ENGINEERING PHYSICS SEMESTER /2012

ENGINEERING PHYSICS Sub Topics ● Charge units ● Electric field ● Electric force & Coulomb’s Law ● Capacitance and unit ● Parallel plate capacitor ● Dielectric constant and it’s function SEMESTER /2012

ENGINEERING PHYSICS Electric Charge Electric charge is a fundamental property of matter; electric charges may be positive or negative. The atom consists of a small positive nucleus is surrounded by a negative electron cloud. SEMESTER /2012

ENGINEERING PHYSICS Is an intrinsic characteristic of the fundamental particles making up those objects; that is, it is a characteristic that automatically accompanies those particles wherever they exist. Charges with the same electrical sign repel each other; and charges with opposite electrical signs attract each other. SEMESTER /2012

ENGINEERING PHYSICS SEMESTER /2012

ENGINEERING PHYSICS Electric Charge - Lightning SEMESTER /2012

ENGINEERING PHYSICS Electric Charges SEMESTER /2012 SI unit of charge: the coulomb, C. All charges are integer multiples of the charge on the electron: Conservationa of charge: The net charge of an isolated system remains constant. Net charge of the universe is constant !!! n = 1, 2, 3,..

ENGINEERING PHYSICS Electrostatic Charging SEMESTER /2012 Conductors  materials in which electric charges move freely Semiconductors are intermediate; their conductivity can depend on impurities and can be manipulated by external voltages. Insulators  materials in which electric charges do not move freely.

ENGINEERING PHYSICS Electrostatic Charging SEMESTER /2012 An electroscope may be used to determine if an object is electrically charged.

ENGINEERING PHYSICS Electrostatic Charging: Friction SEMESTER /2012 Charging by friction: This is the process by which you get “charged up” walking across the carpet in the winter. It is also the process that creates “static cling” in your laundry, and makes it possible for you to rub a balloon on your hair and then stick the balloon to the wall.

ENGINEERING PHYSICS Electrostatic Charging: Conduction SEMESTER /2012 An electroscope can be given a net charge by conduction – when it is touched with a charged object, the excess charges flow freely onto the electroscope.

ENGINEERING PHYSICS Electrostatic Charging: Induction SEMESTER /2012 An electroscope may also be charged by induction, if there is a way of grounding it while charge is being induced.

ENGINEERING PHYSICS Electrostatic Charging: POLARIZATION SEMESTER /2012 Charge may also be moved within an object – without changing its net charge – through a process called polarization. (charge separation by polarization)

ENGINEERING PHYSICS Electric Force SEMESTER /2012 The electric force acting on a point charge q1 as a result of the presence of a second point charge q2 is given by Coulomb's Law: Where ε 0 = permittivity of space The constant of proportionality k appearing in Coulomb's law is often called Coulomb's constant.

ENGINEERING PHYSICS Electric Force SEMESTER /2012 In graphical: r q1q1 q2q2 If there are multiple point charges, the force vectors must be added to get the net force.

ENGINEERING PHYSICS Example 1 SEMESTER /2012 (a) Two point charges of -1.0nC and +2.0nC are separated by a distance of 0.3m, what is the electric force on each particle? 0.3m q 2 = +2nC F 12 F 21 q 1 = -1nC (0, -0.3m) (0, +0.3m) (0, 0.4m) q 1 = +2.5nC q 2 = +2.5nC y x q 3 = +3.0nC r 31 r 32   (b) What is the net electric force on q 3 ?

ENGINEERING PHYSICS Solution SEMESTER /2012 (a) q 1 = +2.5nC q 2 = +2.5nC y x F 32 F 31   F net = F 3 (b)

ENGINEERING PHYSICS Solution SEMESTER /2012 (b) r 31 = r 32 = 0.5m Taking into account the direction of F 31 and F 32 is symmetry – then y – components cancel. Thus, F 3 (the net force on q 3 ) acts along the positive x-axis and has magnitude of

ENGINEERING PHYSICS Example 2 SEMESTER /2012 a) What is the magnitude of the repulsive electrostatic force between two protons in a nucleus? Taking the distance from center to center of these protons to be 3 x m. b) If the protons were released from rest, how would the magnitude of their initial acceleration compare with that of the acceleration due to gravity on Earth’s surface, g ?

ENGINEERING PHYSICS Solution SEMESTER /2012 Given: r = 3 x m; q 1 =q 2 = +1.6 x C ; m p = 1.67 x kg (a) Using Coulomb’s Law; b)

ENGINEERING PHYSICS Electric Field SEMESTER /2012 The electric field at any location is defined as follows: SI Unit: N/C The direction of the field E is the direction the force would be on a positive charge.

ENGINEERING PHYSICS Electric Field SEMESTER /2012 Charges create electric fields, and these fields in turn exert electric forces on other charges. Electric field of a point charge:

ENGINEERING PHYSICS Electric Field SEMESTER /2012 Electric field lines due to very large parallel plates:

ENGINEERING PHYSICS Electric Field SEMESTER /2012 Electric field lines due to like charges: (a) equal charges; (b) unequal charges.

ENGINEERING PHYSICS Electric Field SEMESTER /2012

ENGINEERING PHYSICS Conductors & Electric Field SEMESTER /2012 Electric charges are free to move within a conductor; therefore, there cannot be a static field within the conductor: The electric field is zero inside a charged conductor. Excess charges on a conductor will repel each other, and will wind up being as far apart as possible. Any excess charge on an isolated conductor resides entirely on the surface of the conductor.

ENGINEERING PHYSICS Conductors & Electric Field SEMESTER /2012 There cannot be any component of the electric field parallel to the surface of a conductor; otherwise charges would move. The electric field at the surface of a charged conductor is perpendicular to the surface.

ENGINEERING PHYSICS Conductors & Electric Field SEMESTER /2012 Excess charge tends to accumulate at sharp points, or locations of highest curvature, on charged conductors. As a result, the electric field is greatest at such locations.