# Electromagnetic Induction -- Moving Magnet (1 Viewer)

#### Physics_FTW

##### New Member
Quick Question,
If you have a Bar magnet with its North Pole approaching a solenoid, a current will be induced into the solenoid to create a North Pole at the entrance to the solenoid (Hence South Pole on other side?)

Now, say the Magnet is 10cm long and the solenoid is 20cm long and the magnet actually moves through the solenoid.

I am confused as to what the readings on the galvanometer would be throughout the entire movement of the magnet through the solenoid.

So at these points
- North Pole approaching Left side of Solenoid
- North Pole enters Left side of Solenoid
- Enitre Magnet enters solenoid (South Pole moving away from Left side)
- North Pole approaches Right side of Solenoid (from the inside)
- North Pole exits right side of solenoid
- Entire magnet exits Right side of Solenoid

To me it doesnt make sense, especially when the entire magnet is in the solenoid as South Pole is moving away from one end (Induce current to create a North Pole at this end to try and 'attract it') and North Pole is approaching the other end (Inducing current to create a North Pole also at this end to try and 'repel it')

Much confusion?

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#### Bored Of Fail 2

##### Banned
its easy man, what dont you understand LOL

#### Physics_FTW

##### New Member
Easy for you maybe.. I havent even started Year 11 Physics yet.. But to me it looks boring, ceebz looking over that.
Lets say the initial movement created a positive reading on the galvanometer, i dont understand whether the reading would be positive, negative or zero when the magnet is inside the solenoid, still moving and approaching the other side, as Lenz' Law tells us that North Poles would be created at both ends, hence saying current is travelling in both directions?!?!

#### Bored Of Fail 2

##### Banned
lol i havent done hsc physics in a year but my guess would be that the current would go positive, then decrease to zero in the centre and then become increasingly negative as it came out, but im not sure if thats 100% lol

#### nelsonzheng

##### New Member
Have a watch of this video:

A current will be induced only when there is a change in magnetic field or in this case a relative movement between the solenoid and the magnet.
When the North pole of a magnet moves towards a solenoid, it will induce a current, giving rise to a North Pole at the side of the solenoid that it is approaching, repelling it.
When the North pole of a magnet moves away from a solenoid, it will induce a current in the opposite direction as in the scenario above, giving rise to a South Pole at the side of the solenoid that it is approaching, attracting it.

When the magnet is inside the solenoid the induced current by either end of the magnet will be in equal and opposite directions, thus no overall current will be observed.

#### k02033

##### Member
Theres a simple trick to all of this. And that is to keep Lenz's law in your head.

The law says "changes can induce currents and these currents will set up magnetics field to oppose the change"

Change =North Pole approaching Left side of Solenoid
Then the induce magnetic field will oppose the change, ie push the north pole to the right

change = entire magnet enters solenoid while moving right
Then the induce magnetic field will oppose the change, ie push the magnet to the left

SO just think about what the change is and then configuration of magnetic fields and currents that will oppose it.

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#### Physics_FTW

##### New Member
When the magnet is inside the solenoid the induced current by either end of the magnet will be in equal and opposite directions, thus no overall current will be observed.
LoL, love the funky music

I'm still not clear about the magnet whilst it is inside the solenoid, as for your statement to be true, it would mean that the rate of change of flux at both ends is equal, which is to be unlikely throughout the entire journey of the magnet through the solenoid, ie when the magnet is closer to one side of the solenoid, that side will experience a greater rate of change of flux then the other side until the magnet gets to the exact centre, then the other side will start experiencing a greater rate of change of flux?

I am thinking to much into it? Or is my logic flawed?

SO just think about what the change is and then configuration of magnetic fields and currents that will oppose it.
That's the bit I am finding hard! It's all good whilst the magnet is outside the solenoid, but once it is moving inside the solenoid, it doesnt makes sense to me and I cant find it in any textbooks or on the internet

#### nelsonzheng

##### New Member
I'm still not clear about the magnet whilst it is inside the solenoid, as for your statement to be true, it would mean that the rate of change of flux at both ends is equal, which is to be unlikely throughout the entire journey of the magnet through the solenoid, ie when the magnet is closer to one side of the solenoid, that side will experience a greater rate of change of flux then the other side until the magnet gets to the exact centre, then the other side will start experiencing a greater rate of change of flux?
yeah, i simplified it somewhat but that would be how things would work. Why don't u go get a magnet and a solenoid and play with it?

#### jyu

##### Member
Hope it is right

Not necessarily straight lines, depends on physical size of magnet and solenoid, as well as relative motion.

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