Body capacitance

Storage of electricity in the human body

Body capacitance is the physical property of a human body to act as a capacitor. Like any other electrically conductive object, a human body can store electric charge if insulated. The actual amount of capacitance varies with the surroundings; it would be low when standing on top of a pole with nothing nearby, but high when leaning against an insulated, but grounded large metal surface, such as a household refrigerator, or a metal wall in a factory. When human body capacitance is charged to a high voltage by friction or other means, it can produce undesirable effects when abruptly discharged as a spark. This can result in momentary pain, a startle response that may cause further accidents, or damage to sensitive materials or electronic devices. In exceptional cases, a spark may ignite flammable gas or vapor resulting in a fire. The influence of body capacitance on a tuned circuit may affect the performance of radio receivers. A circuit that senses a change in capacitance can be used as a touch sensitive switch, allowing control of devices without operating a button or toggle.

Some fabrics and friction can charge a human body to about 3 kV. Low potentials may not have any notable effect, but some electronic devices can be damaged by voltages of the order of 100 V. Electronics factories are careful to prevent people from becoming charged. A branch of the electronics industry deals with preventing static charge build up and protecting products against electrostatic discharge. Notably, a combination of footwear with some sole materials, low humidity, and a dry carpet can cause footsteps to charge a person's body capacitance to as much as a few tens of kilovolts with respect to the earth. The human and surroundings then constitute a highly charged capacitor. A close approach to any conductive object connected to earth (ground) can create a shock, even a visible spark. Body capacitance was a significant nuisance when tuning the earliest radios; touching a tuning knob would couple the body capacitance into the tuning circuit, slightly changing its resonant frequency. However, body capacitance is exploited in a Theremin, a musical instrument in which it causes slight frequency shifts of the instrument's internal oscillators. One of them changes pitch, and the other causes loudness (volume) to change smoothly between silence and full amount. Capacitance of a human body in normal surroundings is typically in the tens to low hundreds of picofarads, which is small by typical electronic standards. While humans are much larger than typical electronic components, they are also mostly separated by significant distance from other conductive objects. Although the occasional static shock can be startling and even unpleasant, the amount of stored energy is relatively low, and won't harm a healthy person. The Human Body Model for capacitance, as defined by the Electrostatic Discharge Association (ESDA) is a 100pF capacitor in series with a 1.5kΩ resistor.

The body capacitance can be used to operate pushbutton switches such as for elevators or faucets. Certain voltage tester probes rely on body capacitance. A capacitive touch sensor responds to close approach (but not force of touch) of a part of a human body, usually a fingertip. The capacitance between the device itself and the fingertip is sensed. Capacitive touch screens don't require applying any force to their surfaces, which makes them easier to use and design in some respects.

Radio receivers rely on tuned circuits to select the frequency of a particular desired signal. Design of such circuits intended to be adjusted by a user must prevent interaction of the user's body capacitance with the resonant circuit, so that the resonant frequency is not affected. For example, a metal shield may be placed behind a tuning knob so that the presence of an operator's hand does not affect the tuning.