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9. A 1200 turn coil of wire that is 3.00 cm in diameter is in a magnetic field that drops from 0.750 T to 0 T in 15.0 ms. The axis of the coil is parallel to the field. What is the induced emf in the coil?

10. Computer hard drives are made of aluminum platters coated with a layer of magnetic material. Each tiny magnetic domain represents one bit of information (1 or 0). A) How many such domains must a hard drive of 3 TB have (1 Byte = 8 bits)? B) A typical drive has a round platter of 9.5 cm in diameter with a hole of 2.5 cm diameter at the center. Assuming the domain are squares, what is the length of their sides in such a drive?

11. In the week 7 lab, we used the e/m device to observe the deflection of an electron beam by a uniform magnetic field from a pair of Helmholtz coils. We used a 200 V potential difference to accelerate the electrons such that it moved in a circular path with a 4.00 cm radius in the magnetic field. a. Calculate the magnetic field strength supplied by the coils. b. If you change the accelerating potential to 125 V, what will be the radius of the electron beams circular path?

12. The human body is not as conductive as metals. However, it does conduct electricity. If the body of a patient in a MRI machine experiences too sudden a change in the magnetic flux, the induced current can be significant and harmful. Suppose the cross-sectional area of a human body is 0.070 m2. What is the smallest time interval in which a 5.00 T magnetic field can be switched on and off with the induced emf in the patients body not exceeding 0.250V?

13. The two important constants we used in discussing electric field (0) and magnetic field (0) represent a very important constant in the form of 1/()1/2. Find this constants value and unit. Do you recognize the constant?

Explanation / Answer

9)E = Change Flux / Time induced EMF

Flux = B * A = (.75 - 0) * pi * N * (.03 / 2) ^2 where N = 1200

Flux = .636

E = .636 / .015 = 42.4 V

10)3 Tera Bytes = 3 X 10^12 Bytes

Multiply by 8 for the bits, that is 2.4 X 10^13 bits, which menas we need that many domains

We need 2.4 X 10^13 domains

The total area on the disk is outer area minus inner area

pi(.0475)^2 - pi(.0125)^2 = 6.60 X 10^-3 m^3

The area per domain is 6.60 X 10^-3 / 2.4 X 10^13 = 2.75 X 10^-16 m^2

For the length of the square, take the square root of 2.75 X 10^-16 m and get...

1.66 X 10^-8 m ( which is 16.9 nm)

11)(B :)
  

(R= Radius)
  

(magnetic field strength infty I/R)

  
(magnetic field strength infty V/R)
  

  
(for 100 V )
  

  
(radius = 4m)
  

  
( for 125 V )
  

  
(radius = 125*4/100)
  

  
(radius = 5 cm)

12)EMF=dflux/dt

flux=B.A

so time be t.so,

0.25=5*0.07/t

or t=1.4 s

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