Mechanisms for Transfer of Charge

Restless Electrons

How do objects gain “static” charge?

• Friction– Rubbing two electrically neutral objects together can

charge both objects – both gain OPPOSITE charge

• Conduction– Charged object TOUCHES another object – electrons

jump from one to the other in an attempt to EQUALIZE the charge on both objects

• Induction– Charged object comes near another object WITHOUT

TOUCHING – causes it to POLARIZE

Conductive Properties of Solids

Conductors

allow charge toflow through them

easily

high conductivity

Insulators

inhibit chargeflow

high resistivity

rubber

silicon

wood

pure water

metals

special

metal alloys

SUPER CONDUCTORS SEMI INSULATORS CONDUCTORS CONDUCTORS

sea water

Friction

This works best when two INSULATORS are rubbedagainst one another

ConductionNegative To Neutral

--

Excess electrons exert repulsive forceson one another, causing them tomove apart as much as possible

BOTH now have the same charge!

ConductionPositive To Neutral

+ +

Electrons in the neutral object experiencean attractive force. They neutralize some of the

positive charge, leaving both with a lack ofelectrons

BOTH now have the same charge!

InductionNegative To Neutral

Polarization

+ -

-

Excess electrons exert forceagainst nearby electrons in the

neutral object

InductionPositive To Neutral

Polarization

+ -

+

Electrons in the neutral object experiencean attractive force, which pulls them

closer to the charged object

GroundingSince the Earth is so large it

can equalizecharges from other objects very easily

The Earth can equalize charge by:

absorbing excess electrons from

negatively charged objects

giving freeelectrons to positively

charged objects

In either case, grounding a

chargedobject makes it

neutral

One symbol that is often used to show thatsomething is grounded is:

Electroscope

- -

- -

- - -

An electroscope is a device thatdetects CHARGE

When the scope is CHARGED, theLEAVES at the bottom will REPEL one another

The charged LEAVESexert force on oneanother, forcing

them apart

Electroscope – negative cond.

- -

- -

- - - Step 1

Negative rod touchesneutral scope

Step 2

Electrons spread outand charge plates

Step 3

Scope is now negativelycharged

Step 4

Scope is grounded allowing electrons to

escape

Step 5

Scope is once againneutral – excess electrons

are gone

Electroscope – positive cond.

++

+ +

- - -

- -

-

Step 1

Positive rod touchesneutral scope

Step 2

Electrons leave scopebecause they are attracted

to the positive rod

Step 3

Scope is now positively charged

Step 4

Scope is grounded –electrons from the ground

are attracted to the positivescope

Step 5

Scope is once again neutral – balanced charge

is restored

Electroscope – negative ind.

- + - -

Step 1

Negative rod brought neargrounded scope

Step 2

Scope polarizes as electronsat the top try to move away

from the charged rod.

Step 3

Rod is removed – electronsare attracted back to the top

and “re-neutralize” theelectroscope.

Electroscope – positive induction

+

- -

Step 1

Positive rod is brought nearneutral electroscope

Step 2

Electrons from the bottomof the electroscope are

attracted to thepositive rod – get pulled up.

The leaves get a positivecharge on them.

Step 3

Rod is removed. Electronsare attracted back into the

leaves by the protons,making the electroscope

neutral again.

+ +

Electroscope – ind. with ground

- + -

++ - -

Step 1

Negative rod brought neargrounded scope

Step 2

Excess electrons from therod force electrons from the

scope into the ground.

Step 3

Ground is removed while the rod is still nearby, keepingelectrons from re-entering

the scope

Step 4

Scope now has a charge thatis opposite to the charge on

the rod

Step 5

The rod is taken away and thepositive charge remains

on the scope

Electroscope – ind. with ground

+

- - -

Step 1

Positive rod is brought neargrounded scope

Step 2

Electrons from the groundare attracted to the

positive rod

Step 3

Ground is removed, while therod is still nearby, pulling onelectrons and keeping them

from escaping

Step 4

Scope is now negatively charged – the opposite

charge as that of therod

Step 5

Rod is taken away and the negative charge remains

on the scope