Physics (Part-v) (Railway)

If a resistance is connected in parallel, then total resistance will always be less than the resistance which is connected in parallel as well as the resistance of the wire section.

• Resistance is inversely proportional to the cross-sectional area. So, increasing cross area, decreases resistance.

• So, increasing cross sectional area of the conductor is equivalent to connecting a number of resistors in parallel.

The focal length of convex lens or convex mirror is at the right-hand side of the optical centers, So it is taken as positive.

The magnification of a lens is defined as the ratio of the height of an image to the height of an object.

• It is also given in terms of Image distance and object distance. It is equal to the ratio of image distance to that of object distance.

• It is expressed as m = h'/h = v/u

• where, m= magnification, h'= height of the image, h = height of an object, v is the image distance, and u is the object distance.

The current produced during electromagnetic induction is called Induced current. The magnitude of induced current be increased in a coil by the following ways,

• As we increase the strength of the magnet, the magnetic field increases, and hence induced current increases.

• If we increase the rate of change of flux associated with coil, then iriduced current increases.

• If we take the conductor in the form of wire of many turns of insulated wire, then induced current Increases.

Image formed by convex mirror is always virtual, erect and diminished irrespective of object position.

• The focal length of a convex mirror is at the right-hand side of the optical centers, So it is taken as positive.

The magnitude of the magnetic field depends upon the number of turns in the current-carrying loop.

• Current in each circular turn has the same direction and the field due to each turn adds up.

• If there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn.

the physical conditions of the atmosphere keep changing

• The twinkling of a star gives its apparent position.

• Star Twinkling is due to atmospheric refraction of star light as starlight undergoes continuous refraction before it reaches the earth.

• Since the atmosphere bends the starlight towards normal, the apparent position of the star appears to be slightly different from its actual position.

• This apparent position of the star is hence not constant and predictable and keeps fluctuating.

The Fleming's left-hand rule gives the direction of force experienced by current carrying conductor placed in an external magnetic field.

• According to the Fleming's left-hand rule, when we place the thumb. the index finger and the middle finger mutually perpendicular to each other such that the middle finger points to the direction of flow of current and the index finger points to the direction of magnetic field then the thumb points to the direction of force acting on the conductor.

The concave lens is termed as 'diverging lens' as it diverges the light rays, while the convex lens is termed as 'converging lens' as it converges the light rays.

• The power can be calculated by using the formula: P = 1 / f (in metres)

• 50 cm = 0.5 metres; So, P = 1/0.5 = -2 dioptre

• The focal length of the concave lens is taken as negative; therefore, the Power is also negative for the concave lens.

By the Fleming is right-hand rule, if the right-hand thumb points in the direction of a current, the magnetic field at a point will be given by the direction of curl of fingers at that point.

Hence, the direction of the magnetic field lines as viewed-from the east end will be anticlockwise in a plane perpendicular to the wire.