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Transcript
Fundamental Electrical Theory Definitions • Current (I): flow of electric charges per unit time or “flow rate”, measured in “amperes” or “amps” (A) • Electromotive Force (emf) (E): a potential difference or “electric pressure” which drives the flow of charges, measured in “volts” (V) • Resistance (R): an electrical circuit’s opposition to current flow, measured in “ohms” () • Conductor: a material which offers little resistance to current flow, e.g. silver, copper, iron, etc… • Insulator: a material which offers high resistance to current flow, e.g. wood, paper, plastic, etc... Direct Current (DC) • Current flow is unidirectional and of constant magnitude (Batteries) • Ohm’s Law: current in a circuit is directly proportional to the applied voltage and inversely proportional to the circuit resistance E=IR P = I E = I2 R (W) Basic Circuit Properties • Electrons flow (-) to (+) • Conventional Current flows (+) to (-) • Kirchoff’s Law of Voltages (KVL) – Sum of all voltages in a complete circuit is zero – Choose arbitrary loop of a circuit (CW or CCW) – If encounters (+) terminal 1st then (-), then its a voltage drop. (usually a resistor) – If encounters (-) terminal 1st then (+) which is a voltage increase. (usually a battery) • Sum of current into and out of a node is always zero and constant (KCL) Basic Circuit Properties • Series – Current is constant (constant flow rate of e-) – Voltage drops across each resistor (pressure) – R = R1 + R2 + R3 + etc • Parallel – Voltage drop constant – Iin = I1 + I2 + I3 +etc – 1/R = 1/ R1 + 1/R2 + 1/R3 + etc Example Problem #1 Determine V1, V2, V3, V4, and I. 10 10 5 20 V1 V2 V3 V4 90V - + I Meters, volt-meter and amp-meter Example Problem #2 Determine I1, I2, I3, I4 and total circuit resistance. 20 I4 30 I3 20 I2 75V - + I1 Batteries • Dry-cell batteries: cylindrical zinc container, carbon electrode, and ammonium chloride/water electrolyte • Wet-cell batteries: lead-acid battery is the most common, can be charged by forcibly changing the direction of electrical current Voltage Kills • It is the volume of the current that flows that kills. • 0.001 amps = 1 milliamp Tingles • 0.01 amps = 10 milliamps Severe shock, uncontrolled muscle spasms • 0.1 amps = 100 milliamps DEATH! If the current passes through vital organs such as the heart. FINSIHED FOR THE TIME BEING