# Homework Help: Magnetic bar rotates rather than oscillates when magnetic fi

1. Oct 7, 2015

### Mddrill

1. The problem statement, all variables and given/known data
In my lab experiment we suspended a magnetic bar in between to Helmholtz coils and pointed it north. it then began to oscillate because of earths magnetic field. Then we pointed the apparatus south and when we applied 1 V to the Helmholtz coils the magnetic bar began to spin rather than oscillates

Does anyone know why this is?
2. Relevant equations
$τ= -mBθ$
Tau is torque of the bar, m is magnetic moment, B is magnetic field, Theta is angle of oscillation

$$1/T^2 = CB_{tot} = C[B_h +(0.72Nμ_0I_c)/R]$$

T is period of oscillation, C is a constant which is unknown, B_tot is magnetic field of earth plus magnetic field of the Helmholtz coils, N is the number of turns of the coils (200), mu sub zero is the constant 4π*10^-7 T*m/A I sub c is the current through the coil, and R is the radius of the coils (10.5 cm)

I don't know if these equations are relevant or not.
3. The attempt at a solution
I honestly have no idea

Thank You

2. Oct 7, 2015

### Jeff Rosenbury

This seems to be an harmonic oscillator problem with a twist (literally). Rather than a linear system (linear as in up/down, not the differential equation which is linear for both systems and pretty much the same equation) this one works with angular momentum.

You might consider where your damping force comes from. Also remember spinning is a form of oscillation.

3. Oct 8, 2015

### Jeff Rosenbury

Another hint: What is the impulse response to a magnetic field impinging on a non-perfect conductor?

4. Oct 8, 2015

### Mddrill

Sorry but I have no idea what either of your posts mean. We havnt gotten to learning about magnets in the lecture class, but were doing it in the lab.

I don't know anything about magnets and I cant really figure this out, even with a hint, because I know as much about magnets as someone who has never taken physics in their life.

5. Oct 8, 2015

### Jeff Rosenbury

When a magnetic field impinges on a conductor it sets up electric currents which try to exactly counter the impinging fields. This actually happens for perfect conductors, but happens slower and with losses with poorer conductors.

Because of this when you initially turn the magnet on, the field will seem stronger than it is a few microseconds later. Thus there will be more initial force on the magnet to start it turning than there is to slow it and turn it around. Once the magnet starts spinning a full circle the restoring force starts pulling equally in both directions (divided in time). Thus the magnet spins.

Eventually the eddy currents formed by the impinging magnetic field should slow the spinning (magnetic braking). But this works better at higher speeds, so it could take a while and the friction of your axial support may dominate.