Electric charges and fields Flashcards
Electrostatics
What is static electricity?
It is the build up of charges on an isultator, that produced sparks when discharged
Electrostatics:
Is the study of forces, feilds and potentials of charges at rest.
Electricity comes from the Greek word:
Elektron, which means ‘amber’
What device is used to detect charge on a body?
A gold-leaf electroscope
How does one electrify a neutral body?
By transferring electrons on/off the body.
Initially, all objects in the universe are:
Electrically neutal
What are the two methods of charging?
Induction and Conduction
Conduction = with contact/like charges
Induction = without contact/unlike charges
What are the two types of materials that exist around us?
Conductors and Insulators
What are the two types of charges/polarity?
+ve and -ve
(Like = repel)
(Unlike = attract)
Quantisation of charge can be represented as:
Q = ne
Where e = ± 1.6 * 10 ^ -19 c = charge of 1 electron or proton
How many electrons make up ± 1 C ?
6 * 10^18 proton/electrons
Coulomb’s law:
F = k q1 q2 / r ^ -2
k = 9 * 10^9
ε0 = 8.8 * 10 ^ - 12
What is ε0?
ε0 is the permittivity of free space.
It represents the efficiency of a medium in the transmission of field lines.
Relative permittivity/ Dielectric constant =
ε /ε0
For a conductor, K = infinity
What device did Coulomb use to experimentally arrive at his law?
A torsion balance
(Cavendish used this device to later prove Newton’s law of gravitation)
What is the relation b/w Gravitational and Electrostatic force?
Electrostatic force > Gravitation force
(However, gravitational forces are the leading forces in the universe)
What does the slope of the F and 1/r 2 give?
Slope = y/x = kq1q2 = tan θ
What is the electrical field of a charge ?
The region around a charge within which its influence can be left is called its field.
Electric field =
Force / test charge (q0)
Field lines were first introduced by:
Michael Faradary. He called them ‘lines of focre’.
Charge to mass ratio (emf) ∝ ??
the displacement/degree of deflection
What are field lines?
They are imaginary lines that represent the motion of a unit + ve charge when placed in the field of another charged, when allowed to move freely.
They go from the +ve charge to the -ve charge.
Field lines are:
Continuous curves and NOT closed loops. The tangent at any point represents the direction of the electric field. They DO NOT cross each other at any instant at time.
The magnitude of the field is represented by:
The density of the field lines. More crowed the field lines at a point, stronger the electrical field.
An uniform electric field has:
a constant strenght and magnitude throughout
Electric dipole:
A pair of equal and opposite charges seperated by a very short distance.
The total charge in a dipole = 0
Its direction is conventionall from -ve to +ve
Dipole moment =
2l * q
2l = total distance between the charges
Rel. b/w E(axial) and e (equitorial) =
e (equitorial) = e (axial) /2
OR
e (axial) = 2 * e (equitorial)
Torque in an electric field =
P X E = PESinθ
Work done to rotate a dipole =
- P. E = - PE Cos θ
Surface charge density (sigma) =
q / area
Linear charge density (lambda) =
q/l
Volume charge density/charge density =
q/v
Electric flux =
E. ds or EdsCos θ
Electric flux as per Gauss’ law =
q net / ε0
E for an infinitely charged rod =
λ/2 pi r ε0
E for an infinite charged thin plane sheet=
σ / 2 ε0
E for a thick charged sheet =
σ/ε0
E for a uniformly charged spherical shell =
σ R^2 / r^2 ε0
E inside the shell = 0
E on the surface = σ /ε0
