1 Chapter 20 Electricity Chapter 21 Magnetism Prentice Hall 2006

Electric Charge and Static Electricity A. Electric charge – an electrical property of matter that creates a force between objects. *based on number of electrons and protons in the objects *excess number of electrons creates a negative charge - atom gains electrons *excess number of protons creates a positive charge - atom loses electrons

3 Electric Charge and Force srikant.org/core/node8.html

4 Static Electricity Study of the behavior of electric charges (at rest) including how charge is transferred between objects by –Friction (like walking across a carpet) –By conduction (contact) –By induction (bringing a charged object near a neutral object)

For current to flow: You must have –Source of voltage –Complete path or circuit –Conductor with low resistance –Device to use the energy (i.e., light bulb) Current flows from positive to negative –This is considered conventional current –Direction of positive charge movement that is equivalent to actual motion of charge in the material

6 Types of Current Direct current (DC) - flows in one direction only –Flashlight and car batteries produce DC Alternating current (AC) - flows back and forth –This is produced by an electrical generating power plant that sends electricity to places like businesses, schools, and homes

7 Resistances Conductors have low resistances and allow charges to flow easily –Samples: metals Insulators have high resistances because electrons are tightly bound to its atoms –Samples: plastics, dry wood

8 Ohm’s Law V = IR V, potential difference in volts, v I, current in amperes, a R, resistance in ohms,  * Resistance is affected by a material’s thickness, length, and temperature. *Increasing the thickness of a metal wire will reduce its resistance. V I R

9 Problem samples Find the resistance in a circuit with an 8.0 volt battery and 0.2 amp flowing when the current is on. R = V I R = 8.0 v 0.2 a R = 40. 

10 Practice Problems

11 Practice Problems, answers 1. R=V/I = 24v/0.80a = 3.0 x 10 1  2. R=V/I = 120v/0.50a = 240  3. V = IR = (0.50a)(12  ) = 6.0 v 4. I = V/R = 1.5 v/ 3.5  = 0.43a

Circuits Two main types –Series where there is one path for current flow –Parallel where there is more than one path for current to flow. Most circuits in your home are of this type - parallel. –SeriesParallel

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17 Answers to the previous 4 slides Frame 25: no, no, yes Frame 26: 1-abc; 2-ab; 3-a; 4-abc; 5-abc Frame 27: 1-a; 2-c; 3-a Frame 28: 4-d; 5-d; 6-c

18 Electrical Safety These devices are used make electricity safer: –Circuit breaker (if too much current flows, this opens the circuit - page 609) –Fuse (if too much current flows, the wire in this melts and opens the circuit - page 612) –Ground-fault circuit interrupter (this automatically opens the circuit if it senses unequal currents - see page 613)

19 Schematic Diagrams The figures on the prior slide are schematic diagrams where symbols represent the parts of a circuit. See page 374 in book. lamp Wires, Connected, Crossing Wires, Not Connected, Crossing

20 Power = Current x Voltage P = IV, unit: watt = amp volt An electric space heater requires 29 amp of 120 v current to adequately warm a room. What is the power rating of the heater? P = IV P = 29a (120v) P = 3.5 x 10 3 watt

21 Your assignment for /1-3 math practice 613/5-8

20.4 Electronic Devices Electronic Signal - information sent as patterns in controlled flow of electrons through a circuit. Analog signals are produced by continuously changing voltage or current (page 619) Vacuum tubes can be used to change AC into DC, increase signal strength, turn current on or off. Semiconductors are made from crystalline solids and conduct current under certain circumstances 22

23 Other electrical information Transistors are solid state components with 3 layers of semiconductors Transistors amplify a mobile phone’s incoming signal A diode maintains proper voltage levels in the circuits in a mobile phone An Integrated Circuit is a thin slice of silicon that contains many solid-state components

21.1&2 Magnetism & Electromagnetism Define vocabulary terms in (15) Write the key concepts in (10) 651/1-10 (complete sentences), 14, 15,17, 30 Workbook pages:

&2 Magnetism & Electromagnetism Review: 1. Like magnetic poles _____ 2. Unlike magnetic poles ____ 3. Magnetic field lines run from ___ to ___

26 answers Like poles repel Unlike poles attract Magnetic field flows from N to S

27 Earth’s Magnetic Field

28 Electricity can produce magnetism and … Magnetism can produce electricity!!! When electricity flows through a conductor, a magnetic field is produced Right hand rule for a straight conductor

Electrical Energy Generation and Transmission When a coil of wire moves in a magnetic field, electricity will flow in the coil (aka: electromagnetic induction) This is how our electricity is generated Faraday’s Law - an electric current can be produced in a circuit by a changing magnetic field.

30 Right hand rule used in designing and understanding of generators I current flow B magnetic field direction F resulting force acting on wire

31 Faraday’s Law Just by moving a magnet in a coil of wire or [as in fig. (c), the switch is turned on or off] electricity is produced. This is how electricity is generated.

32 Transformers These change the voltage and current of electricity by having different numbers of turns on the primary and secondary coils Before electricity gets to your house it is stepped down by a passing through a step-down transformer.

33 More ElectroMagnetic Information The stronger the magnetic force of a magnet, the more effect it has on another magnetic or on moving charges Magnetic fields are produced by moving electric charges AC and DC generators are similar except DC generators use commutators to produce direct current Heat from fossil fuels spins a turbine (coil of wire that spins in a magnetic field) that generates electrical energy