Electric Charges and Fields - Class 12 Physics

Physics Unit 1 - Electric Charges and Fields study cards based on the Vigeta Batch lecture transcript.

What branch of physics studies charges that are stationary?

Electrostatics

What is the primary subject of unit electrostatics?

The study of charges at rest

How many types of electric charges exist?

Two types

What are the two specific types of electric charges?

Positive and negative

What subatomic particle is considered to have a positive charge?

Proton

What subatomic particle is considered to have a negative charge?

Electron

What is the convention for the charge of a proton?

Positive

What is the convention for the charge of an electron?

Negative

If a body loses electrons, what happens to its charge?

It becomes positively charged

If a body gains electrons, what happens to its charge?

It becomes negatively charged

What is a positively charged body?

An object that has donated or lost electrons

What is a negatively charged body?

An object that has taken or gained electrons

What symbol is commonly used to denote charge?

$Q$ or $q$

What is the magnitude of the charge of an electron?

$$1.6 \times 10^{-19}\,C$$

What is the charge of a proton in Coulombs?

$$1.6 \times 10^{-19}\,C$$

What is the SI unit of charge?

Coulomb ($C$)

What property states that charges can be simply added together?

Additive property of charge

Can charges be added like scalars?

Yes, they follow the additive property

If you have $+2\,C$, $+3\,C$, and $-2\,C$ at a point, what is the total charge?

$$3\,C$$

What does the conservation of charge state for an isolated system?

Total charge remains constant

Can charge be created or destroyed according to conservation law?

No, it can neither be created nor destroyed

In the silk cloth experiment, if electrons move to the pipe, what charge does the pipe gain?

Negative charge

In the rubbing experiment, if a pipe gains negative charge, what happens to the silk cloth?

It becomes positively charged

What is a neutral object?

An object with an equal number of protons and electrons

What is the net charge of a neutral object?

$$0$$

What property describes that charge exists only in discrete integral multiples?

Quantization of charge

What is the fundamental unit of charge in quantization?

$e$ ($1.6 \times 10^{-19}\,C$)

What is the formula for total charge on a body using quantization?

$$Q = \pm ne$$

In the formula $Q = ne$, what does $n$ represent?

An integer ($1, 2, 3, \dots$)

If an object gains $6$ electrons, what is the value of $n$ in the charge formula?

$$6$$

Can a person be rejected by '10.5' people in a charge analogy?

No, it must be an integer; similarly, charge must be integral multiples of $e$

What is the mass of an electron as mentioned in the lecture?

$$9.1 \times 10^{-31}\,kg$$

What is the charge of an electron used in calculations?

$$1.6 \times 10^{-19}\,C$$

Does the magnitude of charge change with the speed of the body?

No, charge is invariant

What does 'charge is invariant' mean?

Charge remains constant regardless of the speed at which it moves

Which property of matter is variant (changes with high speed)?

Mass (according to Einstein's relativity)

Who provided the theory of relativity regarding mass variance?

Einstein

What does Coulomb's Law state regarding the relationship between force and charge magnitude?

Force is directly proportional to the product of the charges

What is the relationship between electrostatic force and distance between charges?

Force is inversely proportional to the square of the distance

What is the formula for Coulomb's Law in vacuum?

$$F = \frac{1}{4\pi \epsilon_0} \frac{Q_1 Q_2}{r^2}$$

What does $\epsilon_0$ represent?

Permittivity of free space (vacuum)

What is the approximate value of the constant $k$ in vacuum?

$$9 \times 10^9\,N\,m^2\,C^{-2}$$

What is the relationship between like charges?

They repel each other

What is the relationship between unlike (opposite) charges?

They attract each other

Where is the force of attraction or repulsion directed between two point charges?

Along the line joining the centers of the charges

What is relative permittivity also known as?

Dielectric constant ($k$ or $\epsilon_r$)

What is the formula for permittivity of a medium ($\epsilon_m$)?

$$\epsilon_m = \epsilon_0 \times \epsilon_r$$

In the vector form of Coulomb's Law, what does $\mathbf{F_{21}}$ denote?

Force on charge $2$ due to charge $1$

What is a unit vector used for in Coulomb's Law equations?

To define the direction of the force

How do you calculate the number of electrons in $1\,C$ of charge?

$$n = Q/e = 1 / (1.6 \times 10^{-19})$$

What are the three categories of materials based on conductivity discussed?

Conductors, Insulators, and Semiconductors

What is a conductor?

A material that allows electricity to flow through it easily

What is an insulator?

A material that does not allow electricity to flow through it

What is the charging method involving rubbing two objects?

Charging by friction

In charging by friction, what causes the transfer of electrons?

Heat or energy generated by rubbing

What is the charging method involving physical contact between a charged and neutral conductor?

Charging by conduction

What happens in charging by conduction when two identical conductors touch?

Charge is equally distributed between them

What is charging by induction?

Charging a neutral object without physical contact

What process is used in induction to remove unwanted like charges?

Earthing

What is a Gold Leaf Electroscope used for?

To detect the presence and nature of electric charge

What metal is typically used for the disk in a leaf electroscope?

Brass

Why do the leaves of a Gold Leaf Electroscope diverge?

Due to electrostatic repulsion between like charges

What is the Superposition Principle for electrostatic forces?

The net force on a charge is the vector sum of all individual forces from other charges

Does the presence of a third charge change the force between two existing charges?

No, the individual force between two charges remains constant

What is an electric field?

The region around a charge where another charge experiences an electric force

What is electric field strength (intensity)?

The force experienced per unit positive test charge ($E = F/q$)

What is the formula for electric field due to a point charge?

$$E = \frac{1}{4\pi \epsilon_0} \frac{Q}{r^2}$$

What charge is conventionally used as a test charge?

A unit positive charge ($+1$)

What is the direction of electric field lines for a positive charge?

Radially outwards (away from the charge)

What is the direction of electric field lines for a negative charge?

Radially inwards (towards the charge)

Can two electric field lines intersect?

No

Why can electric field lines never intersect?

Because at the point of intersection, there would be two directions for the net electric field, which is impossible

Do electric field lines form closed loops?

No

Where do electric field lines start and end?

They start from positive charges and end at negative charges

What does the tangent at any point on an electric field line represent?

The direction of the electric field at that point

What does a higher density of field lines in a region signify?

A stronger electric field

What is a uniform electric field?

A field represented by parallel, equidistant field lines where intensity is same at all points

What is an electric dipole?

A pair of equal and opposite charges separated by a small distance

What is the formula for dipole moment ($\mathbf{p}$)?

$$\mathbf{p} = q \times 2l$$

What is the direction of the dipole moment vector?

From negative charge to positive charge

What is the SI unit of dipole moment?

Coulomb-meter ($C\,m$)

Wait, the term '2l' in dipole moment represents what?

The total separation distance between the two charges

What is the formula for electric field on the axial line of a dipole?

$$E_{axial} = \frac{1}{4\pi \epsilon_0} \frac{2pr}{(r^2 - l^2)^2}$$

For an ideal (short) dipole, what is the axial electric field formula?

$$E_{axial} = \frac{1}{4\pi \epsilon_0} \frac{2p}{r^3}$$

What is the formula for electric field on the equatorial line of a dipole?

$$E_{equatorial} = \frac{1}{4\pi \epsilon_0} \frac{p}{(r^2 + l^2)^{3/2}}$$

For an ideal dipole, what is the equatorial electric field formula?

$$E_{equatorial} = \frac{1}{4\pi \epsilon_0} \frac{p}{r^3}$$

What is the ratio of axial field to equatorial field for a short dipole at same distance?

$2:1$ ($E_{axial} = 2 E_{equatorial}$)

What is torque on a dipole in a uniform electric field?

$$\tau = pE \sin(\theta)$$

What is the vector expression for torque?

$$\mathbf{\tau} = \mathbf{p} \times \mathbf{E}$$

At what angle is the torque on a dipole in a uniform electric field maximum?

$$90^{\circ}$$

What is the torque when the dipole is parallel to the electric field?

Zero

What is stable equilibrium for a dipole in an electric field?

When the angle between $p$ and $E$ is $0^{\circ}$

What is unstable equilibrium for a dipole in an electric field?

When the angle between $p$ and $E$ is $180^{\circ}$

What is linear charge density ($\lambda$)?

Charge per unit length ($\lambda = Q/L$)

What is surface charge density ($\sigma$)?

Charge per unit area ($\sigma = Q/A$)

What is volume charge density ($\rho$)?

Charge per unit volume ($\rho = Q/V$)

What is the SI unit of linear charge density?

$$C\,m^{-1}$$

What is the SI unit of surface charge density?

$$C\,m^{-2}$$

What is electric flux ($\Phi$)?

Total number of electric field lines passing through a surface normally

What is the formula for electric flux?

$$\Phi = E \cdot A = EA \cos(\theta)$$