6.5 Measuring Electric and Magnetic Fields
195
Humans seem to be rather insensitive even to very large magnetic fields
(thousands of times greater than the Earth’s field). Unless, of course, you have a
steel needle buried in you, or surgically implanted ferric metal.
6.5.8 Static Field Dosimetry
Being critical to the functioning of life systems, there should be no surprise that life
systems can be easily disturbed, harmed, and even killed by strong fixed electric
fields. The direct effects of strong fixed magnetic fields can be far milder. 12
A general rule: Neither voltage nor current kills; but their product over time does.
After all, an electric voltage gives the electric pressure, which does not determine
whether that pressure can harm. Some people are scared by hearing how high a
voltage is. A very high voltage, with very little power, is harmless. It is voltage
times current that determines the power delivered. One can pull 20 extra electrons
from some material, squeeze them tightly onto a conductor to a million volts of
electric pressure and store them there. Even if all those electrons with a million
volts are released toward a person, that person will probably feel nothing. If those
electrons were directed toward a single cell, the cell is likely to survive, perhaps
sustaining damage to a few molecules on its surface. If, instead, 20 Coulomb of
charge (1.25 × 10 20 electrons) is stored at a million volts, then directed toward
a person as a lightning bolt, not only is that person likely to die, but the energy
released is 20 million J, enough to completely vaporize an adult. An analogy with
water pressure and water flow is useful: A hypodermic needle can eject water under
high pressure, but with little volume of water flowing, and harm no one. In contrast,
an open fire hose, with the same water pressure, can knock a person to kingdom
come.
The student volunteers who hold onto the ball of a van der Graaff electrostatic
generator and have their hair stand on end (due to electric repulsion) with a voltage
generated of several hundred thousand volts relative to the ground are not likely to
be damaged, physically. This follows from the facts that there is only a small total
charge and that the electrons, under high pressure, move to the surface of the body,
discharging from there, rather than forming a current through the interior during
discharge.
Because skin and underlying tissue have a relatively high resistance (hundreds
of thousands of ohms from one hand to the other), no damaging current will flow
when the electric pressure is a few volts. Touching the terminals of a 9 V battery to
the tongue is mildly painful. However, with a 100 V sustained by a power company
and put across your two hands, the current becomes on the order of a milliamp,
depositing perhaps a quarter of a millijoule per centimeter per second. This is
12 Dosimetry of electromagnetic waves is also an important topic, but will be postponed until the
character of such waves is elucidated in Chap. 7.
195
Humans seem to be rather insensitive even to very large magnetic fields
(thousands of times greater than the Earth’s field). Unless, of course, you have a
steel needle buried in you, or surgically implanted ferric metal.
6.5.8 Static Field Dosimetry
Being critical to the functioning of life systems, there should be no surprise that life
systems can be easily disturbed, harmed, and even killed by strong fixed electric
fields. The direct effects of strong fixed magnetic fields can be far milder. 12
A general rule: Neither voltage nor current kills; but their product over time does.
After all, an electric voltage gives the electric pressure, which does not determine
whether that pressure can harm. Some people are scared by hearing how high a
voltage is. A very high voltage, with very little power, is harmless. It is voltage
times current that determines the power delivered. One can pull 20 extra electrons
from some material, squeeze them tightly onto a conductor to a million volts of
electric pressure and store them there. Even if all those electrons with a million
volts are released toward a person, that person will probably feel nothing. If those
electrons were directed toward a single cell, the cell is likely to survive, perhaps
sustaining damage to a few molecules on its surface. If, instead, 20 Coulomb of
charge (1.25 × 10 20 electrons) is stored at a million volts, then directed toward
a person as a lightning bolt, not only is that person likely to die, but the energy
released is 20 million J, enough to completely vaporize an adult. An analogy with
water pressure and water flow is useful: A hypodermic needle can eject water under
high pressure, but with little volume of water flowing, and harm no one. In contrast,
an open fire hose, with the same water pressure, can knock a person to kingdom
come.
The student volunteers who hold onto the ball of a van der Graaff electrostatic
generator and have their hair stand on end (due to electric repulsion) with a voltage
generated of several hundred thousand volts relative to the ground are not likely to
be damaged, physically. This follows from the facts that there is only a small total
charge and that the electrons, under high pressure, move to the surface of the body,
discharging from there, rather than forming a current through the interior during
discharge.
Because skin and underlying tissue have a relatively high resistance (hundreds
of thousands of ohms from one hand to the other), no damaging current will flow
when the electric pressure is a few volts. Touching the terminals of a 9 V battery to
the tongue is mildly painful. However, with a 100 V sustained by a power company
and put across your two hands, the current becomes on the order of a milliamp,
depositing perhaps a quarter of a millijoule per centimeter per second. This is
12 Dosimetry of electromagnetic waves is also an important topic, but will be postponed until the
character of such waves is elucidated in Chap. 7.