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Electrostatics


Electrostatics

Electrostatics is the study of static or stationary electric charges. Charging by friction is a simple way to acquire static electric charges.

There are 2 kinds of electric charges:
- positive charge (proton, cation)
- negative charge (electron, anion)

The SI unit for charge is the coulomb, C

The charge of a proton is 1.6 x 10-19 C
The charge of an electron is -1.6 x 10-19 C

All charges obey the law of electrostatics which states that
- like charges repel
- unlike charges attract

The magnitude of the electrical force of attraction or repulsion between charges is increased if
- the quantity of charge is increased
- the distance between the charges is decreased

Explanation for electrostatics

Matter is made up of atoms. In every atom, there are positively charged protons and negatively charged electrons. The electrons orbit around the nucleus which contains the protons (as well as the uncharged neutrons)

When an atom is in the uncharged state, the number of protons and the number of electrons are equal. An atom can only become charged by adding or removing the negatively charged electrons.

Therefore an object becomes negatively charged when it gains electrons and positively charged when it loses electrons.

Electric field

An electric field is a region where electrical forces of attraction or repulsion act on any charged object placed in it

The pattern of the electric field is illustrated by drawing electric lines of force. The direction of the electric field is defined as the direction of the electric lines of force.



Electrostatic induction

Electrostatic induction is a process whereby a conductor becomes charged when a charged body is brought near it but is not in direct contact with it.

Explanation

A positively charged rod, when placed near an uncharged aluminium foil, attrots the free electrons inside the foil. This results in the near side of the foil becoming negatively charged, and the other side positively charged.
In other words, the positive and negative charges in the foil are separated by the positively charged rod. These separated charges are known as induced charges.

Once the rearrangement of the charges is done, following the law of charges, the rod attracts the near side of the foil but repels the far side. However, since the strength of an electrostatic force decreases with distance, there will be an overall attractive force between the rod and the foil.

Any object that allows electric charges by induction method is known as a conductor.

The separated charges due to electrostatic induction are known as induced charges. The presence of different types of induced charges explains why the pieces of paper are attracted to a charged comb brough near to them.

Methods of electrostatic charging

1. Charging by friction

a. rub 2 different materials together and then separate them quickly

b. one of the materials will be positively charged and the other will be negatively charged

2. Charging by electrostatic induction


a. charging a conductor by induction


b. charging two conductors by induction



c. charging an electroscope by induction



3. Charging by contact

When a charged conductor is in contact with another neutral conductor or is sliding on an insulator, electric charges will be shared, though not necessarily equally between the 2 objects in contact.


Conductors, insulators, semi-conductors and superconductors

 conductors   insulators
Allow electric charges to flow through them easily (low resistance) Do not allow electric charges to flow through them easily (high resistance)
eg: metals, acids, bases, salt solutions, graphite, ionized gases    
eg: non-metals, acids, bases and salts in solid states, diamond and non-ionized gases
solid conductors have free-moving electrons as charge carriers. liquid and gaseous conductors have ions as charge carriers
insulators do not have free-moving electrons or ions functioning as charge carriers

Semiconductors

Eg. germanium and silicone

To increase their conductivity, impurities are added to them.


Applications

- For making microchips and transistors used in computers and other electronic appliances like radios

Superconductors

They have near zero resistance at very low temperatures around 0K. Materials which have superconductivity properties include mercury, tin, lead and some ceramic materials

Van de Graaf generator

Friction between the moving rubber belt and the metal roller produces positive charges on the belt.

The positive charges are then carried up to the dome where they attract negative charges to the inside of the dome and repel positive charges to the outside of the dome.

This discharges the belt, but leaves an excess of positive charges on the dome.

As the amount of positive charges accumulate on the dome, its voltage increases.

MCQ Questions

1. Which of the following statements about an electrical insulator is correct?
a. it contains some electrons but more protons
b. it contains some protons but more electrons
c. it contains electrons but they are not free to move at all
d. it contains no electrons at all
e. it contains only neutrons

2. When a plastic rod is charged positively by friction,
a. it gains electrons
b. it gains neutrons
c. it gains protons
d. it loses electrons
e. it loses protons

3. How could the unit of potential difference, the volt, also be written?
a. A s
b. A/s
c. C/A
d. C/J
e. J/C

4. Two suspended light, aluminium coated with balls have been touched by the same charged perspex rod.. They will
a. repel each other but be attacked by the rod
b. repel each other and be repelled by the rod
c. attract each other but be repelled by the rod
d. attract each other and be attracted by the rod

5. A charged rod may be discharged by holding it just above a flame. This is because
a. the rod is ionized when heated
b. the hot gases in the flame are ionized
c. the rod is oppositely charged to the flamed
d. the hot gases strike the rod and remove its charge

6. A negatively charged strip is brought near to (but not touching) an uncharged conductor. If  the conductor is earthed it
a. becomes negatively charged
b. becomes negatively charged on one side and positively charged on the other
c. is attracted by the strip
d. is repelled by the strip

7. What makes metals good conductors of electricity?
a. they contain electrons that are free to move
b. their molecules are free to move
c. they contain excess positive charge
d. their atoms are close together and transfer energy when they vibrate

8. Lightning conductors are used to protect tall buildings from the damages due to lightning. Which one of the following statements about a lightning conductor is not true?
a. its top must be higher than the highest part of the building
b. it has sharp points at its top end
c. its lower end is buried in the ground
d. it must be insulated from the building

9. Four processes are used to charge an isolated metal sphere. 
P: The sphere is earthed by touching it
Q: The earth connection is removed from the sphere
R: A charged rod is brought close to the sphere
S: The charged rod is removed

In which order should these processes be carried out to charge the sphere?
a. P Q R S
b. P R S Q
c. R P Q S
d. R S P Q

MCQ Answers

1. c
2. d
3. e
4. b
5. b
6. c
7. a
8. d
9. c

Structured Question Worked Solutions

1. The diagram shows a metal sphere S mounted on an insulating stand
Describe a simple test you could perform, and which does not alter any charge there may be on the sphere, to determine whether or not the sphere is charged.

Given that the sphere is charged, how could you test whether the charge is positive or negative, without altering the charge on the sphere?


Solution

1. Bring an uncharged conducting sphere P suspended on an insulating thread close to but not touching S. If P does not move towards S, then S is uncharged. If P moves towards S, then S is charged.

Using P with a positive charge, bring it slowly from a far distance towards S. Observe for any deflection of P along the way. If P is deflected away from S, then S is positively charged. If P is attracted towards S, then S is negatively charged.

2. State briefly how you would give an electric charge to
a. a glass rod
b. a copper disc attached to the end of a nylon rod, assuming that you have available a charged polythene tile.


Solution

2a. By rubbing the glass rod with another insulating material such as plastic or a cloth, we can charge the rod by friction.

2b. First place the charged polythene tile close to but not touching the disc and then earth the disc once. The disc now has an induced charge of sign opposite to that of the charged polythene tile.

3. Three copper spheres are placed near each other in air. The large sphere carries a positive charge and the two small spheres touch each other, as shown.


The two small spheres are pulled apart, using their insulated handles, and then taken well away from the large sphere, as shown

a. The charge on the large sphere has been drawn for you. On the diagrams above draw in the charges, if any, on each of the smaller spheres.

b. Explain why energy is needed to separate the two small spheres.


Solution

a.

b. The two small spheres are oppositely charged, so there is a force of attraction between them. Energy is thus needed to overcome this force to separate these two spheres.

4. An electrically charged sphere C is brought near a small uncharged conducting sphere S suspended as shown in Fig. 1. S is first attracted towards C until it touches the surface of C and then repelled to the position shown in Fig. 2

ai. Explain why S is first attracted towards C

aii. Explain why S is repelled after touching the surface of C

b. On Fig 2 mark and label each force acting on S

c. When a bunsen flame is passed beneath S, the sphere falls back towards C. Suggest why this happens


Solution

4ai. Since C is positively charged, it will induce negative charges on the side of S facing C. As unlike charges attract, S is attract towards C.

4aii. Upon touching C, S's negative charges get transferred onto C to neutralise some of the positive charges. S becomes positively charged. The like charges in C and S repel each other.

4b.

4c. The flame ionizes the air surrounding S which neutralises the charges on S. This eliminates the force of repulsion.
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