Alternating current is caused by a generator which does not contain a brush-style connector (anyone with better terminology? :) This results in the potential on the wire changing directions every half rotation of the generator (60 Hz in North America). This creates a voltage plot which is a sine wave. Direct current was originally to be adopted by the US, but alternating current ultimately won out (as transformers are easy with AC).

In American uses of alternating current the voltage on a line from the power company is a sinusoid with a frequency of 60 Hz and an amplitude of a 140 volts; it has an RMS value of 120 volts.

A problem that stumps many medical students (and even some anesthetic and surgical registrars) is how come diathermy (the heating element used in the operating theatre) does not cause ventricular fibrillation even when they are told that it is indeed alternating current.

The problem is that everyone (even stumped medical students) knows that touching a live AC outlet can lead to a shock that can leave your heart in fibrillation (pumping uselessly).

The wrong answer commonly given is "The patient is grounded". True, the patient has a metal plate attached to another part of his/her body but this just acts as a conduit for the current to flow. It has no bearing on why the current flowing through the patient does not cause electrical discharge on the cellular level.

Another commonly wrong answer is that the current is not high enough or "not enough amps are going through". This also flies in the face of common sense as there is enough current to burn tissue and the amount of current that normally passes through from touching an AC outlet is much less than what flows through the diathermy.

The correct answer, as I learned from an anesthesiologist in Nowra was in the frequency of the alternating current used. Household AC runs at around 50-60 Hz, depending on which country you live in. This works out to a period between voltage peaks of between 10 to 20 milliseconds. As it turns out, any astute student of physiology can tell you that the time taken to depolarise one single membrane of a cell is in the range of 3-5 ms, the limiting factor being how fast the charged ions can move across the cell membrane through the ion gates in the cell membrane. The frequency of the AC used in diathermy is much higher, greater than 10 kHz (and indeed up to whatever frequency the machine can generate, even 300 kHz and higher). At this higher frequency, the current changes polarity so often that the ions scarcely have time to move much in one direction before being tugged in exactly the opposite direction.

Thus, while household current at 50 Hz can cause a significant movement of ions back and forth across a cell membrane and could depolarization in the cell, a much higher frequency at > 10 kHz does not. The higher frequency current only manifests in the human body as heat.

Why then does the diathermy only burn at one end and not at the "grounded" plate as well? It has to do with "electrical density" as the cutting edge is a small electrode whilst the grouded plate is relatively large.

Some important points about alternating current

  • The total power carried by an alternating current is calculated using a root mean square formula.This is because of the fact that the average value of a sine wave equals zero if we average over an integral number of cycles.Instead we square the sinewave, average it,and then take the squareroot of that average.Squaring first makes the negative halves of the sinewave add to the average.

The advantages of using alternating current are many.

  1. Its voltage can be changed using transformers which are over 95% efficient.
  2. Induction motors which run on AC are much more long lasting and maintenance free compared with universal motors.
  3. The cycling nature of the AC makes sure that any spark generated in mechanical switching devices do not last longer than a very tiny fraction of a second.DC switches are much more prone to sparking and contact pitting.
  4. Polarity reversal need not cause immense damage as it can with DC.
  5. An alternator powered by a rotating power source (wind / water turbine or engine) can use slip rings intead of brushes which again give them the advantages that induction motors have.

Al"ter*nat`ing cur"rent. (Elec.)

A current which periodically changes or reverses its direction of flow.


© Webster 1913

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