Physics Times – July 2019


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.

23. Energy density

2 2

avg 0 0 0

0

1 1

u E B

2 2

  



24. Intensity of Radiation

2

0 0

I^1 E c

2

  

25. 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











23. Energy density


24. Intensity of Radiation


25. 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

3. 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


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

Magnetic Effects of Current

1. 
   

2. 
   

3.

4.

 along

the circumference. Magnetic field inside the

cylinder is

(a)   0 (b)^2   0

(c)^0

2

 

(d) zero