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Transcript
Electric Forces & Fields Chapter 18 The Origins of Electricity • In the mid 18th century Ben Franklin created the idea on positive and negative electric charge. • It wasn’t until 150 years later the electron was discovered. • Franklin described an electric “fluid” that would flow depending on electric pressure. Electric Charge • In 1909 Robert Millikan discovered charge was “quantized.” • This means there is a smallest amount. • Thing about it like this, In order to have a car all the pieces must be there. If you try to take the engine out, it’s no longer a car. • Electric charge is much the same. • The electron has a set charge, take some away and it’s no longer an electron. The Millikan Experiment • Click here to recreate the Millikan oil-drop experiment Charged Particles • Protons (+e): Mass = 1.673 x 10-27 kg, Charge = 1.60 x 10-19 C • Neutron: Mass = 1.675 x 10-27 kg, Charge =0 • Electron (-e): Mass = 9.11 x 10-31 kg, Charge = -1.60 x 10-19 C e = 1.60 x 10-19 C Neutral Objects • If the number of electrons equals the number of protons the object is said to be electrically neutral. • In general q (charge) = Ne, where N is an integer. • Since proton are much more difficult to remove, most objects are charged by removing or adding electrons. Charged Objects • When two dissimilar materials are rubbed together electron usually go from one to the other. • Look on the triboelectric scale to see which way they go, positive or negative • Also, charge is conserved. • The net charge of an isolated system is constant Like and Unlike charges • Like charges repel each other • Unlike (opposite) charges attract Conductors & Insulators • Materials that have lose valence electrons are conductors • Materials with tightly held valence electrons are insulators • Can you think of some? Charging by Induction (Conductors) Click on the picture to open an applet Polarization (Insulators) Coulomb’s Law • F = 1/(4peo) q1q2 / r2 – F = Force (N) – eo = 8.85 x 10-12 (electric permittivity of a vacuum) – q = charge (Coulombs) – r = distance between charges • 1/(4peo) = k Point Charges • When more than two charges are acting on each other we sum the forces. • Treat each pair independently, then add the forces. r2 r1 q1 q2 q3 Point Charges in 2D • When more than two charges are acting on each other in 2D, sum the forces for x and y dimensions. • Again, treat each pair independently, then add the forces. r2 q3 q1 q2 r1 Electric Field • Just like mass create gravitational fields, charges create electric fields • With gravity the field strength is measure as Newton per kilogram • What do you think Electric fields are measured in? Newton's per Coulomb Measuring the Electric Field • If the unit is Newton's per Coulomb, what is the equation? E = F / qo • Simple enough, right. • E = Electric Field • F = Force • qo = charge producing field Summing electric Fields • It is the surrounding charges that create an electric field at a given point in space. • Look at Example 8 Parallel Plate Capacitors • To store charge a capacitor is used. • The most common type is a parallel plate capacitor. • Charge is spread uniformly, not really, through out the plate • E = q /eoA = s / eo • s = charge Density Picturing the Electric Field Click on the picture to open an applet Electric Field Rules • Fields start at positive and end at negative, or start or end at infinity. • This is by convention. The field is said to predict the movement of a positive charge. • The density of lines should represent the strength of the field. • A positive charge will have a velocity tangent to a field line. • Field lines do not actually exist since the are an infinite number of paths a test charge can take. Electric Fields Inside Conductors • Excess electric charge moves to the surface of a conductor • At equilibrium the electric field inside a conductor is zero – This comes from the fact that free electrons will not move inside the conductor. • So the electric field lines don’t penetrate the conductor • The electric field outside a conductor is perpendicular to the surface Click here for more info Triboelectric Scale • Human hands (usually too moist, though) (Very positive) • Rabbit Fur • Glass • Human hair • Nylon • Wool • Fur • Lead • Silk • Aluminum • Paper • Cotton • Steel (Neutral) • • • • • • • • • • • • • • • Wood Amber Hard rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (like Scotch Tape) Polypropylene Vinyl (PVC) Silicon Teflon (Very negative )