method: The magnitude of the horizontal field can be measured by observing the oscillations of a compass needle. * Proton-Precession Magnetometer: This instrument measures the gyration of protons around Earth's magnetic field, with the gyration period dependent on the field's magnitude. * Flux-Gate Magnetometer: This device measures the three components of the field vector using sensors aligned with each component. * Hall Effect: The Hall effect can detect the presence of a magnetic field, as well as measure the density of current carriers and their mobility.

What objects create magnetic fields?

Magnetic fields are created by moving electric charges and magnetic dipole moments associated with the intrinsic spin of subatomic particles. Materials have varying magnetic properties. Most are diamagnetic and weakly repelled by magnets. Paramagnetic materials, like chromium, are weakly attracted to magnets. Ferromagnetic materials, including iron, nickel, and cobalt, are strongly drawn to magnets and can be permanently magnetized. The most common source of magnetic fields is the electric current loop, such as current in a circular conductor or an orbiting electron in an atom.

How do magnetic fields interact with electric currents?

When a charge is at rest, there is no interaction with a magnetic field. However, a moving charge is subject to a force that is directly proportional to the charge's velocity. The force's direction is perpendicular to both the motion of the charge and the magnetic field. This force acts on moving charges within a wire, which transfers to the wire itself. The magnitude of the force is given by ( idlB \sin \phi ), where ( \phi ) is the angle between the magnetic field ( B ) and the length of the wire ( dl ). The force is maximized when the current and field are perpendicular. The right-hand rule can help determine the direction of the force. The force between two current-carrying wires arises from the interaction of one current with the magnetic field produced by the other.

What does the direction of a magnetic field depend on?

The direction of a magnetic field depends on the motion of electric charges. Magnetic fields are produced by moving charges (i.e., an electric current). The magnetic field does not point along the direction of the source of the field; instead, it points in a perpendicular direction. For a long straight wire, the magnetic field has a direction that encircles the wire on a plane perpendicular to the wire. The direction of the magnetic field is defined by the right-hand rule : the direction of the magnetic field points in the direction of the fingers of the right hand when the thumb points in the direction of the current.

How are magnetic fields used in everyday technology?

Magnetic fields are integral to everyday modern technology. They are present in various applications, from simple devices to complex systems.

magnetic field

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magnets and their associated magnetic field lines

Diagram showing a small copper sample suspended between the north and south poles of an electromagnet, with magnetic field lines concentrated around the sample in an inhomogeneous magnetic field.

Diagram showing magnetic field lines around a small horizontal current loop at the center, with arrows indicating the field direction created by a clockwise current when viewed from above.

Diagram showing a circular loop of wire with an electric current, surrounded by curved blue lines representing magnetic field lines that loop around the wire and pass through its center.

Diagram showing the magnetic field lines around two parallel current loops with currents flowing in opposite directions. The field lines form closed loops, curving outward from each loop and meeting between them, illustrating the opposing magnetic fields.

Diagram showing two parallel current loops with currents flowing in the same direction, surrounded by blue magnetic field lines that form closed loops and connect the two coils, illustrating the combined magnetic field pattern.

Diagram of an electromagnetic wave showing two perpendicular oscillating fields: electric field (E) and magnetic field (B), both at right angles to the direction of wave propagation (c). Wavelength is marked as lambda.

Two diagrams show the magnetic field around a long straight wire carrying current. Panel A presents an end view with concentric circular field lines around the wire, current flowing toward the viewer. Panel B shows a three-dimensional view with the wire angled across the image and circular magnetic field lines surrounding it.

Quick Facts

Key People:: Nikola Tesla; George Ellery Hale; David Thouless; Duncan Haldane; Hans Christian Ørsted

Related Topics:: geomagnetic field; Lorentz force; energy density; magnetostatic field; magnetic pulsation

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Top Questions

What is a magnetic field?
A magnetic field is a vector field near a magnet, electric current, or changing electric field where magnetic forces can be observed. Earth's magnetic fields cause magnets to align with the field's direction, and they force moving, charged particles into circular or helical paths. This force on electric currents in wires within a magnetic field is the basis for electric motors.

Magnetic fields can be visualized using continuous lines of force, also known as magnetic flux, that emerge from north-seeking magnetic poles and enter south-seeking magnetic poles. The density of these lines indicates the strength of the magnetic field. Magnetic fields are measured in teslas (T), and they vary widely in strength, from the fields inside atomic nuclei to those in interstellar space.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How can you detect the presence of a magnetic field?
You can detect a magnetic field in several ways:
* Compass: A compass needle aligns with the magnetic field vector, indicating the field's direction.
* Dip Meter: A dip meter measures the inclination angle by aligning with the total field vector after the device aligns with the magnetic meridian as defined by a compass.
* Oscillation Method: The magnitude of the horizontal field can be measured by observing the oscillations of a compass needle.
* Proton-Precession Magnetometer: This instrument measures the gyration of protons around Earth's magnetic field, with the gyration period dependent on the field's magnitude.
* Flux-Gate Magnetometer: This device measures the three components of the field vector using sensors aligned with each component.
* Hall Effect: The Hall effect can detect the presence of a magnetic field, as well as measure the density of current carriers and their mobility.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What objects create magnetic fields?
Magnetic fields are created by moving electric charges and magnetic dipole moments associated with the intrinsic spin of subatomic particles.

Materials have varying magnetic properties. Most are diamagnetic and weakly repelled by magnets. Paramagnetic materials, like chromium, are weakly attracted to magnets. Ferromagnetic materials, including iron, nickel, and cobalt, are strongly drawn to magnets and can be permanently magnetized.

The most common source of magnetic fields is the electric current loop, such as current in a circular conductor or an orbiting electron in an atom.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do magnetic fields interact with electric currents?
When a charge is at rest, there is no interaction with a magnetic field. However, a moving charge is subject to a force that is directly proportional to the charge's velocity. The force's direction is perpendicular to both the motion of the charge and the magnetic field. This force acts on moving charges within a wire, which transfers to the wire itself.

The magnitude of the force is given by ( idlB \sin \phi ), where ( \phi ) is the angle between the magnetic field ( B ) and the length of the wire ( dl ). The force is maximized when the current and field are perpendicular. The right-hand rule can help determine the direction of the force.

The force between two current-carrying wires arises from the interaction of one current with the magnetic field produced by the other.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What does the direction of a magnetic field depend on?
The direction of a magnetic field depends on the motion of electric charges. Magnetic fields are produced by moving charges (i.e., an electric current). The magnetic field does not point along the direction of the source of the field; instead, it points in a perpendicular direction.

For a long straight wire, the magnetic field has a direction that encircles the wire on a plane perpendicular to the wire. The direction of the magnetic field is defined by the right-hand rule: the direction of the magnetic field points in the direction of the fingers of the right hand when the thumb points in the direction of the current.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How are magnetic fields used in everyday technology?
Magnetic fields are integral to everyday modern technology. They are present in various applications, from simple devices to complex systems.
- Electric Motors: Magnetic fields generated by electric currents in starter motors are used to start automobiles. Permanent magnets are also used in motors for precise speed and position control.
- Data Storage: Magnetic fields are used to store data on magnetic disks, such as hard drives, by magnetizing tiny spots on the disk surface.
- Medical Imaging: Magnetic Resonance Imaging (MRI) utilizes magnetic fields to visualize organs and structures inside the body for diagnostic purposes.
- Power Generation and Transmission: Faraday's law of induction is the basis for the power industry and the transformation of mechanical energy into electric energy. Transformers use magnetic fields to convert electric power from one voltage level to another.
- Maglev Trains: Maglev (magnetic levitation) trains use magnetic fields to levitate and propel trains, eliminating friction from wheels on rails and enabling high speeds.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

magnetic field, a vector field in the neighbourhood of a magnet, electric current, or changing electric field, in which magnetic forces are observable. Magnetic fields such as that of Earth cause magnetic compass needles and other permanent magnets to line up in the direction of the field. Magnetic fields force moving electrically charged particles in a circular or helical path. This force—exerted on electric currents in wires in a magnetic field—underlies the operation of electric motors. (For more information about magnetic fields, see magnetism.

Around a permanent magnet or a wire carrying a steady electric current in one direction, the magnetic field is stationary and referred to as a magnetostatic field. At any given point its magnitude and direction remain the same. Around an alternating current or a fluctuating direct current, the magnetic field is continuously changing its magnitude and direction.

Magnetic fields may be represented by continuous lines of force or magnetic flux that emerge from north-seeking magnetic poles and enter south-seeking magnetic poles. The density of the lines indicates the magnitude of the magnetic field. At the poles of a magnet, for example, where the magnetic field is strong, the field lines are crowded together, or more dense. Farther away, where the magnetic field is weak, they fan out, becoming less dense. A uniform magnetic field is represented by equally spaced parallel straight lines. The direction of the flux is the direction in which the north-seeking pole of a small magnet points. The lines of flux are continuous, forming closed loops. For a bar magnet, they emerge from the north-seeking pole, fan out and around, enter the magnet at the south-seeking pole, and continue through the magnet to the north pole, where they again emerge. The SI unit for magnetic flux is the weber. The number of webers is a measure of the total number of field lines that cross a given area.

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