6.5 Measuring Electric and Magnetic Fields
193
runaway effect, because the effective ‘Bethe drag’ 10 on relativistic electrons is less
than on slower ones. Measured X-ray and gamma-ray emissions support this idea.
A lightning strike can carry 100,000 A and release billions of joules of thermal
energy in a few microseconds. 11 The energy comes from a portion of the work done
by the hot air rising into the cooler upper troposphere, pulling charges apart all the
while. The rising moist hot air releases heat by cooling, but more significantly, the
moist air releases additional heat from the condensation into water droplets and ice.
In this way, the warmed air can push its way up to the edge of the stratosphere.
Lightning strikes to the Earth all over the globe give the Earth a negative charge
relative to the atmosphere. There are also discharges upward from the tops of
stratospheric thunderheads toward outer space into the ionosphere. Lightning strikes
between a cloud and the ground are typically 5 km long, first along a channel called
a plasma tube from cloud to ground, then by one or more return discharges, followed
by two or three down strokes. The rapid heating of the air (to about 40,000 ◦ C in a
tube about 4 cm in diameter) produces a sound shock wave, which we hear first as
a crackling sound of the leader discharges, and then as thunder. The sound wave
persists over seconds as it refracts and reflects off clouds and thermal air gradients.
Most of the lightning energy is release as heat and light, with only a few percent
into sound. Next to the strike, the sound level can be higher than 120 dB, causing
temporary deafness or even rupturing of the eardrum. Of course, more serious is the
passage of current through the body.
Lightning rods are pointed conductors which are attached to structures to either
slowly dissipate built up charge (via “Saint Elmo’s fire”), or, if necessary, quickly
discharge, when a lightning strike releases charge to the Earth along a good
conductor, such as a large diameter copper wire. (A point on a conductor is used
because the electric field at a point is larger than on a blunt shape. A larger electric
field will more likely ionize the air and allow charge to leak, thereby avoiding some
lightning strikes.)
6.5.4 Typical Static Electric and Magnetic Fields
Here are the sizes of the electric and magnetic fields we might encounter:
• Electric fields in homes emanating from wiring: A few volts per meter.
• Electric fields near power lines: A few volts per meter (220 VAC lines) to several
thousand (high voltage lines: up to 765 kV).
• Electric field of an electric eel shock: Up to 430 V/m, releasing about 2 J in 2 ms
(not expected to be lethal to a human).
10 As described in Sect. 8.13.3.
11 One ton of TNT releases 4.184 billion J.
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runaway effect, because the effective ‘Bethe drag’ 10 on relativistic electrons is less
than on slower ones. Measured X-ray and gamma-ray emissions support this idea.
A lightning strike can carry 100,000 A and release billions of joules of thermal
energy in a few microseconds. 11 The energy comes from a portion of the work done
by the hot air rising into the cooler upper troposphere, pulling charges apart all the
while. The rising moist hot air releases heat by cooling, but more significantly, the
moist air releases additional heat from the condensation into water droplets and ice.
In this way, the warmed air can push its way up to the edge of the stratosphere.
Lightning strikes to the Earth all over the globe give the Earth a negative charge
relative to the atmosphere. There are also discharges upward from the tops of
stratospheric thunderheads toward outer space into the ionosphere. Lightning strikes
between a cloud and the ground are typically 5 km long, first along a channel called
a plasma tube from cloud to ground, then by one or more return discharges, followed
by two or three down strokes. The rapid heating of the air (to about 40,000 ◦ C in a
tube about 4 cm in diameter) produces a sound shock wave, which we hear first as
a crackling sound of the leader discharges, and then as thunder. The sound wave
persists over seconds as it refracts and reflects off clouds and thermal air gradients.
Most of the lightning energy is release as heat and light, with only a few percent
into sound. Next to the strike, the sound level can be higher than 120 dB, causing
temporary deafness or even rupturing of the eardrum. Of course, more serious is the
passage of current through the body.
Lightning rods are pointed conductors which are attached to structures to either
slowly dissipate built up charge (via “Saint Elmo’s fire”), or, if necessary, quickly
discharge, when a lightning strike releases charge to the Earth along a good
conductor, such as a large diameter copper wire. (A point on a conductor is used
because the electric field at a point is larger than on a blunt shape. A larger electric
field will more likely ionize the air and allow charge to leak, thereby avoiding some
lightning strikes.)
6.5.4 Typical Static Electric and Magnetic Fields
Here are the sizes of the electric and magnetic fields we might encounter:
• Electric fields in homes emanating from wiring: A few volts per meter.
• Electric fields near power lines: A few volts per meter (220 VAC lines) to several
thousand (high voltage lines: up to 765 kV).
• Electric field of an electric eel shock: Up to 430 V/m, releasing about 2 J in 2 ms
(not expected to be lethal to a human).
10 As described in Sect. 8.13.3.
11 One ton of TNT releases 4.184 billion J.