# When is the flux through a coil from a bar magnet zero?

• aliens123
In summary, the conversation discusses the total flux through a circular closed wire when a finite sized cylindrical bar magnet passes through the origin. The flux is not necessarily zero and can be increased by increasing the wire's radius. The flux as a function of the magnet's center of mass position is not zero due to the formation of closed loops of magnetic flux. The flux through the wire is not zero even in the static case, unless the wire is very large and the magnet is moving close to the speed of light.
aliens123
Suppose I have a finite sized cylindrical bar magnet of radius $a$ and length $b$. It is coaxial with the $x$ axis and moving from $-\infty$ to $\infty.$ A circular closed wire of radius $r>a$ is in the $y-z$ plane with center at the origin. When the bar center of the bar magnet passes through the origin, what is the total flux through the wire?

The answer is not necessarily zero because note that by increasing the radius $r$ we can increase the total flux through the wire. So what does the flux as a function of the position of the center of mass of the bar magnet look like?

Lines of magnetic flux form closed loops. That is to say, the flux inside the bar magnet is not zero. There is a very strong B field pointing the opposite direction from the more accessible parts of the loops outside the bar. So unless your loop of wire is very large extending out to where the magnetic field is almost zero, the flux through your wire is not anything like zero even in the static case. Unless it is moving close to the speed of light, the movement of the bar doesn’t change the snapshot of the magnetic field of the bar magnet at that moment when the bar is in the middle.

## 1. What is a coil and a bar magnet?

A coil is a loop of wire that is wound around a core, while a bar magnet is a permanent magnet with a north and south pole.

## 2. What is flux and how does it relate to a coil and a bar magnet?

Flux is the measure of the flow of magnetic field lines through a surface. In the case of a coil and a bar magnet, the flux is the amount of magnetic field lines passing through the coil due to the presence of the magnet.

## 3. Why does the flux through a coil from a bar magnet become zero?

The flux through a coil from a bar magnet becomes zero when the coil is perpendicular to the magnetic field lines of the magnet. This is because the magnetic field lines are not passing through the coil at this angle, resulting in zero flux.

## 4. How can I calculate the flux through a coil from a bar magnet?

The flux through a coil from a bar magnet can be calculated using the formula: Φ = BA cosθ, where Φ is the flux, B is the magnetic field strength, A is the area of the coil, and θ is the angle between the magnetic field lines and the coil.

## 5. What factors can affect the flux through a coil from a bar magnet?

The flux through a coil from a bar magnet can be affected by the strength of the magnet, the number of turns in the coil, the size and shape of the coil, and the angle between the coil and the magnet. Additionally, any magnetic materials or other magnetic fields in the vicinity can also affect the flux.

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