Page 314 - Physical chemistry understanding our chemical world

INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 281

Electrons
Aluminium metal Silver metal Tooth interior
and nerve
− electrons + electrons
(oxidation) (reduction)

Aluminium ions Silver ions

Aluminium Electrolyte Silver
(‘silver foil’) (saliva) (tooth filling)
Figure 7.1 Schematic illustration of the electron cycles that ultimately cause a sensation of pain
in the teeth in people who have metallic fillings and who have inadvertently eaten a piece of
aluminium (‘silver’) foil, e.g. while eating sweets

Why does an electric cattle prod cause pain?

Themagnitudeofa current

An electric cattle prod looks a little like a walking stick with an attached battery. A
potential from the tip of the stick is applied to a cow’s flank, and the induced current
hurts the animal. The cow moves where prompted to avoid a reapplication of the
pain, thereby simplifying the job of cowherding.
Ohm’s law says that applying a potential V across an electrical resistor R induces
a proportional current I. We can state this relationship mathematically as

V = IR (7.4)

It is reported that the great Victorian scientist Michael Faraday
The word ‘empirical’
discovered a variation of Ohm’s law some 20 years before Ohm
implies a result derived
himself published the law that today bears his name. Faraday, with
from experiment rather
his typical phlegm and ingenuity, grasped a resistor between his
than theory. Similarly,
two hands, immersed them both in a bowl of tepid water, and
a chemist calculates a
applied a voltage between them. It hurt. He found the empirical
compound’s ‘empirical
relationship ‘pain ∝ I × R’. formula’ while fully
The pain Faraday induced was in direct proportion to the magni- aware its value is based
tude of the current passed. He discovered the principle underlying on experiment rather
the action of an electric cattle prod. It is sobering to realize how than theory.
Faraday’s result is today employed through most of the world as
the basis of torture. Despite the explicit banning of torture by the
UN Charter on Human Rights (of 1948), it is common knowledge that giving an
electric shock is one of the most effective known means of causing pain.
In summary, the cattle prod causes pain because of the current formed in response
to applying a voltage.