Electromagnetic Waves — Physics STD 12 Science — Question

Two events take place simultaneously at points A and B as seen in the lab frame. They also occur simultaneously in a frame moving with respect to the lab in a direction:

1. Parallel to AB.
2. Perpendicular to AB.
3. Making an angle of 45^° with AB.
4. making an angle of 135^° with AB.

When a body is weighed on an ordinary balance we demand that the arm should be horizontal if the weights on the two pans are equal. Suppose equal weights are put on the two pans, the arm is kept at an angle with the horizontal and released. Is the torque of the two weights about the middle point (point of support) zero? Is the total torque zero? If so, why does the arm rotate and finally become horizontal?

A steel wire of cross-sectional area 0.5mm² is held between two fixed supports. If the wire is just taut at 20^°C, determine the tension when the temperature falls to 0^°C. Coefficient of linear expansion of steel is 1.2 × 10^-5 °C^-1 and its Young's modulus is 2.0 × 10¹¹N m^-2.

Light is incident normally on a small plane surface. If the surface is rotated by an angle of 30° about the incident light, does the illuminance of the surface increase, decrease or remain same? Does your answer change if the light did not fall normally on the surface? 

If the sum of all the forces acting on a body is zero, is it necessarily in equilibrium? If the sum of all the forces on a particle is zero, is it necessarily in equilibrium?

Name the dielectric whose molecules have:

1. Non-zero,
2. Zero dipole moment.

Among alpha, beta and gamma radiations, which get affected by electric field?

A fan with copper winding in its motor consumes less power as compared to an otherwise similar fan aluminium winding. Explain.