Physics

# Magnetic Force .

المحتويات (Contents)

### Overview.

One surprising discovery in early physics was that electricity and magnetism are two sides of the same phenomenon: electromagnetism. In fact, magnetic fields are generated by moving electric charges or changes in the electric field. As such, magnetic forces act, not just on anything magnetized, but also on moving charges.

### Definition of Magnetic Force .

magnetic force, attraction or repulsion that arises between electrically charged particles because of their motion. It is the basic force responsible for such effects as the action of electric motors and the attraction of magnets for iron. Electric forces exist among stationary electric charges; both electric and magnetic forces exist among moving electric charges. The magnetic force between two moving charges may be described as the effect exerted upon either charge by a magnetic field created by the other.

From this point of view, the magnetic force F on the second particle is proportional to its charge q2, the magnitude of its velocity v2, the magnitude of the magnetic field B1 produced by the first moving charge, and the sine of the angle theta, θ, between the path of the second particle and the direction of the magnetic field; that is, F = q2B1v2 sin θ. The force is zero if the second charge is travelling in the direction of the magnetic field and is greatest if it travels at right angles to the magnetic field.

The magnetic force on a moving charge is exerted in a direction at a right angle to the plane formed by the direction of its velocity and the direction of the surrounding magnetic field.

### How To Find Magnetic Force?

The magnitude of the magnetic force depends on how much charge is in how much motion in each of the objects and how far apart they are.

Mathematically, we can write magnetic force as:

F=q[E(r)+v×B(r)]

This force is termed as the Lorentz Force. It is the combination of the electric and magnetic force on a point charge due to electromagnetic fields.

The interaction between the electric field and the magnetic field has the following features:

▪️ The magnetic force depends upon the charge of the particle, the velocity of the particle and the magnetic field in which it is placed. The direction of the magnetic force is opposite to that of a positive charge.

▪️ The magnitude of the force is calculated by the cross product of velocity and the magnetic field, given by q [ v × B ]. The resultant force is thus perpendicular to the direction of the velocity and the magnetic field, the direction of the magnetic field is predicted by the right-hand thumb rule.

▪️ In the case of static charges, the total magnetic force is zero.

### The Laws of Magnets.

The laws of magnetism have had a profound effect on science and culture. Since the early years of the 19th century, scientists have worked to identify and explain the various physical laws governing the behavior of magnets in a variety of contexts. By 1905, the scientific understanding of magnetism evolved to the point that it helped drive the creation of Einstein’s theory of special relativity. Although a detailed, in-depth understanding of magnetism requires extensive effort, you can gain a broad overview of these fundamental laws relatively quickly.

#### Exploring the First Law of Magnetism.

The laws of magnetism have been developed and refined extensively since the experiments of Orsted, Ampere and other now-famous scientists in the early 1800s. The most fundamental law introduced during this time is the concept that the poles of a magnet each have their own distinct positive or negative charge and only attract oppositely charged poles. For example, it is nearly impossible to keep two positively charged magnetic poles from repelling each other. On the other hand, it is difficult to keep a positively charged and negatively charged magnetic pole from attempting to move toward each other.

Where this concept becomes particularly interesting is when a pre-existing magnet is cut into two different, smaller magnets. Following the cut, each of the smaller magnets has its own positive and negatively charged poles, irrespective of where the larger magnet was cut.

The concept of oppositely charged poles is commonly referred to as the First Law of Magnetism.

#### Defining the Second Law of Magnetism.

The second law of magnetism is slightly more complex and relates directly to the electromotive force of the magnets themselves. This particular law is commonly referred to as Coulomb’s Law.

Coulomb’s law states that the force exerted by the pole of a magnet on an additional pole adheres to a series of strict rules, including:

▪️ The force is in direct proportion to the product of the forces of the pole.

▪️ The force exists in inverse proportion to the square of the middle distance between the poles.

▪️ The force is dependent on the specific medium in which the magnets are placed.

The mathematical formula commonly used to represent these rules is:

F = [K x M1 x M2)/d2]

In the formula, M1 and M2 represent the strengths of the poles, D is equal to the distance between the poles, and K is a mathematical representation of the permeability of the medium in which the magnets are placed.

The Domain Theory of Magnetism provides additional insight into the behavior of magnets. First introduced in 1906 by Pierre-Ernest Weiss, the theory of magnetic domains seeks to explain the changes that occur inside a substance when it becomes magnetized.

Large magnetized substances consist of smaller areas of magnetism, commonly referred to as domains. Within each domain are smaller units referred to as dipoles. The complex nature of magnetic composition allows for the continued presence of magnetism when larger magnetic units are broken or separated.

### Understanding How Demagnetization Occurs.

Magnets do not remain magnetized forever. Deliberate demagnetization can occur through the reorganizing of dipoles within the magnet itself. A variety of processes can be used to make this happen. Heating a magnet past its Curie point, which is the temperature at which it is known to manipulate dipoles, is one popular method. Another method for demagnetizing a substance is to apply alternate current to the magnet. Even without applying any of these methods, a magnet slowly demagnetizes over time as part of a natural degradation process.

### Magnet Properties .

▪️ It has north and south poles. When hanging freely, it heads north and south.

▪️ The magnetic force is concentrated at its poles and decreases in other areas.

▪️ Poles of different kind attract each other, and poles of the same kind repel each other.

▪️ If the magnet is cut from any area in it, it will have two poles and it cannot have a single pole in practice.