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# CHAPTER VII Static Electric Field

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¾ With the modern knowledge about the structure of matter we know that electric charges come from elementary particles: negatively charged electron and positively charged proton. In nuclei there are also neutron with no charge. Interaction between charges: two positive charges or two negative charges repel each other. A positive charge and a negative charge attract each other. This interaction is called the static electric interaction.

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## Nội dung Text: CHAPTER VII Static Electric Field

1. A Course of Physics Physics 211: Lecture 1, Pg 1
2. GENERAL PHYSICS II Electromagnetism & Thermal Physics Physics 211: Lecture 1, Pg 2
3. CHAPTER VII Static Electric Field §1. Electric charges. Coulomb’s law §2. Electric field and electric field vector §3. Electric flux and Gauss’s law §4. Electric potential Physics 211: Lecture 1, Pg 3
4. It is known that in nature there are four kinds of fundamental forces which are caused by: gravitational interaction electromagnetic interaction (between static or moving electric charges) strong interaction (that connects protons and neutrons in nuclei) weak interaction (that drives the beta decay) We consider now the electromagnetic interaction: static electric forces between charges magnetic field between currents (moving charges) Physics 211: Lecture 1, Pg 4
5. §1. Electric charges – Coulomb’s law 1.1 Electric charges: Some history of the concept “electric charge”: In 600 B.C. (Before Christ) ancient Greeks discovered that when they rubbed amber with wool, the amber could then attract light objects (feather,…). Today we say that the amber acquired an electric charge, or has become charged. (the Greek word elektron means amber) Many experiments has shown that there are exactly two kinds of electric charge: » The charge of the same kind with charge on the plastic rod rubbed with fur it is called negative charge » The charge of the same kind with charge on the glass rod rubbed with silk it is called positive charge (then the fur has positive, and the silk has negative charge) The names negative and positive charges were suggested by B. Franklin (1706 – 1790). Physics 211: Lecture 1, Pg 5
6. With the modern knowledge about the structure of matter we know that electric charges come from elementary particles: negatively charged electron and positively charged proton. In nuclei there are also neutron with no charge. Interaction between charges: two positive charges or two negative charges repel each other. A positive charge and a negative charge attract each other. This interaction is called the static electric interaction. Two important principles for charges: The principle of conservation of charge: “The algebraic sum of all the electric charges in any closed systeme is constant” “The magnitude of charge of the electron or proton is a natural unit of charge”. It is also called the element charge, that is the smallest charge which cann’t devided into smaller charges. Physics 211: Lecture 1, Pg 6
7. 2.2 Coulomb’s law: Interaction forces of charges are studied by French physicsist Charles Augustin de Coulomb (1736 – 1806). The Coulomb’s law was established in 1785. Physics 211: Lecture 1, Pg 7
8. Physics 211: Lecture 1, Pg 8
9. Comparison of electric and gravitational forces: Physics 211: Lecture 1, Pg 9
10. Superposition of electric forces: F = F1 + F2 Physics 211: Lecture 1, Pg 10
11. Example 1: Physics 211: Lecture 1, Pg 11
12. Example 2: Physics 211: Lecture 1, Pg 12
13. Physics 211: Lecture 1, Pg 13
14. Physics 211: Lecture 1, Pg 14
15. §2. Electric field and electric field vector: 2.1 The concept of electric field: A B Why two electric charges A and B in empty space can exert force each on other? Modern physics gives the answer: The charged body A, as a result of the charge that it carries, somehow modifies the properties of space around it. Around the body A then there exists a form of matter – the electric field. The field exerts force on the charged body B. Physics 211: Lecture 1, Pg 15
16. More on the concept of FIELD: Besides the electric field we have known the gravitational field Generally, a field is something that can be defined anywhere in space, it can be represented by a function of 3-D spatial position f (x, y, z) : » Temperature field: for each location in space the temperature has a definite value → T = T (x, y, z). Temperature is scalar quantity → the temperature field is a scalar field. » Force fields, as electric field or gravitational field are vector fields. At each point in space, we define a vector E=E(x,y,z) E = E (x, y, z) (x,y,z) Physics 211: Lecture 1, Pg 16
17. 2.2 Definition of electric field vector: Physics 211: Lecture 1, Pg 17
18. Superposition of electric field: How can determine the electric field due to a system of many charges? We know the principle of superposition of forces: the net force which exerts on body is the vector sum of all the component forces the electric field vector for a system of charges is equal to the vector sum of all the electric field vectors due to each charge E = Σi Ei Units of electric field: From the definition, the units of E is N / C . Physics 211: Lecture 1, Pg 18
19. Physics 211: Lecture 1, Pg 19
20. Example 1: Physics 211: Lecture 1, Pg 20