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Electricity and Magnetism

Electricity and Magnetism. Chapter 5. Great Idea : Electricity and magnetism are two different aspects of one force—the electromagnetic force. Static Electricity. Observations Explanation by Newton’s Law: Forces exist between two charges Two types of electrical charges.

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Electricity and Magnetism

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  1. Electricity and Magnetism Chapter 5 Great Idea: Electricity and magnetism are two different aspects of one force—the electromagnetic force

  2. Static Electricity Observations • Explanation by Newton’s Law: Forces exist between two charges Two types of electrical charges Different charges attract each other Same charges repel each other

  3. Benjamin Franklin and Electric Charge • Benjamin Franklin (1706-1790) • One of the pioneers of electrical science as well as a central figure founding United States (writing US Constitution) • All electrical phenomena could be explained by the transfer of a single fluid, and objects could have an excess or a deficiency of this fluid: positive and negative • Kite Experiment (extremely dangerous) • Lightning Rod

  4. The Movement of Electrons • All objects are made up of atoms • Atomic structure: • Nucleus: positive charge • Electrons: negative charge

  5. (continue) Electrons can be removed from atoms • Positive: electrons are stripped off • Negative: extra electrons are added

  6. Coulomb’s Law • Coulomb’s Experiment • Coulomb’s Law: Forces between charges are related to charge and distance. • Charge is measured by the unit of Coulomb. 1 Coulomb = charge of 6.3 x 1018 electrons

  7. Comparison of Two Forces (Science by Numbers) Comparison of Electric Force and Gravitational Force Electric Force Gravitational Force In Hydrogen atom, a single electron circles a single positive particle knows as a proton. The masses of the electrons and protons are and respectively. The charge on the protons is , and the charge on the electrons has the same magnitude but is negative. A typical separation of these two particles in an atom is . What are the values of the electric and gravitational attractions between these two particles?

  8. The Electrical Field • Every charged object exerts forces on its surroundings charges. • The electric filed at a point is defined to be the force that would be felt by a positive 1-coulomb charge if it were brought to that point (force with both strength and direction). • An electric field surrounding a positive charge

  9. Magnetism • Magnetic Force • William Gilbert • (1544-1603) • Dipole magnet • Like repels, unlike attracts • Magnetic Field

  10. Aurora

  11. Pairs of Poles • There is no isolated magnetic poles in nature

  12. Electric Circuits Electric circuit • Unbroken path of material carrying electricity • Electrical conductor Circuit • A source of energy (battery) • A loop of wire • A device to use the electrical energy

  13. Electric Circuits A battery, a wire, and a flashlight bulb. Can you get the bulb to light?

  14. Electric Circuits Which one lights the bulb, and why?

  15. Electric Current and Water Flow Electric Current: Flow of Charges Electric current is analogous to water flow. Which elements correspond in the two systems?

  16. Ohm’s Law • Current: • Unit: Ampere (A) • 1 amp=1 coulomb/sec • Voltage • Analogous to Pressure • Unit: Volts (V) • Typically, • a new flashlight battery produces 1.5 V • A fully charged car battery produces 12 V • A calculator battery produces 9 V • Resistance: • how hard it is to push electrons through wires • Unit: ohms • Ohm’s Law • Current is directly proportional to voltage and inversely proportional to resistance • Equation: V= IR • Unit: 1 V = 1 A x 1 Ohm

  17. Load • The device where useful work is done • Filament of a light bulb • Heating element in hair dryer • Electric coils of wire in an electric motor • Power of Load • P = I x V • Unit: 1 watt = 1 A x 1 V Ex5-1 When you turn on the ignition of your automobile, your 15 vol car battery must turn a 400 amp starter motor. What is the resistance of this circuit, and how much power is required to start your car? Ex5-2 A typical compact disc system has a resistance of 50 ohms. Assuming that this system is plugged into a normal household outlet rated at 115 volts, how much current will flow through the stereo, and what is the power consumption?

  18. Circuit and Its Schematic The battery-and-bulb circuit with its corresponding schematic

  19. Two Kinds of Electric Circuits • Series circuit

  20. Parallel Circuits

  21. Typical Household circuit

  22. Connections between Electricity and Magnetism

  23. Magnetic Effects from Electricity • Oersted • Magnetic field created by motion of electrical charges • Electromagnet • Why magnetic monopoles don’t exist

  24. Electrical Effects from Magnetism • Faraday • Electromagnetic induction • Electrical fields and currents can be produced by changing magnetic fields • Electric Generator

  25. Maxwell’s Equations • Coulomb’s Law • No magnetic monopoles • Magnetic phenomena produced by electrical effects • Electrical phenomena produced by magnetic effects

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