Physics Times 07.2019

(Kiana) #1


E E j E sin k yˆ 0  x- t



(Ex = Ez = 0)

 B B k B sin k zˆ 0  x- tkˆ



(Bx = By = 0)

 The magnitudes of E



and B


are related by

0
0

E E
c or c
B B

 

Electromagnetic waves travel through vacuum with
the speed of light c, where


8
0 0

c =^1 = 3 x 10 m/s
μ

where  0 = permeability of free space
 0 = permittivity of free space

The Poynting vector S E H 


  
=
0

E×B
μ

 

represents the direction of energy flow per unit
area per sec along the direction of wave
propagation.


  1. Energy density


2 2
avg 0 0 0
0

1 1
u E B
2 2

  


  1. Intensity of Radiation


2
0 0
I^1 E c
2

  


  1. Momentum and Radiation Pressure
    The magnitude of the momentum delivered to a
    surface is


p U
c

 (Complete absorption )

where c = velocity of light
U = total energy

p^2 U
c

 (Complete reflection)

When radiation is incident on a surface, Radiation

pressure r

I
P
c

 (total absorption) and







  1. Energy density

  2. Intensity of Radiation

  3. Momentum and Radiation Pressure



2
r

I
P
c


(total reflection back along the incident path)

Magnetic Effects of Current


  1. A uniform magnetic field,

  2. An electron of mass m and charge q is travelling
    with a speed


B B j 0 ˆ exists in space.
A particle of mass m and charge q, is projected
towards x-axis with speed v, from a point ( , 0, 0)a.
The maximum value of v for which the particle
does not hit the yz- plane is

(a)

Bqa
m (b) 2

Bq
am (c) 2

Bqa
m (d)

Bq
am

v along a circular path a radius r at
right angles to a uniform of magnetic field B. If
speed of the electron is doubled and the magnetic

field is halved, then resulting path would have a
radius of

(a)
2

r
(b) 2r (c)
4

r
(d) 4r


  1. Two concentric circular loops of radii r 1 and r 2 carry
    clockwise and anticlockwise currents i 1 and i 2. If
    the centre is a null point, i 1 /i 2 must be equal to
    (a) r 2 /r 1 (b) r 22 /r 12 (c) r 12 /r 22 (d) r 1 /r 2

  2. A hollow cylinder having infinite length and
    carrying uniform current per unit length


Magnetic Effects of Current









3.

4.
 along
the circumference. Magnetic field inside the
cylinder is
(a)   0 (b)^2   0

(c)^0
2

 
(d) zero
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