PHY 102: Lecture 1A 1.1 Scotch Tape Experiment 1.2 Atomic Nature of Electricity 1.3 Insulators / Conductors

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 = Q / e (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