Lecture 26 Chapter 23 Circuits Chapter 24 Magnetism

Transcription

1 Lecture 26 Chapter 23 Circuits Chapter 24 Magnetism Quiz 4: Monday Nov. 1; Chaps. 22,23,24 Electric Circuit Make electric circuit by connecting voltage source and resistive object(s) together in a loop with metal wires that are (nearly) perfect conductors 9V + - Resistance (light bulb) Battery Wires (voltage source) (assumed to be perfect conductors) This is a closed circuit -- there is a continuous path through conducting material from the + terminal to the - terminal of the voltage source

2 Electric Circuit Circuit Any closed path around which current (electrons) can flow. A complete circuit has a voltage source and one or more resistances (such as light bulbs), connected by ideal wires. There may be a switch to open and close the circuit. Ideal wires have no resistance. Water circuit. Electric circuit. Ideal wires Circuit Diagram Could be light bulb, heater, etc. Ideal wires + -

3 Circuits Types of Electric Circuits Connected in two common ways: series one single pathway for current flow from one terminal of the battery (or generator or wall outlet), through each resistance in turn, and back to the other battery terminal. parallel has two or more branches, each of which is a separate path for the flow of current. Series Circuit Resistive objects connected one after the other in only one loop -- in series. Same current passes through each element. Disconnect one of the bulbs and the circuit is broken (other bulbs go out).

4 Parallel Circuit There are several different paths, or branches that current can follow; branches are in parallel. Same voltage on each bulb; current from voltage source splits, with some going through each branch. Disconnect one of the bulbs and the other bulbs stay light with same brightness. Check Yourself How do the brightnesses of the identical light bulbs compare (which is brightest)? Which bulb draws the most current? What happens if bulb A is unscrewed? What happens if bulb C is unscrewed?

5 Overloading a Circuit Electric devices in home connect in parallel. More appliances added to a parallel circuit, the more current flows. A large current can cause significant ohmic heating in the wires, which is a fire hazard. Protect against overloading a circuit by adding a fuse. Fuses & Circuit Breakers Fuse ribbon is designed to melt (due to ohmic heating) when current is too large. Circuit breaker does same job without needing replacement; flip the switch to reconnect. 20A Fuse Circuit Breaker

6 Check Yourself If a 1200 watt hair dryer is connected to a 120 Volt line, how much current will it draw? How many hairdryers can you operate before blowing a 30 amp fuse? Check Yourself If a 1200 watt hair dryer is connected to a 120 Volt line, how much current will it draw? P = IV so I = P/V = 1200W/120V = 10A How many hairdryers can you operate before blowing a 30 amp fuse? Three!

7 Chapter 24 Magnetism Magnetic Field & Force An electric charge creates an electric field around it. A second charge placed in the field feels an electric force. Coulomb s Law describes the force. A moving electric charge also creates a magnetic field around it. A second moving charge placed in the magnetic field feels a magnetic force. Note that a set of moving charges constitutes a current.

8 Magnetic Field & Force Electric current produces magnetic field Magnetic field is a disturbance in space due to a current (current in a wire, or atomic current) No magnetic charges (magnetic monopoles ) have been found. There must be a current to get a magnetic field. Magnetic field and magnetic force are both vectors -- have size and direction. Electric Current & Magnetic Field An electric current in a wire produces a magnetic field. Electric Current Magnetic Field Lines

9 Source of Magnetic Field-Current Magnetic field lines near current-carrying wire and Right Hand Rule for magnetic field lines: Thumb of right hand in direction of current flow; fingers curve along magnetic field lines Magnetic Field from Atomic Current All atoms have moving charges (electrons) Orbital motion Rotational motion ( spin ) Moving charges Form current loops and generate magnetic field Feel magnetic force from external magnetic field Electron Magnetic Field Lines

10 Magnetic Fields CHECK YOURSELF The source of all magnetism is A. electron currents in atoms. B. electron currents in wires C. either or both A and B. D. accelerating charges E. none of the above. Magnetic Metals For most atoms, the magnetic fields of the different electrons cancel out. However, Iron, Nickel, and Cobalt atoms produce a fairly strong magnetic field. Spin of the electron in these metals produces a net magnetic field Iron, Cobalt, Nickel

11 Magnetic Domains The magnetic field of an single iron atom is so strong that interactions among adjacent atoms cause large clusters of atoms, called magnetic domains, to line up their magnetic fields with one another. A microscopic view of magnetic domains in a crystal of iron. Each domain consists of billions of aligned iron atoms. The blue arrows pointing in different directions tell us that these domains are not aligned. Magnetic domains can be aligned by external magnetic field. The whole object becomes magnetized.

12 Permanent Magnet (Magnetized piece of iron, nickel, or cobalt) Lines of magnetic field emerge from North pole (N) of magnet and return into South pole. (S) Magnetic Poles Two types of poles North (N) and South (S). N pole somewhat like + electric charge S pole somewhat like - electric charge In all magnets can t have one type pole without the other No single pole ( monopole ) known to exist Example: simple bar magnet poles at the two ends horseshoe magnet: bent U shape poles at ends S N

13 Magnetic Force between Poles As with electric charges, like magnetic poles (N&N, S&S) repel and opposites (N&S) attract. As in electric polarization, both types of poles can attract unmagnetized iron, steel, nickel or cobalt objects by forming induced magnets Induced Magnets Unmagnetized iron can be induced to align by an external magnetic field, forming induced magnets. S Strong Magnet N N S Strong Magnet N Strong magnetic poles attract unlike induced magnet poles.

14 Permanent/Temporary Magnets Difference between permanent magnet and temporary magnet Permanent magnet alignment of domains remains once external magnetic field is removed Temporary magnet (induced magnet) alignment of domains returns to random arrangement once external magnetic field is removed Demo: Magnetizing Iron Magnetic domains in iron nails are induced to align by proximity of the strong magnet Each nail becomes itself a magnet, which in turn magnetizes the nail below it, forming a chain. When the strong magnet is removed, most of the domains un-align and nail lose most of their magnetization.

15 Magnetic Poles CHECK YOURSELF A weak and strong magnet repel each other. The greater repelling force is by the A. stronger magnet. B. weaker magnet. C. Both the same. D. None of the above. Electric Currents and Magnetic Fields Wire with loop

16 Electromagnets Electric current in a coil of wire creates a magnetic field similar to a bar magnet. N S N S N Current passing through loops of coiled wire S Electric Currents and Magnetic Fields CHECK YOURSELF An electromagnet can be made stronger by A. increasing the number of turns of wire. B. increasing the current in the coil. C. Both A and B. D. None of the above.

17 Practical Electromagnets Electromagnet created by passing current through a coil of wire. Electromagnet is stronger when an iron bar is inserted within the coil. N Wire Coil S Connect to battery or power supply Physics 1 (Garcia) SJSU Iron Bar Check Yourself When an object is charged with static electricity, is that object also magnetized? But isn t iron is magnetized when the electrons are aligned. If iron has electrons, then why isn t it charged?

18 Magnetic Data Storage Hard disk drive: Magnetic ink on checks (& paper money) Videotape, credit cards,... Series Circuits Key Points of Lecture 26 Parallel Circuits Fuses and overload Magnetic Field Magnets: permanent, induced, electromagnets Magnetic force between atomic currents (poles) Magnetic Force on Current in Wire Magnetic Data Storage Before Friday, read Hewitt Chap. 24 (first half). Homework #18 due by 11:00 PM Friday Oct. 29. Homework #19 due by 11:00 PM on Sunday Oct. 31.