Electromagnetic waves are produced when
a) an electric charge vibrates or accelerates
b) there is exertion of magnetic forces on charged particles
c) there is alignment of an electric field and a magnetic field
d) a magnetic change occurs in a medium

The correct answer is a.

Explanation:

Electromagnetic wave is a wave in which the magnetic field and the electric field are perpendicular to each other.

Both fields are directed in the direction of the propagation of the electromagnetic wave. It travels with the speed of the light.

The motion of the electric field produces both electric field and the magnetic field.

The electromagnetic waves are produced when an electric charge vibrates or accelerates.

Therefore, the correct option is (a).

explanation:

earth's surface is mainly heated by the absorption of infrared radiation from the sun.

evaporation occurs because molecules in liquid water are warmed ( usually by the sun) and vibrate quicker, becoming less dense and rise.

condensation occurs because molecules in water vapor are cooled and therefore vibrate more slowly thus becoming more dense and eventually fall to earth.

lines of equal barometric pressure on a weather map are called: isobars : lines of equal temp on a weather map are called: isotherms.​

answer: 802.3 kilo joules of energy represents 191.75 kilo calorie of energy.

explanation:

energy released during combustion of one mole methane = 802.3 kilo joules

if 1 kilo joule = 0.239006   kilo calorie

then ,802.3 kilo joule will be = 802.30.239006 kilo calorie

802.3 kilo joule = 191.75 kilo calorie

hence, the energy in kilo calorie is 191.75 kilo calorie.