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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.