Magnetic Field

Purpose:  We take a first look at how magnets behave and how the magnetic field lines look like. We will take a look at some of the fundamental principles of magnets and their relationships with moving charges. It will turn out that these guidelines will mirror what we saw in static electricity.

The Magnetic Field Lines Of A Magnet

Professor Mason shows us on a projector the behavior of the magnetic field lines on a magnet. This is done by using iron shavings. The iron shavings react to the magnetic field and basically give a visual representation of the magnetic field in 2-D. We see that the magnetic field, in a way, circles the magnet.

Our group makes an attempt on how these shavings look using arrows. This proves not to be a good representation for the magnetic field.

We alter the lines of the previous picture to give a better idea of how the magnetic field lines go on the magnet. The field lines actually create circular loops that go through the magnet. If the field lines go in one end of the magnet, they go out the other end. We call these ends, poles. One end is the North pole and the other end the South pole.

What Occurs When A Magnet Is Cut Into Two Pieces

When a magnet is cut into two pieces, we see that we do not create monopoles. Instead, the cut pieces because new magnets we north and south poles. Professor Mason demonstrates this by cutting a magnet.

Using Gauss' Law To Interpret Magnetism

We take a look at the idea used in Gauss' Law for drawing field lines to find the flux of the magnetic field. We see that the amount of lines that go out go back in. This means that the flux is always equal to zero.

Effects Of A Magnet On An Oscilloscope

Professor Mason demonstrates the effects a magnet has on an oscilloscope. We see the dot shift. The dot shifts according to the definition of the magnetic force, which is basically the cross product between the velocity the charge moves with the electric field.

We draw on our white board what occurs from a side view inside the oscilloscope. The horizontal line indicates the velocity of the beam. If we put a magnetic field across the beam, we can see that the force vector is perpendicular to the velocity vector and the magnetic field vector.

As an activity, we calculate the acceleration along the force axis. The cross product give us the force, and Newton's Second Law helps us attain the acceleration.

Motion Of A Moving Charge Under A Magnetic Field

Our board shows that if we have a charge moving at some velocity in a direction perpendicular to the magnetic field, the charge goes in a circular motion, as the force and the velocity continually changes.

Magnetic Force Along a Current

Professor Mason demonstrates that when we have a current running through a wire and this wires is exposed to some magnetic field, it will experience a force perpendicular to the length of the wire and the magnetic field.

After seeing the wire with current experience a force, we rewrite the equation for the force cause by the magnetic field in a way that describes a current carrying wire. The definition comes from the combination of the drift velocity and the number of charges contained within the wire. We run into the the definition for the current which replaces most of the variables that describe the wire.

Magnetic Force On A Circular Current

We see that if we have a circular current exposed to a magnetic field, the loop experiences a force that cause a torque. The loop does not continue to spin because after the loop rotates 90 degrees, the torque immediately goes the opposite direction. This cause the loops to spin 90 degrees and then stop.

The white board shows our prediction of what would happen to the loop when it experiences a magnetic field. We were right. We knew that there would be a counter torque, and while it is true that there is some angular momentum on the loop, the momentum is not large enough to keep the loop spinning.

Force On a Semicircular Loop

On excel, we are given the task to find the force that the loop experiences. We decide to approach the problem from a length of theta and calculate these individual forces. We know that the loop experiences the maximum force at the very top, as the vector length is perpendicular to the magnetic field. It has no force at the ends because the vector length is parallel to the magnetic field.

Calculating the different forces at different thetas gave us many forces in which we add to give us the net force that the loop experiences from the magnetic field.

Conclusion:  We discuss how magnetic fields behave and that there are no monopoles in existence. We also talk about how the movement of the charge and the direction of the magnetic field causes a force to be applied on the charge. This mirrors the ideas used in static electricity, where the force was dependent of a motionless charge and the electric field. We were also able to make a relationship for current and the force on the current. This idea is based on the fact that a current consists of many moving charges along a wire. Using the force of magnetism, we are able to apply the force equation to things that involve torque, which is fundamental to electric motors.