Electrostatics

Structure of the atom All matter is made up of atoms Atoms consist of a small central nucleus, containing positively charged protons and neutral neutrons, surrounded at relatively large distances by negative electrons Atom Nucleus Protons(+)Neutrons Electrons(- )

Electrical charge In an uncharged system/atom there must be the same no. of + protons and – electrons In any system it is only the electrons which are free to move Thus to give an object an electrical charge it is necessary to gain or lose electrons. Electric charge is measured in coulombs (C)

If an object is negatively charged it means it has gained electrons If an object is positively charged, it means that it has lost electrons and so is left with more protons than electrons, and so has a positive charge

Conductors and Insulators Conductors = substances through which electric charge can flow (i.e. when you place a charge on the object the charge can move around) Insulators=substances through which electric charge cannot flow (i.e. when you place a charge on the object the charge stays where you put it)

To charge an object An object becomes electrically charged when it either gains or loses electrons There are two ways to charge an object (a)by contact (b) by induction

Charge by Contact If a polythene rod is rubbed with a woollen cloth some of the electrons from the cloth rub onto the rod. Thus the rod has acquired extra negative charges and is negatively charged. The cloth has lost an equal no. of electrons and so is positively charged to the same extent

(a)A and B are initially neutral. (same no. of protons and electrons) (b)A and B are brought into contact. Some of the loosely held electrons from A transfer to B. (c)When A and B are separated A now contains more protons than electrons and so A is positively charged. Similarly B is negatively charged

Charging by induction 1Bring a rod of the opposite charge to the one desired near, but not touching the object. (this causes the + charges in the object to be attracted and the – charges to be repelled) 4 step process Object for charging Charging rod

2.Connect the object to earth. (the negative charges in the object, which want to get as far away from the charging rod as possible, travel down the earth connection to ground) Charging rod Object for charging

3.Break the earth connection – without removing the charging rod. 4. Remove the charging rod- object is charged in the opposite way Charging rod Object charged by induction

NOTE: IT IS ALWAYS THE ELECTRONS WHICH MOVE When charging an object positively the electrons from the object flow away through the earth connection to ground. When charging an object negatively electrons from the ground travel up through the earth connection to neutralise (i.e. effectively remove) the positive charges in the object

The gold leaf electroscope Used to detect the presence of electric charge, and indicate its size. Metal cap Metal rod Insulator Metal Case Gold leaf Glass window

Note: The divergence of the leaves is due to 2 factors − The repulsion of the same charges on the two leaves − The attraction between the leaves and the opposite charge induced on the inside of the case The divergence of the leaves measures the potential difference between the leaves and the case

To detect charge using an electroscope If a charged rod is brought near the cap of the electroscope the leaves diverge -the neg. charges in the rod repel the neg. charges in the electroscope, which travel down the leaves which repel -the size of the divergence shows the size of the charge

To test the sign of a charge Charge the electroscope either + or – Bring the object to be tested near to the cap If the leaves collapse then the object has the opposite charge If the divergence increases the charge is the same sign

Distribution of charge on a conductor If positive (or negative) charges are placed on the surface of a conductor, they will move until they are as far away from each other as possible When they have stopped moving (i.e. are static) it is found that All static charges reside on the outside of a conductor (where they are furthest apart) Static charges tend to accumulate where the conductor is most pointed

Van de Graaff generator A machine for storing large static charges Charge from a point travels up the belt and is picked off by another point to be stored on the dome. If the dome is then connected to earth a current will flow to earth

Point Effect Charge tends to accumulate around pointed objects This generates a large electric field in the air around the point This causes the air to ionise and then the ions in the air are attracted or repelled by the point -as these move they hit other atoms and cause further ionisation

Oppositely charged ions move towards the point and neutralise it. Thus a pointed object will not retain as big a charge as a rounded object. This loss of charge is called the point effect Same charged ions move away from the point, creating an electric wind. e.g. Put a candle near a point on a charged van de Graaff-gets deflected

Effects of static electricity Lightning When clouds pass over each other they build up static charge Lightning occurs when these charges discharge, either to each other or to earth Lightning will choose the easiest path to earth-through a tall object

Lightning conductor Long metal rod attached to the side of the building. The top of the rod ends in a point (which is higher than the building) and the bottom ends in a large metal plate buried in the earth- this rod provides the easiest path to earth.

The cloud induces an opposite charge on the rod. This charge accumulates around the point and so a big electric field builds up around the point. This ionises the air around the point and so provides an easy path for the cloud to discharge

Other effects of static electricity TV screen collects dust-picture on screen is caused by beams of electrons, so screen becomes charged, so attracts dust particles. Aeroplane gets charged in flight-by friction with air. Must be discharged before refuelling in case a spark would ignite fuel-usually discharged via the conducting rubber of the wheels

Force between charges (Coulomb’s Law) Coulomb’s law states that the force of attraction or repulsion between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their distance apart F  Q 1 Q 2 / d 2

Coulomb’s Law F  Q 1 Q 2 / d 2 F= constant Q 1 Q 2 / d 2 Constant =1/4  F= (1/4  ) Q 1 Q 2 / d 2 Inverse square law-force proportional to 1/square of distance

Coulomb’s Law calculations Example Three charges, each of +100  C, are equally spaced along a straight line, successive charges being 3 m apart. Calculate (i) the resultant force acting on one of the end charges, (ii) the resultant force on the central charge.

Permittivity   = permittivity of the medium – this is a constant for a particular medium. Usually the medium is air or free space, in which case the permittivity is given by  0 = 8.9x F m -1

Relative permittivity  r If the medium is not free space the permittivity is sometimes given in terms of relative permittivity  r which relates the permittivity of the medium to that of free space. The permittivity of the medium can then be calculated from  =  r  0 Do questions p229

Electric Field An electric field is a region where an electric charge experiences a force-this force is caused by the presence of other static charges in the vicinity Electric field can be represented by electric field lines –Imaginary lines showing the direction in which a positive charge would move if placed in the field

Electric field lines Always point out from + charges Always point into negative charges + -

Field lines due to combinations of charges Field line pattern due to unlike point charges Field pattern due to like point charges Field pattern due to parallel plates

To show electric field patterns (Experiment) Place some cooking oil in a container with 2 electrodes Sprinkle fine powder (semolina) on the oil The semolina particles will align themselves along the field pattern

Electric Field Strength The electric field strength (E) at a point is defined as the force per unit positive charge at that point (measured in volts/m) E = F/Q Since F is given by coulomb’s law  E = Q/4  d 2 See text p232

Electric field intensity calculations

Applications Photocopier- The drum is charged electrostatically. Light is reflected off the blank bits of the page, and this reflected light knocks charge off the drum  the charge pattern on the drum represents the print pattern on the page

Toner is then sprinkled over the drum and sticks to the charged bits - The pattern thus gets transferred to the new page

Applications (ctd) Electrostatic precipitators -device for removing dust particles from air. It charges the dust particles using the point effect. These are then attracted to oppositely charged metal plates. Once a certain amount of dust has built up on these plates, they can then be removed and cleaned.-used for smoke removers etc.