A discharge. Although charge exchange can happen whenever any two surfaces come into The familiar phenomenon of
a static 'shock' is caused by the neutralization of charge.
to ground), or a region with an excess charge of the opposite polarity (positive or negative).
contact and separate, a static charge will only remain when at least one of the surfaces has a high resistance to
electrical flow (an electrical insulator). The effects of static electricity are familiar to most people because we can see,
feel and even hear the spark as the excess The familiar phenomenon of a static 'shock' is caused by the neutralization of
The materials we observe and interact with from day-to-day are formed from atoms and molecules that are electrically
neutral, having an equal number of positive charges (protons, in the nucleus) and negative charges (electrons, in shells
surrounding the nucleus). The phenomenon of static electricity requires a separation of positive and negative charges.
Contact-induced charge separation
Electrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them,
while materials with sparsely filled outer shells tend to gain them. This is known as the triboelectric effect and results in
one material becoming positively charged and the other negatively charged. The polarity and strength of the charge on a
material once they are separated depends on their relative positions in the triboelectric series. The tribo electric effect is
the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations
involving rubbing different materials together (e.g. fur and an acrylic rod). Contact-induced charge separation causes
your hair to stand up and causes static cling.
Pressure-induced charge separation
Applied mechanical stress generates a separation of charge in certain types of crystals and ceramics molecules.
Heat-induced charge separation
Heating generates a separation of charge in the atoms or molecules of certain materials. All pyroelectric materials are
also piezoelectric. The atomic or molecular properties of heat and pressure response are closely related.
Charge-induced charge separation
A charged object brought into the vicinity of an electrically neutral object will cause a separation of charge within the
conductor. Charges of the same polarity are repelled and charges of the opposite polarity are attracted. As the force due
to the interaction of electric charges falls off rapidly with increasing distance, the effect of the closer (opposite polarity)
charges is greater and the two objects feel a force of attraction. The effect is most pronounced when the neutral object is
an electrical conductor as the charges are more free to move around.
Careful grounding of part of an object with a charge-induced charge separation can permanently add or remove
electrons,leaving the object with a global,permanent charge. This process is integral to the workings of the Van de Graaf
Generator, a device commonly used to demonstrate the effects of static electricity.
The spark associated with static electricity is caused by electrostatic discharge, or simply static discharge, as excess
charge is neutralized by a flow of charges from or to the surroundings.
The feeling of a static electric shock is caused by the stimulation of nerves as the neutralizing current flows through the
human body. Due to the ubiquitous presence of water in places inhabited by people, the accumulated charge is
generally not enough to cause a dangerously high current.
Despite the apparently innocuous nature of static electricity as we generally experience it, there can be significant risks
associated with it in circumstances where large charges may accumulate in the presence of sensitive materials or
Lightning is a dramatic natural example of static discharge. While the details are unclear and remain the subject of
debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds.
In general, significant charge accumulations can only persist in regions of low electrical conductivity (very few charges
free to move in the surroundings), hence the flow of neutralizing charges often results from neutral atoms and molecules
in the air being torn apart to form separate positive and negative charges which then travel in opposite directions as an
electric current, neutralizing the original accumulation of charge. Air typically breaks down in this way at around 30,000
volts-per-centimetre (30 kV/cm) depending on humidity. The discharge superheats the surrounding air causing the
bright flash, and produces a shockwave causing the clicking sound. The lightning bolt is simply a scaled up version of
the sparks seen in more domestic occurrences of static discharge. The flash occurs because the air in the discharge
channel is heated to such a high temperature that it emits light by incandescence. The clap of thunder is the result of the
shock wave created as the superheated air rapidly expands.
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