circuits1 - Electric Current Whenever electric charges of...

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1 Circuits 1 Current and Resistance Electric Current Whenever electric charges of like signs move, an electric current is said to exist a current is the result of the presence of an electric field Look at the charges flowing across a surface of area A The current is the rate at which the charge flows through this surface, average or instantaneous The SI unit of current is Ampere (A) 1 A = 1 C/s Q I t I= dQ dt Electric Current, cont’d The direction of the current is the direction positive charge would flow This is known as conventional current direction In a common conductor, such as copper, the current is due to the motion of the negatively charged electrons It is common to refer to a moving charge as a mobile charge carrier g a charge carrier can be positive or negative (could be an ion, and not just an electron or proton) doesn’t have to be in a wire – e.g. could be in a beam water flowing into the nction ust flow Water and Pipeline Analogy for Circuits junction a must flow out charge flowing into junction must flow ut at same rate out at same rate
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2 Clicker: The figure shows a portion of a circuit. What is the magnitude and direction of the current I? 1. 5A , right 4 A l e f t 2. 4A , left 3. 8A , right 4. 8A , left 5. 10A , left Clicker: The figure shows a portion of a circuit. What is the magnitude and direction of the current I? 1. 5A , right 12345 0% 0% 0% 0% 0% 0 of 5 2. 4A , left 3. 8A , right 4. 8A , left 5. 10A , left 20 Ex: A steady current of 2.5 A exists in a wire for 4 min. How much total charge passes a given point during those 4 min? To how many electrons does this correspond? Current and Drift Speed sometimes we refer to the current density J = I/A, which measures how much charge flows per unit area Charged particles move through a conductor of cross-sectional area A n is the number of charge carriers per unit volume n A Δ x is the total number of charge carriers The total charge is the number of carriers times the charge per carrier, q Δ Q = (n A Δ x) q The drift speed, v d , is the speed at which the carriers move v d = Δ x/ Δ t Rewritten: Δ Q = (n A v d Δ t) q Finally, current, I = Δ Q/ Δ t = nqv d A
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3 Charge Carriers in a Conductor: No E field present If a conductor is isolated no external E field is ( present), the electrons move randomly in all directions with a distribution of speeds e have ndom we have random thermal motion
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This note was uploaded on 03/22/2011 for the course MAC AEPH114 taught by Professor Knutt during the Spring '11 term at McGill.

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circuits1 - Electric Current Whenever electric charges of...

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