Content Strategy Manager
Electric Charge is the characteristic of a unit of matter that determines the extent to which it has more or fewer electrons than protons. Electric Charge helps to measure the way by which we can measure how much electric force an object feels. The observation of electrostatic phenomena lead to the concept of electric charge. Electric charge is also called: electrical charge, or electrostatic charge and is symbolized as q.
Key Terms: Electric Charge, Electromagnetic field, Electromagnetic force, Coulomb’s law, Proton, Electron, Electrostatic phenomena, Electrostatic Charge, Electric force
Electric Charge
[Click Here for Sample Questions]
Electric charge is the physical property carried by the matter that governs it to experience a force when it is kept in an electric or magnetic field. An electric charge is generated with an electric field whereas a magnetic field is generated by the moving electric charges. The interaction between the electric and magnetic fields is responsible for the formation of the electromagnetic force.
The field that is formed with the combination of electric field and magnetic field and where the electromagnetic field works are known as the electromagnetic field.
Charges are denoted by the symbol ‘q’ and the S.I. unit for measuring electric charge is Coulomb (C).
Electric Charge is a Scalar Quantity
The electric charge has both magnitude and direction but still, it is a ‘scalar quantity. The reason behind why it is termed as a scalar quantity is that apart from having a magnitude and direction if a quantity has to be termed as a vector then it should also obey the laws of vector addition such as the parallelogram law of vector addition.
But it is not possible in the case of an electric current. When two currents meet at a junction then the total sum of these currents is the algebraic sum and not the vector sum. Therefore, an electric current is a scalar quantity.
Here's a brief overview of important concepts related to Electric Charges:
| Parameters | Description |
|---|---|
| Definition | Electric charge is a basic property of matter. It is the force experienced by a mass when kept in an external field. |
| Symbol | q |
| Formula | q = I.t |
| SI Unit | Coulomb (C) |
| Other Units | Faraday, Ampere-Hour |
Also check:
| Important Concepts | ||
|---|---|---|
| Electroscope | Unit of Electric Flux | Difference Between Earthing and Grounding |
| Physical Significance of Electric Field | Gaussian Surface | Electric Field Formula |
Coulomb's Law
[Click Here for Previous Year Questions]
Coulomb is the SI unit of charge and it is named after Charles-Augustin Coulomb who was a French physicist.
Definition of one Coulomb-
One Coulomb can be defined as the quantity of charge transferred by one ampere current for one second.
It is represented as-
q= I.t
Where q= charge
I= Current
t= time duration
One Coulomb of charge contains around 6 × 1018 electrons.
Protons and Electrons
In an atom, the majority of charges are contained within the protons and electrons. The positive charge is carried by the protons while electrons carry the negative charge. The positive and negative charges i.e. the protons and the electrons repel each other as they are opposite which results in the development of electric fields.
Types of Electric Charges
[Click Here for Sample Questions]
There are two kinds of electric charge-
- Positive charge
- Negative charge
- Positive Charge (+): When an object has a positive charge then it means that it has more protons than electrons.
- Negative Charge (-): When an object has a negative charge then it means that it has more electrons than protons.
Note- If a matter has the same number of positive and negative charges then the charges will cancel out with each other and the matter would become neutral.
Read More: Gauss's Law
Properties of Electric Charge
[Click Here for Previous Year Questions]
- Electric charges are additive - The charges behave like scalars and can be added directly. For example, if there are two charges in a system which are named q1 and q2, the total charge of that system can be found by adding the two charges -
For n number of charges-
- A charge is a conserved quantity - Which means that an electric charge can not be created or destroyed. However, it can be transferred from one body to another with various methods such as conduction or induction.
- Quantization of charge - It implies that an electric charge is a quantized quantity and it can be expressed as an integral multiple of the basic unit of a charge.
Where,
- q = electric charge
- e = charge on one electron
- n = number of electrons.
Methods of Charging
[Click Here for Sample Questions]
Charging is the process of gaining electrons from an object or losing electrons from an object. There are 3 ways to charge an object:
- Using friction
- Using conduction
- Using induction
Charging by Friction
In this method, the electrons are transferred from one uncharged object to another by rubbing the two objects together. One of the objects loses electrons while the other object gains electrons. The one that loses electrons attains a positive charge. The one that gains electrons attains a negative charge. This method of charging is commonly known as electrification by friction.
Charging by Conduction
In this method, charging an uncharged object is done by bringing it close to a charged object. The number of protons and electrons in a charged conductor is unequal. When an uncharged object is brought near it, it transfers its electrons to the uncharged object and stabilizes itself. This process is known as charging by conduction.
Charging by Induction
When the object is charged by bringing it in the vicinity of a charged body without bringing it in contact with the charged body, the process is known as charging by induction.
Read More: Electrolytic Capacitor
Forces Between Multiple Charges
[Click Here for Previous Year Questions]
According to the parallelogram law of addition, the force on any charge concerning several other charges is the vector sum of all the forces on that charge due to the other charges at that time. The individual forces are unaffected due to the presence of other charges. This is also known as the principle of superposition.
Let a system of three charges q1, q2, and q3.
The force acting on charge q1 due to two other charges q2 and q3 can be obtained by the vector addition of the forces.
So, if the force on charge q1 due to q2 and q3 respectively is given by
Thus the total F on q1 due to the two charges q2 and q3 is given as
F= F12 + F13
The equation of force can be generalized for the n number of charges.
The above equation of force can be generalized for the n number of charges.
The principle of superposition says that in a system of n number of charges say, q1, q2, …, qn, the force on q1 due to q2 is the same as given by Coulomb’s law which means it will remain unaffected in the presence of the other charges q3, q4, …, qn.
So the total force F1 of charge q1 is the vector sum of the forces F12 + F13…...F1n.
The vector sum is obtained by the parallelogram law of the addition of vectors.
Things to Remember
- Electric charge is the number of electrons or energy transferred from one body to another through different modes such as induction, conduction, etc.
- Universally, the charge is denoted by ‘q’, and its unit is called Coulomb.
- Properties of electric charge are: Electric charge is a scalar quantity, Electrical charge is connected with mass, Electrical charge is transferable, Electrical charge is quantized, and Electrical charge is invariant.
- There are three methods of charging a body: Charging by friction, Charging by conduction, and Charging by induction.
- The electric field is a mathematical representation of the magnitude and direction of the total electrical force shown by one unit of electric charge.
Also check:
Sample Questions
Ques 1: Name the S.I. Unit of Charge. (All India, 2014, 2 Marks)
Ans: The S.I. of charge is Coulomb (C).
Ques 2: Mention any three properties of charges. (All India, 2014, 2018, 2019, 2 Marks)
Ans:
- Electric charges are additive - If there are two charges in a system which are named q1 and q2 the total charge of that system can be found by adding the two charges -
\(q = q_1 + q_2\)
For n number of charges-
\(q_n = q_1 + q_2 + q_3 + q_4 + ..........q_n\)
- A charge is a conserved quantity - An electric charge can not be created or destroyed. However, it can be transferred from one body to another with various methods such as conduction or induction.
- Quantization of charge - It implies that an electric charge is a quantized quantity and it can be expressed as an integral multiple of the basic unit of a charge.
q = ne
Where,
q = electric charge
e = charge on one electron
n= number of electrons.
Ques 3: A positive point charge (+ q) is kept in the vicinity of an uncharged conducting plate. Sketch electric field lines originating from the point onto the surface of the plate.
Derive the expression for the electric field at the surface of a charged conductor. (All India, 2015, 3 Marks)
Ans: Electric Field due to positive charge can be represented as-
Electric Field due to positive charge can be represented as
Ques 4: Two identical metallic spherical shells A and B having charges + 4 Q and – 10 Q are kept a certain distance apart. A third identical uncharged sphere C is first placed in contact with sphere A and then with sphere B, spheres A and B are brought in contact and then separated. Find the charge on spheres A and B. (All India, 2011, 3 Marks)
Ans: When sphere A and sphere C are brought in contact and then separated, the charge on A and C will become 2Q on each nas the charge will be divided between them.
Now when B and C are brought in contact with each other, their total charge will become 2Q + (– 10 Q )= – 8 Q
After the equal division of charge, the charge on each will be – 4Q
Now when A and B came in contact with each other then their total charge will be + 2Q – 4Q = –2Q
Therefore the charge on each sphere A and sphere B will become – Q
Ques 5: Two point charges + 3q and – 4q are placed at the vertices ‘B’ and ‘C’ of an equilateral triangle ABC of side ‘a as given in the figure. Obtain the expression for
(i) the magnitude and
(ii) the direction of the resultant electric field at vertex A due to these two charges. (Comptt. All India 2014, 3 Marks)
Ans:
Ques 6: Two-point charges having equal charges separated by 1 m distance experience a force of 8 N. What will be the force experienced by them, if they are held in water, at the same distance? (Given Kwater= 80). (AI 2011, Comptt, 3 Marks)
Ans: The force acting on two-point charges is given by-
Ques 7: (a) Derive an expression for the electric field E due to a dipole of length ‘2a’ at a point distant r from the center of the dipole on the axial line.
(b) Draw a graph of E versus r for r >> a.
(c) If this dipole were kept in a uniform external electric field diagrammatically represent the position of the dipole in stable and unstable equilibrium and write the expressions for the torque acting on the dipole in both cases. (Outside Delhi 2017, 5 Marks)
Ans:
(a) Expression for the electric field-
(b) Only the faces perpendicular to the direction of the x-axis contribute to the Electric Flux. The remaining faces of the cube give zero electric flux.
(c) Graph between E Vs r
Ques. 8: Two-point charges q and – 2 q are kept d distance apart, find the location of the point relative to charge to q at which potential due to this system is zero. (All India 2014, 3 Marks)
Ans: Given that two charges q and – 2 q are placed at a distance ‘d’
Ques 9: An electric dipole moment p is held in a uniform electric field E.
(i)Prove that no translation force acts on the dipole.
(ii)Hence, prove that the torque acting on the dipole is given by pE sin 0 indicating the direction along which it acts. (Foreign 2008, 5 Marks)
Ans: (i) When the dipole is placed in a uniform electric field E, then force on +q charge due to electric field E will be
Ques 10: Deduce the expression for the torque acting on a dipole of dipole moment p in the presence of a uniform electric field E. (All India 2014; Delhi 2008, 5 Marks)
Ans: Consider an Electric Dipole consisting of two charges – q and + q placed in a uniform external electric field E. The length of the electric dipole is 2l. The dipole moment p makes an angle θ with the direction of the electric field.
Ques 11: Explain the meaning of the statement ‘electric charge of a body is quantized’. (1 mark)
Ans: Electric charge cannot be transferred infractions. It will always be an integral multiple of charge on a single electron.
Ques 12: Why can one ignore quantization of electric charge when dealing with macroscopic i.e., large-scale charges? (1 mark)
Ans: In large-scale charges, the magnitude of the charge is very large (as compared to a unit charge). Hence, although the charge is quantized, it is considered to be continuously flowing.
Ques 13: Charges appear on both when a glass rod is rubbed with a silk cloth. A similar phenomenon is observed with many other pairs of bodies. Explain how this observation is consistent with the law of conservation of charge. (1 mark)
Ans: In the cases described above, the charges are conserved. This is because if one body loses charge (i.e. if it becomes positively charged on rubbing), the other body gains electrons (i.e. it becomes negatively charged). Hence, the overall amount of charge on the 2 bodies combined does not change.
Ques 14: How are electric charges distributed within an atom? (1 mark)
Ans: All the subatomic particles have electric charges. The electric charges are distributed in the form of electrons and protons. The negative charges are called the electrons whereas the positive charges are called the protons which are present inside the nuclei of atoms.
Ques 15: Why is an electric charge a scalar quantity? (2 marks)
Ans: The electric charge has both magnitude and direction but still it is a ‘scalar quantity. The reason behind why it is termed as a scalar quantity is that apart from having a magnitude and direction if a quantity has to be termed as a vector then it should also obey the laws of vector addition such as the parallelogram law of vector addition.
But it is not possible in the case of an electric current. When two currents meet at a junction then the total sum of these currents is the algebraic sum and not the vector sum. Therefore, an electric current is a scalar quantity.
Ques 16: Two equal and opposite charges +q and -q are separated by a small distance ‘2a’. (4 Marks) [2011]
a) Name this arrangement.
b) Define its moment. What is its direction?
c) If the above system is placed in a spherical shell, wh would be the net electric flux coming out of it?
d) The above system of two charges is placed in an external electric field E, at an angle q with it. Obtain a relation for the torque acting on it.
Ans: a) Electric dipole
b) The electric dipole moment is the product of the magnitude of any one charge and dipole length = P =q2a. The direction of dipole moment is from negative charge to positive charge.
c) Zero
d)
Consider an electric dipole of dipole moment P = 2aq kept in a uniform external electric field, inclined at an angle O to the field direction.
torque = any one force x perpendicular distance t=qE x2asinø
Since P = 2aq
Ques 17: Two identical metallic spheres of exactly equal masses are taken. One is given positive charge q coulombs and the other an equal negative charge. Are their masses charging equal? (2 Marks)
Ans: No. The positive charge of a body is due to a deficit of electrons while the negative charge is due to a surplus of electrons. Hence the mass of the negatively charged sphere will be slightly more than that of the positively charged spheres.
Ques 18: How does a positively charged glass rod attract a neutral piece of paper? (2 Marks)
Ans: The positively charged rod induces a negative charge on the closer end and a positive charge on the farther end of the paper. The rod exerts greater attraction than repulsion on the paper because the negative charge is closer to the rod than the positive charge. Hence the rod attracts the piece of paper.
Ques 19: An ebonite rod held in hand can be charged by rubbing with flannel but a copper rod cannot be charged like this. Why? (2 Marks)
Ans: Ebonite rod is insulating. Whatever charge appears on it due to rubbing, stays on it. Copper is a good conductor. Any charge developed on it flows to the earth through our body. So copper rod cannot be charged like this. It can be charged by providing it with a plastic or rubber handle.
Ques 20: A comb run through one's dry hair attracts small bits of paper. Why? What happens if the hair is wet or if it is a rainy day? (2 Marks)
Ans: When the comb runs through dry hair, it gets charged by friction. The molecules in the paper get polarized by the charged comb, resulting in a net force of attraction. If the hair is wet, or if it is a rainy day, friction between hair and the comb reduces. The comb does not get charged and thus it will not attract small bits of paper.
Previous Year Questions
- In the given circuit, what will be the equivalent resistance between the points…. [JIPMER 2006]
- How to adjust a system of three identical capacitors to get high electrostatic energy with the given battery… [UPSEE 2006]
- The surface charge density of the Earth…. [DUET 2009]
- A point dipole with dipole moment... [KEAM]
- The time of fall of the electron, in comparison to the time of fall of the proton is….[NEET 2018]
- An electric dipole of dipole moment is placed in a uniform external electric field…. [KEAM]
- An infinite number of charge, each of charge…
- Infinite charges of magnitude q each are lying at… [JKCET 2004]
- Two long current carrying thin wires, both with current… [JEE Main 2015]
- Identify the wrong statement… [KEAM]
- An electron enters a uniform electric field maintained by parallel plates and… [KEAM]
- When a soap bubble is charged… [KCET 2020]
- Two pith balls carrying equal charges are suspended from a common point by… [NEET 2013]
- A glass rod rubbed with silk is used to change a gold leaf electroscope… [JEE Advanced 2011]
- A copper rod AB of length l is rotated about end A with a constant angular… [VITEEE 2021]
- Two charges of equal amount +Q are placed on a line… [WBJEE 2016]
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments