Chapter 12: Electric Field Lines and Electric Charges

J. PulickeelNovember 2008

SPH 3U1

Contents of this PresentationIn this presentation we will learn

What are Electric Force Fields?What are the properties of Electric Force

Fields?How do we calculate the magnitude of the

Force between two charged particles?How do we measure the charge of a particle?

Electric Force FieldsA field force is sometimes referred to as an

action at a distanceThe space surrounding a charged object is

affected by the presence of the force or charge. Gravitational Fields Electrical FieldsMagnetic Fields

Electric Field LinesElectric Field Lines are sometimes called

lines of force. The lines are drawn to show the direction of the force, due to the electric field, as it would

act on a test charge.Generally we state that the field starts at the

positive charge and goes to the negative charge

Electric Field LinesAll charged objects create electric field lines.

Like charges will repel each other.Similar charges attract each other

Drawing Electric Field LinesField Lines always go from positive to negative

Field lines NEVER cross. They represent the net force on a test charge. If they were to cross it would mean that the charge had two different net forces with different directions. This is not possible. The charge will experience a single net force in the direction of the field.

The number of field lines leaving the (+) is proportional to the magnitude of the charge.

The density of field lines in a given area can represents the relative strength of the field

Field Strength A measure of the

strength of a field is how dense the field lines in a certain area are.

As you get closer to the source of the field the force gets stronger, and the field line density increases.

Drawing Electric Field LinesThe number of field lines leaving the (+) is proportional to the magnitude of the charge. Thus there are more field lines around the 4q charge that the q charge.

The density of field lines does not always indicate relative strength. In this case, color indicates the relative strength of the field.

Calculating the Charge…How do we calculate the charge (Q) for each

particle?The charge of 1 electron is 1.60 x 10-19 C

Therefore the charge of a particle is

Where N is the number of electrons

Coulombs (C)The charge of a particle is measured in

Coulombs.

A Coulomb is approximately the amount of electric charge that passes through a 100 light bulb in 1 second

The earth stores approximately 400 000 C of charge

A person walking across a woollen rug can store 10-8 C

Drawing Electric (and Magnetic) Field Lines

Recall: Gravitational Force FieldsThe Universal Law of Gravitation states that

all objects with mass exert a force of attraction on others.

The force is dependant on the Mass of the two objects (F m)

The distance between them (F 1/d)

Electric and Gravitational ForcesGravitational Forces Electric ForcesA Force of attraction

between two objects.A force between two

charged particles. It can be attractive or repulsive.

Electric Force FieldsCharles de Coulomb (19th Century) discovered a

similar relationship between charged particlesCoulomb’s Law states that all objects with

charge exert a force of attraction on others.The force is dependant on the

Charge of the two objects (F q)

The distance between them (F 1/d)

1. If electrons are added to an object, it will be charged negatively.

2. A negatively charged ebonite rod is brought near a neutral, metallic-coated pith ball. Some of the electrons are repelled by the ebonite rod and move to the far side of the pith ball. This process is called charging by induction

True/ False Questions

True False Answer

True False Answer

True/ False Questions3. Coulomb’s law states that the electric force is directly proportion to the

product of the charges on each sphere and inversely proportional to the distance between the charges.

4. When the distance between two charged spheres is decreased by a factor of 2, then the electric force decreases by a factor of 2.

5. One of the ways in which Newton’s law of universal gravitation differs from Coulomb’s law is that gravitational force can only attract, whereas the electric force can only repel.

True False Answer

True False Answer

True False Answer

Multiple ChoiceA negatively charged rod is held near, but does not touch the knob of an electroscope. The leaves of the electroscope move apart from one another. A wire is connected to the knob and to a water tap with the negatively charged rod staying in the same position. Which of the following would occur?

A. Electrons flow from the earth through the wire to the electroscope.

B. No electron flow takes place.C. The leaves of the electroscope remain still.D. The leaves of the electroscope move closer

together.E. Electrons flow from the electroscope

through the wire to the earth.My Answer Official Answer

Multiple Choice When a charge separation has been induced on an object, the charge on the object

A. is opposite to the charge on the charging object

B. is proportional to the size of the object being charged

C. is permanentD. is the same as the charge on the charging

objectE. is inversely proportional to the size of the

object being chargedMy Answer Official Answer

Multiple Choice To charge an electroscope positively by induction you need

A. a negatively charged rodB. two objects of the same chargeC. a negatively charged rod and a groundD. two objects with opposite chargesE. a positively charged rod and a ground

My Answer Official Answer

Multiple Choice The law of electric charges states that opposite charges

A. attract each other, similar charges attract neutral objects, and charged objects repel one another

B. repel each other, similar charges attract neutral objects, and charged objects attract one another

C. attract neutral objects, similar charges repel each other, and charged objects attract one another

D. attract each other, similar charges repel one another, and charged objects attract some neutral objects

E. attract neutral objects, similar charges attract each other, and charged objects repel one another

My Answer Official Answer

Sample Question 1How many electrons have been removed from a positively charged pith ball electroscope if it ahs a charge of 7.5 x10-11C?

Q = 7.5 x10-11 Ce = 1.6 x10-19CN= ?

Q = Ne

N= 4.7 x 108 electrons

Sample Question 2 Two charged spheres are 2.00 m apart. One sphere has a charge of 5.7 x10-5C and the other sphere has a charge of 1.6 x10-1C . What is the electric force between the two spheres?

The electric force between the two spheres is 2.05 x 104 N [APART].