EE201 Lecture 1 P 1 Charge Current Circuit elements Charge F Glass rods rubbed with silk repel each other F Glass Glass Lucite Lucite Glass Lucite Repulsion Repulsion Attraction Charge on glass Charge on lucite Positive Negative EE201 Lecture 1 P 2 Empirical Observation Similar charges repel Opposite charges attract Atomic theory Charge is an electrical property of matter Electrons negative Protons positive Basic unit of charge Coulomb C Charge of proton 1 602 x 10 19 C Charge of electron 1 602 x 10 19 C Notation for charge Q fixed in time q or q t time dependent EE201 Lecture 1 P 3 Field around test charge F Force on unit charge gives rise to Charge in motion Essence of course Learning how to control charge Control is achieved using circuits Simple circuit elements include sources resistors capacitors inductors and operational amplifiers EE201 Lecture 1 P 4 Current The net flow of charge across any area of matter constitutes a current through the area Basic unit of current Ampere A 1 Ampere equals the net movement of 1 C of charge per second 1A 1C sec Convention The direction of current flow is in a direction opposite to the flow of electrons or along the flow of positive charges Examples using generalized circuit elements current current positive charges negative charges EE201 Lecture 1 P 5 Note current can be positive or negative Current flow is easy through conductors Metals are good conductors Current flow is difficult through insulators Ceramics are good insulators Example In 1 sec 6 24 x 1017 electrons pass from left to right through the left end of the copper wire shown a What direction does the current flow b What is the magnitude of the current Cu wire e EE201 Lecture 1 P 6 Solution a Current flows from right to left b Charge of electron 1 602 x 10 19 C or 1 C charge of 6 24 x 1018 electron x A x C sec 6 24 x 1017 electrons 0 1 C Therefore current 0 1 A L to R 0 1 A R to L If a net charge q crosses a boundary in t sec then I q t where I denotes a constant current that is independent of time EE201 Lecture 1 P 7 The instantaneous or time dependent current is i t dq t dt where q t is the amount of charge crossing a boundary in interval to t The integral form of this equation is q t t i t dt Current that is constant with time is called direct current dc Current that varies sinusoidally with time is called alternating current ac i t A sin t where A is the amplitude of the sin wave is the angular frequency and is the phase angle EE201 Example Lecture 1 6 i t A P 8 wire 4 2 2 4 6 8 10 t sec 2 For the time dependent current shown find the charge transferred through a wire over the time intervals a 0 t 4s and b 0 t 10s Solution q t t i t dt to a By inspection q t 12 C b q t 12 C 8 C 4 C
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