Informative line

Properties Of Charge

Learn formula of electric charge & charge of a proton in coulombs and positive, negative charge with electroscope definition, practice to calculate charge transfer of proton in physics.

Transfer of Charge and its Properties

  • When a plastic comb is rubbed against dry hair it gets electrically charged and thus it attracts tiny bits of paper when brought near it.
  • Similarly, when a glass rod is rubbed with silk and rubber rod with fur, both get electrically charged.

  • In figure (a), two charged rods attract each other and in figure (b) two charged rods repel.

Conclusion: Like charges repel and opposite charges attract.

  • Rubber rod possessed charge similar to electron while glass rod possessed charge similar to proton.
  • Charge on electron is considered to be negative while on proton, it is considered to be positive.
  • Sign of charge is chosen arbitrarily.
  • If magnitude of both positive and negative charge are equal ,then none of them is inferior to other.
  • To show additive nature like numbers, they are given positive or negative values.
  • If a metal sphere is transferred charge by two similarly charged rods, then it will have twice the amount of charge,as comparison to two when only one rod transferred charge to it.
  • But if an equal amount of positive and negative charge is given to a metal sphere, then

Total charge = q + (–q) = 0, i.e. it is neutral.

Note: 

  • Assigning of charge as positive or negative is done arbitrarily. Hence, glass rod is given positive and rubber rod is given negative charge.
  • S.I. unit of charge is Coulomb (C).

Illustration Questions

Which of the following charge is greater?

A –10 C

B 8 C

C 5 C

D –6 C

×

–10 C charge is greater. Negative sign only indicates opposite nature.

Which of the following charge is greater?

A

–10 C

.

B

8 C

C

5 C

D

–6 C

Option A is Correct

Repulsion is Sure Test of Charge

Case-I:

  • Consider two bodies in which one is positively charged. Find the kind of charge on other body when given that body A repels body B.

  • Since both the charges  repel each other, therefore they are like charges. It means B is positively charged.

Case-II:

  • Consider another case where one is charged negatively. Find the charge on another body when given that body A repels body B.

  • Since both the charges repel each other. So, they are like charges. It means B is negatively charged.

 

 

  • Now let us consider another situation. (Here B can be Negative, Positive, Neutral)

Case-1: Neutral

  • Due to presence of A, the nearer side of B gets negatively charged i.e. A attracts negative charge of B towards its side.
  • The other side of B gets positively charged.
  • Attraction force between positive charge of A and negative charge of B is greater than the repulsive force between positive charge of both A and B due to the distance of separation.
  • So, net effect will be attraction.

Case-2: Positive

  • Let us consider B to be neutral and remove few negative charges from polarized B.
  • Now, net charge on B is positive, even though it is attracted by A as the attractive force is dominant. 

Case-3: Negative

  • Since polarities of both A and B are different therefore, they attract each other.

Conclusion: Repulsion is a conclusive proof charge on a body.

Illustration Questions

We have three bodies A, B and C. A is positively charged and it attracts C but repels B. Comment on sign of charge on B and C.

A C may be negative, neutral or even positive and B is positively charged.

B Both are positively charged

C Both are negatively charged

D Both are neutral

×

Since A repels B and like charges repel each other. So, B is positively charged.

image

Since A attracts C. So, C can be negative, neutral or even positive.

image

We have three bodies A, B and C. A is positively charged and it attracts C but repels B. Comment on sign of charge on B and C.

A

C may be negative, neutral or even positive and B is positively charged.

.

B

Both are positively charged

C

Both are negatively charged

D

Both are neutral

Option A is Correct

Charge Calculation

Q.    Why is water molecule neutral?

Ans. Molecular formula for water is H2

2 Hydrogen 2 electrons
1 Oxygen 8 electrons
Total 10 electrons
  • Charge on \(1e^-\rightarrow1.6\times10^{-19}C\)
  • Total charge in a water molecule due to electrons

= Total electrons × charge on 1 e

 \(=10\times (-1.6\times10^{-19}C)\)

\(=-1.6\times10^{-18}C\)

  • But it consist of 10 proton and charge on proton is same as electron. So, charge on 10 proton  \(=10\times1.6\times10^{-19}C\)

\(=1.6\times10^{-18}C\)

  • Net charge in a water molecule

= Total charge of proton + Total charge of electron

\(=1.6\times10^{-18}+(-1.6\times10^{-18})C\) = 0

  • This is why H2O molecule is neutral.

NOTE: A neutral body is not chargeless. It has equal amount of both positive and negative charges. 

Illustration Questions

Calculate charge due to proton in CO2. Atomic number of carbon - 6 Atomic number of oxygen - 8

A 2.95 × 10–19 C

B 3.52 × 10–19 C

C 3.52 × 10–18 C

D 3.52 × 1019 C

×

Total number of protons in C = 6,

Total number of protons in O = 8

 

Total protons in CO2 = 6 + 8 × 2  \(\Rightarrow\)6 + 16 = 22

Charge on 1 proton = 1.6 × 10–19 C

Total charge = 22 × 1.6 × 10–19 C

= 3.52 ×10–18 C

Calculate charge due to proton in CO2. Atomic number of carbon - 6 Atomic number of oxygen - 8

A

2.95 × 10–19 C

.

B

3.52 × 10–19 C

C

3.52 × 10–18 C

D

3.52 × 1019 C

Option C is Correct

Transfer and removal of charge

  • Rubbing action causes charging of a body.
  • Excess of electrons in a body causes body to be negatively charged.
  • Deficiency of electrons causes body to be positively charged.

    Charge on electron = –1.6 × 10–19 C = e

    Charge on proton = 1.6 × 10–19 C = e+

  • If a body has net charge –q, then number of excess electron in body \(\dfrac {|\;-q\;|}{1.6\times10^{-19}C}\)
  • If a body has net charge +q, then number of electrons removed \(\dfrac {|\;q\;|}{1.6\times10^{-19}C}\) 

or \(|\;q\;|=n|\;e^-\;|\)

 

Illustration Questions

10 electrons are removed from a body. Calculate net charge on the body.

A – 1.6 × 10–19C

B 1.6 × 10–18C

C 1.6 × 10–19C

D – 1.6 × 10–18C

×

Total charge is given as \(|\;q\;|=n|\;e^-\;|\)

where q = total charge, n = number of electrons, e = charge on electrons

So, | q | = 10 × | – 1.6 × 10–19C |

= 1.6 × 10–18

10 electrons are removed from a body. Calculate net charge on the body.

A

– 1.6 × 10–19C

.

B

1.6 × 10–18C

C

1.6 × 10–19C

D

– 1.6 × 10–18C

Option B is Correct

Distribution of Charge on Metallic Sphere

Case-I

  • When a positive rod is brought near a metallic sphere, the nearer side of sphere gets negatively charged and farther side gets positively charged.

 

Case-II

 

  • Similarly if a negative rod is brought near metallic sphere, the nearer side gets positively charged and farther side gets negatively charged.

Illustration Questions

A positive charge is brought near a metallic sphere. Choose the correct statement.

A Electrons move to left and left part gets positively charged.

B Electrons move to left and left part gets negatively charged.

C Electrons move to right and left part gets positively charged.

D Electrons move to right and right part gets positively charged.

×

When a positive charge is brought near a metallic sphere, the nearer side of sphere gets negatively charged and farther side gets positively charged.

Hence, option (B) is correct.

A positive charge is brought near a metallic sphere. Choose the correct statement.

image
A

Electrons move to left and left part gets positively charged.

.

B

Electrons move to left and left part gets negatively charged.

C

Electrons move to right and left part gets positively charged.

D

Electrons move to right and right part gets positively charged.

Option B is Correct

Electroscope

  • An electroscope is a device consisting of a metal ball, attached by a conductor to thin leaves of metal foil in a jar protected from air disturbances.

  • Electroscope is useful for both, charge detection and measuring.
  • The more the charge transferred to the ball, the more the leaves get diverged.
  • As charged rod brought near to metal ball it get opposite charge due to attraction and leaves got same type of charge as of rod.
  • Hence,the leaves get diverged.

Illustration Questions

A charged body of 5 C is brought in contact with electroscope. Now 5 C body is replaced by 10 C. The spreading in leaves will be

A Less

B More

C May decrease or may increase

D

×

More the charge transferred, more will be the divergence in leaves.

A charged body of 5 C is brought in contact with electroscope. Now 5 C body is replaced by 10 C. The spreading in leaves will be

A

Less

.

B

More

C

May decrease or may increase

D

Option B is Correct

Practice Now