5/7 Magnetic forces and fields

Lab Day 5/7/15 : Magnetic forces and fields

Purpose: We look at magnetic fields due to magnets and also due to moving charges. Today's experiments will analyze the magnetic force that comes from a moving charge and the magnetic field in which relates to the magnetic forces in electric circuits that have a flow of current.  

Magnet and Magnetic field

We began looking at magnetic fields around a magnet that has a north and south pole. The magnetic field direction is determined to go from the north to the south creating field lines similar to that of electric fields. The picture displayed shows the magnetic field lines that appeared by adding iron fillings.  By using a compass, we also determined the direction of the north pole by moving it around a magnet to find the direction of the magnetic field attractions and it was as expected pointing away from the north toward the south pole. The magnetic field lines represent the interactions of things magnetically where we find a magnetic equivalent to Gauss's law. 

Magnetic Field Lines using a Compass 

Next, we redefined electric flux into magnetic flux where we created surfaces on the magnetic field lines created by the magnet to find the net magnetic flux. Three different surfaces were defined: one that included one pole, one that include both poles, and one that include no poles. The results showed that the magnetic flux is equal to the magnetic field lines in and the magnetic field lines out. A net flux was calculated when there is one pole and no net flux was calculated when the surface contained two poles. Therefore, the magnetic flux using Gauss's law allowed us to identify the magnetic flux to be equal to zero.   

U-shaped magnet

We were able to look at the u-shaped magnet which showed that a magnet always consist of two poles where this is just a regular magnet bent. Even if we break it apart, the magnet still has a north and a south pole that creates magnet fields.

Next, we looked at the magnetic field on a moving charge using an oscilloscope. We found that an electric force is acting upon the particle shown on the screen in which the force will always be perpendicular to the moving charge particle and the magnetic field.

As a result we saw that as the north pole of the magnet is brought perpendicular to the charged particle, it moved to the left, and when we flipped the magnet to the opposite south pole, the particle moved in the opposite direction. When the magnet(north side) was brought from the right parallel to the charged particle it moved down and opposite when the poles were switched (south). This shows how the force acting on the particle is always perpendicular of the magnetic field caused by the magnet.

The units of magnetic field was found which is also known as Teslas.

By using the right hand rule, we learned to find the direction of the force by using the thumb as a force vector, index as a velocity vector, and middle as the magnetic field. The force was found using the cross product and Lorentz force as seen in the calculations in the picture below.

We are also able to use the centripetal force and the magnetic force to find the radius for our calculation of angular frequency seen below.

The results using angular frequency was used to calculate the magnetic field.

In this setup we have a magnet that interferes our circuit wire above and when we turn the switch on we observe the wire moving which shows that there is a force due to the magnet in the wire.

It is seen that the force is the cross product of the current times the length with the magnetic field. This is similar to when we are looking at a moving charged particle and its magnetic field. The right hand rule allows us to find the direction the wire would move by using the index finger to identify the current. 

Magnetic Force on a Current Loop

In our current loop, when the magnetic field is into the board, we see that there is no net torque and the loop will not move as the forces cancel out. when we have a magnetic field going left to right with a clockwise current the left side of the wire will have a force going in and the right side will have a force going out which causes it to rotate. The top and bottom has a force of zero because the current is parallel to the magnetic field. Based on this we can see that there would be a movement where our group predicted that the loop caused by the magnet would spin counterclockwise and continue spinning, however our predictions were wrong because the loop spun 90 degrees and then stopped. This was due to the the force going back at 90 degrees position because of torque.

Magnetic Forces on a Semicircuclar wire

We calculated the net force on a semicircular segment of current-carrying wire in a uniform magnetic field. This part showed that the magnetic field is going upward in the z axis. The equation we derived for the net force due to the magnetic field in the whiteboard below. 

We broke the wire into 15 segments to perform a numerical calculation of the net force using excel. The current, magnetic field, and the length was identified. The angle theta was changing in multiples of 12 of to the 15th segment.

F=IrBsintheta

Conclusion:

We were able to analyze magnetic fields due to magnets and found that a wire carrying a current can be affected by a magnetic field. When this happens, we can use the cross product and torque to find the magnetic force on a wire with a current and a charged particle. These two concepts are useful when looking at the effects of a magnetic field on wires and charges.