Final akshay
- 1. Electric Charges, Forces, and Fields
- 2. TOPICS • Electric Charge • Insulators and Conductors • Coulomb’s Law • The Electric Field • Electric Field Lines • Electric Flux and Gauss’s Law
- 3. Electric Charge Electric charge is the physical property of matter that cause it to experience a force when placed in an electromagnetic field. There are two types of charge: positive and negative
- 4. Electric Charge The effects of electric charge were first observed as static electricity: After being rubbed on a piece of fur, an amber rod acquires a charge and can attract small objects.
- 5. Electric Charge Charging both amber and glass rods shows that there are two types of electric charge; like charges repel and opposites attract.
- 6. Electric Charge The electrons in an atom are in a cloud surrounding the nucleus, and can be separated from the atom with relative ease.
- 7. Electric Charge Some materials can become polarized – this means that their atoms rotate in response to an external charge. This is how a charged object can attract a neutral one.
- 9. If a system contains two point charges q1 and q2, the total charge of the system is obtained simply by adding algebraically q1 and q2 , i.e., charges add up like real numbers or they are scalars like the mass of a body. If a system contains n charges q1, q2, q3, …, qn, then the total charge of the system is q1 + q2 + q3 + … + qn . whereas a charge can be either positive or negative. Additivity of charge
- 10. Proper signs have to be used while adding the charges in a system. For example, the total charge of a system containing five charges +1, +2, –3, +4 and –5,in some arbitrary unit, is (+1) + (+2) + (–3) + (+4) + (–5) = –1 in the same unit Charge Charge has magnitude but no direction, similar to the mass. However, there is one difference between mass and charge. Mass of a body is always positive
- 11. Electric charge is conserved. Also, electric charge is quantized in units of e. The atom that has lost an electron is now positively charged – it is a positive ion The atom that has gained an electron is now negatively charged – it is a negative ion
- 12. The electric charge, q, is said to be quantized q is the standard symbol used for charge as a variable Electric charge exists as discrete packets q = ±Ne N is an integer e is the fundamental unit of charge |e| = 1.6 x 10-19 C Electron: q = -e Proton: q = +e Quantization of Electric Charges
- 13. Charging Objects by Induction • Charging by induction requires no contact with the object inducing the charge • Assume we start with a neutral metallic sphere – The sphere has the same number of positive and negative charges
- 14. • A charged rubber rod is placed near the sphere – It does not touch the sphere • The electrons in the neutral sphere are redistributed
- 15. • The sphere is grounded • Some electrons can leave the sphere through the ground wire
- 16. • The ground wire is removed • There will now be more positive charges • The charges are not uniformly distributed • The positive charge has been induced in the sphere
- 17. • The rod is removed • The electrons remaining on the sphere redistribute themselves • There is still a net positive charge on the sphere • The charge is now uniformly distributed
- 18. Insulators and Conductors Conductor: A material whose conduction electrons are free to move throughout. Most metals are conductors. Insulator: A material whose electrons seldom move from atom to atom. Most insulators are non-metals.
- 19. Insulators and Conductors Semiconductors have properties intermediate between conductors and insulators; their properties change with their chemical composition. Photoconductive materials become conductors when light shines on them.
- 20. Coulomb’s Law Coulomb’s law gives the force between two point charges: The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if the charges are like.
- 21. Coulomb’s Law The forces on the two charges are action- reaction forces.
- 22. Coulomb’s Law If there are multiple point charges, the forces add by superposition.
- 23. Coulomb’s Law Coulomb’s law is stated in terms of point charges, but it is also valid for spherically symmetric charge distributions, as long as the distance is measured from the center of the sphere.
- 24. The Electric Field Definition of the electric field: Here, q0 is a “test charge” – it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges.
- 25. The Electric Field If we know the electric field, we can calculate the force on any charge: The direction of the force depends on the sign of the charge – in the direction of the field for a positive charge, opposite to it for a negative one.
- 26. The Electric Field The electric field of a point charge points radially away from a positive charge and towards a negative one.
- 27. The Electric Field Just as electric forces can be superposed, electric fields can as well.
- 28. Lines that indicate the strength and direction of the electric field. The more dense the lines, the stronger the field. Electric field vectors are tangent to the curve. Properties of electric field
- 29. Electric Flux and Gauss’s Law Electric flux is a measure of the electric field perpendicular to a surface:
- 30. Electric Flux and Gauss’s Law Gauss’s law states that the electric flux through a closed surface is proportional to the charge enclosed by the surface:
- 31. Electric Flux and Gauss’s Law Gauss’s law can be used to find the electric field in systems with simple configurations.
- 32. Summary • Electrons have a negative charge, and protons a positive charge, of magnitude • Unit of charge: Coulomb, C • Charge is conserved, and quantized in units of e • Insulators do not allow electrons to move between atoms; conductors allow conduction electrons to flow freely
- 33. Summary • The force between electric charges is along the line connecting them • Like charges repel, opposites attract • Coulomb’s law gives the magnitude of the force: • Forces exerted by several charges add as vectors
- 34. Summary • A spherical charge distribution behaves from the outside as though the total charge were at its center • Electric field is the force per unit charge; for a point charge: • Electric fields created by several charges add as vectors
- 35. Summary • Electric field lines help visualize the electric field • Field lines point in the direction of the field; start on + charges or infinity; end on – charges or infinity; are denser where E is greater • Parallel-plate capacitor: two oppositely charged, conducting parallel plates • Excess charge on a conductor is on the surface • Electric field within a conductor is zero (if charges are static)
- 36. Summary • A conductor can be charged by induction • Conductors can be grounded • Electric flux through a surface: • Gauss’s law: