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Electricity and Magnetism Level 2

A metal sphere is charged up to a total charge of 1 mC. It then touches an identical metal sphere without charge briefly and moves away. After it touches, what will be the total charge on the sphere in mC ?


The answer is 0.5.

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David Mattingly by David Mattingly

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2 solutions

Discussions for this problem are now closed

Rutvik Paikine
Dec 14, 2013

When a charged body touches another body which is uncharged i.e. neutral, it loses half its charge.

HENCE, 1mC/2 equals 0.5mC. The answer!!!

14 Helpful 0 Interesting 0 Brilliant 0 Confused

i thought the charge of the first body would be lost :( and trasfered to another body haha . im jst a student.

Jemuel Babina - 7 years, 5 months ago

Why does it happen? What law do you use?

Edward Jeremy Lo - 7 years, 5 months ago

Since charges flows from higher to lower potential, they will flow from the charged to the uncharged sphere till their potential is same.

Utsav Vadgama - 7 years, 5 months ago
Shashank Tiwari
Dec 18, 2013

A Metal Sphere Having Charge 1mC Is Touched To A Identical Sphere So The Tendency Of Charges Will Be To Spread Evenly To The Available Surface Area So...........The Charge On Any Of Sphere Will be Half Of That Of Original=1/2mC

1 Helpful 0 Interesting 0 Brilliant 0 Confused

does time of contact has no importance here ?

oindrila ghosh - 7 years, 5 months ago

But the total charge on a sphere can also be assumed to be concentrated at the center of the sphere. In that case how do you propose to apply the uniform distribution property? (P.S. Correct me if I'm wrong).

Kou$htav Chakrabarty - 7 years, 5 months ago

I Would Like To Clarify Your Doubt In Two Parts....& Would Suggest You To Skip The One You Know……. And Directly Move To Your Doubt Clarification Below 1)Electric Field Inside A Conductor Is Zero 2)Two Like Charges Repel Each Other Is Such A Way Which Ensures Greater Separation.

1)We Know That Electric Field Inside A Conductor Is Zero,If This Were Not So The Field Would Exert Forces On Conduction(Free) Electrons Which Are Always Present In A Conductor And Thus Current Would Always Exist Within A Conductor But It Is Not So There Is No Such Current Which Proves There Is No Electric Field Which Further Implies No Electric Flux Inside Conductor.It Contradicts The Fact Of Gauss Law.......So We Conclude That Any Volume Inside Conductor Is Neutral So The Charge Injected Anywhere Must Come Over The Surface.(It Hold True Even For Conductor Having Cavity)

2)It Would Be Known To You That Two Like Charges Repel……..So As To Attain A Minimum Potential They Try To As Far From Each Other As Possible{For Example Two Like Charges Will Arrange Themselves Linearly So That Angle Between Them Is 180 Degrees Similarly Three Charges Take Shape Of Triangular Planar With 120 Degrees Between Them And Four Charges A Tetrahedral Shape With Appx. 109.4 Degrees Between Them} So If A Charged Sphere is Touched To Neutral Sphere They Get Extra Space For Spreading {Of course They Cannot Leave Conductor Unless A External Force} So They Spread Evenly To Available Space For Effectiveness.

Now knowing These Two Facts, It Should Clarify Your Doubt. This Concept Is Used In Earthing To Allow The Excess Charge To Flow Into Earth To Avoid Shock. From You Query I Doubt That You Are Confusing With Charged Conductors. Any Charged Conductor Can Considered As A Point Charges At The Centre Or Any Concentric Circle To The Conductor FOR THEOROTICAL PURPOSES AND EASE, IT DOES NOT MEAN PRACTICALLY {A SPHERE WITH VOUME CANOOT BE CONSIDERED AS POINT SIZED PHYSICAL IDENTITY If Not Related With Something.} If This Were Case Earth Would Have Been Point Sized And We Were Not Been Able To Stand Upon It!!!!!

Any More Sensible Doubts Are Welcome And Sorry For Such A Long Clarification

Shashank Tiwari - 7 years, 5 months ago

Thanks! I'm glad you explained it so precisely. And yes I guess I forgot the question is from electrostatics and in electrostatics, there's a property that the charge on a conductor is always on the surface. Any alteration from this fact can cause us to deviate from the electrostatic model and go to the electrodynamics model(flowing charges). Now everything's fine.

Once again thanks!

P.S. I don't worry if the explanation was long or short as long as it can clarify my doubts ;)

Kou$htav Chakrabarty - 7 years, 5 months ago

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