Electricity and Magnetism

1. Electricity and Magnetism Electric Charges and Forces Electric Charge Coulomb’s Law

2. Objectives • Describe and calculate the forces between like and unlike electric charges. • Identify the parts of the atom that carry electric charge. • Apply the concept of an electric field to describe how charges exert force on other charges. • Sketch the electric field around a positive or negative point charge. • Describe how a conductor shields electric fields from its interior. • Describe the voltage and current in a circuit with a battery, switch, resistor, and capacitor. • Calculate the charge stored in a capacitor.

3. Vocabulary Terms • charge • electrically neutral • static electricity • positive charge • negative charge • electric forces • charge by friction • electroscope • protons • neutrons • electrons • gravitational field • charged • induction • Coulomb’s law • capacitor • parallel plate capacitor • microfarad • coulomb • electric field • capacitance • charge • polarization • shielding test • charge • farad • field inverse • square law • discharged field • lines

4. Key Question: How do electric charges interact? Electric Charge

5. Electric Charge • All ordinary matter contains both positive and negative charge. • You do not usually notice the charge because most matter contains the exact same number of positive and negative charges. • An object is electrically neutral when it has equal amounts of both types of charge.

6. Electric Charge • Objects can lose or gain electric charges. • The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges. • A tiny imbalance in either positive or negative charge on an object is the cause of static electricity.

7. Electric Charge • Electric charge is a property of tiny particles in atoms. • The unit of electric charge is the coulomb (C). • A quantity of charge should always be identified with a positive or a negative sign.

9. Electric forces • Electric forces are created between all electric charges. • Because there are two kinds of charge (positive and negative) the electrical force between charges can attract or repel.

10. Electric forces • The forces between the two kinds of charge can be observed with an electroscope.

11. Electric forces • Charge can be transferred by conduction.

12. Electric current • In conductive liquids (salt water) both positive and negative charges carry current. • In solid metal conductors, only the electrons can move, so current is carried by the flow of negative electrons. • The direction of current was historically defined as the direction that positivecharges move. • Both positive and negative charges can carry current.

13. I = q t Electric current • Current is the movement of electric charge through a substance. Charge that flows (coulombs) Current (amps) Time (sec)

14. Calculate current • Two coulombs of charge pass through a wire in five seconds. • Calculate the current in the wire.

15. Conductors and insulators • All materials contain electrons. • The electrons are what carry the current in a conductor. • The electrons in insulators are not free to move—they are tightly bound inside atoms.

16. Conductors and insulators • A semiconductor has a few free electrons and atoms with bound electrons that act as insulators.

17. Conductors and insulators • When two neutral objects are rubbed together, charge is transferred from one to the other and the objects become oppositely charged. • This is called charging by friction. • Objects charged by this method will attract each other.

19. F = K q1 q2 r2 Coulomb's Law • Coulomb’s law relates the force between two single charges separated by a distance. Constant 9 x109 N.m2/C2 Force (N) Charges (C) Distance (m)

21. Coulomb's Law • The force between two charges gets stronger as the charges move closer together. • The force also gets stronger if the amount of charge becomes larger.

22. Coulomb's Law • The force between two charges is directed along the line connecting their centers. • Electric forces always occur in pairs according to Newton’s third law, like all forces.

23. Coulomb's Law • The force between charges is directly proportional to the magnitude, or amount, of each charge. • Doubling one charge doubles the force. • Doubling both charges quadruples the force.

24. Coulomb's Law • The force between charges is inversely proportional to the square of the distance between them. • Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4). • This relationship is called an inverse square law because force and distance follow an inverse square relationship.

25. Calculating force • Two balls are each given a static electric charge of one ten-thousandth (0.0001) of a coulomb. • Calculate the force between the charges when they are separated by one-tenth (0.1) of a meter. • Compare the force with the weight of an average 70 kg person.

26. 1) You are asked to calculate the force and compare it to a person’s weight. • 2) You are given the charges and separation, and the mass of the person. • 3) Use Coulomb’s law, F= -Kq1q2/d2, for the electric force and F=mg for the weight. • 4) Solve: • F = (9×109 N•m2/C2)(0.0001C)(.0001C) ÷ (0.1 m)2 = 9,000 N • The weight of a 70 kg person: F = mg = (70 kg)(9.8 N/kg) = 686 N • The force between the charges is 13.1 times the weight of an average person (9,000 ÷ 686).

27. Fields and forces • The concept of a field is used to describe any quantity that has a value for all points in space. • You can think of the field as the way forces are transmitted between objects. • Charge creates an electric field that creates forces on other charges.

28. Fields and forces • Mass creates a gravitational field that exerts forces on other masses.

29. Fields and forces • Gravitational forces are far weaker than electric forces.

31. Drawing the electric field

32. Electric fields and electric force • On the Earth’s surface, the gravitational field creates 9.8 N of force on each kilogram of mass. • With gravity, the strength of the field is in newtons per kilogram (N/kg) because the field describes the amount of force per kilogram of mass.

33. Electric fields and electric force • With the electric field, the strength is in newtons per coulomb (N/C). • The electric field describes the amount of force per coulomb of charge.

35. Accelerators • An electric field can be produced by maintaining a voltage difference across any insulating space, such as air or a vacuum. • Electric fields are used to create beams of high-speed electrons by accelerating them. • Electron beams are used in x-ray machines, televisions, computer displays, and many other technologies.

36. Electric shielding • Electric fields are created all around us by electric appliances, lightning, and even static electricity. • These stray electric fields can interfere with the operation of computers and other sensitive electronics. • Many electrical devices and wires that connect them are enclosed in conducting metal shells to take advantage of the shielding effect.

37. Coulomb’s Law Key Question: How strong are electrical forces?

38. Capacitors • A capacitor is a storage device for electric charge. • Capacitors can be connected in series or parallel in circuits, just like resistors.

39. Capacitors • A capacitor can be charged by connecting it to a battery or any other source of current. • A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

40. Capacitors The current flowing into or out of a particular capacitor depends on four things: • The amount of charge already in the capacitor. • The voltage applied to the capacitor by the circuit. • Any circuit resistance that limits the current flowing in the circuit. • The capacitance of the capacitor.

41. How a capacitor works inside • The simplest type of capacitor is called a parallel plate capacitor. • It is made of two conductive metal plates that are close together, with an insulating plate in between to keep the charges from coming together. • Wires conduct charges coming in and out of the capacitor.

42. How a capacitor works inside The amount of charge a capacitor can store depends on several factors: • The voltage applied to the capacitor. • The insulating ability of the material between the positive and negative plates. • The area of the two plates (larger areas can hold more charge). • The separation distance between the plates.

43. Capacitance The ability of a capacitor to store charge is called capacitance (C). Capacitance (coulombs/volt) Charge (C) q = C V Voltage (volts) Cameras use capacitors to supply quick bursts of energy to flash bulbs.

44. Capacitance • Capacitance is measured in farads (F). • A one-farad capacitor can store one coulomb of charge when the voltage across its plates is one volt. • One farad is a large amount of capacitance, so the microfarad (μF) is frequently used in place of the farad.

45. Calculate capacitance • A capacitor holds 0.02 coulombs of charge when fully charged by a 12-volt battery. • Calculate its capacitance and the voltage that would be required for it to hold one coulomb of charge.

46. Capacitors Key Question: How does a capacitor work? *Students read Section 21.3 BEFORE Investigation 21.3

47. Application: How a Television Works