# What is Global Warming? Physics | Grade XII

## 1. What is global warming? Give two examples of its effect.

Ans: Global warming is the increase in the average temperature of atmosphere near the earth surface and oceans. The main cause of global warming is the burning of fossil fuels such as coal, oil and natural gas which releases carbondioxide and other substances known as green house gases. Due to such gases, heat provided by the sun cannot escape from the earth’s atmosphere. As a result, the temperature o the earth increases.

Two examples of its effect are as follows:

i. Global warming evaporates terrestrial water and leads to shortage of drinking water.

ii. It reduces crop yields by reducing the soil moisture.

## 2. Write down expressions of acceleration of a moving charge Q in parallel and perpendicular magnetic fields.

Ans: The magnetic force on a charged particle is given by

F = BQv sinθ

When the charge Q is moving in parallel to the magnetic field, then

θ=0°

∴ F=0

As the charge does not experience force, it’s acceleration, a = 0

When the charge Q is moving perpendicular to the magnetic field, then θ = 90º.

∴ F = BQv

or, ma = BQv

∴ a = BQv/m

This is the required expression of acceleration.

## 3. What are the characteristic elements associated with a diffraction grating? How is plane transmission grating constructed?

Ans: A diffraction grating is a large number of close equidistant parallel lines (slits) ruled on a optically plane glass plate or metal plate. The ruled lines act like opaque wires and thus obstruct the passage of light through them. These are called opticities. Light passes through the spaces in between the lines. These are called transparencies. A series of equidistant opacities and transparencies are the characteristic elements associated with a diffraction grating.

A plane transmission grating is constructed by ruling a large number of fine, equidistant and parallel lines with a diamond point on an optically plane glass plate, the ruled widths are opaque to the light, while the space between any two lines is transparent.

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