ElectrostaticsLevel 1 Physics

Objectives and Essential Questions

OBJECTIVES

At the end of this unit, the student should be able to: Define electrostatics and

the nature of electric charge

State Coulomb’s Law and its equation to calculate the electrostatic force between two charges

Explain charging of an object by contact and induction

Distinguish between conductors and isulators

ESSENTIAL QUESTIONS How are objects charged? What are the principles of

charging by induction? What is Coulomb’s Law?

Electric Charge – Just the facts

Charge is a property of subatomic particles

Facts:• Two types of charge – Positive (protons) and Negative (electrons)• LIKE charges REPEL and OPPOSITE charges ATTRACT•Similar to fluids – Can be in 2 states; STATIC and DYNAMIC

Electric ChargeThe magnitude of the charge on the proton exactly equals the magnitude of the charge on the electron.

•The symbol for charge is q•The unit for charge is Coulomb (C)•A single charged particle (1 electron or proton) is called an elementary particle (symbol is e).

Particle Charge Mass

Proton +1.6 x 10-19 C 1.67 x 10-27 kg

Electron -1.6 x 10-19 C 9.11 x 10-31 kg

Neutron 0 1.67 x 10-27 kg

Charged ObjectsIt is possible to transfer electric charge from one object to another.

• Usually, electrons are transferred•The body that loses electrons has an excess of positive charge.•The body that gains electrons has an excess of negative charge.

When an ebonite rod isrubbed against animal fur,electrons from atoms of thefur are transferred to the rod.This gives the rod a negativecharge and leaves a positive charge on the fur

Conservation of ChargeWhen an ebonite rod is rubbed with animal fur:

Process serves to separate electrons and protons already present in the materials.

Protons/Electrons are not created or destroyed. As an electron is transferred to the rod, a proton is left

behind on the fur. Both have identical charges (magnitudes are the same,

signs are opposite). Transfer does not change the net charge of the fur/rod

system. Each material contains an equal amount of protons and

electrons to begin with, net charge of the system is zero initially and will remain zero during the entire process.

Law of Conservation of Electric Charge

During any process, the net electric charge of an isolated system remains constant (is conserved).

Conductors and InsulatorsNot only can charge exist on an object, but it can move through an object.The movement of charge is limited by the substance the charge is trying to pass through. There are generally 2 types of substances.

Conductors: Substances that readily conduct electric chargeInsulators: Substances that conduct electric charge poorly

(resist the movement).

ChargingThere are three ways to charge an object:

FRICTION CONDUCTION

INDUCTION

FrictionThe only reason that we are able to use electricity in our modern world is thatit is possible to separate positive and negative charges from each other.

•Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths•As they pass over each other, the electrons with weaker bonds are“stripped” off of that material and collect on the other material.

Example – Rub a piece of ebonite across a piece of animal fur.The fur does not hold on to its electrons as strongly as ebonite.Some of the electrons will be ripped off of the fur and stay on the ebonite. Now the fur has a slightly positive charge and theebonite is slightly negative.

NET CHARGE IS STILL ZERO CONSERVATION OF ELECTRIC CHARGE

ConductionThe two objects will come into physical contact with each other (sometimes called “charging by contact”).

Figure 2a: Negatively charged metal object and an uncharged metal sphere

Figure 2b: Bring the twoobjects close together, separation of charge starts occur

Figure 2c: The two objectstouch. Some negative chargewill transfer over to unchargedmetal object

Figure 2d: When negative objectis removed, it will not be as negativeas it was. Both objects have some negative charge.

InductionTo charge a conductor without coming in direct contact with it.

Figure 3a: Metal sphere is on insulating stand and hasa ground wire attached.

Figure 3b: Bring a negativeobject nearby. Causes a separation of charge in sphereand electrons will travel down the wire.

Figure 3c: Keep negative object nearby. Either cut or remove the ground wire. The electrons have no way to travelback up the wire

Figure 3d: Now, remove the negative object. the sphere has a net positive charge

Electric ForceThe electric force is similar to the gravitational force. Remember

MmFg 2

1

rFg

Coulomb’s LawThe magnitude F of the electrostatic force exerted by one point charge q1 onanother point charge q2, is directly proportional to the magnitude of thecharges and inversely proportional to the square of the distance r between them

LawsCoulombr

qqkF

C

Nmx.k

kr

qqF

rFqqF

E

EEE

'

10998 constant Coulomb

alityproportion ofconstant

1

221

2

29

221

221

For calculation ease, one may use 9.00 x 109 Nm2/C2 for the constant

Electric Forces and Newton’s Laws

Electric Forces obey Newton’s Laws. Example: An electron is released above the surface of the Earth. A second electrondirectly below it exerts an electrostatic force on the first electron just great enough to cancel out the gravitational force on it. How far below the first electron is the second?

e

e

mg

Fe

r = ?

)8.9)(1011.9(

)106.1()99.8(

31

2199

212

21

x

x

mg

qqkrmg

r

qqk

mgFE

5.1 m