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Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science

Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science. Chapter 7 ELECTRICITY AND MAGNETISM. This lecture will help you understand:. Electric Force and Charge Coulomb’s Law Electric Field Electric Potential Conductors and Insulators Voltage Sources Electric Current

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Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science

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  1. Hewitt/Lyons/Suchocki/YehConceptual Integrated Science Chapter 7 ELECTRICITY AND MAGNETISM

  2. This lecture will help you understand: • Electric Force and Charge • Coulomb’s Law • Electric Field • Electric Potential • Conductors and Insulators • Voltage Sources • Electric Current • Electrical Resistance • Ohm’s Law • Electric Circuits • Electric Power • Magnetic Force • Magnetic Fields • Magnetic Forces on Moving Charges • Electromagnetic Induction

  3. Electric Force and Charge Electric force can attract some objects and repel others Electric charge: • the fundamental quantity that underlies all electric phenomena • comes in two kinds: • positive such as protons • negative such as electrons

  4. Electric Force and Charge Fundamental rule for electricity: Like charges repel; unlike charges attract.

  5. Electric Force and Charge The charging of bodies relates to the structure of atoms. Fundamental facts about atoms: • Every atom is composed of a positively charged nucleus that contains protons. • Each atomic nucleus is surrounded by negatively charged electrons. • All electrons are identical with the same amount of negative charge; all protons are identical with the same amount of positive charge, equal in amount to the negative charge of an electron. • Protons and neutrons compose the nucleus. Protons are about 1800 times more massive than electrons; neutrons, with no charge, have slightly more mass than protons.

  6. Electric Force and Charge Normally, an atom is electrically neutral—it has the same number of electrons outside the nucleus as protons in the nucleus. In the atoms of metals, outer electrons are loosely bound and can move freely and are available to join or flow by other atoms.

  7. Electric Force and Charge Atom losing 1 or more electrons  positive ion Atom gaining 1 or more electrons  negative ion The amount of work required to pull an electron away from an atom varies for different substances.

  8. Electric Force and Charge CHECK YOUR NEIGHBOR When you brush your hair and scrape electrons from your hair, the charge of your hair is A. positive. • negative. • both A and B. • neither A nor B.

  9. Electric Force and Charge CHECK YOUR ANSWER When you brush your hair and scrape electrons from your hair, the charge of your hair is A. positive. • negative. • both A and B. • neither A nor B. Comment: And if electrons were scraped off the brush onto your hair, your hair would have a negative charge.

  10. Electric Force and Charge Conservation of Charge When electrons are transferred from one material to another—none are created or destroyed.

  11. Coulomb’s Law Coulomb’s Law For a pair of charged objects that are much smaller than the distance between them, the force between them varies directly as the product of their charges and inversely as the square of the separation distance.

  12. Coulomb’s Law Unit of charge is measured in coulombs, C. The charge of an electron is the fundamental charge = 1.6  10-19 C. K is the proportionality constant 9.0  109 N • m2/C2 that converts units of charge and distance to force. The size of force depends on the product of charges of the two objects. For like signs of charge, the force is repulsion. For unlike signs of charge, the force is attraction.

  13. Coulomb’s Law Differences and similarities between gravitational and electrical forces: • Gravitational forces are only attractive, and electrical forces may be either attractive or repulsive. • Both can act between things that are not in contact with each other. • Gravitational forces act in a straight-line direction between masses, and electrical forces act in a straight- line direction between charges. • A force field surrounds any mass (gravitational field) and any charged object (electric field).

  14. Coulomb’s Law CHECK YOUR NEIGHBOR According to Coulomb’s law, a pair of particles that are placed twice as far apart will experience forces that are A. half as strong. • one quarter as strong. • twice as strong. • four times as strong.

  15. Coulomb’s Law CHECK YOUR ANSWER According to Coulomb’s law, a pair of particles that are placed twice as far apart will experience forces that are A. half as strong. • one quarter as strong. • twice as strong. • four times as strong.

  16. Coulomb’s Law Charge Polarization Molecules can’t move from their relatively stationary positions, but their “centers of charge” can move. This distortion of charge in the atom or molecule is electric polarization.

  17. Electric Field Electric field: • is the space that surrounds any charged object • is a vector quantity having magnitude and direction • magnitude of field at any point is force per unit charge • obeys the inverse-square law for a point source

  18. Electric Field Field lines: • used to visualize electric field • show direction of electric field—away from positive and toward negative • show intensity of electric field: bunched together  field is strongest lines farther apart  field is weaker

  19. Electric Potential Electric Potential Energy is the energy possessed by a charged particle or other object due to its location. If particle is released, it accelerates away from the sphere, and its electric PE changes to KE.

  20. Electric Potential Electric Potential Energy Batteries and generators pull negative charges away from positive ones, doing work to overcome electrical attraction. The amount of work done depends on the number of charges and separation distance. Work done by the battery and generator becomes available to a circuit as electrical potential energy.

  21. Electric Potential Electric potential: • electric potential energy per charge • energy that a source provides to each unit of charge Electric potential = electric potential energy charge

  22. Electric Potential Electric potential and voltage are one and the same. Unit of measurement is the volt.

  23. Electric Potential CHECK YOUR NEIGHBOR Electric potential energy is measured in joules. Electric potential, on the other hand (electric potential energy per charge), is measured A. in volts. • in watts. • in amperes. • also in joules.

  24. Electric Potential CHECK YOUR ANSWER Electric potential energy is measured in joules. Electric potential, on the other hand (electric potential energy per charge), is measured A. in volts. • in watts. • in amperes. • also in joules.

  25. Conductors and Insulators Electric Conductors are materials that allow charged particles to pass through them easily. Atoms of metals have free electrons that conduct through a metallic conductor when a potential difference exists. The result is electric current. Electric Insulators are materials having tightly bound electrons.

  26. Conductors and Insulators CHECK YOUR NEIGHBOR When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons A. must be supplied by you, just as water must be supplied for a water pipe. • are already in the wire. • may fall out, which is why wires are insulated. • none of the above.

  27. Conductors and Insulators CHECK YOUR ANSWER When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons A. must be supplied by you, just as water must be supplied for a water pipe. • are already in the wire. • may fall out, which is why wires are insulated. • none of the above.

  28. Conductors and Insulators Semiconductors are materials that are neither good conductors nor good insulators, whose resistance can be varied. Superconductors are certain metals that acquire infinite conductivity (zero resistance) at temperatures near absolute zero.

  29. Voltage Sources Voltage Sources Potential difference exists when the ends of an electrical conductor are at different electric potentials. Batteries and generators are common voltage sources. Charges in a conductor tend to flow from the higher potential to the lower potential. The flow of charges persists until both ends reach the same potential. Without potential difference, no flow of charge will occur.

  30. Electric Current Sustained electric current requires a suitable voltage source, which works by pulling negative charges apart from positive ones (available at the terminals of a battery or generator). This energy per charge provides the difference in potential (voltage) that provides the “electrical pressure” to move electrons through a circuit joined to those terminals.

  31. Electric Current Electric current: • is the flow of electric charge • in metal—conduction electrons • in fluids—positive and negative ions • Is measured in amperes One ampere is the rate of flow of 1 coulomb of charge per second or 6.25 billion billion electrons per second. The actual speed of electrons is slow through the wire, but an electric signal travels near the speed of light.

  32. Electric Current CHECK YOUR NEIGHBOR Which of these statements is true? A. Electric current is a flow of electric charge. • Electric current is stored in batteries. • Both are true. • Neither are true.

  33. Electric Current CHECK YOUR ANSWER Which of these statements is true? A. Electric current is a flow of electric charge. • Electric current is stored in batteries. • Both are true. • Neither are true. Explanation: Voltage, not current, is stored in batteries. The voltage will produce a current in a connecting circuit. The battery moves electrons already in the wire, not necessarily those in the battery.

  34. Electric Current Electric current may be • DC—direct currentcharges flow in one direction • AC—alternating currentcharges alternate in direction Accomplished in a generator or alternator by periodically switching the sign at the terminals

  35. Electrical Resistance Electrical resistance: • describes how well a circuit component resists the passage of electric current • is defined as the ratio of the voltage of the energy source to the current moving through the energy receiver • is measured in ohms after 19th century German physicist Georg Simon Ohm.

  36. Electrical Resistance Factors affecting electrical resistance: • thin wires resist electrical current more than thicker wires • long wires offer more electrical resistance • materials of wire: • copper has a low electrical resistance, so it is used to make connecting wires • rubber has an enormous resistance, so it is used in electrical insulators • temperature: higher temperature (greater jostling of atoms), greater resistance

  37. Ohm’s Law Ohm’s Law relationship between current, voltage, and resistance Current in a circuit varies in direct proportion to the potential difference (voltage) and inversely with the resistance:

  38. Ohm’s Law CHECK YOUR NEIGHBOR When you double the voltage in a simple electric circuit, you double the A. current. • resistance. • both of the above. • neither of the above.

  39. Ohm’s Law CHECK YOUR ANSWER When you double the voltage in a simple electric circuit, you double the A. current. • resistance. • both of the above. • neither of the above. Explanation: This is a straightforward application of Ohm’s Law. Current =

  40. Ohm’s Law Electric Shock Damaging effects of electric shock are the result of current passing through the body: • tissue damage due to conversion of electrical energy to heat • nerve damage due to disruption of normal nerve functions Resistance of one’s body depends on its condition. To receive a shock, there must be a potential difference between one part of the body and another part.

  41. Ohm’s Law Prongs on electric plugs and sockets: • two flat prongs for the current-carrying double wire, one part live and the other neutral • third prong is longer and the first to be plugged into socket; path to ground prevents harm to user if there is an electrical defect in the appliance

  42. Electric Circuits Electric circuits: any closed path along which electrons can flow for continuous flow—no gaps (such as an open electric switch)

  43. Electric Circuits Devices connect to a circuit in one of two ways: • Series • Parallel

  44. Electric Circuits Series: • A single-pathway circuit is formed for electron flow between the terminals of the battery, generator, or wall socket. • A break anywhere in the path results in an open circuit; electron flow ceases. Total resistance adds, current decreases as more devices are added. • Main disadvantage: If one device fails, the entire circuit ceases, and none of the devices will operate.

  45. Electric Circuits Parallel: • A branched pathway is formed for the flow of electrons through a circuit, connected to the terminals of a battery, generator, or wall socket. • A break in any path does not interrupt the flow of charge in the other paths. • A device in each branch operates independently of the others. • Total current in the branches adds.

  46. Electric Circuits CHECK YOUR NEIGHBOR When two identical lamps in a circuit are connected in parallel, the total resistance is A. less than the resistance of either lamp. • the same as the resistance of each lamp. • less than the resistance of each lamp. • none of the above.

  47. Electric Circuits CHECK YOUR ANSWER When two identical lamps in a circuit are connected in parallel, the total resistance is A. less than the resistance of either lamp. • the same as the resistance of each lamp. • less than the resistance of each lamp. • none of the above. Explanation:Resistors in parallel are like extra lines at a checkout counter. More lines means less resistance, allowing for more flow.

  48. Electric Circuits CHECK YOUR NEIGHBOR Consider a lamp powered by a battery. Charge flows A. out of the battery and into the lamp. • from the negative terminal to the positive terminal. • with a slight time delay after closing the switch. • through both the battery and the lamp.

  49. Electric Circuits CHECK YOUR ANSWER Consider a lamp powered by a battery. Charge flows A. out of the battery and into the lamp. • from the negative terminal to the positive terminal. • with a slight time delay after closing the switch. • through both the battery and the lamp. Explanation: Remember, charge is already in all parts of the conducting circuit. The battery simply gets the charges moving. As much charge flows in the battery as outside. So charge flows through the entire circuit.

  50. Electric Power Electric power is the rate at which electrical energy is converted into another form, equal to the product of current and voltage. Electric power = current  voltage = IV 1 watt = 1 ampere  1 volt

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