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 / d2 where Kc = 9.0 x 109 N m2/ C2 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 = qV where V is the potential difference or voltage For point charges E=S Kc Q source /d2 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 = I2 R = DV2/ 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 = R1 + R2 + R3 … 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/R1 + 1/R2 +1/R3 … or R total= (R1 x R2)/ (R1+R2) 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 Coil of 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 Transistor High temperature 4th 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