Physics 4B Bli: lab21--5-14 (Faraday'sLaw and Lenz's Law)

The shape of the B vs. t graph is shown below for two complete cycles of magnetic field oscillations. apply Faraday's law to explain it.

Activity: The magnetic field in a loop

Turn on the current to measure the magnetic field change due to one

loop. Next measure the magnitude of the field in milli-teslas [mT] as you coil the

insulated wire into more loops. Use the ammeter to measure the current through

the wire. Record the measurements in logger pro

Conclusion: The magnetic filed quantity is proportional to the number of loops. The more loops, the larger the magnetic field.

B is proportional to NI

Next, we explore the magnetic flux in two situations;GIVEN surface area=ab

a: Magnetic flux through a surface between two parallel fields

b:Magnetic flux if a number of magnetic field lines that pass perpendicularly through a surface.

a: Flux=B.A=B.A.cos@=B.ab.cos(90)(normal vector of surface perpendicular to field)=0

b: Flux=B.ab.cos(0)=Bab

Prof Mason is demonstrating the magnetic field due to a certain amount of copper loops, as we can see, the magnitude of magnetic field is directly proportional to the number of loops, when the current and voltage are constant

Since we know B is proportional to NI, we can max current by 4 ways;

Next, we are introduced to Lenz's Law in the next two experiments;

a.)The bulb is lighten. as prof Mason place metal close

b.)Professor Mason added an aluminum coil, the silver coil levitated higher because the density was lower than the copper coil.The next one was a steel ring with a gap in between, which means that current can't flow, so nothing will happen.

c.)Place two metal inside the tube, suppose they will drop through the tube at the same time. when we use plastics, they drop at the same time; when we use alum they don't.

Question: Why the other one drop slowly?

Answer: The induced current in a loop causes additional magnetic field and flux in the area bounded by the loop.The direction is in the opposite of that of the original mag field so it tends to cancel the effects of that field.As the magnet was sliding down it generated a current moving counterclockwise in the aluminum that slowed the magnet down since there was a magnetic field going up as a consequence.

Whenever the plane of a coil of area A is perpendicular to the magnetic field vector, then the dot product can be dropped and the equation can be simplified to emf=-NA(dB/dt)

Consider a pickup coil consisting of a coil of wire with N loops of radius R. Suppose it is placed perpendicular to a uniform magnetic field B that varies sinusoidally with time with an angular frequency w so that

B = B0 sin w t

where B0 is a constant representing the maximum magnetic field at the site of the pickup coil.

Thus, we derive the flux and emf.