# Magnetic field around a moving charge

• krishkoushik
In summary, a moving charge creates a magnetic field around it because of relativity. The electrons in motion appear closer together to an observer at rest, causing the positively charged particle to be attracted to it. This is known as electromagnetism and is explained by Einstein's theory that a force in one reference frame must exist in another. For a detailed explanation, refer to Edward Purcell's book.
krishkoushik
why does a moving charge create a magnetic field around it?
can you please explain the clear mechanism?

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The answer comes from relativity so I'll give you Edward Purcell's explanation. Imagine a wire with a current moving through it. Since the current is moving steadily the wire is neutral. Now let's say your positively charged particle is moving along at the same speed as the current. If we only knew of electric fields and had never read anything about magnetism, we would say there is no force here. However, now imagine the postive particle is at rest. We know from relativity that objects in motion appear shorter to observers at rest. Since the electrons (negative particles) are moving and the postive particles are not, the electrons will appear closer together from the point of view of the positive particle. Therefore, even though we know the wire is neutral, our friend the positive particle will see a negative charge and be attracted to it as it would to any negative electric field.

Einstein said that a force in one reference frame must exist an another, so the two are one and the same. This is why we use the term electromagetism. Depending on how fast the particle is moving, it can see only an electric field or a combination of an electric and magnetic field. This may be hard to visualize so check out the book by Purcell if you have trouble.

## 1. What is a magnetic field?

A magnetic field is a region in space where a magnetic force can be detected. It is created by moving electric charges, such as electrons, and can be visualized as invisible lines of force that form loops around the charges.

## 2. How does a moving charge create a magnetic field?

When an electric charge, such as an electron, moves, it creates a magnetic field around it. This is because the movement of the charge causes a disturbance in the space around it, which in turn creates a magnetic force that can be detected by other charges.

## 3. How does the strength of a magnetic field change with distance from a moving charge?

The strength of a magnetic field decreases with distance from a moving charge. This is because the magnetic field lines spread out as they move away from the charge, resulting in a weaker force being exerted on other charges.

## 4. What is the direction of the magnetic field around a moving charge?

The direction of the magnetic field around a moving charge is perpendicular to the direction of the charge's motion. This means that if the charge is moving in a straight line, the magnetic field will form circles around it.

## 5. How can the magnetic field around a moving charge be calculated?

The magnetic field around a moving charge can be calculated using the right-hand rule. This involves pointing the thumb of your right hand in the direction of the charge's motion, and curling your fingers. The direction in which your fingers curl is the direction of the magnetic field.

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