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  • Derive the equation relating the total charge Q that flows through a search coil (Conceptual Example 29.3 in Section 29.2) to the magnetic field B. The search coil has N turns, each with area A and the flux through the coil is decreased from its initial value to zero in a time Dt. The resistance of the coil is R and the total charge is Q=I Dt where I is the average current induced by the change in flux
from example 29 3
From Example 29.3
  • A search coil is a practical way to measure magnetic field strength. It uses a small, closely wound coil with N turns. The coil, of area A, is initially held so that its area vector A is aligned with a magnetic field with magnitude B. The coil is quickly pulled out of the field or rotated.
  • Initially the flux through the field is F=NBA. When it leaves the field or rotated, F goes to zero. As F decreases, there is a momentary induced current which is measured. The amount of current is proportional to the field strength.
  • EMF=-DF/Dt
    • DF=F2–F1
      • Where F1 =NBA and F2 =0
      • DF=-NBA
    • EMF=NBA/Dt
  • V=iR where V=EMF
    • EMF=NBA/Dt=iR
    • i=NBA/RDt
  • Q=iDt=NBA/R
part b
Part B
  • In a credit card reader, the magnetic strip on the back of the card is “swiped” past a coil within the reader. Explain using the ideas of the search coil how the reader can decode information stored in the pattern of magnetization in the strip.
  • The card reader contains a search coil.
  • The search coil produces high EMF and low EMF as the card is swiped.
  • A high EMF is treated as a binary 1 and a low EMF is treated as a binary 0
  • Ascii information (character codes between 1 and 64) can be stored there in a few bits (6-bits or 26).

A circular loop of flexible iron wire has an initial circumference of 165 cm but its circumference is decreasing at a rate of 12 cm/s due to a tangential pull on the wire. The loop is in a constant, uniform magnetic field oriented perpendicular to the plane of the loop and with a magnitude of 0.5 T

  • Find the EMF induced in the loop at the instant when 9 s have passed.
  • Find the direction of the current in the loop as viewed looking in the direction of the magnetic field.
emf df d t b d a d t
  • F=BA
    • dF/dt=BdA/dt
    • c=2pr so A=pr2
      • r=c/2p so A=c2/4p
    • dA/dt=d/dt(c2/4p)= (2c/4p)dc/dt
    • dF/dt= B(c/2p)dc/dt
      • Where B=0.5
      • dc/dt=0.12 m/s
      • At 9s, c=1.65-.12*(9s)=0.57 m
    • dF/dt=5.44 x 10-3 V
  • Since F is decreasing, the EMF is positive. Since EMF positive, point thumb along A and look at fingers.
    • They curl counterclockwise.

Suppose the loop in the figure below is

  • Rotated about the y-axis
  • Rotated about the x-axis
  • Rotated about an edge parallel to the z-axis.

What is the maximum induced EMF in each case if A=600 cm2, w=35 rad/s, and B=.45T?

case b rotating about the x axis
Case b—rotating about the x-axis
  • The normal to the surface is in the same direction as B during this rotation. Thus BA=constant
  • d/dt(constant)=0 so EMF=0

The figure below shows two parallel loops of wire having a common axis. The smaller loop (radius r) is above the larger loop (radius R) by a distance x>>R. Consequently, the magnetic field due to current i in the larger loop is nearly constant throughout the smaller loop. Suppose that x is increasing at a constant rate, v.

  • Determine the magnetic flux though area of the smaller loop as a function of x.
  • In the smaller loop find
    • The induced EMF
    • The direction of the induced current.





magnetic field of a circular loop
Magnetic Field of a Circular Loop

By the RH Rule, the field is upward in the small ring

direction of current
Direction of current
  • Assume that normal to smaller loop is positive upwards
  • B, then, is in positive direction
  • But F is decreasing or has a negative d dF /dt
  • A negative * negative=positive so EMF is +
  • RH Thumb in up direction, fingers curl counter clockwise!