# Magnetic field of a solenoid problem

When a magnet is pushed into a solenoid, the solenoid will have an induced emf and hence induced current since it is a closed circuit. But when the magnet is outside does, how does the magnetic field of the solenoid look like? I mean is it combined with the magnet's magnetic field or does it exist on its own with two separate magnetic fields? Then when the magnet is partially inside the solenoid then what would the magnetic field look like (it is not at the middle as it won't have a magnetic field in the middle)? I'm not sure if the magnetic field of the magnet and the solenoid will combine with each other. But I'm unsure about it..

Then when the magnet is inside the solenoid will there be an induced emf (it is not in the middle of the solenoid). Is the magnetic field strength decided by how much of the magnetic field passing through the loops, so when it is inside the field but not in the middle then the direction of infused current be the same just that it is weakens as it goes on until it reaches the middle then there is no current. Once it goes past the middle, then the current will start to increase in the other direction?

Lastly, in a solenoid with a power source, when I push an unmagnetised iron bar through a it it will have the induced poles right? So until I push it in all the way it will have the same poles right? Eg if I have a solenoid at the left it is north and at the right it is south when I push the bar from the right it will be north at the end facing the right and south at the other end. As I push it in, the poles will remain the same until the other end.where it acts as a iron core. But as I push it in, won't it have the same effect as a induction of emf scenario? Since I'm pushing in the "magnet" so the south will induce a north on the solenoid? Or is the effect too insignificant?

Jano L.
Gold Member
When a magnet is pushed into a solenoid, the solenoid will have an induced emf and hence induced current since it is a closed circuit. But when the magnet is outside does, how does the magnetic field of the solenoid look like? I mean is it combined with the magnet's magnetic field or does it exist on its own with two separate magnetic fields? Then when the magnet is partially inside the solenoid then what would the magnetic field look like (it is not at the middle as it won't have a magnetic field in the middle)? I'm not sure if the magnetic field of the magnet and the solenoid will combine with each other. But I'm unsure about it..

Sgstudent, because Maxwell's equations are linear, total field is a sum of partial fields due to all present bodies. The partial field is the field which would be measured as total field if no other bodies were present. So, the total field in your question is a sum of the field produced by the coil and the field due to magnet.

Then when the magnet is inside the solenoid will there be an induced emf (it is not in the middle of the solenoid). Is the magnetic field strength decided by how much of the magnetic field passing through the loops, so when it is inside the field but not in the middle then the direction of infused current be the same just that it is weakens as it goes on until it reaches the middle then there is no current. Once it goes past the middle, then the current will start to increase in the other direction?

Induced emf is there only of the magnet moves - when the magnetic field of the magnet changes in time. The dependence should be as you say, turnpoint being when the magnet is at the centre.

Lastly, in a solenoid with a power source, when I push an unmagnetised iron bar through a it it will have the induced poles right? So until I push it in all the way it will have the same poles right? Eg if I have a solenoid at the left it is north and at the right it is south when I push the bar from the right it will be north at the end facing the right and south at the other end. As I push it in, the poles will remain the same until the other end.where it acts as a iron core. But as I push it in, won't it have the same effect as a induction of emf scenario? Since I'm pushing in the "magnet" so the south will induce a north on the solenoid? Or is the effect too insignificant?

The effect is there, the bar will behave similarly as the magnet. How much significant the effect is depends on the ability of the bar to magnetize in the field of the coil. Iron and ferrites get magentized a lot, and they have strong effect on the emf in the coil. If the bar is made of aluminum, the effect will be opposite and very weak (diamagnetism), and if it is made of wood, there will be no appreciable effect.

Sgstudent, because Maxwell's equations are linear, total field is a sum of partial fields due to all present bodies. The partial field is the field which would be measured as total field if no other bodies were present. So, the total field in your question is a sum of the field produced by the coil and the field due to magnet.

Induced emf is there only of the magnet moves - when the magnetic field of the magnet changes in time. The dependence should be as you say, turnpoint being when the magnet is at the centre.

The effect is there, the bar will behave similarly as the magnet. How much significant the effect is depends on the ability of the bar to magnetize in the field of the coil. Iron and ferrites get magentized a lot, and they have strong effect on the emf in the coil. If the bar is made of aluminum, the effect will be opposite and very weak (diamagnetism), and if it is made of wood, there will be no appreciable effect.

Hi thanks for the help. Could you attach a link with the image of the solenoid with a magnetic field in it? I can't really find any on the internet. Thanks again!

Jano L.
Gold Member
I can't find any good picture either. The simplest version would be to calculate the sum of two dipolar fields and plot the resulting field. Maybe there is some software that can do that.

Oh okay, will we be asked to draw it in the O levels? Thanks for the help!

Jano L.
Gold Member
I do not know about the tests. Glad to be of help,
Jano