Dropping bar magnet through loop of wire

[ninjadrummer8]

Ok I have 2 questions about this.

So I know that a current is induced when the magnetic field changes near the wire. So say I dropped the magnet North end up and South end down through this wire, the current would increase as it was nearing it, and decrease as it leaves it.

1) is this correct?
2) what happens in the short time period that the magnet is still passing through the coil? is the current 0?

 

[Bob S]

If your signal is going into a low impedance ammeter, the current would go positive (or negative depending on polarity of ammeter), go back to zero while the magnet is passing through the wire, then go negative (opposite polarity) as the other pole passes through the loop. It is a bipolar signal. The reason it goes to zero in the middle of the magnet is because dB/dt through the loop is zero.

If you built a voltage integrator with an operational amplifier, the integrated voltage would stay positive until the other end of the magnet passed through the loop.

 

[astrokreng]

1) Yes, your understanding is correct. When a magnet is dropped through a loop of wire, the changing magnetic field induces a current in the wire. The direction of the current depends on the direction of the changing magnetic field and follows Faraday's law of induction. As the magnet approaches the wire, the current will increase, and as it leaves the wire, the current will decrease.

2) In the short time period that the magnet is still passing through the coil, the current will not be zero. The current will continue to flow as long as the magnetic field is changing near the wire. However, the strength of the current may decrease as the magnet moves further away from the wire. This can be explained by Lenz's law, which states that the induced current creates a magnetic field that opposes the changing magnetic field that caused it. So as the magnet moves away, the induced current creates a magnetic field that opposes the magnet's field, causing the current to decrease.