Circuit Theory Tashfeen Khan

Basic components and electric circuits We will be defining the currents, voltages and powers. Be careful about the + and – signs. They may change your answers.

Units and Scales Length – m Mass – kg Current – A Voltage – V Power – W

Prefixes Deci – 10^-1 Milli – 10^-3 Micro – 10^-6 Nano – 10^-9 Decka – 10^1 Kilo – 10^3 Mega – 10^6 Giga – 10^9

Charge Positive charge – Proton – (+1.6*10^-19) Negetive charge – Electron – (-1.6*10^-19) Constant charge – Q Instantaneous charge – q(t)

Current Transfer of charge or charge in motion I = dq(t)/dt Units is amperes Current can be alternating current, direct current or exponential current.

Voltage Measure of work required to move a charge through the element. Therefore units are J/C, which is same as V. Voltage can be alternating voltage, direct voltage or exponential voltage.

Power P = V * I (Product of voltage and current) Follow passive sign convention When current enters the positive terminal and leaves the negative terminal then the power absorbed is positive. When current enters the negative terminal and leaves the positive terminal then the power suplied is positive.

Independent sources Independent sources supply voltage or current independent of any external factor. Independent current source can be ac or dc – Is = 10 A or Is = 10cos(10t) A Independent voltage source can be ac or dc – Vs = 10 V or Vs = 10cos(10t) V

Dependent Sources These sources depend on external variables. Following are four types of dependent sources. Voltage controlled current source Voltage controlled voltage source Current controlled current source Current controlled voltage source

Ohms Law Ohms law states that voltage across any conducting material is directly proportional to the current flowing through it. V = I * R (where R is the constant of proportionality with units ohms)

Series and Parallel Circuits When electrical components are connected in series the current passing through them is same. When electrical components are connected in parallel the voltage across them is same.