Introduction
Electric charges and fields are the key to most modern electrical devices, from batteries to computers. They make up a large part of both chemistry and physics. Without these familiar concepts you wouldnt be able to put things into motion or make sense of our world. How are these charges produced and what are the laws that govern them? You will find answers to these questions below.
Electric Charge
An electric charge is a property of matter that is usually associated with electrons, and other subatomic particles. These exert force on each other. They can be categorized into positive and negative charges (Benjamin Franklin).
The reason that electric charges can be measured experimentally is because they provide the mechanism to control things like currents, moving charges around, and how much force they exert on objects.
Points to Ponder:
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Like charges repel and unlike charges attract.
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Unit of Charge is Coulomb (C).
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Proton has positive charge of 1.602192 * 10^-19 C.
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Electron has negative charge of -1.602192 * 10^-19 C.
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When number of electrons is equal to number of protons then the object is electrically neutral.
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When the number of electrons is higher than the material is negatively charged and when the number of protons is higher the material becomes positively charged.
Conductors: Materials that help in the transfer of electric charge are conductors. Example- Metals.
Insulators: Materials which resist the flow of charge are called insulators. Example- non-metals such as glass, rubber, etc.
Methods of Charging the Body
1. By Rubbing (Friction)- When you rub your hands together, electrons are transferred from the skin to your hand. The hand becomes positively charged and the other – negatively charged (opposites attract).
2. By Conduction- When an uncharged body is made in contact with charged body, electrons from the charged body flows to uncharged body.
3. By Induction- When you walk over a rug or any other material for that matter, the electrons on the rug move towards your shoes and leave a positive charge behind in the rug. You become slightly positive due to this process. It is an indirect process.
Point Charges
A point charge is a charge that is concentrated at a specific point. Point charges are assumed to be small charges that are situated at very large distances compared to their size.
Properties of Charges
Q= ne, where n= integer, e= 1.6*10^-19 C
Coulomb’s Law
Coulomb’s law suggests that the force between two charges (q1 and q2) is directly proportional to their magnitude and inversely proportional to the square of their separation distance r.
F = K*q1*q2/R^2, where K is called the Coulomb’s constant. It is experimentally measured to be about 9x10^9 N*m^-1.
K= 1/(4π ε 0) where, ε 0 = 8.854×10-12 C2N-1m-2
1 Coulomb
When q1 = q2 = 1 C, r = 1 m, F = 9 × 10^9 N
Then, 1 C is the charge that experiences an electrical force of repulsion of magnitude 9 *10^9 N when placed 1 m apart from another charge of the same magnitude in vacuum.
Electric Field
An electric field is a space in which electric force is exerted.
F=qE where q is charge and E is electric field.
Electric Field Due to Point Charge
E=1/(4π ε 0) * (q/r^2)
In a system of charges net electric field is the vector sum of all the charges.
For calculating electric field with the help of vector law addition and superposition principle, do not hesitate to visit this YouTube channel https://youtube.com/channel/UCoqI7C9rI2UbFPITF2bPgnQ.
Electric Field Lines
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It is a pictorial representation of electric field.
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A curve drawn in such a way that the tangent to it at each point is in the direction of the net field at that point is called an electric field line.
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They do not form closed loops.
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Intensity is determined by number of field lines.
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Two field lines never intersect each other.
Question: Why do electric field lines never intersect each other?
Answer: If you want to clear your doubts regarding electric field lines, visithttps://youtube.com/channel/UCoqI7C9rI2UbFPITF2bPgnQ
Electric Dipole
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An electric dipole is formed when a positive and negative charge are separated by a finite distance.
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A dipole can be formed by symmetric placement of charges around an axis,
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2 Dipoles, each carrying +q and -q separated by distance R generate an Image force F due to their charges. Due to this field, the resultant vector is at an angle α with the dipole axis.
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Due to this field, the resultant vector is at an angle α with the dipole axis.
Dipole Moment
p= q* 2a where q=charge and 2a= distance between the charges. Direction is from negative to positive charge. SI Unit of Dipole moment is Cm (Coulomb meter).
Question: Dipole moment exist differently on different axis and have different properties. So, what is the dipole moment due to an axial electric field?
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Electric Flux
Electric flux is the field lines passing through a surface.
Φ= EA cos θ Nm^2/C
Charge Density
Surface Charge Density
Volume Charge Density
Gauss’s Law
Gauss’s law is used to find the electric flux in a closed surface.
Φ= q/ ε0
Characteristics of Gauss’s Law
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The flux passes through a closed surface regardless of shape and size.
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When the system has some symmetry, Gausss law can be used to compute the electric field.
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Gauss law is based on the Coulombs laws inverse square dependence on distance.
Gauss’s law can be used to calculate electric flux inside a straight charged wire, on a plane sheet of charge, inside a spherical body and shells. For finding more about the applications of Gauss’s law and its significance, visit https://youtube.com/channel/UCoqI7C9rI2Ub