Class 11 Physics Electric Charges Notes

Class 11 Physics Electric Charges Notes
UNIT 5
CLASS 11 PHYSICS ELECTRICITY

Electric Charges

Chapter 19 • Scan header: Chapters 19–21

Original Scanned PDF – View Notes

Open PDF Full Screen
Open PDF in Google Drive
Source scope: The supplied scan is marked Unit 5, Chapters 19–21. The seventh page begins “Electric Field” and “Test Charge”; those notes are retained here because they appear in the supplied PDF.

Electrostatic

The branch of physics which deals with electricity at rest is called electrostatic.

Electric Charge

The physical property of matter which causes it to experience a force when placed in an electromagnetic field is called electric charge. Its SI unit is coulomb.

Properties of Charge

  1. Like charges repel whereas unlike charges attract each other.
  2. Electric charges are quantized:
    q = ne
    where, n = ±1, 2, 3, …
    e = 1.6 × 10−19 C (charge of electron)
  3. Electric charge is a scalar quantity.
  4. Electric charge is a conserved quantity.
  5. The magnitude of charge on a body is not affected by the speed of the body.

Electrostatic Induction

When a charged body is brought near an uncharged body, equal and opposite charge is induced at the near end, but similar charge is induced at the far end. Such a process is known as electrostatic induction. Charge at the near end is called bound charge and charge at the far end is called free charge.

Electrostatic induction A positively charged body near a neutral conductor induces negative bound charge at the near end and positive free charge at the far end. + + + − − − + + + Bound charge Free charge Electrostatic induction
Electrostatic induction showing bound charge and free charge.

Induced Charge and Inducing Charge

The two kinds of charge that appear on the two ends of a conductor due to induction are called induced charge, but the charge present on the charged body which causes the other body to be charged is called inducing charge.

Inducing charge and induced charge A positively charged inducing body is placed near a conductor whose near end becomes negative and far end becomes positive. + + + Inducing charge − − + + Induced charge
Inducing charge and the induced charges on a conductor.

Note: Positive charge can be developed when a glass rod is rubbed with silk.

Negative charge can be developed when a rubber rod is rubbed with cloth or wool.

Charging of a Body by Electrostatic Induction

Charging a Body Negatively by Induction

To charge a body negatively by induction, a glass rod rubbed with silk is brought near it. During the charging of a body negatively by induction, the following steps should be done.

Charging a body negatively by electrostatic induction Four steps show a positively charged glass rod near conductor AB, earthing at B, removal of earth, and finally removal of the rod leaving negative charge spread over the conductor. Step I +++ AB −−− ++ Insulating stand Step II +++ AB −−− Step III +++ AB −−− Step IV AB −−−−
Charging a body negatively by induction: the four source steps.

Step I

Suppose an uncharged body AB is fitted on an insulating stand. If a positively charged glass rod is brought near the uncharged body AB, then end A acquires negative charge while end B acquires positive charge, as shown in figure (a).

Step II

Now end B of the uncharged body AB is earthed with the help of a metal wire. Hence charge at B moves to earth, as shown in figure (b).

Step III

Now the earthing is removed, keeping the glass rod still in its initial position. Then only negative charge remains on the conductor, as shown in figure (c).

Step IV

Finally, the positively charged glass rod is removed away from AB and negative charge spreads over the conductor, as shown in figure (d).

Result: In this way, we can charge a body negatively by induction.

Charging a Body Positively by Induction

To charge a body positively by induction, a rubber rod rubbed with cloth or wool is brought near it. During the charging of a body positively by induction, the following steps should be done.

Charging a body positively by electrostatic induction Four steps show a negatively charged rubber rod near conductor AB, earthing at B, removal of earth, and finally removal of the rod leaving positive charge spread over the conductor. Fig. (a) −−− AB +++ −− Fig. (b) −−− AB +++ Fig. (c) −−− AB +++ Fig. (d) AB ++++
Charging a body positively by induction: figures (a)–(d) from the scan.

Step I

Suppose an uncharged body AB is fitted on an insulating stand. If a negatively charged rubber rod is brought near the uncharged body AB, then end A acquires positive charge while end B acquires negative charge, as shown in figure (a).

Step II

Now end B of the uncharged body AB is earthed with the help of a metal wire. Hence charge at end B moves to earth, as shown in figure (b).

Step III

Now the earthing is removed, keeping the rubber rod still in its initial position. Then only positive charge remains on conductor AB, as shown in figure (c).

Step IV

Finally, the negatively charged rubber rod is removed away from AB and positive charge spreads over the conductor, as shown in figure (d).

Result: In this way, we can charge a body positively by induction.

Coulomb’s Law

The force of attraction or repulsion between two electric charges is:

  1. directly proportional to the product of their magnitudes, and
  2. inversely proportional to the square of the distance between them.

Derivation

Let us consider two charges q1 and q2 separated by distance r as shown in the figure. According to Coulomb’s law, the force experienced by them is given by:

Two charges separated by distance r Charge q1 is on the left, charge q2 is on the right, and the separation between them is labelled r. q₁ q₂ r Coulomb’s law
Two charges q1 and q2 separated by distance r.
F ∝ q1q2(i)
F ∝ 1/r2(ii)

Combining equation (i) and equation (ii):

F ∝ (q1q2)/r2
F = K(q1q2)/r2(iii)

Here, K is a proportionality constant whose value depends on the nature of the medium and the system of unit chosen.

Special Cases

Case (a): In SI System

(i) For Air Medium

K = 1/(4πε0) = 9 × 109 N m2/C2

Where, ε0 = 8.85 × 10−12 C2/N m2 is called the permittivity of free space or vacuum.

With this value, equation (iii) becomes:

F = [1/(4πε0)](q1q2/r2) (iv)

(ii) For Medium Other Than Air

K = 1/(4πε)

Where, ε is permittivity of the medium.

With this value, equation (iii) becomes:

F = [1/(4πε)](q1q2/r2) (v)

Case (b): In CGS System

In CGS system and air medium, K = 1. With this value, equation (iii) becomes:

F = q1q2/r2 (vi)

Relative Permittivity

The permittivity of any medium with respect to the permittivity of free space (or vacuum) is called the relative permittivity of that medium. It is denoted by εr and is given by:

εr = (permittivity of medium)/(permittivity of free space)
∴ εr = ε/ε0

The relative permittivity of the medium is also known as the dielectric constant of the medium. It is denoted by K.

K = εr = ε/ε0

Thus, for SI system and in the medium other than air:

F = [1/(4πε0εr)](q1q2/r2)

OR

F = [1/(4πε0K)](q1q2/r2)

Permittivity

The ability of a medium to pass the electric charge through that medium is called permittivity of that medium.

Electric Field

The space around the electric charge where the electric force of attraction or repulsion exists is called electric field.

Test Charge

The positive charge having unit magnitude is taken as test charge in electrostatics. It is denoted by q0.

q0 = +1 C
KEEP LEARNING View All Posts

Discussion

Share a helpful question, idea, or explanation with other students.

Leave a Comment

Write a clear question, answer, or helpful explanation.
Your email will not be published.

Download Our Offline App

Study class-wise notes even when internet is not available. Get the app from Play Store.

Nepal eNotes offline app preview
Get it on Google Play