Physics (Part-ix) (Railway)

According to Ohm's law, the current, I, flowing through a circuit is directly proportional to the voltage, V, and inversely proportional to the resistance, R.

• The Ohm's law can be expressed as I = V/R, where I is current in ampere, V is potential difference in volt and R is resistance in ohm.

For a current-carrying wire, force is felt in the presence of an external magnetic, field. The magnetic force on a current in a magnetic field is expressed as F=BILsinθ, where F is the force, B'Is the flux density, L is the current. L is the length of the conductor, and 9 is the angle that current makes with the magnetic field.

• The force exerted on a current-carrying wire placed in a magnetic field is zero when the angle between the wire and the direction of magnetic field is either 0° or 180 degrees.

• This is because the value of sin 180 degrees is 0.

The total resistance of a Parallel Circuit is NOT equal to the sum of the resistors (like in a series circuit).

• The total resistance in a parallel circuit is always less than any of the branch resistances.

• Adding more parallel resistances to the paths causes the total resistance in the circuit to decrease.

When light passes from a less dense to a more dense substance, (for example passing from air into water), the light is refracted (or bent) towards the normal.

• The normal is a line perpendicular (forming a 90-degree angle) to the boundary between the two substances.

• The bending occurs because light travels more slowly in a denser medium.

The net resistance of the resistors connected in parallel is given as:

1/Rₚ = 1/R₁ + 1/R₂

Or Rₚ = R₁R₂/R₁ + R₂ = 20 ............................(i)

When two resistors connected in series, the total resistance' is given as:

Rs = R₁ + R₂ = 80 ...........................(ii)

From equations (i) and (ii),

R₁R₂/8 = 2 or, R₁R₂ = 16, or R₂ = 16/R₁

Now, since: R1 = R2 = 8

Or R₁ + 16/R₁  =8

Or (R₁)₂ + 16 = 8R₁

Or (R₁)₂ - 8R₁  + 16 = 0

Or R₁ - 4R₁ - 4R₁ + 16 = 0

Or (R₁ -4) (R₁ -4) = 0

So, R₁ = 40; R₂  = 40

When an object is at infinity, then the rays coming from it is almost parallel to principal axis.

• Hence the image formed is at focus.

• The image formed is highly diminished, real and inverted,

The magnetic field produced by a current-carrying wire at a given point depends directly on the current passing through it.

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

• This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.

Bending of light is inversely proportional to the wave length of the light.

• Higher wavelengths bend less, whereas shorter wavelength bend more.

• Out of all violet has the least wavelength and hence it bends the most and red has the highest wavelength, so it bends the least.

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror.

• This point is called the principal focus of the concave mirгог.

• When a parallel beam of light not parallel to the principal Axis strikes the mirror, then they appear to meet at the for cal plane.

E = VIT

= 12 x 3 x 0.25h

= 9 x 10-³ kwh

= 0.009 kwh