ELECTRIC CHARGES AND FIELDS Flashcards
1
Q
What is electricity?
A
The property of rubbed substances due to which they attract light objects.
2
Q
What is frictional or static electricity?
A
The electricity developed by rubbing or friction.
3
Q
What are the substances that show the property of static electricity called?
A
Electrified or electrically charged.
4
Q
What is an electric charge? Is it Scalar or Vector? Name its S.I unit.
A
Electric charge is an intrinsic property of elementary particles of matter (Eg: Electrons and Protons) which gives rise to electric forces of attraction or repulsion between various objects. It is a scalar quantity. It’s S.I unit is Coulomb (C).
5
Q
What is the value of e?
A
The value of e is 1.6 x 10^-19 C.
6
Q
Large-scale matter that consist of equal number of electrons and protons are said to be ____________
A
Electrically neutral.
7
Q
What does static mean?
A
Anything that does not move or change with time.
8
Q
What is electrostatics?
A
Electrostatics is the study of electric charges at rest.
9
Q
What is the property that differentiates the two kinds of charges?
A
The polarity of charge.
10
Q
How do we electrify a neutral body?
A
We add or remove a certain kind of charge.
11
Q
Show experimentally that there are two kinds of charges.
A
- Rub a glass rod with silk and suspend it from rigid support by means of a silk thread. Bring another similarly charged rod close to it. They both would repel each other.
Bring a plastic rod rubbed with wool close to the charged glass rod. The two rods would attract each other.
Now rub a plastic rod with wool and suspend it from a rigid support. Bring another similarly charged plastic rod close to it. They will repel each other.
- If a glass rod, rubbed with silk, is made to touch two small pith balls which are suspended by silk threads, then the two balls repel each other. Similarly, two pith balls touched with a plastic rod rubbed with fur are also found to repel each other. However, it is seen that a pith ball touched with a glass rod attracts another pith ball rubbed with a plastic rod.
12
Q
What is the fundamental law of electrostatics?
A
Like charges repel and unlike charges attract each other.
13
Q
Substances that readily allow the passage of electricity through them are called __________. Some examples are _____________. Does charge get readily distributed over its entire surface once transferred?
A
Conductors. Metals, Human and Animal bodies, and Earth. Yes.
14
Q
Substances that offer high resistance to the passage of electricity through them are called __________. Some examples are ____________. Does charge get readily distributed over its entire surface once transferred?
A
Insulators. Most non-metals like glass, porcelain, plastic, nylon, or wood. No.
15
Q
What’s the process of removal or addition of electrons?
A
When materials are rubbed, only a small fraction of electrons from the transferring body are transferred (loosely bound electrons).
16
Q
Forces that hold atoms and molecules in nature are all __________ in nature.
A
Electrical.
17
Q
Unlike charges brought in contact __________ each other’s effect.
A
Neutralize or Nullify.
18
Q
What is positive charge?
A
The charge developed on a glass rod when rubbed with silk is called positive charge.
19
Q
What is negative charge?
A
The charge developed on a plastic rod when rubbed with wool is called negative charge.
20
Q
What are the basic properties of electric charge?
A
Additivity, Conservation, and Quantization.
21
Q
What is meant by the additive nature of electric charge? Give its equation as well.
A
It means that the total charge of a system is the algebraic sum of all the individual charges located at different points inside the system.
q = q1 + q2 + … + qn (Negative charges are added along with the negative charge).
22
Q
What is meant by quantization of charge? Give its equation as well.
A
It means that all free charges are integral multiples of a basic unit of charge denoted by e. Thus charge q on a body is given by:
q = ne (Where n is any integer, positive or negative)
23
Q
When are charges assumed to be point charges?
A
If the sizes of charged bodies are very small compared to the distance between them, we consider them to be point charges.
24
Q
What does magnetism arise from?
A
Magnetism arises from charges in motion.
25
What is the cause of charging? Are electrons and protons both transferred?
The transfer of electrons from one material to another due to rubbing is the cause of charging. No new charges are created or destroyed. Only electrons are transferred.
26
What is the difference between mass and charge?
The difference between mass and charge is that mass is always positive while charge can be negative or positive.
27
By convention, what is the charge on an electron and a charge on a proton? How are they written as?
By convention, the charge on an electron is taken as negative and the charge on a proton is taken as positive. The charge on an electron is written as -e and the charge on a proton is written as +e.
28
Define 1 coulomb.
1 coulomb is the charge flowing through a wire in 1s if the current is 1A. It is also defined as the amount of charge that repels an equal and similar charge with a force of 9 x 10^9 N when placed in a vacuum at a distance of 1m from it.
29
What is a quantum?
The minimum amount by which a physical quantity can change is called a quantum.
30
What is quantization of a physical quantity?
The quantization of a physical quantity means that it cannot vary continuously to have any arbitrary value but it can change discontinuously to take any one discrete set of values.
31
What is the charge of an alpha-particle?
The charge of an alpha particle is +2e.
32
How is the quantization of charge an experimentally verified law?
The quantization of charge was first suggested by the experimental laws of electrolysis discovered by Faraday. It was experimentally demonstrated by Milliknan.
33
When can quantization of charge be ignored?
Quantization of charge can be ignored when dealing with macroscopic charges as the basic charge of e is very small and n is very large in most practical situations.
34
What does the law of conservation of charge state?
The law of conservation of charge states the following postulates:
1. The total charge of an isolated system remains constant.
2. The electric charge can neither be created nor destroyed. They can only be transferred from one body to another.
35
What is the relation of mass with speed?
The mass of a body increases with its speed.
36
State Coulomb's law in electrostatics? Is it valid for all charges? Express it in terms of S.I units too.
The force of attraction or repulsion between two stationary point charges is (i) directly proportional to the product of the magnitudes of the two charges and (ii) inversely proportional to the square of the distance between them.
It is only valid for point charges.
F = (kq1q2)/(r^2)
37
What is K? What is the value of K in S.I units?
K is the constant of proportionality called the electrostatic force constant.
K = (1)/(4piepsilonknot) = 9 x 10^9 Nm^2C^-2.
38
What is a torsion balance?
A torsion balance is a sensitive device used to measure force.
39
What is epsilon knot? What is its value in S.I units?
Epsilon knot is the permittivity of free space. Its values in S.I units is 8.85 x 10^-12 C^2N^-1m^-2.
40
Does Coulomb's law agree with Newton's third law? How so?
Yes, it does. Since r21=r12 and F21=F12.
40
What is a unit vector equal to?
A unit vector is equal to (vector)/(magnitude).
41
Why are Coulombian forces known as central forces?
They are known as central forces since the forces act along the line joining the two charges
42
What is permittivity?
Permittivity is a property of the medium which determines the electric force between two charges situated in that medium.
43
What is epsilon? What is (epsilon)/(epsilon knot)
The quantity epsilon is the absolute permittivity. The ratio is known as the relative permittivity or dielectic constant of the given medium.
44
Define dielectric constant. What is the value of kappa in air, vacumn and water?
The dielectric constant of a medium may be defined as the ratio of the force between two charges placed some distance apart in free space to the force between the same two charges when they are placed the same distance apart in a given medium. The value of kappa in the air is 1.00054, in a vacuum is 1 and in water is 80.
45
What are electrostatic force and gravitational force? Give two similarities and differences.
The electrostatic force is the force of attraction or repulsion between two charges at rest while gravitational force is the force of attraction between two bodies by virtue of their masses.
- Both forces obey the inverse-square law.
- Both forces are proportional to the product of Masses or charges.
- Gravitational forces are attractive while repulsive forces may be attractive or repulsive.
- Electrostatic forces are much stronger than gravitational forces.
46
Define principle of superposition? What is the value of total force exerted?
The principle of superposition states that when a number of charges are interacting, the total force on a given charge is the vector sum of the forces exerted on it due to all other charges.
F^2 = F1^2 + F2^2 + 2F1F2Costheta
47
Write Coulomb's law in vector form.
Coulomb's law in vector form is as follows:
(Kq1q2)(r unit vector)/(r^2)
48
Derive the principle of superposition.
49
Represent Coulomb's law in vector form.
50
What is q and q knot?
q knot is the small positive test charge while q is the source charge.
51
Define electric field. Is it a vector quantity? What is its S.I unit?
Electric field or electric intensity or electric field strength at a point is defined as the force experienced by a unit positive test charge placed at that point, without disturbing the source charge. Yes, it is. It is represented as N/C or V/M and its dimensions are [MLT^-3A^-1].
52
What is F equal to?
F is equal to qe.
53
What is a source charge and test charge?
A source charge is a charge which is producing the electric field while a test charge is a charge that tests the effect of the source charge.
54
What does the term 'field' mean in Physics?
The term 'field' in physics refers to a quantity that is defined at every point in space and may vary from point to point.
55
What is the physical significance of electric field?
The physical significance of an electric field is that it differs from point to point and isn't just a single vector, but a set of infinite vectors.
56
What is the relation between density of field lines and electric field?
Density of field lines is directly proportional to the electric field.
57
What are field lines? What are the properties of field lines?
A field line is a space curve. Some important general properties of field lines are as follows:
- Field lines start from positive charges and end at negative charges. If there is a single charge, they may start or end at infinity.
- In a charge-free region, electric field lines can be taken to be continuous curves without any breaks.
- Two field lines can never cross each other. (If they did, the field at the point of intersection will not have a unique direction)
- Electrostatic field lines do not form any closed loops. This follows from the conservative nature of the electric field.
58
What is continuous charge distribution? What is the general formula of force and electric field exerted on a test charge due to continuous charge distribution?
A charge distribution in which a charge is spread in a region in a continuous manner is called continuous charge distribution. Its general formula is:
F = (q knot)(integral dq/r^2)(r unit vector)/(4 pi epsilon knot)
E = (1)(integral dq/r^2)(r unit vector)/(4 pi epsilon knot)
59
What are the three different types of charge distributions?
Volume, Surface and Line charge distribution.
60
What are volume charge distribution and volume charge density? Give its formula and S.I unit.
It is a charge distribution spread over a three-dimensional volume of space. Volume charge density at any point in this volume is the charge contained per unit volume at that point.
p = (dq)/(dV)
p = (q)/(4/3pi R^3) Cm^-3
60
What are surface charge distribution and surface charge density? Give its formula and S.I unit.
It is a charge distribution spread over a two-dimensional surface of space. Surface charge density at any point in this surface is the charge contained per unit area at that point.
sigma = (dq)/(dS)
sigma = (q)/(4pi R^2) Cm^-2
61
What are line charge distribution and line charge density? Give its formula and S.I unit.
It is a charge distribution spread over a one-dimensional curve or line. Line charge density at any point in this line is the charge contained per unit length at that point.
lambda = (dq)/(dL)
lambda = (q)/(2piR) Cm^-1
62
What is an electric dipole? Define dipole moment and give its S.I unit and formula.
A pair of equal and opposite charges separated by a small distance is called an electric dipole. The dipole moment of an electric dipole is a vector whose magnitude is either charge times the separation between the two opposite charges and the direction is along the dipole axis from the negative to the positive charge.
Its formula and S.I unit are: p = 2aq Cm.
63
What is an Ideal or point dipole?
A dipole of negligibly small size is called an ideal or point dipole.
64
What is an Ideal or point dipole?
A dipole of negligibly small size is called an ideal or point dipole.
65
What is a dipole field?
An electric field produced by an electric dipole is called a dipole field.
66
What is the total charge of a dipole?
Zero.
67
What is an electric field at an axial point of dipole? Derive it.
(2kp)/r^3
68
What is an electric field at an equatorial point of dipole? Derive it.
(kp)/r^3
69
What is the comparison between electric field at axial and equatorial point?
E (axial) = 2E (equatorial)
70
Derive an expression for torque on an electric dipole placed in a uniform electric field. What is the net translating force on a dipole in a uniform electric field.
Zero.
71
What is the formula of torque? Represent it in vector form as well.
t = pE sin(theta)
t = p x E
72
What is an electric line of force?
An electric line of force may be defined as the curve along which a small positive charge would tend to move when free to do so in an electric field and the tangent to which at any point gives the direction of the electric-field at that point.
73
Positive point charge: They are directed radially outwards because a
small positive charge would be accelerated in the outward direction. They extend to infinity. The field is
spherically symmetric i.e., it looks same in all directions, as seen from the point charge.
Negative point charge: Like that
of a positive point charge, the electric field of a negative point charge is also spherically symmetric but the lines of force point radially inwards as shown in. They start from infinity.
Two equal and opposite point charges: They start from the positive charge and end on the negative charge. The lines of force seem to contract lengthwise as if the two charges are being pulled together. This explains attraction between two unlike charges. The field is cyllindrically symmetrical about the dipole access.
Field lines of two equal and opposite point charges: They seem to exert a lateral pressure as if the two charges are being pushed away from each other. This explains repulsion between two like charges. The field E is zero at the middle point N of the join of two charges. This point is called neutral point from which no line of force passes. This field also
has cylindrical symmetry.
Field lines of a positively charged plane conductor: A small positive charge would tend to move normally away from the plane conductor. The lines of force are parallel and normal to the surface of the conductor
74
How is area vector specified?
The direction of a planar area vector is specified by the normal to the plane.
75
What is a flux and what is electric flux? What is its formula? Is it a scalar quantity? Define it in terms of its S.I units. What is flux density?
The term flux implies some kind of flow. Flux is the property of any vector field. The electric flux through a given area held inside an electric field is the measure of the total number of electric lines of force passing through that area.
Delta phi E = E delta S cos theta = E . delta S
Yes, it is a scalar quantity. Nm^2C^-1
Flux density is equal to total flux by unit area.
76
What is Gauss theorem? Express it mathematically.Deduce Coulomb's law from Gauss theorem.
Gauss theorem states that the total flux through a closed surface is 1/epsilon knot times the net charge enclosed by the closed surface.
Mathematically, it can be expressed as:
Phi E = integral over a closed loop (E.dS) = q/epsilon knot
77
What is a gaussian surface? What is the net flux through a closed surface due to a charge lying outside the closed surface?
Any hypothetical closed surface or
enclosing a charge is called the Gaussian surface of that charge. Zero.
78
Apply Gauss's theorem to calculate the electric field of a thin infinitely long straight line of charge, with a
uniform charge density of Lambda Cm^-1
E = (Lambda)/(2 epsilon knot pi r)
79
Apply Gauss's theorem to calculate the electric field due to an infinite plane sheet of charge.
E = (Sigma)/(2 epsilon knot)
80
Two infinite parallel planes have uniform charge densities of sigma1 and sigma2. Determine the electric field at points (i) to the left of the sheets, (ii) between them, and (iii) to the right of the sheets.
81
Two infinite parallel planes have uniform charge densities ± a. Determine the electric field in ti) the region between the planes, and (ii) outside it.
82
Apply Gauss's theorem to show that for a spherical shell, the electric field inside the shell vanishes, whereas outside it, the field is as if all the charge had been
concentrated at the centre.
