Physics-Chapter 1-"Electric charges and fields"

Electric charge

is a fundamental property of matter that causes particles to experience a force when kept in a electrical or magnetic field

Additivity of Charge

states that the total electric charge is the sum of all individual charges regardless of their signs.

Conservation of electric charge

Charge can neither be created nor destroyed, and the total charge before and after creation is a constant.

Quantization of charge

The fact that the electric charge is always an integral multiple of e.

Mathematical expression for quantization of charge

q=ne

What is the value of “e”?

e=1.602 × 10^-19 C

1 Coulomb of charge means?

I C of charge means there is exactly 1 coulomb of charge flowing through a wire in 1sec if the current is 1 A.

State Coulombs law?

States that the force between two charges is directly proportional to the product of magnitude of two charges and is inversely proportional to the square of the distance between them

Expression for coulombs Law?

F= K.(|q₁ . q₂|)/r²

what is the value of K?

K=1/4piE₀ = 9×10⁹ Nm²/C

value of E₀ ?

E₀=8.85 × 10^-12 N^-1 m^-2

Principle of Superposition?

The force on any charge due to a number of other charges is the vector sum of all the forces on the charge due to the other charges, taken one at a time. The individual forces are unaffected due to the presence of other charges.

Electric field

The region or space around a charged particle where its effect/force can be felt.

Expression for electric field

E=f/q

Electric field intensity

The electric force experienced per unit positive charge at a given point.

Expression for electric field intensity

E=(1/4.pi.E_o) x (Q)/r²

What is the direction of electric field lines?

Electric field lines start from [positive] charges and end on [negative] charges.

Can two electric field lines ever cross/intersect each other? Why?

No, If they crossed, the field would point in two different directions at that single point, which is impossible.

Do electrostatic field lines form continuous closed loops? Why or why not?

No, electric field lines form [discontinuous] loops because they cannot start and end on the same charge.

How to give the direction of an electric field?

Draw a tangent to the field line at a point.

How does the crowding (density) of field lines relate to field strength?

Crowded=Stronger electric field
Loose=Weaker electrical field

Property of field lines in a uniform electrical field

Field lines of forces are parallel

Electric flux

measures the total number of electric field lines passing through a given surface area in an electric field.(

Expression for electric flux

phi= E.ds

Electric dipole

it is a pair of two equal and opposite charges (q and-q) separated by a distance of 2a.

what is the direction of an electric dipole?

it is from [negative] to [positive]

Explain dipole moment?

Defined as the product of magnitude of one of the 2 charges and the distance between them

Expression for Dipole moment

p=2aq

Axial field

any location that lies directly along the axis (or extended axis) of a physical system.

Expression for axial field

E=(1/4.pi.E_o)x 2p/r³

Equatorial plane

means a plane perpendicular to the dipole passing through the dipole.

Expression for equitorial plane

E=-p/4.pi.E₀ r³

relation between axial field and equatorial field

E_axial = 2E_equitorial

Polar molecules

The molecules in which the centers of negative charges and of positive charges do not coincide. Hence having a permanent electrical dipole moment even in the absence of an electric field, eg:-H20

Non-Polar molecules

The molecules in which the centers of +ve charges and of -ve charges lie at the same plane. Dipole moment is 0 for non-polar molecules in the absence of electric field.
Eg-CO_2, CH₄

What is the net translational force(F_net ) on a dipole placed in a uniform electric field?

Zero,The forces on +q and -q are equal in magnitude but opposite in direction, so they cancel out.

What is the formula for the torque experienced by a dipole in a uniform electric field?

T=pxE or T=pEsinthetha

What is the direction of torque?

The direction of torque is perpendicular and into the plane containing p and E.

Dipole in a uniform field!

Dipole in a uniform field experiences rotational motion due to the formation of torque.

In a dipole parallel to the uniform field…

There is no [translational motion] or [Rotational motion]

Value of torque when dipole is parallel

T=0 due to thetha being zero

When a dipole is perpendicular to the electrical feild

The dipole experiances rotational motion

Value of torque when dipole is perpendicular?

T=pE due t thetha being 90, sinthetha=1

When dipole is anti parallel to electric field

the dipole experiences static motion(no motion)

Value of torque for antiparallel to dipole?

T=0 due to thetha being 180