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Electrostatics, the study of electric charges The evidence of electricity has been noted since 2700 BC Lets review some basics: Like charges repel Opposite charges attract An insulator is an object that a charge or energy can not move easily through A conductor is an object that a charge or energy can move easily through. The Atom Composed of Protons (+), Electrons (-) and Neutrons Protons are found in the nucleus, along with Neutrons. Electrons orbit the nucleus. In a neutral atom, the number of electrons equals the number of protons Electric charges exert a force on other charges at a distance This force is stronger when the charges are closer together The charge possessed by a single electron and proton are equal at 1.60 x 10-19 C. The magnitude of the charge on an electron is called the elementary charge C = 1 Coulomb the SI unit for charge 1 Coulomb of charge is equal to 6.24 x 1018 electrons, about 2000 Amps A typical lightning bolt has 5 – 350 C A nickel can have up to 106 C of charge balanced by positively charge protons. Typically two non conductive or neutral objects in contact with each other will result in the transfer of electrons. Air is non conductive but lightning and sparks fly across it. If the unbalance of electrons or protons is large enough, almost anything can become a conductor. In the case of air electrons are forced to move across it creating a plasma of charged atoms and freely moving electrons we see as lightning. Electric charges can be move through induction or conduction. Induced charges are charges brought on without physical contact. If a source of electrons or negative charge is brought next to two objects in contact the electrons will move from one to the other creating a positively charged object and a negatively charged one The second way to charge something is via INDUCTION, which requires NO PHYSICAL CONTACT. We bring a negatively charged rod near a neutral sphere. The protons in the sphere localize near the rod, while the electrons are repelled to the other side of the sphere. A wire can then be brought in contact with the negative side and allowed to touch the GROUND. The electrons will always move towards a more massive objects to increase separation from other electrons, leaving a NET positive sphere behind. Polarization is the separation of positive and negative charges in a neutral object when a charged object is brought close. When any object/atom gains or loses a charge it is ionized, aka an unequal amount of electrons and protons. This separation creates a dipole, any separation is a dipole, created by another charge is called an induced electric dipole. Charging by conduction is where charges are brought by physical contact. A source of negative charge brought in contact with another object will transfer those excess electrons. Charges will separate based on their polarity. Electrostatic forces can be useful and destructive. As stated earlier. Force depends on distance and is the inverse square of the distance F = 1/r2 Force depends on charge F = qa qb where q represents charge and (a) and (b) are charge if two objects Coulomb was a French physicist around 1785 and established these relationships which are summarized in the following equation: F = K qaqb/ r2 Which states the force (F) between two objects (q) is equal to coulombs constant (K) times the product of the two charges (q) in Coulombs, divided by the square of the distance between them (r) in meters K = 9.0 x 109 N – m2/C2 Remember this electric forces are vector quantities with magnitude and direction, sorry, more trig. Compare Coulombs law with Newton’s Law of Universal Gravitation These both equate force and will be treated the same from a kinematics point of view. Red neck speak = opposite forces cancel, same or like forces add, forces at angles have vector components Electric Fields and Forces are ALL vectors, thus all rules applying to vectors must be followed. Consider three point charges, q1 = 6.00 x10-9 C (located at the origin),q3 = 5.00x10-9 C, and q2 = -2.00x10-9 C, located at the corners of a RIGHT triangle. q2 is located at y= 3 m while q3 is located 4m to the right of q2. Find the resultant force on q3. Which way does q2 push q3? 4m q2 q3 Which way does q1 push q3? 3m q1 q Fon 3 due to 1 5m Fon 3 due to 2 q= tan-1(3/4) q3 q = 37 Electricity behaves much like gravity, the same equations actually just different variables. As stated yesterday, the electric force varies inversely with the square of the distance. If a small charged object has an electric force on it then there is an electric field, basically an electric field is a force on a charged object. Faraday theorized that the electric force changes the properties of space and that the object acted upon experiences this change in space That object (charge) has to be considered a point source. The electric field strength or force can be summarized by the following: E = F on q/q E is in N/C E is the electric field, divided by the force on q divided by the strength of q Lets try one: An electric field is measured using a positive test charge of 3.0 x 10-6 C. The test charge experiences a force of 0.12 N. What is the magnitude of the electric field strength on the test charge Just like vectors, arrows can be used to represent the magnitude and direction of the electric field. The electric field is represented by field lines. These are drawn perpendicular to the point source or charge. Positive charges have field lines pointing away from the source. Negative charges have field lines pointing towards the source JANKY PICTURES, I KNOW LIMITED BUDGET Field lines extend out straight unless there are two or more charges then the field becomes the vector sum of the fields , the field lines become curved and more complex Remember, field lines are simply a way of representing the electric field around an object or charge. What happens to the potential energy of an object as you lift it into the air? In the same way, as you perform work on a charge by pulling it away from another one of opposite charge you increase its potential energy. The larger the test charge the greater the potential energy because more work is required. V = W on q/q This is the difference in electrical potential, the ration of the work done on a charge/by the strength of that charge The electric potential difference is measured in J/C also known as a volt = 1 joule/1 coulomb Can you move a charge to two different positions and have the same potential difference? If the charge is moved in a circle. The distance remains the same. Equipotential is when the electrical potential difference is the same in two or more positions. Only differences in electric potential can be measured Putting different equations together we get the following : V = Ed , remember V = W on q/q as well, basically W/q Where V is the potential difference, E is electric field intensity, and d is the distance the charge moves. E is in N/C and d is in m putting the two together we get J/C which is 1 volt. Two charged parallel plates are 1.5 cm apart with a single proton between them. The magnitude of the electric field between the plates is 1800 N/C What is the potential difference between the plates? What work is required to move from the negative to the positive plate? Uniform electric fields allowed for the charge of an electron to be established which means that an object can have only a charge with a magnitude that is some integral multiple of the charge of an electron (1.6 x 10-19 C). Robert Millikan did this in 1909 with the oil drop experiment. REAL DEAL EASIER VERSION All systems come to equilibrium when the energy of the system is at a minimum. If there excess charges on one charged object they will migrate to another object in contact with it until the balance of charges is equal on both sides. After equilibrium is reached, work would have to be performed to move any more charges. At equilibrium there is no potential difference. Two metal objects (spheres) of different sizes would result in an imbalance of charges at equilibrium due the proximity of charges and their repulsive forces Any potential difference results in the movement of electrons (charges) aka electricity. This is the purpose of grounding. Providing a path for a potential difference to flow. The book references gas trucks and electrical devices. Sort of common sense, but charges on a conductive surface will spread apart as far as possible. A solid sphere or a hollow sphere for example will have all the accumulated charges migrate to the outside. This explains why a car (hollow metal sphere) shields a driver from a lightning strike or downed power line. Also because the outside provides the easiest path to ground. Charges accumulate more in smaller areas on an irregularly shaped object, a point or protrusion of some kind. For this reason smooth, regularly shape objects are used to reduce the electric fields and thus reduce the chance of a spark. Lighting rods are pointed in order to maximize the electric field and the subsequent potential difference to form the stepped leader (conducting path) for lightning. A capacitor is a device used to store electrical charge. First created in 1746 by a Dutch dude, the device was called the Leydon Jar. Capacitance is the ratio of the stored charge in an object to the electrical potential difference C = q/ V Capacitors vary widely in size and capacitance are independent of the charge. If charge goes up so does the potential difference. Capacitors are nothing more than two conducting plates of equal and opposite charge separated by an insulator, usually some type of foil separated by thin plastic. The capacitance is varied by manipulating the amount of surface area of the conductors, the distance between them, and the insulating material used. Ceramic, polyester, and air are common dielectrics or insulating materials. Capacitance is measured in farads (F) which is one coulomb per volt. A farad is a relatively large unit and is most often measured in very small fractional amounts.