Atoms are electrically neutral. They consist of protons, electrons, and neutrons.

Transcription

1 ELECTROSTATICS Electrostatics (ie. static electricity) is the study of electrical forces between stationary charges or charges that are hardly moving. Electric charge is a concept that is defined in terms of the effect it produces. It has no mass, color, length, or width. It is quantified based on multiples of a fundamental charge e -. Examples of electric charge are the nervous system of the human body and lightning.

2 Atoms are electrically neutral. They consist of protons, electrons, and neutrons. Electrons in an atom occupy different orbits. Those in orbits near the nucleus are tightly bound to it by their electrical attraction. Those farthest from the nucleus are loosely bound. Since the outer electrons are loosely bound they may be transferred to other atoms (chemically or by other means). If an atom has lost an electron, it becomes overall positively charged and is called a cation. If an atom has gained an electron, it becomes overall negatively charged and is called an anion.

3 Transfer of Electric Charge An atom holds on to its electrons by the force of electrical attraction to its oppositely charged nucleus. The electrostatic series table lists many of the substances that can be charged by friction. The substance that is higher in the table becomes negatively charged, while the other substance becomes positively charged. Electrostatic Series - holds electrons tightly sulphur brass copper ebonite paraffin wax silk lead fur wool glass + holds electrons loosely

4 Separation of Charge Induced charge separation is caused by the presence of a negative or positive distribution of charge on an object. Negative distribution of charge: pith ball metal leaf electroscope There is a repulsion between the object and the similar charge on the far side. However, the strength of the electric forces between similar and opposite charges depends on the distance between the charges. As the distance increases, the magnitude of the force of attraction or repulsion decreases.

5 Charge by Contact When a negatively charged object (ebonite rod) is touched to a neutral object, some of the excess electrons on the rod, repelled by their neighbouring electrons, move over to the neutral object. Both objects have some of the excess electrons and both become negatively charged. If the object is positively charged (glass rod), some of the electrons on the neutral object are attracted onto the glass rod to reduce its deficit of electrons. Both objects share the deficit of electrons and both become positively charged. An object charged by contact has the same sign as the charging rod.

6 Charge by Induction An object charged by induction has a charge opposite to that of the charging rod. (grounding)

7 Charge Conservation if a glass rod is rubbed with silk, a positive charge appears on the rod. Measurement shows that a negative charge of equal magnitude appears on the silk. this suggests that rubbing does not create charge but only transfers it from one body to another. the hypothesis of charge conservation was put forward by Benjamin Franklin and no exception has been found either at the macroscopic or subatomic level.

8 The existence of electric charge and of charge transfer can be demonstrated in the lab or classroom. These activities suggest the opposite charges attract and like charges repel. This is the basic law of electrostatics. The SI unit of charge is called the Coulomb. 1 C = the charge on 6.25 x protons therefore, the charge on one proton is: e = 1.60 x C Coulomb used a torsion balance similar to that used by Cavendish in his study of gravitational forces. Coulomb would bring small charged spheres towards a charged sphere on the torsion balance and it would rotate. The magnitude of the electrical force that one charge exerts on another charge was determined in 1788: kq 1q 2 F e = 2 r This force is also known as the Coulomb force. k = 9.0 x 10 9 Nm 2 /C 2 q = charge in Coulombs r = distance in m

9 Ex. An object A has a positive charge of 6.0 x 10-6 C. Object B, carrying a positive charge of 3.0 x 10-6 C is m away. a) calculate the force on A. b) what would be the force if the charge on B were negative? Ex. An object A with +6.0 x 10-6 C charge, has two other charges nearby. Object B, -3.0 x 10-6 C is m to the right. Object C, +1.5 x 10-6 C, is m below. What is the net force on A?

10 Electric Fields (p.643) When you bring an ebonite rod close to a neutral pith ball, the pith ball is attracted to the rod - charge separation. This is an example of an action - at - a - distance force. SO, how does a charge become aware that there is another charge near it? Faraday ( ) proposed the concept of a FIELD. A field is a property of space. An object influences the space around it setting up an electric field. This field in turn exerts a force on other objects located within it.

11 The direction of the electric field at any point in space is: the direction of the force on a positive test charge at that point.

12 Example. - P + P

13 An electric field is defined in terms of the electric force that acts on a charged particle at a point in space. ε = q F+ ε: the electric field (N/C or V/m) F: the force (N) q + : a small imaginary test charge (C) The electric field is a property of the space that surrounds the charge.

14 Electrical Fields Inside and Surrounding a Conductor Conductor: materials through which charged particles move readily. Charges on a conductor are spread apart as far as possible until they come to rest in electrostatic equilibrium. The result is that all the charges are on the surface of a conductor and there is no net field on the inside of the conductor.

15 If the net electric field inside the conductor is zero, there will be no net electric forces acting on the charges along the surface. The electric field will be perpendicular to the surface at all points. If the shape of the surface is not symmetrical, the charges will be concentrated near the surfaces that are more pointed. The electric fields are greater at these points. If the fields become very large, the molecules of air near the sharp edge become ionized and a spark occurs.

16 Electrical Potential Energy Two or more like charges have more electric potential energy when pushed together than when they are apart because work has been done to bring them together. Oppositely charged particles have more electric potential energy when pulled apart than when they are close together. Electric potential energy: = kq 1q r E e 2 units: Nm (J) Electric potential difference, V, is the change in electrical potential energy that occurs when a test charge, q + is in the field of any other charge. E V = e = q + kq r Units - J/C or V (scalar)

17 1 volt is the change in electrical potential difference that occurs when 1 Coulomb of charge experiences a change in electrical potential energy of 1 Joule. If A and B are two points in an electric field and a test charge moves from A to B, then, regardless of the actual path taken by q, the change in electric = the work required to potential energy of q, in move q from A to B, moving from A to B against the electric field E e = W = Fd V ε = d qv B qv A This is the expression for the magnitude of the electric field at any point in the space between two large parallel plates. The electric field direction is from the + plate to the - plate, in the direction of decreasing potential.

18 Volta (1800) developed the voltaic pile. A zinc disk and silver disk were used with a piece of cardboard soaked in salt water. When a wire is connected to the bottom zinc disk to the top silver disk, the pile produced repeated sparks. Before the pile, these sparks were created by friction (this involved work). The voltaic pile provided a continuous source of charge flow. Volta's pile is widely regarded as the first battery.

19 Batteries generate a difference in electric potential between two points. The potential difference is the difference in voltage. IE. A 12 V battery generates an electric field usually via a chemical process. The potential difference V + V - Between the positive and negative terminals is 12 V. In order to move a positive charge of 1 C from the - to the + terminal of the battery, 12 J of work must be done against the electric field.

20 Conventional current Electron flow Current flows from the positive terminal to the negative terminal. Electrons flow from the negative terminal to the positive terminal. e - : negative, therefore, it attracts to the positive terminal.

21 When Ben Franklin arbitrarily chose + and - charges, he also determined the direction of current. Current flows from regions of + charge to regions of - charge. This is the convention followed to this day. When JJ Thomson discovered the electron, more was known about electric charge. Only e - flow in a current carrying wire. The directions of current and electron flow are OPPOSITE!

22 Current Electricity When a conductor acquires an excess or deficit of electrons, we say it has an electric charge. For this to happen, electrons must be able to move. When electric charges move from one place to another, we say that they constitute an electric current. In metals, electric current is defined as the amount of charge that moves past a given point in a conductor per second. When a total charge, Q, flows through an area in a time of t, the electric current, I, through the wire is: q I = t Units - Amperes (A) 1 A is the electric current when 1 C of charge moves past a point in a conductor in 1 s.

23 Electric Potential Electrons move through a conductor and thus constitute and electric current. What causes them to move through the conductor? What pushes or pulls them? Voltage pushes the current though the wire. If there are two oppositely charged particles near one another, work must be done on the particle to overcome the electric force and pull it away from the oppositely charged particle. Charged particles moving in the presence of an electric field and converting electric potential energy into some other form of energy constitute an electric current. The amount of work that is done on an electric charge to move it through an electric field is equal to the increase of electric potential energy, E, of the charge. W V = q Electric potential difference, V, is often referred to as "voltage".

24 It is easier to measure current, I, through a potential difference, V, for a time, t. Therefore, electrical potential energy, E is represented by: E q E = = = qv It VIt To measure the potential difference, V, a voltmeter is used. As charges move from one point to another through a conductor, they lose energy. This is the electric potential difference. The charges experience a decrease in electric potential difference.