ElectrostaticsElectrostatics

ElectrostaticsElectrostatics

The study of electrical charges that can be The study of electrical charges that can be collected and held in one place.collected and held in one place.

To understand electricity we must To understand electricity we must understand the atom.understand the atom.

Protons – positive charge, held in the Protons – positive charge, held in the nucleus by the strong nuclear force.nucleus by the strong nuclear force.

Electrons – negative charge, found outside Electrons – negative charge, found outside of the nucleusof the nucleus

Neutrons – neutral charge, also found in Neutrons – neutral charge, also found in the nucleus.the nucleus.

Elementary chargeElementary charge

The charge on both a proton and an The charge on both a proton and an electron is known as the elementary electron is known as the elementary charge (e). (cover of reference tables)charge (e). (cover of reference tables)

Protons carry a positive elementary Protons carry a positive elementary chargecharge

Electrons carry a negative elementary Electrons carry a negative elementary charge. charge.

Charged ParticlesCharged Particles

Most atoms contain an equal number of Most atoms contain an equal number of protons and electrons. Therefore they protons and electrons. Therefore they are electrically neutral. are electrically neutral.

An atom with more protons than An atom with more protons than electrons will have a positive net electrons will have a positive net charge.charge.

An atom with more electrons than An atom with more electrons than protons will have a negative net charge.protons will have a negative net charge.

Obtaining ChargesObtaining Charges

The transfer of electrons to or from an The transfer of electrons to or from an object, causing a deficiency or excess object, causing a deficiency or excess of electrons. of electrons.

An object loses electrons it will be +An object loses electrons it will be + An object gains electrons it will be –An object gains electrons it will be – Grounding an object allows for excess Grounding an object allows for excess

electrons be released or more electrons be released or more electrons to be gained to achieve a electrons to be gained to achieve a neutral charge.neutral charge.

Charges InteractingCharges Interacting Two objects that are both Two objects that are both

positively charged (+) (+), or both positively charged (+) (+), or both negatively charged (-) (-), negatively charged (-) (-), repelrepel one another. one another.

Two objects that have opposite Two objects that have opposite charges respectively, (+) (-), or (-) charges respectively, (+) (-), or (-) (+), (+), attractattract one another. one another.

Neutral ObjectsNeutral Objects

A positively charged object will A positively charged object will attract either a negatively charged attract either a negatively charged object or a object or a neutral objectneutral object. .

A negatively charged objects will A negatively charged objects will attract a positively charged object or attract a positively charged object or a a neutral objectneutral object. .

This is caused by an induced This is caused by an induced charge…..more to come on that charge…..more to come on that later.later.

Conservation of ChargeConservation of Charge

Your hair and a comb are both neutral. Your hair and a comb are both neutral. Running the comb through your hair Running the comb through your hair

causes electrons to be stripped off of your causes electrons to be stripped off of your hair and added to the comb.hair and added to the comb.

Your hair’s charge is now _____Your hair’s charge is now _____ The comb’s charge is now ______The comb’s charge is now ______ The number of electrons lost by your hair The number of electrons lost by your hair

is equal to the number of electrons gained is equal to the number of electrons gained by the comb.by the comb.

eelost lost = e= egainedgained

Insulators and ConductorsInsulators and Conductors Conductor – material which readily transfers Conductor – material which readily transfers

charge.charge. Insulator – material which does not transfer Insulator – material which does not transfer

charge easily. charge easily. Insulators and conductors can be Insulators and conductors can be charged by charged by

contact. contact. This occurs when there is a transfer of This occurs when there is a transfer of electrons between two objects which are rubbed electrons between two objects which are rubbed together. together.

Example: Example: Rubber and fur. Rubber and fur. Fur has a low affinity for electrons.Fur has a low affinity for electrons. Rubber has a high affinity for electrons.Rubber has a high affinity for electrons. Electroscope – a device that can detect the Electroscope – a device that can detect the

presence of an electric charge and the charges presence of an electric charge and the charges sign. (ex. Pith ball)sign. (ex. Pith ball)

Charging by ConductionCharging by ConductionNeutral pith ball Neutral pith ball

Positively charged rod moves toward pith ball. Positively charged rod moves toward pith ball.

Upon contact, the electrons flow from the pith ball into Upon contact, the electrons flow from the pith ball into

the rod.the rod.

Now both objects have a net positive charge.Now both objects have a net positive charge.

Transferring ChargeTransferring Charge

Object A

+14e

Add the two charges and divide by two to find the final Add the two charges and divide by two to find the final charge.charge.

If A comes in contact with B what will each charge be? If A comes in contact with B what will each charge be? A = B= A = B=

If B comes in contact with C what will each charge be? If B comes in contact with C what will each charge be? B = C=B = C=

If C comes in contact with A what will each charge be? If C comes in contact with A what will each charge be? C = A=C = A=

Object B

-46e

Object C

Neutral

Charging By InductionCharging By Induction Induction – charging an object by Induction – charging an object by

bringing it near another charged object. bringing it near another charged object. Two neutral conducting spheresTwo neutral conducting spheres

The negatively charged PVC pipe repels The negatively charged PVC pipe repels the electrons in the sphere to one side.the electrons in the sphere to one side.

Moving the spheres apart, sphere A Moving the spheres apart, sphere A is left with a positive net charge.is left with a positive net charge.

Measuring ChargeMeasuring Charge Charge (q)Charge (q) – measured in a unit called – measured in a unit called

the Coulomb (C). The smallest unit of the Coulomb (C). The smallest unit of charge called the charge called the elementary chargeelementary charge is is equal to the charge on a single electron equal to the charge on a single electron (e).(e).

e = -1.6 x 10e = -1.6 x 10-19-19 C C A proton has a charge of + 1.6 x 10A proton has a charge of + 1.6 x 10-19-19 C C ****A charge will always be a multiple ****A charge will always be a multiple

of this number since we can’t have ½ of this number since we can’t have ½ of an electron or proton****of an electron or proton****

Coulomb’s LawCoulomb’s Law

Two charged objects may experience motion Two charged objects may experience motion toward or away from one another. toward or away from one another.

Therefore there must be a force acting on Therefore there must be a force acting on these objects.these objects.

The electrostatic force is as follows:The electrostatic force is as follows: FFe e = = k (qk (q11qq22)) rr22

where where k k is the is the electrostatic constantelectrostatic constant = 8.99 x = 8.99 x 101099 N m N m22/C/C22

The electrostatic force is dependant upon the The electrostatic force is dependant upon the magnitude of the charges as well as the magnitude of the charges as well as the distance between the objects.distance between the objects.

What equation does this appear similar What equation does this appear similar too?too?

Fe >>Fg Fe >>Fg Fe always acts on a straight line Fe always acts on a straight line

between objects. between objects. Electric force must be a field force since Electric force must be a field force since

there is no contact between the objects.there is no contact between the objects.

PracticePractice Example Problem:Example Problem: Find the electrostatic Find the electrostatic

force between these two point charges. The force between these two point charges. The distance between them is 2.3 x 10-3 m.distance between them is 2.3 x 10-3 m.

FFee = = k (qk (q11qq22))

rr22

FFee = 12760 N Attraction = 12760 N Attraction

Magnitude and DirectionMagnitude and Direction

This force, like other forces is a vector. This force, like other forces is a vector. (magnitude and direction)(magnitude and direction)

When finding the magnitude of the When finding the magnitude of the force, ignore the positive or negative force, ignore the positive or negative charge. charge.

The direction will have to be found by The direction will have to be found by interpreting the charges relations. interpreting the charges relations.

FF1-21-2 in the previous question is to the in the previous question is to the left because 1 is attracting 2.left because 1 is attracting 2.

Practice #1Practice #1

How many excess electrons are on a How many excess electrons are on a sphere with a charge of -9.20x10sphere with a charge of -9.20x10-17-17C?C?

575 electrons575 electrons

Practice #2Practice #2

Two charges q1 and q2 are a distance d Two charges q1 and q2 are a distance d apart and exert a force F on each other. apart and exert a force F on each other. What will the new force be if:What will the new force be if: q1 is doubled and q2 is cut in half.q1 is doubled and q2 is cut in half. FF q1 is tripled and q2 is doubled.q1 is tripled and q2 is doubled. 6F6F q2 is cut in half and d is tripled.q2 is cut in half and d is tripled. 1/18 F1/18 F

Practice #3Practice #3

Two negative charges of -24Two negative charges of -24C each C each are separated by 6.0cm. What force are separated by 6.0cm. What force exists between the charges?exists between the charges?

1400N repulsive force1400N repulsive force

Practice #4Practice #4

Determine the magnitude of the Determine the magnitude of the electrostatic force between a proton electrostatic force between a proton and an electron that are separated and an electron that are separated by a distance of 7.5x10by a distance of 7.5x10-8-8mm

4.1 x 104.1 x 10-14-14NN