March Guass law

By drawing electric field line, we observe the relationship between electric flux and the enclosed surface.

We draw some electric field line, and we count how many electric line pass through the surface as our electric flux. Based on this, we will observe electric flux deeper.

The top of the equipment is given charge. 2 paper are lifted in side and outside the can. After we start the equipment, the 2 paper reject each other cause there is same type charge on them.

We write the formula to show the relationship between charge, flux.

This is our prediction about the experiment. After watch the experiment, we write our explanation for why it works like this.

We perform a question to use calculus to solve for electric flux.

We put a CD in the microwave, and it begins to burn strongly. Since microwave gives radix to the CD, so that there is current that cause CD burning.

We can see the broken line on the CD, that is where the current flows when it is in the microwave burning.

We perform calculation to determine the electric flux inside a cylinder. Together with find flux outside this cylinder at a certain position, we will find how useful guass law is.

Summarize: eventhough guass law is just a simple formula: Flux = Q/e0, however, it is the most useful formula when we solve for electric flux. 

May 15 1015

In left and right side of a wire that has current pass through, the direction of magnetic field is opposite. We can find the direction by righ hand rule. At this point, we draw the magnetic force applied on the wire to determine their motion.

We perform the experiment to prove our prediction is correct. They attract each other, which matches our prediction.

Later, we write the expression of magnetic on each wire due to the magnetic field cause by the other wire.

We discussed about magnetic field produced by an AC current. Since the direction is changing all the time, so the magnetic field is reversing all the time. As a result, there is no magnetic field in an AC current.

An experiment to record magnetic produced by the current. There is a new equipment that is applied here to detect the magnetic field. We graph he magnetic field respect to time axis, and we got a trig function graph.

In our group we perform same experiment to observe the relationship between number of loops and magnetic field. We predict that the more loops we have, the larger magnetic field at that poiont.

When we record data by logo pro and graph it, the graph generally matches our prediction. after 3 loops, the graph is more linear. We believe that for loop 1 and loop 2 the graph is less linear because we didn't wait for the magnetic go to completely stable to record it.

We draw the magnetic flux through the closed surface. This practice tells us that when we do the cross product, the area vector is vector normal to the surface, not the surface itself.

Not only electric can produce magnet, magnet can also produce electric. This experiment shows that a closed loop moving in a magnetic field causing its flux will produce electric.

Since both magnetic and electric can produce each other. When we put steal ring in it, the motion of the steal in the magnetic whhich produced by the current cause current in the steal ring, and it also produce magnetic field, which cause the motion of the steal ring. If we put some other ring that is non conductive or it is not complete that can't form a closed surface, nothing will happen.

One is Al and the other is glass. if we drop a nonconductive cylinder in the al and magnet cylinder in the glass, they will fall at the same time due to free fall. What is more, they didn't produce current and magnetic field.

This time, the magnet cylinder drop from the al tune is much slower. cause during its fall, it caused flux change which produce current, and current produce magnetic field. Due to the magnetic force, it falls much slower

In this graph, we draw the magnetic and force applied in each tunnel, and we write the flux change in the al tunnel. The reason that magnetic produce electric, and caused emf is when the flux change in the system.

We draw the graph of emf and magnetic field change respect to time. they are both trig function. The reason that the graph looke like this is beacuse during the motion, angle between field and surface changes. Since we applied dot product to determine flux, the change of flux also contains trig function.

Summarize: Magnetic can produce electric due to the change of flux in the field and motion of the wire loop. The emf caused by this is change during the spining of the loop. Electric can also produce magnetic field. Thus, when we do problems that contains both magnetic and electric, we need to consisder both the magnetic force and emf due to their relationship.

Apr 16 2015

Along an infinite lane, we calculate the electric field along  each distribution, and we add all distribution up to calculate electric field along a infinite long line.

We use calculus to calculate the electric field of a point respect to ring. The method and logic is same as we did for a infinite long line.

This is the detail about how we set up and approach to the problem. It doesn't matter how the point around the ring change, the key is to find the correct range of your distance distribution, and we need to care about if the angle will change.

We perform a calculation of electric field at a point along a bar. Calculation of a bar is much easier that rings. These types of question show the importance of math skills in solving physic problem.

We draw three types of equal potential surface in three different types of electric field. Constant electric field, single charge and charge system.

In this carbon paper, we measure the distance and potential difference. By moving a distance away, we got new potential difference each time. We will need this distance and potential difference to calculate the work done from the electric field.

This is the data we collect, and we used those data calculate the work need to be done to move 1C charge by a certain distance in the electric field. This method is useful if the electric field is not constant.

Summarize: When we deal with electric that is not caused by a single charge, we need to use calculus to perform the determination. Also, when we deal with work done in a non constant electric field, we cannt just use w = Eqd, but applying potential difference: W = Vdq

May 12 2015

We list some method to destroy a magnet. 

We draw the force diagram for a loop in a magnetic field, and we also find its net torque. We derive a formula for the net torque of a loop, and it is related to current, area vector and magnetic field. Also, it is a cross product.

We list some method to increase the magnetic force for a loop. Instead of increase current and magnetic field, we can increase the number of loops.

We are given a small electromotor. By changing current and position of pole(changing direction of magnetic field), we observe the change of motion, such as spin direction and the speed it spins.

We also make our own electromotor. If we increase the voltage, it will spin faster. later we use another loop which has more number of loops, and it also spins faster.

This lab shows us that current can produce magnetic field. It is intially turned off, after we turned it on, the campass will spin.

This is our prediction that we make for the direction those campass pointing to after we turned it on.

After we turned it on, the direction for the campass pointing to forms a circle, which doesnt match our prediction.

At this time, we use small campass to detect the magnetic field at some positions around a circuit. We have a conclusion that for a wire, we can add magnetic up to get total magnetic. Different direction of current will cause opposite magnetic, that is the reason we have 0 magnetic field at some point.

For a wire, we are introduced a new formula for calculate the change of magnetic field based on length vector, current and position vector. Again, it is a cross product, thus we have to do it in 3D

Summarize: Wire that has current through in it will be applied magnetic force if it is in a magnetic field. Based on this truth, we develop electromotor. Also, electric can cause magnetic field which has been proved by the spining of campass when we make the wire has current. 

May 7 2015 first day of magnetic

We put a small compass around a magnet to figure the direction of the magnetic field at that point. By defination, the direction of a magnet is from north pole point to south pole. 

we drawn 3 closed surface around the magnet, and then we calculate the net magnetic flux through the closed surface. We reach a conclusion that the net magnetic flux through a closed surface around a magnet is always 0.

Putting a large magnet around the wire electron gun. We can observe a clear change on the screen. It tells us that magnetic flux can influence electron, and magnetic and electric are not independent.

We draw those diagram to show how the charge will influenced by the magnetic field. In general, the force apply on the charge due to its motion in the magnetic field is the cross product of qv and magnetic field.

This experiment shows that there is also force apply to a wire has current from magnetic field, and the direction is depended on the magnetic field direction and current direction.

We perform a simple question apply formula F = qvbsin(a) to solve for the charge of a moving particle in a magnetic field.

This graph shows the moving particle in magnetic field in 3D. The motion of the particle is more like a circular motion. Also, we can apply formula from circulation motion. Magnetic force is always perpendicular to its velocity, thus, it plays a role of centripetal force.  

We solve a simple question for the magnetic given its velocity. As well as F = qvBsin(a), we used Fc = mv^2/R to solve the question.

Similiar to moving particle, the force on the wire has current pass throught is also a product of length vector, current and magnetic field. Above is the formula we derived.

Since there is force from magnetic field apply on the wire, if we make it a loop, it gonna spin, and during its spining, the torque changes.

We draw a diagram to show the netic force and net torque on a loop. Based on this, we believe that the loop will spin up and stop.

Now we are dealing with non-linear wires, thus, we applied calculus. The force is integral of IdL cross B(magnetic field)

Summarize: The first thing for today is we now that magnetic and electric have relationship, and this relationship is shown by the force that wire and moving particle are applied in magnetic field. The magnetic force is a cross product, thus, right hand rule is applied.