ELECTRICITY 1 : STATIC ELECTRICITY By the end of this presentation, you should be able to: • Describe the two types of charge and the particle that carries it. • Describe the process of charging by friction in terms of the movement of electrons. • Recall that opposite charges attract and similar charges repel. • Describe how electrostatics is useful in the photocopier and smoke precipitator. • Describe situations where static electricity is dangerous and precautions taken in these situations.
ELECTRICITY There are two areas that can be studied ELECTROSTATICS CURRENT ELECTRICITY This section deals with stationary charge. This section deals with moving charge.
Charge is represented by Q. • It is measured in coulombs (C). • Charge comes from particles in the atom.
Can you remember what an atom looks like? This is where charged particles are found.
Structure of the atom nucleus Proton (+) neutron Electrons (-)
An atom is neutral (total charge = 0) if it has the same number of protons as electrons If electrons (-) are removed, the atom becomes positively charged, since there is now an excess of protons (+). If electrons are added, the atom will become negatively charged. Note that static charge is not created by movement of protons! These are “trapped” in the nucleus and are difficult to disturb.
ELECTROSTATIC FORCES Similarly charged objects will repel each other. + + The larger the charge the greater the force of repulsion. + + Oppositely charged objects will attract each other. - +
Charging by frictionLightning and other dangers of static electricityAttraction of uncharged objects.
Charging by friction Perspex tends to lose its electrons easier than cloth, so ends up positive. The cloth now has more electrons than protons and ends up with a net negative charge. Initially both the cloth and the perspex are neutral, i.e they have equal numbers of protons and electrons. As the two objects are rubbed together it becomes possible to move some electrons around
Other materials with different properties may gain electrons when they are rubbed, like polythene.
In order to create a static charge on an object, the charge must be unable to flow away. Insulators have no free electrons, so charge cannot flow through it. If charged by friction, only the area that was rubbed will become charged. Holding the other end with your hand will not discharge it, since the charge cannot flow to your hand.
With an insulator • It is initially neutral (number of protons = number of electrons). • If electrons are added to one end, that end gets an extra negative charge which remains where it was put. • If it is earthed, the rod still cannot discharge. - - - - - - + - + - + - + - + + - + - + - + - + -
If a conductor is given a charge, it will flow away if connected to the Earth. To keep a static charge on a conductor it must be on an insulated stand to prevent discharge. Also, the charge that is placed on a conductor will not all remain in one place. Since charge can flow through a metal (due to free electrons), forces of repulsion between similar charges will cause a redistribution of electrons in such a way, that the static charge spreads out.
With a conductor • Consider a metal sphere which is initially neutral, placed on an insulating stand. • If negative electrons are put onto a particular spot…. - - -
The force of repulsion between these charges will cause them to move through the conductor so that the excess charge is evenly distributed - - -
If this conductor is earthed… Anything that is charged will be neutralised when it is connected to the Earth. Either by the electrons flowing away or towards the object. The sphere will discharge, since the electrons can easily flow to the ground. - - -
A positively charged sphere will discharge, by the electrons flowing from the Earth, to restore the balance between protons and electrons. The charge flows because of a potential difference between the two points. Electrons flow to an area that has a more positive (or less negative) potential. Conventional current (+) flows towards a lower potential. + + + + + + _ _ _ _ _ _
Discharging • The greater the charge on an object, the greater the voltage (potential difference) between the object and the Earth. • If the voltage becomes high enough, a spark may jump across the gap.
Lightning strikes somewhere on the surface of the earth about 100 times every second.
Each flash contains about one billion volts of electricity. That's enough energy to light a 100-watt bulb for three months.
A lightning flash can happen in half a second. In that instant, the lightning flash superheats the surrounding air to a temperature five times hotter than that on the surface of the sun. Nearby air expands and vibrates, creating the sound that we hear as thunder. Sound travels more slowly than light, so it seems that thunder occurs later.
+ + + + - - - - - - - - Strong winds in the cloud causes friction between particles and charges begin to accumulate. An opposite charge is induced on the surfaces of the Earth. Very pointy objects will have a very high charge density on their surfaces. ++ ++ + + + + + + + + + + + + + + + ++ ++ + ++ + + + + +
+ + + + - - - - - - - - As the charge builds up, the electric field between the bottom of the cloud and the ground becomes higher and higher. The charge on the bottom of the cloud then discharges as a bolt of lightning surges to a positive area on the ground. ++ ++ + + + + + + + + + + + + + + + + + ++ ++ + ++ + + + + +
Other Examples of Charging by Friction… • The tumbling action of clothes in dryer causes a build up of electric charge that may make them crackle when pulled apart. • Combing your hair (comb charges negatively, hair charges positively) could make it stand on end. More dangerous! • Gasoline rushing out of a hose at a gas station: Sparks could ignite petrol fumes so the flow rate is controlled. • Air rushing over surface of car or airplane: Sparks could ignite fuel fumes so planes are earthed with a metal wire on landing. • Surgical equipment being pushed around on a hospital trolley will attract dust and germs: Trolley is earthed with a metal wire.
Attraction of uncharged objects Temporary dipoles are created. There is now an attractive force between the positive pole and the negatively charged rod - - - _ _ _ _ _ _ - - - - + - - - - Once the charged rod is removed, the forces of repulsion between the electrons will restore the shape and charge distribution of the electron cloud. The atom is initially neutral If a charged polythene rod is brought close by …
Uses of electrostatics The photocopier The smoke precipitator
The photocopier ++++++++++++++++++++++++++++++++++++++++++ A rotating drum is given a positive charge.
The photocopier A light shines onto the paper to be copied, and the reflection falls on the drum. ++++++++++++++++++++++++++++++++++++++++++ X
The photocopier ++++++++++++++++++++++++++++++++++++++++++ The drum is made of a special material that conducts when light falls on it. So the positive charge leaks away, where the light falls. X Only this area will be left with a positive charge.
The photocopier Black powder that is negatively charged is sprayed across the drum (toner). The toner is attracted only to the charged area. X * * * * *
The photocopier A sheet of paper is passed over the drum. The toner is transferred to the paper, and is heated to fuse it to the paper. X X
The Electrostatic Smoke Precipitator. The burning of fossil fuels pollutes the air with waste gases and smoke. The smoke can be removed from the waste gases before they pass into the atmosphere by using the principles of electrostatics.
The smoke particles pass through the grid and pick up a negative charge. These plates are positively charged Chimney As the charged smoke particles move up through the chimney, they are attracted to the oppositely charged plates. This grid is negatively charged. When the plates are covered with particles, they are tapped so that the particles fall to the bottom and can be cleared away.