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Unformatted text preview: Dr. Bruce McCann 1 Exam 1 Review Systems of Units We will use many other units of measure, but they can all be expressed in terms of these fundamental 6. Engineers tend to use powers divisible by 3- (engineering notation) Standard prefixes: micro 10-6 milli m 10-3 kilo k 10 3 mega M 10 6 giga G 10 9 Suppose a calculation results in a value of 10-2 amps. What is the value in engineering notation? 10mA In engineering notation, 10 x 10-3 amps or 10 milliamps (10 mA) 2 Engineers Use Units! The current in the resistor is 5 Five what? Amps, mA, A? Any numeric quantity must have units or it is meaningless . The answer to any problem must have units or it is wrong . 3 Current Current (i) is the rate at which charge passes through a cross sectional area: i = q/ t where i = the current in Amperes q = the charge in Coulombs t = the time in seconds = the change in a quantity 1 Ampere = 1 Coulomb/second Charge is conserved. This is a result of conservation of mass (charge). 4 Direction of Current Flow Current is a signed quantity Physicists: Current flow in a conductor is due to electrons flowing from the negative terminal to the positive terminal of a voltage source. Electrical Engineers: Use the convention that current is due to positive charges flowing from the positive terminal to the negative terminal of a voltage source. 5 Voltage Energy is expended to move the charges in an electric circuit The energy required to move one unit of charge between two points is called voltage . v = w/ q where v = the voltage in Volts w = the energy in Joules q = the charge in Coulombs 1 Volt = 1 Joule/Coulomb 6 Dr. Bruce McCann 2 Voltage continued The voltage, or potential difference , between two points in a circuit indicates the energy required to move charge from one point to the other . Voltage is the result of separation of charge....
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- Fall '06