Behaviour of Charges at Rest
The branch of physics which deals with electric effect of static charge is called electrostatics.
Electric Charge
Electric charge is an intrinsic characteristics of the fundamental particles which when acquired by some body it gets electrified, i.e. Charge of a material body or particle is the property (acquired or natural) due to which it produces and experiences electrical and magnetic effects. Some of naturally charged particles are electron, proton, a-particle etc.
Only two types of charge has been discovered :
1.Positive charge that is present on the proton .
2. Negetive charge present on the electrons .
Methods of Charging of a Body
1.Charging by friction (frictional electricity):
When two bodies are rubbed firmly against one another, due to the friction between the surfaces, electrons are transferred from one body to the other. In this process the bodies aquire equal and opposite charges. The cause of charging in this case is due to the transfer of electrons from a material whose work function is lower to the material whose work function is higher.
2.Charging by induction:
Charging a body by putting it close to another oppositely charged body (without bringing any actual contact).The positive charge is gained by the body due to transfer of electrons from the body to earth and negetive charge is due to the gain of electrons .If the body is made of an insulating material, instead of metal, the of method of producing a net charge by induction does not work, because very little charge can actually flow from the insulating material to the ground.
3.Charging by Contact (conduction):
When an uncharged body is brought in contact with another charged body ,it (the former) acquires the charge due to the transfer of electron from one body to another.Charge is not equally distributed. It is distributed directly proportional to the capacitance of the bodies in contact.
Thus a net charge is generated on a body only when the body either looses or gains electrons.
4. Charging by Thermionic emission:
When the metal is heated at a high temperature then some electrons of metals are ejected. The metal gets positively charged.
5. Charging by Photoelectric effect :
When light of sufficiently high frequency is incident on metal surface then some electrons come out and metal gets positively charged.
6. Charging by Field emission :
When electric field of large magnitude is applied near the metal surface then some electrons come out from the metal surface and hence the metal gets positively charged.
Finally, what conclusion we derive from these charging methods?
“Charge is generated on a body, only because to the transfer of electrons from one body to another”.
What is Charge? – Video Lecture
Fundamental Properties of Charge
Fundamentals Properties of Charge
1.Charge is transferable :
Charging a body implies transfer of charge (electrons) from one body to another. Positively charged body means loss of electrons, i.e., deficiency of electrons. Negatively charged body means excess of electrons. This also shows that mass of a negatively charged body is more than mass of a positively charged identical body.
2.Charge is always associated with mass :
Charge can not exist without mass though mass can exist without charge. The particle such as photon or neutrino which have no (rest) mass can never have a charge. As charge can not exist without mass, the presence of charge itself is a convincing proof of existence of mass.
3. Principle of Quantisation of charge :
Net amount of charge present on a body is integral multiple of electronic charge.
i.e. q = n e
where n =± 1, ±2, ±3, ………… and e = 1.6 × 10-19 C
(Millikan’s oil drop experiment proved the quantization of charge)
Note : Recently, the existence of particles of charge -e/3(down quark) and +2e/3 (up quark) has been postulated. These particles are called quarks, but still this is not considered as the quantum of charge because these are unstable (They have very short span of life).
4. Principle of Conservation of Charge:
The total amount of charge present in an isolated system remains constant.
( The charge may however be transformed from one part of the system to another.)
For example in case of alpha decay:
92U238 → 90TH234 + 2He4 (Process of alpha decay)
[Q = 92e] → [Q1 = 90e] + [Q2 = 2e]
5. Aditivity of Charges:
The total charge on a body is the algebraic sum of all the charges located any where on the body.
i.e. while taking the sum of the charges ,their signs (±)must be taken into account. This is because Charge is a scalar quantity.
* Like point charges repel each other while unlike point charges attract each other.
* A charged body may attract a neutral body or an oppositely charged body but it always repels a similarly charged body.
Note : Repulsion is a sure test of electrification whereas attraction may not be.
If two particles are attracting one another due to electrostatic force, there are two possibilities: (i) both the particles are charged with charges of opposite sign.
(ii) One of the particle is charged and other is uncharged.
But, if the particles repel one another, we infer that the particles are definitely oppositely charged.
