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6 Electric and Magnetic Fields in Life
the negative gradient of the electric potential: E = −∇V . For example, an active
pump carrying a sodium ion across a cell membrane (with a typical thickness of
10 nm) against an electric field of 0.07 V per 10 nm expends 0.07 eV of energy.
(The hydrolysis of ATP to ADP, which supplies energy to the pump, makes about
30.5 kJ/mole = 7.29 kcal/mole ≈ 0.326 eV per ATP molecule.)
6.5 Measuring Electric and Magnetic Fields
6.5.1 Instruments
There are a wide variety of instruments for detecting electric and magnetic fields.
They all take advantage of the fact that these fields are defined by their effect on
test charges. Specialized instruments can be used to measure biologically generated
electric and magnetic fields.
Electroscope
When charges are separated, we characterize the surrounding space with an electric
field. If a test charge is placed in the region, it will experience an electric force.
Suppose a set of negative test charges are taken as electrons on the end of a probe
connected to a conducting wire. If the probe is put in an electric field produced by
a negative charge, the electrons will move from the probe into the wire. The field
from a positive charge will pull probe electrons into the probe from wire.
Now imagine connecting the wire to a pair of conductors suspended in the Earth’s
gravity and hinged together at the top. When charged, the two conductors will push
each other apart. The angle of separation can be used to calibrate the strength of the
field. This is an ‘electroscope’.
Voltmeter
An ordinary ‘voltmeter’ allows a small amount of charge to flow from one end
of an electric probe (a conducting pointed material) touching one point on an
object to a second point that allows charges to move toward or away from the
first point. The resulting current (if small enough) is proportional to the potential
difference between the two locations. The small current can be measured by using
the current’s magnetic field to deflect a magnetized needle. Of course, our intent
is not to significantly change the system we are trying to measure. This limits the
amount of current we can use. These days, very small currents can be amplified to
much larger currents which in turn deflect needles or are calibrated digitally and this
information is stored.
6 Electric and Magnetic Fields in Life
the negative gradient of the electric potential: E = −∇V . For example, an active
pump carrying a sodium ion across a cell membrane (with a typical thickness of
10 nm) against an electric field of 0.07 V per 10 nm expends 0.07 eV of energy.
(The hydrolysis of ATP to ADP, which supplies energy to the pump, makes about
30.5 kJ/mole = 7.29 kcal/mole ≈ 0.326 eV per ATP molecule.)
6.5 Measuring Electric and Magnetic Fields
6.5.1 Instruments
There are a wide variety of instruments for detecting electric and magnetic fields.
They all take advantage of the fact that these fields are defined by their effect on
test charges. Specialized instruments can be used to measure biologically generated
electric and magnetic fields.
Electroscope
When charges are separated, we characterize the surrounding space with an electric
field. If a test charge is placed in the region, it will experience an electric force.
Suppose a set of negative test charges are taken as electrons on the end of a probe
connected to a conducting wire. If the probe is put in an electric field produced by
a negative charge, the electrons will move from the probe into the wire. The field
from a positive charge will pull probe electrons into the probe from wire.
Now imagine connecting the wire to a pair of conductors suspended in the Earth’s
gravity and hinged together at the top. When charged, the two conductors will push
each other apart. The angle of separation can be used to calibrate the strength of the
field. This is an ‘electroscope’.
Voltmeter
An ordinary ‘voltmeter’ allows a small amount of charge to flow from one end
of an electric probe (a conducting pointed material) touching one point on an
object to a second point that allows charges to move toward or away from the
first point. The resulting current (if small enough) is proportional to the potential
difference between the two locations. The small current can be measured by using
the current’s magnetic field to deflect a magnetized needle. Of course, our intent
is not to significantly change the system we are trying to measure. This limits the
amount of current we can use. These days, very small currents can be amplified to
much larger currents which in turn deflect needles or are calibrated digitally and this
information is stored.