Lecture 1 Circuit Variables ECE 205 Prof Ali

Lecture 1 Circuit Variables ECE 205 Prof. Ali Keyhani

Symbols and Units • In this course international system (SI) of units is used • SI system has six fundamental units: – – – Meter Kilogram Second Ampere Kelvin Candela Shown as: m Shown as: kg Shown as: s Shown as: A Shown as: K Shown as: cd • Other units are derived from the six fundamental units

• Symbols in Electrical Engineering:

• Numerical values in engineering range over many orders of magnitude. • Example:

Circuit Variables • Charge and Energy are two variables in study of electronic systems • There are two kind of charges: Positive and Negative • Two important rules: – Like charges repel – Unlike charges attract one another

Charge & Current • In SI shown by q and measured in coulombs (C) • Electron’s charge: • It is not easy to measure the electrical charges • The charges are moving all the time

Charge & Current • By having the amount of charge passing a point current i is defined: • The unit of current in SI is ampere (A): • 1 ampere=1 coulomb/second • The direction of current is defined as the direction of the net flow of positive charge

Voltage • Change in energy as a result of a charge passing through a circuit: – W: energy (joules) or (J) – V: voltage (volt) or (V): • 1 joule/coulomb= 1 volt

Power • Power is defined as the time rate of change of energy: – Unit in SI: watts • 1 watt= 1 joule/second • Power is also defined as “Volt. Amperes”:

Passive Sign Convention • Engineering problems are stated in terms of voltage, current and power. • Voltage is not measured at a single point but is measured between two points in a circuit.

• The arrows define the positive direction of the current with respect to the voltage across a device • Ground: A reference point for the voltage • The voltage of all other points in the circuit are defined based on the common reference point • The symbol for the ground:

Example 1 • The current through a circuit is 20 m. A. Find the total charge and the number of electrons transferred in 200 ns.

Example 2 Power dissipation of an electrical device is 200 W. How much energy is delivered to this device in 20 sec? Solution: W=200× 20=4000 J=4 k. J