Electric charges and fields Flashcards
(13 cards)
Define electric charge.
—> It is the fundamental property of matter where an another charged substance/particle experiences a force of attraction or repulsion is defines as an electric charge.
—> SI units : Coulombs (C)
—> It is the source of electric force and is conserved in nature.
Explain the properties of electric charges.
—> The properties of charges are given below :
1. Additivity of charges
The total number of charges on the surface equals the algebraic sum of the individual charges present.
Let n number of charges be present on a given surface, then
qnet = q1+ q2+ q3+ q4+ …….. + qn
- Quantisation of charges
The charge of a body is an integral multiple of a basic charge, which is the electronic charge.
Q = ±ne
where Q is the charge, n is the integral multiple, and E is the charge of an electron ( i.e., 1.6 * 10^-19 ) - Conservation of Charge
It defines that the total charge of an isolated system remains constant. It is not possible to create or destroy net charge carried by an isolated system, although the charge-carrying particles may be created or destroyed in a process. - Association with mass
The mass of a positive charge is slightly less than that of a neutral charge, and similarly mass of a negative charge is slightly greater than that of a neutral charge due to the addition/removal of an electron from the atom. - Interaction between charges
Like charges repel each other, and unlike charges attract each other.
State and explain coulomb’s law.
.Coulomb’s law states that the electrostatic force of attraction or repulsion between two point/stationary charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them.
Mathematically,
F = kq1q2/r^2
where {k = 1/4πεo}
Define one coulomb charge.
One coulomb is the amount of electric charge transferred by a current of one ampere in one second.
or
If a current of 1 ampere flows through a conductor for 1 second, it transfers a charge of 1 coulomb.
What are conductors and insulators?
Conductors are substances that allow electricity
to be passed through them. E.g.. Metals, human and animal bodies, and the earth are
conductors.
The substances that offer high resistance to the passage of electricity through them are called Insulators. E.g.. glass, porcelain, plastic, nylon, wood.
Differentiate between conductors and insulators.
- They have electric charges (electrons) that are comparatively free to move inside the material.
- When some charge is transferred to a conductor, it readily gets distributed over the entire surface of the conductor.
- If some charge is put on an insulator, it stays in the same place.
- Conductors provide low resistance, whereas insulators provide high resistance to the charge supplied.
Define electric field.
It is defined as the region around the electric charge where its effect can be experienced.
SI Unit: Newton per Coulomb (N/C)
E = F/q
It is a vector quantity
Define electric field lines and also mention their properties.
An electric field line is 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.
▪ Electric Field lines start from a positive charge and end at a negative charge.
▪ Electric field lines of a positive charge are radially outwards, and
those of a negative charge are radially inwards
▪ Electric field lines do not form closed loops.
▪ In a charge-free region, field lines are continuous.
▪ Two field lines never intersect.
(Two directions for the electric field are not possible at a point.)
▪ Field lines are parallel, equidistant, and in the same direction in a uniform electric field.
Define electric flux.
Electric flux is a measure of the number of electric field lines passing through a given surface. It represents how much of the electric field “flows” through that surface.
It is a scalar quantity
SI unit: Nm^2/C
Electric flux through a surface is defined as:
𝛟=∫𝐄⋅ⅆ𝐒
𝛟=∫𝐄ⅆ𝐒 𝐜𝐨𝐬𝛉
( 𝛉 𝐢𝐬 𝐭𝐡𝐞 𝐚𝐧𝐠𝐥𝐞 𝐛𝐞𝐭𝐰𝐞𝐞𝐧 𝐄 𝐚𝐧ⅆ 𝐧𝐨𝐫𝐦𝐚𝐥 𝐭𝐨 ⅆ𝐒)
Where:
* ΦE is the electric flux,
* E is the magnitude of the electric field,
* A is the area of the surface,
* θ is the angle between the electric field and the normal (perpendicular) to the surface,
What is a continuous charge distribution, and mention its types.
When electric charge is spread over a body or region uniformly or non-uniformly rather than being located at discrete points, it is called a continuous charge distribution.
1. Linear Charge Distribution
Charge is distributed along a line (e.g., a thin wire).
𝝀=𝒒/𝒍
The unit of 𝝀 is C/m
Example: A charged rod.
- Surface Charge Distribution
Charge is distributed over a surface (e.g., a metal sheet).
𝝈=𝒒/𝑺
The unit of 𝝀 is C/m^2
Example: A charged metal plate. - Volume Charge Distribution
Charge is distributed throughout a volume (e.g., a charged sphere).
𝛒=𝒒/𝑽
The unit of 𝝀 is C/m^3
Example: A charged cloud or solid sphere.
Define an electric dipole and electric dipole moment.
a. An electric dipole is a system of two equal and opposite point charges separated by a small fixed distance.
Definition:
An electric dipole consists of:
A charge +q and a charge –q are separated by a distance 2a.
The total charge of the system is 0.
b. Dipole Moment:
It is the product of the magnitude of one of the charges and the distance between the charges.
The strength of an electric dipole is characterized by its electric dipole moment (p),
p = q * 2a
Where:
q is the magnitude of one of the charges,
2a is the vector pointing from the negative charge to the positive charge.
p is a vector quantity, and points are directed from –q to +q along the dipole axis.
State Gauss’ law.
Gauss’s law states that the total electric flux through a closed surface is
equal to 𝟏/𝜺𝟎 times the total charge enclosed by the surface.
𝝓=∮𝑬⋅ⅆ𝑺=𝒒/𝜺0
Features of Gauss’s Law
* Gauss’s law is true for any surface, irrespective of size and shape.
* The charge includes the sum of all charges enclosed by the surface.
* The surface that we choose for the application of Gauss’s law is called the Gaussian Surface.
* Gauss’s law applies to both symmetric and asymmetric systems, but it will be much easier if the system has some symmetry.
* Gauss’s law is based on the inverse square dependence on distance contained in Coulomb’s law.
Explain the principle and mechanism of gold leaf electroscope in detail.
Gold Leaf Electroscope
A gold leaf electroscope is a simple scientific instrument used to detect the presence and nature of electric charge on a body. It operates on the principle of electrostatic repulsion.
Principle:
The gold leaf electroscope works on the principle that:
Like charges repel each other.
When a charged object touches or comes near the conducting top of the electroscope, the charge spreads down the conductor to the leaves, causing them to repel and diverge if they receive like charges.
Construction:
A typical gold leaf electroscope consists of:
1. Metallic Rod – A vertical conductor with a metal disc or knob at the top.
2. Gold Leaves – Two very thin gold (or aluminum) leaves attached to the lower end of the rod inside a glass enclosure.
3. Glass Case – Protects the gold leaves from air currents.
4. Insulating Stopper – Holds the metal rod in place and insulates it from the case.
5. Earthing Strip – A metal plate at the base to allow safe discharge.
Working Mechanism:
1. Charging the Electroscope:
There are two main ways:
* By Conduction (Contact):
○ A charged object touches the metal knob.
○ Charge is transferred to the rod and gold leaves.
○ The leaves receive the same type of charge and repel each other.
○ The divergence indicates that the electroscope is charged.
* By Induction (No Contact):
○ A charged object is brought close to the knob.
○ Opposite charge is induced on the knob and similar charge on the leaves.
○ Leaves diverge temporarily.
○ If the electroscope is earthed during this process, it retains the induced charge even after removing the external object.
2. Discharging the Electroscope:
* Touching the knob with a finger allows the charge to flow into the earth (grounding).
The leaves collapse due to a loss of charge.
Conclusion:
The gold leaf electroscope is a classical yet effective device for observing electrostatic phenomena, and it beautifully demonstrates basic principles of electric charge and induction.
