Is A Magnetic Field A Force Or Energy?

Magnetism is a physical phenomenon that arises from the motion of electric charges. It manifests as attractive and repulsive forces between objects that have magnetic properties. Magnetic fields are the invisible lines of force that surround magnets and exert influence on other magnets or magnetic materials that come close to them.

The Earth itself contains a giant magnetic field that aligns roughly with its geographic north and south poles. This field allows compasses to point north and helps protect the planet from solar wind and cosmic radiation. On a smaller scale, magnets have a wide range of applications in everyday objects, engineering devices, medicine, and scientific research.

This article will provide an overview of magnetism and magnetic fields. We will explore the properties of magnetic fields, how they interact with matter, and their important applications. Understanding magnetism is key to harnessing this invisible but powerful force for human benefit.

Definition of a Magnetic Field

magnetic fields surround magnets.
A magnetic field is a region around a magnetized material or a moving electric charge where magnetic forces can be detected. Magnetic fields are created by the motion of electric charges, such as electrons, protons, or ions. They exert forces on other nearby moving charges.

Magnetic fields are often visualized using magnetic field lines, which show the strength and direction of the field at different points in space. The density of the field lines indicates the magnitude of the magnetic field – areas with high density have a strong magnetic field, while areas with low density have a weak magnetic field. Field lines are continuous and directed from the north pole to the south pole of a magnet.

The strength of a magnetic field is measured in units of tesla (T) in the International System of Units (SI). A magnetic field can be generated by permanent magnets made of ferromagnetic materials like iron, cobalt and nickel. Electromagnets produce magnetic fields using electric current, and the Earth itself produces a magnetic field that resembles that of a simple bar magnet.

Properties of Magnetic Fields

Magnetic fields have some unique and important properties that determine how they behave and interact with other objects. Understanding the properties of magnetic fields provides insight into how they exert forces and potentials on moving charges or magnets placed within them.

Some key properties of magnetic fields include:

  • Magnetic fields exert forces on moving charges – When charges like electrons or protons move through a magnetic field, they experience a force perpendicular to their direction of motion. This is how electric motors work.

  • Magnetic fields exert forces on other magnets – Just as electric charges respond to magnetic fields, magnets are attracted or repelled by magnetic fields. This force depends on the orientation of the magnets.

  • Magnetic fields can induce currents – When magnetic fields change, they can induce circulating electric currents in nearby conductors through electromagnetic induction. This is the basis for electric generators.

  • Magnetic fields can store energy – The magnetic field stores energy in the form of magnetic potential energy. This stored magnetic energy is proportional to the strength of the magnetic field.

  • Magnetic fields are directional – Magnetic fields exert their forces and effects in a specific direction at every point in space. Field lines are used to visualize the magnetic field’s direction.

  • Magnetic monopoles don’t exist – All magnets have a north and south pole. Isolated magnetic north or south poles (monopoles) have never been observed.

These properties determine how magnetic fields can generate forces and voltages, store energy, and exert directional effects on materials placed in them. Understanding the behavior of magnetic fields is key to harnessing their capabilities in electric motors, generators, electromagnets, MRI machines, and other applications.

Magnetic Force

Magnetic fields exert forces on moving charges such as electrons. When an electrically charged particle moves through a magnetic field, it will experience a force perpendicular to both its velocity and the magnetic field lines. This is known as the Lorentz force law:

F = q(v x B)

Where F is the magnetic force, q is the electric charge, v is the velocity of the particle, and B is the magnetic field. The x represents the vector cross product, indicating the force is perpendicular to both v and B.

The magnitude of the magnetic force increases with the strength of the magnetic field, the charge of the particle, and its velocity. The direction of the force depends on the direction of the magnetic field and the charge of the particle. Positively charged particles will experience a force in one direction, while negatively charged particles will experience a force in the opposite direction.

This magnetic force allows magnetic fields to exert torque on current carrying loops, deflect moving charges, and more. Magnetic forces and interactions enable electric motors, particle accelerators, mass spectrometers, and other applications.

Magnetic Potential Energy

Magnetic fields are able to store potential energy. This stored energy is proportional to the strength of the magnetic field. The potential energy comes from the work required to align magnetic dipoles against the external magnetic field.

For example, consider a ferromagnetic material like iron placed in an external magnetic field. The magnetic dipoles (tiny atomic-level magnets) within the iron will align themselves parallel to the external field. However, this requires work to go against their random thermal motion. So the iron gains potential energy when it aligns with the field. This potential energy is released as heat if the external magnetic field is removed and the dipoles return to their random orientations.

The potential energy density stored in a magnetic field B is given by:

u = B^2 / (2μ0)

Where μ0 is the permeability of free space constant. This shows that the potential energy density is proportional to the square of the magnetic field strength. Stronger fields are able to store more potential energy.

This magnetic potential energy has many uses. For example, some motor designs take advantage of the potential energy stored in magnetic fields. The fields are created by electromagnets which are turned on and off at specific times to attract or repel the motor’s rotor magnets. The rotor gains kinetic energy as the stored potential energy is released.

Magnetic Fields vs Electric Fields

Magnetic fields and electric fields share many similarities but also have some key differences.

Both magnetic and electric fields are invisible fields that surround an object and can impart forces on other objects placed in the field. They both exert forces that can push or pull on charged particles.

However, magnetic fields are generated by moving charges or intrinsic magnetic dipoles, while electric fields are generated by stationary charges. Magnetic fields are produced by the motion of electrons, while electric fields are produced by electric voltage. Also, magnetic fields can exert forces on moving charges, while electric fields exert forces on stationary charges.

Additionally, magnetic fields are dipolar in nature and surround their source in closed loops. Electric fields are monopolar and point away or towards a single electric charge. Magnetic fields can pass through non-magnetic materials like wood or plastic easily, while electric fields are easily shielded by conductive materials like metal.

In summary, magnetic and electric fields have similarities in that they both exert forces on charges and mediate electromagnetic interactions. But they have key differences in terms of their sources, directionality, and how they interact with materials. Understanding both types of fields is important across many areas of physics and engineering.

Measuring Magnetic Fields

Magnetic fields are measured using devices called magnetometers. There are several different types of magnetometers that can measure magnetic field strength:

  • Hall effect sensors – These use a semiconductor material called a Hall effect element. When a current passes through the element perpendicular to an applied magnetic field, a voltage is generated that can be measured. This voltage is proportional to the magnetic flux density.
  • Fluxgate magnetometers – These contain a ferromagnetic core surrounded by two coils of wire. Alternating current in one coil magnetizes the core, and the second coil detects changes in magnetic flux density.
  • SQUID (Superconducting Quantum Interference Device) – These extremely sensitive magnetometers rely on superconducting loops to detect tiny changes in magnetic fields.
  • Proton precession magnetometers – These use hydrogen rich fluid and measure the precession frequency of the protons in an applied magnetic field.

Portable handheld magnetometers can measure magnetic field strength on the order of nanoteslas to milliteslas. More sensitive stationary laboratory magnetometers can detect fields as low as femtoteslas. The Earth’s magnetic field strength is around 25-65 microteslas.

Applications of Magnetic Fields

Magnetic fields have many important applications in our everyday lives and across various industries. Some key examples include:

  • Electric motors – Magnetic fields interact with electric currents to generate rotational force which powers motors. Motors are used in everything from small appliances to vehicles to industrial machinery.

  • Generators – Rotating magnetic fields can induce electric current, allowing generators to transform mechanical energy into electrical energy.

  • Transformers – Transformers use magnetic fields to step voltage up or down for efficient power transmission over long distances.

  • Maglev trains – Magnetic levitation uses strong magnetic fields to suspend train cars above the track, eliminating friction for high speed transportation.

  • MRI machines – Magnetic resonance imaging applies magnetic fields to detect water molecules in the body and generate detailed internal images for medical diagnosis.

  • Hard drives – Data on hard disk drives is stored and retrieved using rapidly alternating magnetic fields.

  • Credit cards – The magnetic stripe on credit cards stores data using tiny magnetic fields which are then read by card readers.

  • Speakers – Audio speakers contain electromagnets that move and vibrate to produce sound waves when fed an audio signal.

In summary, magnetic fields are a fundamental force of nature that engineers have harnessed in many ways for energy generation, power transmission, transportation, data storage, medical imaging, and more.

Magnetic Fields in Nature

Magnetic fields are present throughout nature, from the Earth itself to stars and galaxies lightyears away. Here are some examples of magnetic fields in the natural world:

The Earth has a magnetic field that surrounds the planet and extends out into space. This field is generated by electric currents produced by the churning of molten iron alloys in the Earth’s outer core through a process called the geodynamo. The Earth’s magnetic field helps protect the planet from solar winds and cosmic radiation.

The Sun has a very complex magnetic field that changes polarity approximately every 11 years. The Sun’s magnetic field drives solar activity like sunspots and solar flares while also expelling the solar wind. Similar to Earth, the Sun’s magnetic field is produced by the motion of conductive plasma inside the star.

Neutron stars have some of the strongest magnetic fields observed in the universe, estimated to be a quadrillion (1015) times stronger than Earth’s. These incredibly dense, rotating stars produce massive magnetic fields that can accelerate particles to near the speed of light.

Galaxies also exhibit magnetic fields extending throughout the galactic disk and into the galactic halo. These fields may arise from the motions of ionized gases or conduction electrons in the interstellar medium. The magnetic fields in galaxies help regulate star formation and drive galactic outflows.

Magnetars are a type of neutron star with magnetic fields trillions of times stronger than Earth’s that can cause huge gamma ray flares detectable across the galaxy. Only a couple dozen magnetars have been observed in our Milky Way galaxy.


To conclude, magnetic fields are neither a force nor a form of energy on their own. A magnetic field is a region of space surrounding magnets and electric currents where magnetic forces can be detected. While magnetic fields themselves do not exert a force, they influence moving electric charges and magnets within the field to experience a magnetic force. Magnetic fields also contain magnetic potential energy that can be converted to other forms of energy when interacting with matter. The key distinction is that magnetic fields facilitate magnetic forces and energy, but the fields themselves are not forces or energy. They serve as an intermediary that enables magnetic interactions to occur. In summary, magnetic fields are best understood as an invisible map of magnetic influence rather than a direct force or form of energy. Their role is shaping magnetic forces and energy demonstrates the close relationship between these phenomena while maintaining their distinct identities.

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