EM & friction problems

Question

1what is the relationship between electricity and magnetism? Do they occur
simultaneously?

2What is the effect of self-induction towards its own coils?(regarding its voltage and
current) What happen when mutual induction occurs?

3How to determine the rate of change of magnetic field in electromagnetic induction

4What effects do frictions have in a rolling motion .e.g. driving car.

Plz elaborate the answers by your own words

Answer 1

1. There is a saying that [電動生磁，磁動生電。]
When charges move (i.e. a cuurent), magnetic field is produced (Ampere's law).
When there is a change of magnetic field, (induced) current is produced. (Faraday's law).

2. Both self induction and mutual induction are due to induced emf caused by a change of magnetic flux. According to Lenz' Law, the induced emf always opposes the original current in the coil.

3. It can be done by a small coil connected to a sensitive voltmeter. By placing the coil in the changing magnetic filed, the induced emf E, using Faraday's Law, is:
E = -d(NAB)/dt
where N is the no. of turns in the coil, A is the area of the coil and B is the magnetic flux density

thus E = - NAdB/dt (since both N and A are constant)
i.e. dB/dt = -E/(NA)
from which dB/dt, the rate of change of magnetic flux can be found.

4. Friction prevents the rotating wheels from slipping, hence provides a forward reaction force to drive the car.