An electric field mill measures
Answer: C
A rotating shutter modulates the field at a sensing plate to give a measurable signal. Field mills are used to warn of imminent lightning risk at launch sites and airfields. They report conditions rather than completed flashes.
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Assertion (A): Field mills are used to issue lightning warnings before any flash occurs.
Reason (R): They detect the rising surface field as charge accumulates overhead.
Answer: D
Both are true and the reason gives the anticipation, and this is why they are installed at rocket launch facilities. Detection networks by contrast report only flashes that have already happened. Anticipation is what makes the field mill valuable.
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The potential of the electrosphere (ionosphere) is positive with respect the Earth and its magnitude is about :
MH SET - 2018, SET Exam - 2018,
Answer: C
The ionosphere's potential relative to the earth is about 300 kV.
In the global atmospheric electric circuit, thunderstorms act as generators that charge the upper atmosphere (the electrosphere) positively with respect to the ground. The resulting potential difference is about 250-300 kV, which drives a small fair-weather current downwards and produces the surface electric field of about 100 V/m.
1000 kV and 2000 kV are several times too large, and 10 kV is far too small for this ionospheric potential.
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................................ maintains the ionospheric potential (Vi) of approximately ~ + 300 kV with respect to the Earth surface.
MH SET - 2020, SET Exam - 2020,
Answer: C
The ionospheric potential is maintained by thunderstorm and lightning activity.
In the global electric circuit, the roughly 2,000 thunderstorms active at any moment act as generators. They carry positive charge upward to the ionosphere and negative charge to the ground (largely through lightning), keeping the ionosphere at about +250 to +300 kV relative to the Earth. This drives a weak fair-weather current downward everywhere else, which would otherwise discharge the system within minutes.
Monsoon rain, cyclones and anticyclones, and earthquakes and tsunamis do not act as charge separators on the global scale needed to sustain this potential.
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The total electrical resistance for a unit area of the Atmospheric column from the earth surface to the ionosphere is called :
MH SET - 2021, SET Exam - 2021,
Answer: A
It is called the columnar resistance.
In the global atmospheric electric circuit, the resistance of a vertical column of air of unit cross-section from the ground to the ionosphere is the columnar resistance, of the order of 1017 ohm square metres. With the ionospheric potential of about 250 kV it fixes the fair-weather air-earth current density.
Columnar potential and field are not names for a resistance, and conductivity is the reciprocal quantity: the atmosphere's conductivity increases with height, so most of the columnar resistance lies in the lowest few kilometres.
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The electric potential of the electrosphere is positive with respect to Earth and its magnitude is about :
MH SET - 2023, SET Exam - 2023,
Answer: C
The potential is about 300 kV.
In the global atmospheric electric circuit, thunderstorms act as generators that charge the upper atmosphere (the electrosphere, at the base of the ionosphere) positively relative to the ground. Measurements put this potential difference at roughly 250-300 kV, and fair-weather current leaks back down through the conducting air.
30 kV is an order of magnitude too low, and 1000 kV and 3000 kV are several times too high.
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At any given time, there are an estimated ........... thunderstorms in progress over the globe.
MH SET - 2025, SET Exam - 2025,
Answer: D
An estimated 2,000 thunderstorms are in progress at any given time.
The widely quoted estimate, based on early global surveys, is about 1,800-2,000 thunderstorms active at any moment, adding up to some 16 million a year. They are concentrated over tropical land, and together they produce lightning about 40-50 times per second worldwide and maintain the global electric circuit.
50,000, 30,000 and 20,000 are far too high for storms active at one moment; figures of that order describe the number of thunderstorms over a day or so, not at any given instant.
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