Stories from Physics for 11-14 14-16
- Surviving the shock − different magnitudes of 60 Hz alternating current have different effects on the human body:
While currents of between 100 mA and 200 mA can be lethal, victims who experience currents in the range of 200 mA to 1 A have a good chance of survival as the current causes the heart muscle to contract so fibrillation cannot occur.
|1 mA||Barely perceptible|
|5-10 mA||Painful shock|
|100 mA||Minimum current that will cause ventricular fibrillation|
|2 A||Causes cardiac standstill and damage to internal organs|
- Wet and dry resistance − a large proportion of the body’s resistance occurs due to the outer layer of dead skin. For example, a calloused, dry hand may have a resistance as high as 100,000 Ω. By contrast, the resistance of the internal body, where tissues are wetter, is only around 300 Ω. Cuts and abrasions on skin can, therefore, reduce the resistance of the injured part of the body. When experiencing a shock, sweating and the formation of blisters may reduce body resistance, increasing the current that flows in a positive feedback loop. At potential differences greater than around 500 V, the high resistance of the outer layer of skin may break down, allowing current to flow into the body. Current can enter the body through a pinhead-sized wound and such shocks may lead to significant deep tissue damage with minimal burning to the skin.
- Pins, needles and buzzes − in direct current shocks, the victim experiences a shock only when contact is made or broken, but with alternating current, a continuous feeling of shock is reported. The experience of a shock may be more severe in someone who has tensed their muscles, or if the site of the shock is close to a nerve. The sensations induced by electric shocks are subjective. Victims encountering the same low-level electrical shock, have variously reported buzzing, tingling (pins and needles), a jolt or a burning sensation.
- A shock to the heart − the two most dangerous paths for current to take through the human body are from hand-to-hand and from hand-to-foot (in particular left hand to left foot) since the current will travel through the chest and, possibly, the heart.
- Letting it go − when receiving a shock above a certain magnitude of alternating current, known as the let-go threshold, the person experiencing the shock may struggle to release a conductor they have grasped because of contractions to their muscles. Exposure to currents above the let-go threshold may not be harmful if the person only experiences the shock for a short period of time. However, prolonged exposure may lead to breathing difficulties and asphyxiation. The let-go thresholds are different for men and women: the maximum reasonably safe current is 9 mA for men and 6 mA for women because of differences in the average strength of muscles in their hands.