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

1 Bring 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 rodInsulator

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 theelectroscope, 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 Q1Q2 / d2

Coulomb’s Law

F Q1Q2 / d2

F= constant Q1Q2 / d2

Constant =1/4

F= (1/4 ) Q1Q2 / d2

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 = 8.9x10-12 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/QSince F is given by coulomb’s law

E = Q/4d2

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.