TOPIC 6.2: Electric Fields and ForcesThese notes were typed in association with Physics for use with the IB Diploma Programme by Michael Dickinson

6.2 Electric Force and Field6.2.1 State that there are two types of electric charge.

Have you ever removed a wool sweater and heard a strange crackling noise? Have you ever opened a band aid and seen a flash?When ever two objects rub against each other there is friction. This friction can cause “electrification” effect as tiny charged particles are forced to move form one region to another.

6.2 Electric Force and Field6.2.1 State that there are two types of electric charge.

These charged particles are electrons.

Negative chargeOutside of atomCan easily be stripped away by friction.“Orbit” the nucleus.

The nucleus containsProtons, positive chargeNeutrons, no charge.

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6.2 Electric Force and Field6.2.1 State that there are two types of electric charge. UNITS

Electric charge = q or Q. Measured in Coulombs/C 1 Coulomb of charge = the charge carried on 6.25 x 1018 electrons

ORThe charge on 1 electron(e) = 1.6 x 10-19Coulombs

IB GIVEN DATA

Charge on electron – e = 1.60 x 10-19C Electron rest mass – me = 9.11x10-31kg = 0.000549u = 0.511MeV/c2

Proton rest mass – mp = 1.673x10-27kg = 1.007276u = 938MeV/c2

Neutron rest mass – mm = 1.675x10-27kg = 1.008665u = 940MeV/c2

6.2 Electric Force and Field6.2.1 State that there are two types of electric charge.

Two types of charges:Positive and negative.

Uncharged/neutral objects have equal amounts of positive and negative charges.Proton and electron have equal amounts of chargeBut proton is about 1800 times more massive.

6.2 Electric Force and Field6.2.1 State that there are two types of electric charge.

Electrification occurs because electrons move.This creates a negative area and leaves behind an equally positive region. Net charge is produced when there is an imbalance in the number of protons and electrons.

6.2 Electric Force and Field6.2.1 State that there are two types of electric charge.

Some objects naturally become positively charged. Others naturally become negatively charged.

Example: Rub an acrylic plastic ruler on a piece of cloth. The friction between the plastic and the cloth causes some of the electrons to be stripped from the surface of the plastic and deposite don the wool. This gives an overall positive charge to the plastic and a negative charge to the cloth.

6.2 Electric Force and Field6.2.2 State and apply the law of conservation of charge. IB Definition

Charge cannot be created or destroyed but it can move from one place to another or from one object to another.

Charges can move from one place to another. Charges can NOT be created from nothing.

6.2 Electric Force and Field6.2.2 State and apply the law of conservation of charge. Practice 6

A metal sphere carrying a charge of +4.2µC comes into contact with an identical second sphere which is neutral. Calculate the charge on each sphere.Answer = 2.1µC

6.2 Electric Force and Field6.2.2 State and apply the law of conservation of charge. Practice 7

Two identical metal spheres carry charges of –3.2µC and –4.8µC respectively. The two spheres are brought together and the charge evenly distributes itself so that each sphere carries equal charge. Determine the charge on each sphere. Answer = –4µC

6.2 Electric Force and Field6.2.2 State and apply the law of conservation of charge. Practice 8

Two metal spheres of different size carry charges of –3.0µC and +4.8µC respectively. The two spheres are brought together and the charge distributes itself so that one sphere carries twice as much charge as the second sphere. Determine the charge on each sphere.Answer = +1.2µC and +0.6µC

6.2 Electric Force and Field6.2.3 Describe and explain the difference in the electrical properties of conductors and insulators.

Conductors – materials that allow a free movement of electrons both within the substance and on its outer surface.Insulators – do not allow electrons to move freely. Do allow the build-up of electrons over its outer surface. If a charged insulator is held, it will keep its charge.If a conductor is held it will lose its charge. Electrons will flow from the conductor to the person and then to the earth.

Rubber soled shoes don’t allow this.

6.2 Electric Force and Field6.2.3 Describe and explain the difference in the electrical properties of conductors and insulators.

Examples of ConductorsAll metals – gold, silver, copper, ect.Carbon in the form of graphite

Examples of IsulatorsPlastics – perspex, poythene, PVCRubberWoodGlass

6.2 Electric Force and Field6.2.4 State Coulomb’s Law.

Two charged objects are brought together and an electrical force is created between them.

6.2 Electric Force and Field6.2.4 State Coulomb’s Law.

Opposites attract/likes repel ***See diagram on board***Two charged objects are brought together and an electrical force is created between them.

6.2 Electric Force and Field6.2.4 State Coulomb’s Law.

Size of force depends upon size of each charge and the distance between them. This electrical force is directly proportional to the product of the two charges. This electrical force is inversely proportional to “the square of the distance between them”.

6.2 Electric Force and Field6.2.4 State Coulomb’s Law. IB Definitions/Data

Coulomb’s Law – the force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. The direction of the force is along the line connecting the two charges.

Coulomb constant = k = 8.99 x 109 Nm2/C2

Permittivity of free space = ε0 = 8.85 x 10-12 C2/Nm2

6.2 Electric Force and Field6.2.4 State Coulomb’s Law.

F ∝ q1q2/r2

F = kq1q2/r2

IB Definitions/Data

Coulomb’s Law

F = kq1q2/r2 = 1 / 4πε0 * q1q2/r2 (write this out)

k = a constant of proportionality called Coulomb’s constant

k = 1/4πε0

ε0 = permittivity of free spaceε0 = 1/4πk

6.2 Electric Force and Field6.2.4 State Coulomb’s Law. Practice 9

A +4µC charge is placed 20cm away from a –6µC charge. Calculate the force between these charges. Is this an attractive or repulsive force?Answer: - 5.39 Newtons

***Note the negative sign means the force is an attractive force

6.2 Electric Force and Field6.2.5 Define electric field strength.

We’ve already looked at the idea of a filed when we looked at gravitational fields. A field is a region of space where a force acts without contact.Around any electric charge, there exists an “electric field”

A region of space where a force would exist if another charge entered the field.

IB Definition

Electric Field – the area around an electric charge where a force exists if another charge enters the field.

6.2 Electric Force and Field6.2.5 Define electric field strength.Point Charge

We use the idea of a “test charge” or “point charge” to investigate electric field strength.A test charge is considered to be a positive charge whose charge is so small that I does not alter the electric field in any way. (The charge can be ignored.)Check out this simulation to better under stand a test charge. http://phet.colorado.edu/en/simulation/electric-hockey

6.2 Electric Force and Field6.2.5 Define electric field strength.

Field strength of the electric field is proportional to the magnitude of the force at that point and inversely proportional to the charge. Also notice the units are N/C or NC-1

IB Definition: E = F/q

Electric Field Strength(E) – the force per unit charge on a tiny positive test charge.

E = F/q → F = Eq And since F = kq1q2/r2

Eq = kq1q2/r2

E= k(q/r2)

6.2 Electric Force and Field6.2.6 Determine the electric field strength due to one or more point charges.Problem 10

Calculate the strength of the electric field 0.4m away form a point charge of magnitude +20µC. Answer: E=1.12 x 106 N/C

6.2 Electric Force and Field6.2.6 Determine the electric field strength due to one or more point charges.

Remember that electric field has both magnitude and direction. This means that we can use vector addition to solve of the net electric field.

Problem 11The arrangement shows three point charges q1, q2, and q3. The sizes of these charges are +6µC, +4µC and -3µC respectively. They are separated by the distances shown.Calculate the magnitude and direction of the resulting force on q1

Answer 21.1N @ 71.6º

6.2 Electric Force and Field6.2.6 Determine the electric field strength due to one or more point charges.Problem 12

Calculate the magnitude and direction of the electric field for a test charge placed at the position shown due to the two point charges q1 and q2 of magnitude +5µC and -8µC. The distance from the test charge to the two individual charges is shown. Answer: 142 x 103 N/C @ 62º

6.2 Electric Force and Field6.2.7 Draw the electric field patterns for different charge configurations.

Radial field - The electric field lines are directly towards or away from the point charge. The closer the field line the larger the relative magnitude of the field.As the distance away fro the charge increases, the strength of the field diminishes and the gap between the field lines increases.

6.2 Electric Force and Field6.2.7 Draw the electric field patterns for different charge configurations.

Notice electric field lines are directed away from the positive charge and toward the negative.

6.2 Electric Force and Field6.2.7 Draw the electric field patterns for different charge configurations.

Which charge is the strongest? Weakest?

6.2 Electric Force and Field6.2.7 Draw the electric field patterns for different charge configurations.Guidelines for drawing field lines:

Electric field lines never cross each other.Electric field lines are most dense around objects with the greatest amount of charge.At locations where electric field lines meet the surface of an object, the lines are perpendicular to the surface.

Equipotential LinesAn equipotential is a line or surface over which the potential is constant.Electric field lines are perpendicular to equipotentials. The surface of a conductor is an equipotential.

Equipotential Lines

6.2 Electric Force and Field6.2.8 Solve problems involving electric charges, forces and fields..***See previous problems***Problem 16

Erin Agin drew the following electric field lines for a configuration of two charges. What did Erin do wrong? Explain.

6.2 Electric Force and Field6.2.8 Solve problems involving electric charges, forces and fields..Problem 17

Consider the electric field lines shown in the diagram below. From the diagram, it is apparent that object A is ____ and object B is ____.

6.2 Electric Force and Field6.2.8 Solve problems involving electric charges, forces and fields..Problem 18

Consider the electric field lines drawn at the right for a configuration of two charges. Several locations are labeled on the diagram. Rank these locations in order of the electric field strength - from smallest to largest.Answer: DAECB (with the order of C and B being in question)