Electrostatics

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A Bit of History Ancient Greeks – Observed electric and magnetic phenomena as early as 700 BC Found that amber, when rubbed, became electrified and attracted pieces of straw or feathers Magnetic forces were discovered by observing magnetite attracting iron

A Bit More History William Gilbert – 1600 – Found that electrification was not limited to amber Charles Coulomb – 1785 – Confirmed the inverse square relationship of electrical forces

History Final Hans Oersted – 1820 – Compass needle deflects when placed near an electrical current Michael Faraday – A wire moved near a magnet, an electric current is observed in the wire

Properties of Electric Charges Two types of charges exist – They are called positive and negative – Named by Benjamin Franklin Like charges repel and unlike charges attract one another

Question #2 The charge on sphere 2 is three times the charge on sphere 1. Which force diagram is correct? (e) is none of the others.

Answer #2: (d) The charge on sphere 2 is three times the charge on sphere 1. Which force diagram is correct? (e) is none of the others.

More Properties of Charge Nature’s basic carrier of positive charge is the proton – Protons do not move from one material to another because they are held firmly in the nucleus Nature’s basic carrier of negative charge is the electron – Gaining or losing electrons is how an object becomes charged

More Properties of Charge Electric charge is always conserved – Charge is not created, only exchanged – Objects become charged because negative charge is transferred from one object to another

Properties of Charge, final Charge is quantized – All charge is a multiple of a fundamental unit of charge, symbolized by e – Electrons have a charge of –e – Protons have a charge of +e – The SI unit of charge is the Coulomb (C) e = x C

Conductors Conductors are materials in which the electric charges move freely – Copper, aluminum and silver are good conductors – When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

Insulators Insulators are materials in which electric charges do not move freely – Glass and rubber are examples of insulators – When insulators are charged by rubbing, only the rubbed area becomes charged There is no tendency for the charge to move into other regions of the material

Semiconductors The characteristics of semiconductors are between those of insulators and conductors Silicon and germanium are examples of semiconductors

Charging… Three ways – Friction – Conduction (or Contact) – Induction

Charging by Friction Self-explanatory… (demo)

Charging by Conduction A charged object (the rod) is physically touches the other uncharged, object (the sphere) The same type of charge is CONDUCTED from the rod to the sphere

Charging by Induction

1.NO physical contact between between charged & uncharged object 2.OPPOSITE charged is INDUCED

Question #3 An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. The force on the electron is: a)Greater than that on the alpha particle b)Less than that on the alpha particle c)Same as that on the alpha particle d)I haven’t a clue…

Answer #3: (c) Same The force on the electron the same as that on the alpha particle - Newton’s Third Law.

Question #4 An alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. The particle with the most acceleration is the a)Alpha particle b)Electron c)Neither - they have the same acceleration d)I haven’t a clue…

Answer #4: (b) Electron The particle with the most acceleration is the ELECTRON. Newton’s Second Law (F=ma)

Question #5 An alpha particle with two positive charges and a less- massive electron with a single negative charge are attracted to each other. As the particles get closer to each other, each experiences an increase in: a)force b)speed c)acceleration d)All of these e)None of these

Answer #5: (d) ALL As the particles get closer, the FORCE  and thus the ACCELERATION  and also the SPEED 

Coulomb’s Law Governs forces and charges, k e is called the Coulomb Constant – k e = 8.99 x 10 9 N m 2 /C 2 Typical charges can be in the µC range Remember that force is a vector quantity

Question #6 If q1 = +20  C and q2 = +10  C and the two charges are 3 meters apart, what is the MAGNITUDE of the force between them? a)0.2 N b)0.6 N c)22.22 N d)2.0 x N e)I don’t have a clue

Answer #6: (a) 0.2 N

Vector Nature of Electric Forces Two point charges are separated by a distance r The like charges produce a repulsive force between them The force on q 1 is equal in magnitude and opposite in direction to the force on q 2

Vector Nature of Forces, cont. Two point charges are separated by a distance r The unlike charges produce a attractive force between them The force on q 1 is equal in magnitude and opposite in direction to the force on q 2

Question #7 If q1 = +20  C and q2 = +10  C and the two charges are 3 meters apart, what is the DIRECTION of the force between them? a)Away from each other b)Towards each other c)One chases the other d)Nothing - they don’t move at all e)I don’t have a clue

Answer #7: (a) Away If q1 = +20  C and q2 = +10  C and the two charges are 3 meters apart, what is the DIRECTION of the force between them? Like charges repel

Electrical Field An electric field is said to exist in the region of space around a charged object – When another charged object enters this electric field, the field exerts a force on the second charged object

Electric Field, cont. A charged particle, with charge Q, produces an electric field in the region of space around it A small test charge, q o, placed in the field, will experience a force

Electric Field Mathematically, The electric field is a vector quantity

Question #9 What is the magnitude of the electric field 0.50 meters away from a -3  C point charge? a)1.08 x 10 5 N/C b)-1.08 x 10 5 N/C c)5.4 x 10 4 N/C d)-5.4 x 10 4 N/C e)I don’t have a clue…

Answer #9: (a) 1.08x10 5 N/C What is the magnitude of the electric field 0.50 meters away from a -3  C point charge? a)1.08 x 10 5 N/C b)-1.08 x 10 5 N/C c)5.4 x 10 4 N/C d)-5.4 x 10 4 N/C e)I don’t have a clue…

Question #9 What is the magnitude of the electric field 0.50 meters away from a -3  C point charge? a)1.08 x 10 5 N/C b)-1.08 x 10 5 N/C c)5.4 x 10 4 N/C d)-5.4 x 10 4 N/C e)I don’t have a clue…

Direction of Electric Field The electric field produced by a negative charge is directed toward the charge – A positive test charge would be attracted to the negative source charge

Direction of Electric Field, cont The electric field produced by a positive charge is directed away from the charge – A positive test charge would be repelled from the positive source charge

Question #10 What is the electrostatic force acting on a 2 nC charge placed in a 335 N/C electric field? a)0 N b)6.7 x N c)6.7 x N d)6.7 N e)I don’t have a clue…

Answer #10: (c) 6.7 x N What is the electrostatic force acting on a 2 nC charge placed in a 335 N/C electric field?

Electric Field Lines A convenient aid for visualizing electric field patterns is to draw lines pointing in the direction of the field vector at any point These are called electric field lines and were introduced by Michael Faraday

Electric Field Lines, cont. The field lines are related to the field by – The electric field vector, E, is tangent to the electric field lines at each point – The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region

Electric Field Line Patterns Point charge The lines radiate equally in all directions For a positive source charge, the lines will radiate outward

Electric Field Line Patterns For a negative source charge, the lines will point inward

Electric Field Line Patterns An electric dipole consists of two equal and opposite charges The high density of lines between the charges indicates the strong electric field in this region

Electric Field Line Patterns Two equal but like point charges At a great distance from the charges, the field would be approximately that of a single charge of 2q The bulging out of the field lines between the charges indicates the repulsion between the charges The low field lines between the charges indicates a weak field in this region

Electric Field Patterns Unequal and unlike charges Note that two lines leave the +2q charge for each line that terminates on -q

Electric Field Lines

Electrostatics The End…