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The top of the atmosphere is at about 400 kV with respect to the surface of the earth, corresponding to an electric field that decreases with altitude. Near the surface, the field is about $100V{{m}^{-1}}$. Why then do we not get an electric shock as we step out our house into the open?

Answer
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Hint:For an electric current to pass through a material, the material must be a good conductor of electricity. If a potential difference is produced across a bad conductor, then there will be no current passing through it.

Complete step by step answer:
Let us first understand when we get an electric shock.

When we say that we got an electric shock, it means that some electric current has passed through us. Electric current possesses energy, we feel this energy in the form of shock.
In other words, our body acts as a conductor, which allows the electric current to pass through it.

However, we know that a current passes through a material (conductor) only when there is
potential difference across the material.

The current always flows from a point of higher potential to a point of lower potential.
In the case mentioned in the question, it is said that there is some potential difference between the top layer of the atmosphere and the ground. As a result, there must be a current passing through the atmosphere (from the atmosphere to the ground) and if we come in the path of the current, it will pass through us and we will get a shock. However, we know that this does not happen.

The reason for this is the atmosphere is not a good conductor of electricity. Since it is not a good conductor of electricity, it offers a very high resistance and even in presence of potential difference there is no current.

Note:During the rainy season, due to the black clouds the top layer of the atmosphere is at a very large positive potential. The electric field between the atmosphere and the ground is so strong that it ionises the air molecules and many positive and negative charges are produced.

As a result, the atmosphere becomes a conductor and electric current passes from the atmosphere to the ground, which we call as lighting.