physics//electromagnetism//electric circuit//electric current//electric shock

An electric shock is the physiological effect of an electric current passing through a body, and it is what electrical safety is designed against: insulation, earthing, residual-current breakers and the limits on electric fences all exist to keep the current through a person below the levels that harm. What harms is the current and its path; voltage matters only because it drives current through the body's resistance.

An electric shock is the physiological effect of an electric current passing through a body, and it is what electrical safety is designed against: insulation, earthing, residual-current breakers and the limits on electric fences all exist to keep the current through a person below the levels that harm. What harms is the current and its path; voltage matters only because it drives current through the body's resistance.

The effects climb with current, for mains-frequency alternating current through the trunk. Around 1 mA it is felt as a tingle. At about 10 mA the muscles of the hand clamp and the person cannot let go of the conductor, the let-go threshold. From roughly 30 to 100 mA through the chest for a second or two, the heart's rhythm can collapse into ventricular fibrillation, a chaotic quiver that pumps no blood. Burns from Joule heating dominate only from about an ampere upwards.

The heart fails long before anything burns.

The heart is an electrically paced pump, and milliamperes in the wrong moment are enough to break its rhythm; heat needs a thousand times more current.

Timing matters as much as size. The heart is most vulnerable during part of each beat (the rising slope of the T wave), and a shock that lasts longer than one heartbeat is far more likely to catch that window, which is why the threshold for fibrillation falls as the shock lasts longer.

Mains frequency is the worst case. Alternating current around 50 to 60 Hz is more dangerous than direct current or high-frequency current at the same level, both for gripping the hand and for disturbing the heart.

An electric fence is built on these thresholds. It gives pulses of up to about 10 kV, but each lasts well under two milliseconds, carries a few joules at most and comes about once a second: the animal feels a sharp blow and pulls away between pulses, and no pulse lasts long enough to catch the heart's vulnerable moment.

The same physics, aimed and dosed, is therapy: a defibrillator stops fibrillation with one strong, brief shock that resets every heart cell at once.