Static electricity - spark
What is this thing called "static electricity"? If we accumulate a number of
positiveor negative charges in a place which is electrically isolated from its
surroundings, these charges cannot move, they cannot "flow" as is the case with
electric current. These charges create an electric potential higher then that existing
in its neighborhood. In other words we create a potential difference between the place
with charges and its surroundings. The potential difference between two points is also
called a voltage.
The exact formula for this potential is
where Q is the electric charge, and C - electrical capacitance. If you do not know what this capacitance is, assume for now that it is a quantity roughly proportional to the dimensions of the object on which the charge Q is located. From Eq.1 we see that the more charges we put on a given object, the higher potential V will be and the smaller the object with given amount of electric charges, the higher the potential created. From other sources we know that the higher the potential V, the better the isolation which is required to prevent the flow of the electric current, which will discharge the place.
This flow, if it occurs in atmosphere, will be seen as a spark. Depending on the value of the potential created before the spark occurs, it may be very tiny, like the one observed in darkness while combing one's hair, or huge like the lightning during a storm. These two sparks are of exactly the same nature, only their "size" differs by millions of times.
What exactly is this spark, regardless its size? It is a short flow of electric current through the air possible due to ionization of the gases constituting the atmosphere. You can ask, in turn, what ionization is? Ion is an atom with one or more electrons removed or added. Ionization is then a process in which ions are created. Ions may have a positive charge (electron removed) or a negative one (electron added).
Without going into details, a spark is observed when the potential difference between the object with electric charges and any object in the vicinity is greater then the value required to start the ionization of the gas in which both objects are "immersed", usually the Earth's atmosphere.
What is the potential difference which will ionize the Earth’s atmosphere? In normal conditions, when the pressure is about 1000 hPa and humidity 40% it is about 3.0 MV/m. What does this 3.0 MV/m mean? It means a potential difference of 3.0 millions of Volts at the distance of 1.0 meter. In other words, to start ionization of the air and get a spark between two objects located 1.0m apart, the potential difference between them should be 3 millions Volts. It is a huge voltage, compared to main line 120 V in the USA or 230 V in Europe.
If we have two objects at a distance of 0.5mm, the potential difference required to start ionization is 1500V. It is quite a high voltage, but if it is created during the combing of one’s hair or walking on a carpet, it is completely safe and it can not do any harm to our body. It is only the source of an unpleasant feeling when the spark jumps from our finger to the knob of the door. In some circumstances we can observe a spark 2 or 3 millimeters long. This means that the voltage of 6 kV to 10 kV was created before a discharge occurred. But still it is not dangerous to our body.
Why is such a high voltage safe, while the 230V or even 120V present in our houses can kill a person? The answer lies in the power delivered to our body. Power P in case of electricity is a product of voltage V and current I, P=V I. With V in Volts and I in Amper power is in Joule’s . Current I is defined as a quantity of electric charges per second passing through a given path. In case of electrostatic phenomena the charge accumulated before the spark is observed is very small. For example, an ordinary 100 W bulb conducts a current of 0.43 A at 230V, but the current in a spark of the type described above is of the order of 1.0x 10-7 A, that is one ten millionth of an Amper. The duration of this tiny current is also very short, about one thousandth of the second. So all together, such a discharge from our finger to an external object may be unpleasant, but is completely safe.
There is no need to mention how dangerous lightning is. Why is it so dangerous to be in an open area during the storm, or why should we not stay close to high objects like tree? Large amounts of electric charges are accumulated in a cloud. A very high potential difference (tens of millions of Volt) between the cloud and the Earth’s surface is created. The spark, that is lightning, "will go" to the point nearest to the cloud as this path will require the lowest potential difference to start ionization. So be careful not to create such a point with your own body and avoid places close to potential targets of a lightning.