Electricity Review Electric Charge and Force Electric charge

Electricity Review

Electric Charge and Force • Electric charge is property of object due to an excess (negative) or deficiency of electrons (positive). • Charge is measured in Coulombs = charge of 6. 24 x 1018 protons or electrons. • Like charges repel, opposite charges attract • Electric force is attractive or repulsive force between electric charges • Felectricity = Kc Qsource Qtest / d 2 where Kc = 9. 0 x 109 N m 2/ C 2 • Electric force is much stronger than gravitational force. • Conductors allow electrical charge to freely move while insulators do not. Once charge is placed on an insulator it is stuck.

Electric Force Fields • Electric charge changes space around it so other electrical charges feel a force and have a potential energy. Change in space is called electrical field. • For all electrical fields • Electricity = qtest E where E is the electrical force field variable • PE = q. V where V is the potential difference or voltage • For point charges E=S Kc Q source /d 2 • For opposite plate charges F and E are constant between the plates. • Pictorially represent electric force fields with force line diagrams where arrows indicate force on a positive test charge and density of lines indicates magnitude of field.

Circuit Variables and relationships • Current (I) is the flow of electric charge; measured in amperes or amps. • Voltage difference (DV) is the electrical pressure or energy per charge measured in volts or joules/coulomb • Resistance (R) is the opposition of a material to the flow of electrical charge; measured in ohms • Power (P) is rate at which electrical energy turned into a useful form of energy. • I=DV / R (Ohm’s Law) • Power = I DV = I 2 R = DV 2/ R • Current into and out of junction must be same (conservation of charge) • Voltage must be used up in any complete path (conservation of energy)

Series and Parallel Circuits • Only one complete current path • Rtotal = R 1 + R 2 + R 3 … • Current constant through each part of the circuit is the same. • Voltage drop divides among components (DV = IR) • Break in one part of circuit stops current in all parts • More than one complete current path • 1/Rtotal = 1/R 1 + 1/R 2 +1/R 3 … or R total= (R 1 x R 2)/ (R 1+R 2) • Voltage drop across each branch is the same. • Current splits among branches may not be equal (I=DV/R) • Break in one branch has no effect on current flowing through other branches of circuit.

Magnetism • Attractive or Repulsive force caused by accelerating electric charge. • Spinning electrons give permanent magnets their magnetism • Linearly accelerating charge will cause a magnetic field to be formed around a current carrying wire • Looping a current carrying wire will increase the strength of the magnetic field inside the loop. • Magnetic poles always come in pairs (North and South) resulting in magnetic force often being an aligning force. • Magnetic field is from North to South by convention and acts on other magnets and MOVING electric charges (Lorentz Force). • Heating, hitting can weaken magnets by misaligning domains.

Electromagnetism • Current carrying wire • Creates magnetic field in shape of concentric loops • Right Hand Rule #2 shows direction of magnetic field • Coil of Current carrying wire • Charges changing direction so have centripetal as well as linear acceleration. • Magnetic field strongest in center of coil • RHR #1 gives direction of N pole of field • Lorentz force • Magnetic force is perpendicular to both direction of charge movement and magnetic field. • Strongest when magnetic field is perpendicular to direction of charge motion. Zero when field and direction of motion are parallel. (RHR #3)

Electromagnetic Induction • Changing magnetic field induces a voltage difference in a closed loop. If the loop is a conductor this results in a current. The voltage difference is proportional to the number of loops and the rate at which the change occurs (Faraday’s Law). • The induced current from above will create a second magnetic field which will oppose the change that caused the current in the first place (Lenz Law). Restatement of conservation of energy. • Electric and Changing magnetic fields regenerate each other at speed of light (light is an EM wave). Maxwell’s corallary.

Odds and Ends • Plasma • High temperature 4 th state of matter • Made of free electrons and ions • Conducts electricity • Transistor • Component that conducts from emitter to collector when base is exposed to certain voltage. • Used for amplifiers, oscillators and switches