PHY 102: Lecture 1 1.1 Scotch Take Experiment 1.2 Atomic Nature of Electricity 1.3 Insulators / Conductors 1.4 Charging Objects 1.5 Force between Charges

PHY 102: Lecture 1 Electric Charge 1.1 Scotch Take Experiment

Isaac Newton (1643 – 1727) Force causes acceleration, F = ma Force is a push or a pull Acceleration is a change in velocity (motion) Mass generates a force called “Gravity” Mass is a property of matter Some properties of gravitational force are: force acts between masses force is only attractive action-at-a-distance (masses need not touch)

Do Scotch Tape Experiment - 1 Place 1 foot piece of Scotch Tape on table Label this tape B for bottom Place 1 foot piece of Scotch Tape on top of B tape Label this tape T for top Repeat for a separate set of B and T tapes

Do Scotch Tape Experiment - 2 Remove 1st set of B and T tapes from table Separate the B and T tapes Hold them near each other What happens? Remove 2nd set of B and T tapes from table Hold the 1st B tape near the 2nd B tape

Results of Scotch Tape Experiment B tape and T tape attract B tape and B tape repel T tape and T tape repel Tapes lift up away from Earth

Force Exists Between Tapes B and T tapes accelerate towards each other B and B tapes accelerate away from each other According to Isaac Newton, a force causes this acceleration

Force is Not Gravity The force on the tapes can’t be gravity: The mass of the tapes are so small that gravity between tapes is almost zero The force must be greater than gravity between Earth and tape because the tapes lift up away from the Earth The force is both attractive and repulsive, while gravity is only attractive

New Force – Electric Force The force between the tapes is a new force called the Electric Force Electric comes from the Latin word electrum for amber and the Greek word elektron for amber The Greeks first noticed this force when they rubbed amber with a cloth

Source of Electric Force Electric Charge - 1 The electric force is associated with another property of matter, Electric Charge Because the force can attract and repel, there must be two types of electric charge The two types of charge are named positive and negative

Source of Electric Force Electric Charge - 2 What is the source of the electric charge on the tapes? Created when tapes are pulled apart, or Already exists in tapes and are separated when tapes pulled apart The 2nd is the source of charge The electrical charge of matter is inherent in atomic structure

PHY 102: Lecture 1 Electric Charge 1.2 Atomic Nature of Electricity Charged Objects

Atom An atom consists of a small, relatively massive nucleus Nucleus contains proton, and neutron particles Surrounding the nucleus is a diffuse cloud of orbiting particles called electrons Mass of particles are: Proton: 1.673 x 10-27 kg Neutron: 1.675 x 10-27 kg Electron: 9.11 x 10-31 kg

Charges of Particles in Atom Proton has a positive charge (“+”) Electron has a negative charge (”-”) Neutron has no electric charge (0) The magnitude of the charge on the proton exactly equals the magnitude of the charge on the electron The proton carries a charge +e, and the electron carries a charge of –e The SI unit for the magnitude of an electric charge is the coulomb (C) e has the value 1.60 x 10-19 C

Net Charge on Atoms In nature, atoms are normally found with equal numbers of protons and electrons Usually, an atom carries no net charge When an atom, or any object, carries no net charge, the object is said to be electrically neutral

Charged Objects - 1 The charge on an electron or a proton is the smallest amount of free charge that has been discovered Protons and neutrons contain quarks Quarks have charges of ±1/3 and ±2/3 of an electron Quarks are not usually found to be free

Charged Objects – 2 Charges of larger magnitude are built up on an object by adding or removing electrons A charge of magnitude q is integer multiple of e q = Ne Where N is an integer Because any electric charge q occurs in integer-multiples of elementary, indivisible charges of magnitude e, electric charge is said to be quantized

Charged Objects - 3 It is possible to transfer electric charge from one object to another Usually electrons are transferred and the body that gains electrons acquires an excess of negative charge The body that loses electrons has an excess of positive charges

Charged Object - 4 Separation of charge occurs often when two unlike materials are rubbed together Hard, black rubber rod is rubbed against animal fur Some of the electrons from atoms of the fur are transferred to the rod The rubber becomes negatively charged The fur becomes positively charged

Charged Object - 5 A glass rod is rubbed with a silk cloth Some of the electrons are removed from the atoms of the glass and deposited on the silk The silk becomes negatively charged The glass becomes positively charged

Charged Object - 6 A rubber rod is rubbed with animal fur The rubbing process separates electrons and protons already present in the materials No electrons or protons are created or destroyed Whenever an electron is transferred to the rod, a proton is left behind on the fur

Charged Object - 7 The charges on the electron and proton have identical magnitudes but opposite sign The algebraic sum of the two charges is zero The transfer does not change the net charge of the fur/rod system If each material contains an equal number of protons and electrons to begin with, the net charge of the system is zero initially and remains zero at all times during the rubbing process

Law of Conservation of Electric Charge Electric charges are involved in chemical reactions, electric circuits, and radioactive decay A great number of experiments have verified that in any situation, the law of conservation of electric charge is obeyed Law of Conservation of Electric Charge During any process, the net electric charge of an isolated system remains constant (is conserved)

Problem 1 How many electrons must be removed from an electrically neutral silver dollar to give it a charge of +2.4C? Q = Ne N = e / Q (number of electrons) N = 2.4 x 10-6 C / 1.60 x 10-19 C/electron N = 1.5 x 1013 electrons

PHY 102: Lecture 1 Electric Charge 1.3 Insulators / Conductors

Insulators / Conductors Electric charge can exist on an object Electric charge can move through an object Materials differ vastly in their abilities to allow electric charge to move (conducted) through them Electrical Conductors - Substances that conduct electric charge Metals such as copper, aluminum, silver, and gold Electrical Insulators - Materials that conduct electric charge poorly Rubber, many plastics, ceramics, and wood

Conductor Conductors and insulators differ in their atomic structure Electrons in the outer orbits experience a weaker force of attraction to the nucleus than do those in the inner orbits Outermost electrons can be dislodged more easily than inner ones In a good conductor, some electrons become detached from a parent atom and wander more or less freely throughout the material, belonging to no one atom in particular The exact number of electrons detached from each atom depends on the nature of the material, but is usually between one and three When one end of a conducting bar is placed in contact with a negatively charged object and the other end in contact with a positively charged object, “free” electrons are able to move readily away from the negative and toward the positive end

Insulator In an insulator there are very few electrons free to move through the material Virtually, every electron remains bound to its parent atom Without the free electrons, there is very little flow of charge when the material is placed between positively and negatively charged bodies

PHY 102: Lecture 1 Electric Charge 1.4 Charging Objects

Charging by Contact - 1 When a negatively charged rubber rod is rubbed on a metal object some of the excess electrons from the rod are transferred to the metal object When the rod is removed the sphere is left with a negative charge distributed over its surface When a positively charged rod is rubbed on a metal object some of the electrons from the metal object are transferred to the rod When the rod is removed the sphere is left with a positive charge distributed over its surface The process of giving one object a net electric charge by placing it in contact with another object that is already charged is known as charging by contact

Charging by Contact - 2

Charging by Induction - 1 A negatively charged rod is brought close to, but does not touch, a metal sphere In the sphere, the free electrons closest to the rod, move to the other side The part of the sphere nearest the rod becomes positively charged and the part farthest away becomes negatively charged These positively and negatively charged regions have been “induced” to form because of the repulsive force between the negative rod and the free electrons in the sphere If the rod were removed, the free electrons would return to their original places, and the charge regions would disappear

Charging by Induction - 2 The earth is a good electrical conductor When a metal wire is attached between the sphere and the ground some of the free electrons leave the sphere and distribute themselves over the much larger earth If the ground wire is then removed, followed by the rubber rod, the sphere is left with a positive net charge The process of giving one object a net electric charge without touching the object to a second charged object is called charging by induction

Charging by Induction - 3

Charging by Induction - 4 If the sphere were made from an insulating material like plastic, instead of metal, the method of producing a net charge by induction would not work Very little charge would flow through the insulating material and down the ground wire The electric force would cause the positive and negative charges in the molecules of the insulating material to separate slightly, with the negative charge being “pushed” away from the negative rod Although no net charge is created, the surface of the plastic does acquire a slight induced positive charge and is attracted to the negative rod

Charging by Induction - 5

PHY 102: Lecture 1 Electric Charge 1.5 Force between Charges

Forces on Electric Charges Two electrically charged objects exert a force on one another This is the Electric Force Like charges repel and unlike charges attract each other

Charging by Rubbing - 1 Balloon is electrically neutral Ten + and – charges Sweater is also electrically neutral

Charging by Rubbing - 2 Rub balloon on sweater Electrons (-) transfer from sweater to balloon Balloon is negatively charged Sweater is positively charges

Charging by Rubbing - 3 Separate the balloon and sweater It is clear that the balloon is negatively charged It is clear that the sweater is positively charged Let the balloon go

Charging by Rubbing - 4 The negatively charged balloon is attracted to the positively charged sweater Opposite charges attract

Charging by Induction - 1 Neutral balloon is brought close to wall Nothing happens

Charging by Induction - 2 Negatively charged balloon is brought close to the wall The negative charge on the balloon repels the negative charge on the wall The wall is still neutral but the negative and positive charges separate slightly The negative balloon is attracted to the closer positive charge on the wall

Charles Coulomb (1736 – 1806) 1784 French Engineer Did static electricity experiments Built from the work of Benjamin Franklin Measured strength of electric force

Coulomb’s Law The magnitude F of the electrostatic force exerted by one point charge q1 on another point charge q2 is r is the distance between charges k = 8.99 x 109 Nm2/C2 in SI units Sometimes k = 1/(40) 0 is called the permittivity of free space 0 = 1/(4k) = 8.895 x 10-12 C2/(Nm2) Electrostatic force is directed along the line joining the charges It is attractive if the charges have unlike signs It is repulsive if the charges have like signs

Coulomb’s Law Force on a Point Charge Due to Two or More Other Point Charges We have discussed the electrostatic force on a point charge (magnitude |q1|) due to another point charge (magnitude |q2|) Suppose that a third point charge (magnitude |q3|) is present What is the net force on q1 due to both q2 and q3? First, find the magnitude and direction of the force exerted on q1 by q2 (ignoring q3) Then, determine the force exerted on q1 by q3 (ignoring q2) The net force on q1 is the vector sum of these forces

Problem 2 Two spheres are separated by distance of 1.80 x 10-3 m The spheres are initially electrically neutral and are very small compared to the distance between them Each sphere gets same negative charge due to addition of electrons Each sphere experiences an electrostatic force that has a magnitude of 4.55 x 10-21 N How many electrons did it take to the produce charge on one sphere? F =kqq/r2 4.55 x 10-21 = 8.99 x 109 x q2 / (1.80 x 10-3)2 4.55 x 10-21 x (1.80 x 10-3)2 / 8.99 x 109 = q2 1.64 x 10-36 = q2 q = 1.28 x 10-18 q = Ne N = number of electrons = 1.28 x 10-18 / 1.60 x 10-19 N = 8

Problem 3 Two objects, whose charges are +1.0 C and -1.0 C, are separated by 1.0 km Find the magnitude of the attractive forces that either charge exerts on the other Equivalent to weight of 900 kg mass Equivalent to weight of 2000 pounds

Problem 4 In the Bohr model of the hydrogen atom, the electron (charge=-e) is in a circular orbit about the nuclear proton (charge=+e) at a radius of 5.29 x 10-11 m (0.0529 nm) The mass of the electron is 9.11 x 10-31 kg Determine the speed of the electron Centripetal force = Coulomb’s Force

Problem 5 - 1 Three point charges lie along the x axis Determine the magnitude and direction of the net electrostatic force on q1

Problem 5 - 2 Magnitude of the forces Direction of net force