Introduction to Electromagnetism and Electric Charges

1. Chapter20 – 23 Electricity, Circuits, and Everything Else In Between

2. Introduction • The field of electromagnetism is as far-reaching as the field of mechanics. • Electromagnetism is manifested in many ways in our daily lives, both seen and unseen. • Observed electric and magnetic phenomena as early as 700 BC. • Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers. • Electric comes from the Greek word elecktron, which is amber. • James Clerk Maxwell (1831 – 1879) is a key figure in the development of the field of electromagnetism. • His contributions to the field of electromagnetism is comparable to Newton’s contribution to the field of mechanics.

3. Properties of Electric Charges • Two types of charges exist: • They are called positive and negative. • Like charges repel. • Unlike charges attract one another. • Nature’s basic carrier of positive charge is the proton. • Protons do not move from one material to another because they are held firmly in the nucleus.

4. Properties of Charge • Nature’s basic carrier of negative charge is the electron. • Gaining or losing electrons is how an object becomes charged. • Electric charge is always conserved. • Charge is not created, only exchanged. • Objects become charged because negative charge is transferred from one object to another. • This is just like conservation of energy.

5. Properties of Charge • Charge is quantized, meaning charge occurs in discrete bundles. • All charge is a multiple of a fundamental unit of charge, symbolized by e. • Electrons have a charge of –e. • Protons have a charge of +e. • e = 1.6 x 10-19 C • The SI unit of charge is the Coulomb (C).

6. Creating A Charge • When two unlike materials are rubbed charge has a tendency to be transferred. • A negative charge is transferred in the form of electrons passing between the materials being rubbed. • The triboelectric series gives us an idea of which materials will be more likely to gain electrons and which is more likely to lose electrons. • This phenomenon occurs to some degree when almost any two non-conducting substances are rubbed. • This phenomenon is what causes static electricity.

7. Example: Creating A Charge • Rubbing a glass rod on a silk cloth. • Glass is higher up on the triboelectric series and is more likely to be positive and thus gives up its electrons to the silk which is lower down on the series.

8. Conductors • Conductors are materials in which the electric charges move freely. • Copper, aluminum and silver are good conductors. • When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material.

9. Insulators • Insulators are materials in which electric charges do not move freely. • Glass and rubber are examples of insulators. • When insulators are charged by rubbing, only the rubbed area becomes charged. • There is no tendency for the charge to move into other regions of the material.

10. Charging by Conduction • Conduction is the process by which charge is transferred from one object to another. • A charged object (the rod) is placed in contact with another object (the sphere). • Some electrons on the rod can move to the sphere. • When the rod is removed, the sphere is left with a charge. • The object being charged is always left with a charge having the same sign as the object doing the charging.

11. Charging by Induction • Induction is the process by which a charge is created but the objects involved do not touch one another. • A neutral sphere has equal number of electrons and protons. • A negatively charged rubber rod is brought near an uncharged sphere. • A grounded conducting wire is connected to the sphere. • The wire to ground is removed, the sphere is left with an excess of induced positive charge.

12. Coulomb’s Law • Coulomb shows that an electrical force has the following properties: • It is directed along the line joining the two particles and inversely proportional to the square of the separation distance, r, between them. • It is proportional to the product of the magnitudes of the charges, |q1|and |q2|on the two particles. • It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs. 1736 – 1806

13. Mathematical Expression of Coulomb’s Law • Mathematically: • ke is called the Coulomb Constant: • ke = 8.987 5 x 109 N m2/C2 • Typical charges can be in the µC range. • Remember that force is a vector quantity. • Applies only to point charges. • The distance between the charges (r) is fixed.

14. Characteristics of Particles

15. Vector Nature of Electric Forces • Two point charges are separated by a distance r. • The like charges produce a repulsive force between them. • The force on q1 is equal in magnitude and opposite in direction to the force on q2. • The unlike charges produce a attractive force between them. • The force on q1 is equal in magnitude and opposite in direction to the force on q2. • Therefore, electrical forces also adhere to Newton’s 3rd law.

16. Electrical Forces are Field Forces • This is the second example of a field force. • Gravity was the first. • Remember, with a field force, the force is exerted by one object on another object even though there is no physical contact between them. • There are some important similarities and differences between electrical and gravitational forces.

17. Electrical Force Compared to Gravitational Force • Both are inverse square laws. • The mathematical form of both laws is the same: • Masses replaced by charges • G replaced by ke • Electrical forces can be either attractive or repulsive. • Gravitational forces are always attractive. • Electrostatic force is stronger than the gravitational force.

18. Electrical Field • Field forces, like electric and gravitational forces, are capable of acting over a given space. • These forces are able to produce an effect even when there is no physical contact. • Faraday developed an approach to discussing fields. • In this approach forces are said to exert a field in a region of space around an object (gravitational field) or a charge (electrical field). • An electric fieldis said to exist in the region of space around a charged object. • When another charged object enters this electric field, the field exerts a force on the second charged object.

19. Electric Field • A charged particle, with charge Q, produces an electric field in the region of space around it. • A small test charge, qo, placed in the field, will experience a force.

20. Electric Field • Mathematically, • SI units are N / C. • Use this for the magnitude of the field. • The electric field (E) is a vector quantity. • The direction of the field is defined to be the direction of the electric force that would be exerted on a small positive test charge placed at that point.

21. Direction of Electric Field: “Negative Field” • The electric field produced by a negative charge is directed toward the charge. • A positive test charge would be attracted to the negative source charge.

22. Direction of Electric Field: “Positive Field” • The electric field produced by a positive charge is directed away from the charge. • A positive test charge would be repelled from the positive source charge.

23. Important Final Note About Electric Fields An electric field at any given point depends only on the charge (q) of the object setting up the field and the distance (r) from that object to a specific point in space.

24. Sample Problem An electric field is measured using a positive test charge of 3.0x10-6 C. This test charge experiences a force of 0.12 N at an angle of 15o north of east. What are the magnitude and direction of the electric field strength at the location of the test charge?

25. Electric Field Lines • A convenient aid for visualizing electric field patterns is to draw lines pointing in the direction of the field vector at any point. • These are called electric field lines and were introduced by Michael Faraday. • The field lines are related to the field in the following manners: • The electric field vector is tangent to the electric field lines at each point. • The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region.

26. Electric Field Line Patterns • The lines radiate equally in all directions from a positive charge toward a negative charge. • Radiates in 3 dimensions. • For a positive source charge, the lines will radiate outward. • For a negative source charge, the lines will point inward.

27. Electric Field Line Patterns #1 • An electric dipole consists of two equal and opposite charges. • The high density of lines between the charges indicates the strong electric field in this region.

28. Electric Field Line Patterns #2 • Two equal but like point charges. • At a great distance from the charges, the field would be approximately that of a single charge of 2q. • The bulging out of the field lines between the charges indicates the repulsion between the charges. • The low field lines between the charges indicates a weak field in this region.

29. Electric Field Line Patterns #3 • Unequal and unlike charges. • Note that two lines leave the +2q charge for each line that terminates on –q. • Lines leaving the greater (+) charge greater than the lines arriving at the (-) charge. • At a great distance from the charges, the field would be approximately that of a single charge of q.

30. Conductors in Electrostatic Equilibrium • When no net motion of charge occurs within a conductor, the conductor is said to be in electrostatic equilibrium. • An isolated conductor has the following properties: • The electric field is zero everywhere inside the conducting material. • Any excess charge on an isolated conductor resides entirely on its surface. • The electric field just outside a charged conductor is perpendicular to the conductor’s surface. • On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points).

31. Electric Energy And Capacitance

32. Review of Potential Energy • PE is the energy associated with the position of an object within a system relative to some reference point. • PE is defined for a system rather than an individual object. • PE can also be used to do work or be converted to KE. • Conservation of energy • W = ∆KE • W = -∆PE

33. Electric Potential Energy • The electrostatic force is a conservative force. • Work done by the force is path independent. • It is possible to define an electrical potential energy function with this force. • This PE shares the same characteristics as other PE. • Work done by a conservative force is equal to the negative of the change in potential energy. • W = -∆PE

34. Uniform Electric Field • Two metal plates possessing charges equal in size but opposite in charge.

35. Potential Difference • The potential difference (∆V) between two points A and B is defined as the change in the potential energy (final value minus initial value) of a charge q moved from A to B divided by the size of the charge. • ΔV = VB – VA = ΔPE / q = W/q

36. Potential Difference • Another way to relate the energy and the potential difference: • W= ΔPE = q ΔV • Both electric potential energy and potential difference are scalar quantities. • Units of potential difference: volts • V = J/C • A special case occurs when there is a uniform electric field: • DV = VB – VA= -Ed • Gives more information about units: N/C = V/m

37. Potential Energy vs. Potential Difference • Potential difference is notthe same as potential energy. • Electric potential is characteristic of the field only. • Independent of any test charge that may be placed in the field. • Electric potential energy is characteristic of the charge-field system. • Due to an interaction between the field and the charge placed in the field.

38. Energy and Charge Movements • A positive charge gains electrical potential energy when it is moved in a direction opposite the electric field. • If a charge is released in the electric field, it experiences a force and accelerates, gaining kinetic energy. • As it gains kinetic energy, it loses an equal amount of electrical potential energy. • A negative charge loses electrical potential energy when it moves in the direction opposite the electric field.

39. Energy and Charge Movements • When the electric field is directed downward, point B is at a lower potential than point A. • A positive test charge that moves from A to B loses electric potential energy. • It will gain the same amount of kinetic energy as it loses in potential energy.

40. Summary of Positive Charge Movements and Energy • When a positive charge is placed in an electric field: • It moves in the direction of the field. • It moves from a point of higher potential to a point of lower potential. • Its electrical potential energy decreases. • Its kinetic energy increases.

41. Summary of Negative Charge Movements and Energy • When a negative charge is placed in an electric field: • It moves opposite to the direction of the field. • It moves from a point of lower potential to a point of higher potential. • Its electrical potential energy increases. • Its kinetic energy increases. • Work has to be done on the charge for it to move from point A to point B.

42. Potentials and Charged Conductors • Since W = -q(VB – VA), no work is required to move a charge between two points that are at the same electric potential. • W = 0 when VA = VB • All points on the surface of a charged conductor in electrostatic equilibrium are at the same potential. • Therefore, all points on the surface of a charged conductor in electrostatic equilibrium are at the same potential.

43. Conductors in Equilibrium • The conductor has an excess of positive charge. • All of the charge resides at the surface. • E = 0 inside the conductor. • The electric field just outside the conductor is perpendicular to the surface. • The potential is a constant everywhere on the surface of the conductor. • The potential everywhere inside the conductor is constant and equal to its value at the surface.

44. Capacitance • A capacitor is a device used in a variety of electric circuits. • The capacitance, C, of a capacitor is defined as the ratio of the magnitude of the charge on either conductor (plate) to the magnitude of the potential difference between the conductors (plates).

45. Capacitance • Units: Farad (F) • 1 F = 1 C / V • A Farad is very large • Often will see µF or pF • V is the potential difference across a circuit element or device. • Q is the magnitude of the charge on either conductor/plate.

46. Parallel-Plate Capacitor • The capacitance of a device depends on the geometric arrangement of the conductors. • For a parallel-plate capacitor whose plates are separated by air:

47. Parallel-Plate Capacitor • The capacitor consists of two parallel plates. • Each have area A. • They are separated by a distance d. • The plates carry equal and opposite charges. • When connected to the battery, charge is pulled off one plate and transferred to the other plate. • The transfer stops when DVcap = Dvbattery.

48. Sample Problem A parallel plate capacitor has an area of A = 2.00 cm2 (2.00x104 m2) and a plate separation of d=1.00 mm. Find the capacitance.

49. Capacitors in Circuits • A circuit is a collection of objects usually containing a source of electrical energy (such as a battery) connected to elements that convert electrical energy to other forms. • These objects can be placed in series or in parallel arrangements. • The equivalent capacitance and the equivalent potential difference can be determined in each case. • A circuit diagram can be used to show the path of the real circuit.

50. Capacitors in Parallel • The total charge is equal to the sum of the charges on the capacitors. • Qtotal = Q1 + Q2 • The potential difference across the capacitors is the same. • And each is equal to the voltage of the battery. • ∆V = ∆V1 = ∆V2