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Methods used for charging - class-VIII

Description: methods used for charging
Number of Questions: 22
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Tags: forces active without contact static electricity physics
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Can conductors be charged by friction?

physics static electricity exchange of electrons methods used for charging electric charge

  1. Yes

  2. No

  3. It is not possible to charge conductor at all

  4. None of these

Show answer
Correct Option: B
Explanation:

Conductor cant be charged by friction.
They are charged by
$(1)$ Induction
$(2)$ Conduction

State whether the given statement is True or False :
It is possible to charge objects without ever touching the object to be charged to the one used to charge it. 

physics static electricity exchange of electrons methods used for charging electric charge

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

The statement given is that "it is possible to charge objects without ever touching the objects to be charged". This statement is true. This charging process is called charging by induction. It is a process by which a charged body makes a conductor electrically charged without physical contact. The charges on a body by the process of induction are known as induced charges.

Rubber has greater electron affinity than fur. A rubber object is charged by friction with animal fur. The rubber object is than used to charge an object A by the method of contact. Then object A will be charged

physics static electricity exchange of electrons methods used for charging electric charge

  1. positive

  2. negative

  3. neutral

  4. none of these

Show answer
Correct Option: B
Explanation:

The rubber object becomes negatively charged since it would acquire electrons from fur. It would then charge then object A with negative charge since contact charging result in charging an object with same type of charge.

An object A is charged by friction using animal furs. Animal fur has greater electron affinity than object A. What would be charge of object A?

physics static electricity exchange of electrons methods used for charging electric charge

  1. positive

  2. negative

  3. neutral

  4. none of the above

Show answer
Correct Option: A
Explanation:

Due to friction, objects gets charged. The electrons are transferred between two objects. The material with greater electron affinity will receives electrons, and the other will lose electrons. Thus, object will A loses electrons and will acquires positive charge.

Which depicts charging by conduction?

physics static electricity exchange of electrons methods used for charging electric charge

  1. When we to touch an uncharged conductor with a charged conductor, the charged conductor shares the current with uncharged conductor

  2. When we to touch an uncharged conductor with a charged conductor, the charged conductor shares the charge with uncharged conductor

  3. When we to touch an uncharged conductor with a charged conductor, the charged conductor shares the mass with uncharged conductor

  4. None of the above

Show answer
Correct Option: B
Explanation:

When we to touch an uncharged conductor with a charged conductor, the charged conductor shares the charge with uncharged conductor. This is called charging by conduction.

A neutral metal sphere is touched to a negatively charged metal rod. As a result the sphere will be_____ and the metal rod will be ________ . Fill in the gaps

physics static electricity exchange of electrons methods used for charging electric charge

  1. Positively charged

  2. Negatively charged

  3. Neutral

  4. None

Show answer
Correct Option: B
Explanation:

When the neutral metal sphere is touched with the negatively charged metal rod, both of them try to achieve same potential and hence some negative charge flows from rod to sphere for both of them to have negative charges.
Since the total charge is conserved, the sum of the negative charges in them would be equal to the originally present negative charge in the rod.

 Identify how the electrical energy is conducted through a material.

physics static electricity exchange of electrons methods used for charging electric charge

  1. free electrons impacting and transferring energy to each other

  2. the conductivity of the atoms in the material

  3. an energetic electron passing all the way through the material

  4. free electrons dissipating energy through friction

  5. atoms that hold on tightly to their electrons

Show answer
Correct Option: A
Explanation:

Free electrons in a material increases the mobility of flowing electrons into the material. The free electrons impact and transfer energy to each other. It increases the number of electrons that flow when same potential difference is applied.

Two identical conducting balls having positive charges $q _1$ and $q _2$ are separated by a distance r. If they are made to touch each other and then separated to the same distance, the force between them will be

physics static electricity exchange of electrons methods used for charging electric charge

  1. less than before

  2. same as before

  3. more than before

  4. zero

Show answer
Correct Option: C
Explanation:

If they are made to touch each other, the electrostatic conduction will occur. As a result the total charge will get distributed equally on both the balls because both the balls are identical in nature. 


Initially, $F \propto q _1q _2$      (Since $F = \dfrac{kq _1q _2}{r^2}$)


Now after making contact, charge on each ball will become $Q _1=Q _2=\dfrac{q _1+q _2}{2}$
Now, $F' \propto (\dfrac{q _1 + q _2}{2})^2$
(Because they are separated to the same distance) 


From above we can say that $F' > F$

An uncharged insulated conductor A is brought near a charged insulated conductor B. Then for charge and potential of B :

physics static electricity exchange of electrons methods used for charging electric charge

  1. both will remain constant

  2. both will change

  3. the charge will remain constant, but potential will decrease

  4. the charge will remain constant, but potential will increase

Show answer
Correct Option: C
Explanation:

By induction negative charge developes on surface of A close to B and positive charge developes on surface of A away from B, (when B carries positive charge). Therefore, overall potential of B decreases, charge of B remains unchanged.

A positively charged rod is brought near an uncharged conductor. If the rod is suddenly withdrawn, the charge left on the conductor will be :

physics static electricity exchange of electrons methods used for charging electric charge

  1. positive

  2. negative

  3. zero

  4. cannot say

Show answer
Correct Option: C
Explanation:

Positively charged rod induces negative charge on the side of the sphere closer to the rod and an equal positive charge on the side away from the rod. When the rod is withdrawn, the negative charges move to neutralise the equal positive charge. The charge left is zero.

A positive point charge $Q$ is brought near an isolated metal cube. Then :

physics static electricity exchange of electrons methods used for charging electric charge

  1. the cube becomes negatively charged

  2. the cube becomes positively charged

  3. the interior becomes positively charged and the surface becomes negatively charged

  4. the interior remains charge free and the surface gets nonuniform charge distribution

Show answer
Correct Option: D
Explanation:

The metal cube is a conductor. The charge inside the conductor is zero. So the interior remains charge free. As it is isolated metal so when positive charge is brought near it , surface charge will become non-uniform.

A positively charged rod is brought near an uncharged conductor. If the rod is then suddenly withdrawn, the charge left on the conductor will be

physics static electricity exchange of electrons methods used for charging electric charge

  1. positive

  2. negative

  3. zero

  4. not sure

Show answer
Correct Option: C
Explanation:
When a charged rod is brought near an uncharged conductor, it will attract its charges and induce an opposite charge on conductor.
As the charged rod removed, the attraction forces disappear so as the charge on conductor.

 A conducting wire is connected between two conducting spheres of equal size have a charge of -3C and +1C respectively. Find out the new charge on each sphere ? 

physics static electricity exchange of electrons methods used for charging electric charge

  1. -4C

  2. +4C

  3. -1C

  4. +1C

  5. Zero

Show answer
Correct Option: C
Explanation:

When the two spheres are connected by a wire, the charge will flow between them till the potential between them will be same. Thus, the charge on each sphere is the average charge i.e $Q _{av}=\dfrac{-3+1}{2}=-1 C$

When a charged aluminum plate is touched to a neutral metal sphere then neutral metal sphere gets charged up. This happens due to which method of charging ?

physics static electricity exchange of electrons methods used for charging electric charge

  1. Charging by conduction

  2. Charging by induction

  3. Charging by friction

  4. None

Show answer
Correct Option: A
Explanation:

When a charged aluminum plate is touched to a neutral metal sphere then neutral metal sphere gets charged up. This happens due to which method of charging. This happens due to charging by conduction method.


Option A is correct.

What is known as charging by induction method?

physics static electricity exchange of electrons methods used for charging electric charge

  1. charge created through influence of charged object and not by contact

  2. charge created through contact of charged object and not by influence

  3. charge not generated

  4. None

Show answer
Correct Option: A
Explanation:

When charging a conductor by induction, a charged object is brought close to but does not touch the conductor. In the end the conductor has charge of the opposite sign as the charge on the object.


Option A is correct.

Saran wrap has a larger electron affinity then Nylon. If Nylon is rubbed againest Saran wrap which would end up with the excess negative charge?

physics static electricity exchange of electrons methods used for charging electric charge

  1. Saran Wrap

  2. Nylon

  3. Both

  4. None

Show answer
Correct Option: A
Explanation:

Those with larger electron affinity will be more nagatively charged as it takes electrons from nylon.

Therefore, Saran wrap will get excess negative charge.

Option A is correct.

State whether given statement is True or False :

Charging by conduction includes contact of charged object with the neutral object.

physics static electricity exchange of electrons methods used for charging electric charge

  1. True

  2. False

Show answer
Correct Option: A
Explanation:

True.

Charging by conduction means charged object is in contact with neutral object.

Option A is correct.

A conducting sphere with radius 4.0 cm has a net positive charge on it. The sphere is then connected with a metal wire to a second, uncharged, sphere of radius 1.0 cm.
Which statement below BEST describes the flow of electrons when the metal wire connects the two spheres?

physics static electricity exchange of electrons methods used for charging electric charge

  1. Electrons flow from the 4.0-cm radius sphere to the 1.0-cm radius sphere, leaving the 4.0-cm radius sphere with a net charge of zero.

  2. Electrons flow from the 1.0-cm radius sphere to the 4.0-cm radius sphere, until both spheres have the same net positive charge.

  3. Electrons flow from the 4.0-cm radius sphere to the 1.0-cm radius sphere until both spheres have the same net positive charge.

  4. Electrons flow from the 1.0-cm radius sphere to the 4.0-cm radius sphere until the surfaces of both spheres are at the same electric potential.

  5. Electrons do not flow along the wire, because the system is already in equilibrium.

Show answer
Correct Option: A
Explanation:

Given a conducting sphere, radius $4\, cm$ has positive charge on it. This sphere is then connected to uncharged sphere of radius $1\, cm$. So, when the two spheres are connected through a metal wire, the entire charge flow from charged to uncharged sphere i.e, electrons flow from $4\, cm$ sphere to$1\, cm$ sphere, leaving $4\, cm$ radius sphere with net charge zero.

 Which of the following charging methods result in charging an object opposite to the charge on the object used to charge it?

physics static electricity exchange of electrons methods used for charging electric charge

  1. charging by friction

  2. charging by contact

  3. charging by induction

  4. both A and C

Show answer
Correct Option: D
Explanation:
When two electrically neutral objects are rubbed against each other, some of the electrons get transferred from surface of one object to the other and thus the two objects become electrically charged with opposite polarity. This process is known as charging by friction. 
When a charge $Q$ is brought near an uncharged conductor, the charge $Q$ induces a charge of opposite polarity on the facing surface of the conductor and the same polarity on the rearer surface. This process of inducing a charge on the conductor is known as induction.
Charging by contact induces charge of same polarity on the neutral conductor.
Thus charging by induction and charging by friction are the two methods which results in charging an object opposite to the charge on the object used to charge it.

A solid sphere of radius R has a charge Q distributed in its volume with a charge density, $\rho ={ kr }^{ a }$, where k and a are constants and r is the distance from its centre. If the electric field at $r=\dfrac { R }{ 2 } $ is $\dfrac { 1 }{ 8 } $ times that at r=R, then the value of a is 

physics static electricity exchange of electrons methods used for charging electric charge

  1. 2

  2. 4

  3. 6

  4. 7

Show answer
Correct Option: D

The linear charge density of a thin metallic rod varies with the distance $'x'$ from one end as $\lambda  = {\lambda _0}{x^2}\left( {0 \leqslant x \leqslant l} \right).$ The total charge on the rod is:

physics static electricity exchange of electrons methods used for charging electric charge

  1. $\dfrac{{{\lambda _0}{l^3}}}{3}$

  2. $\dfrac{{{\lambda _0}{l^4}}}{3}$

  3. $\dfrac{{2{\lambda _0}{l^3}}}{3}$

  4. $\dfrac{{{\lambda _0}{l}}}{2}$

Show answer
Correct Option: D
Explanation:
$\lambda =\dfrac {\lambda _0 x}{L}$
$Q=\displaystyle \int _0^L \lambda \ dx$
$=\displaystyle \int _0^L \dfrac {\lambda _0 x}{L}dx$
$=\dfrac {\lambda _0}{L} \dfrac {x^2}{2}\displaystyle \int _0^L$
$=\dfrac {\lambda _0}{L}\times \dfrac {L^2}{2}$
$=\dfrac {\lambda _0L}{2}$

A hollow metal sphere, Sphere A, sits on an insulating stand. Sphere A has a diameter of 4 inches, and a net charge of magnitude $Q _0$. A second hollow metal sphere, Sphere B, also sits on an insulating stand, but has a diameter of 8 inches and zero net charge. The two spheres are brought close so that they touch, then they are separated.
In terms of $Q _0$, what is the final charge on Sphere A?

physics static electricity exchange of electrons methods used for charging electric charge

  1. $\cfrac{Q _0}{5}$

  2. $\cfrac{Q _0}{4}$

  3. $\cfrac{Q _0}{2}$

  4. $Q _0$

  5. $4Q _0$

Show answer
Correct Option: A
Explanation:

$R _B=2R _A$


Let final charge on spheres be $Q _A$ and $Q _B$.

On touching the spheres charge density becomes same.

And, charge density, $\sigma=\dfrac{Q}{4\pi R^2}$

$\implies Q\propto R^2$ for same $\sigma$

Hence, $\dfrac{Q _B}{Q _A}=\dfrac{R _B^2}{R _A^2}=2^2$

$\implies Q _B=4Q _A$

And, total charge$=Q _B+Q _A=Q _o$

$\implies 4Q _A+Q _A=Q _o$

$\implies Q _A=\dfrac{Q _o}{5}$

Answer-(A)

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