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.