Understanding the units associated with the physical quantities associated with electric circuits is imperative. We will use the International System of Units (SI) that are used throughout the world.

The six basic SI units:

Conveniently, SI units allow for prefixes that allow us a shorthand notation for describing the quantity. For example, 20 km = m = m.

Common SI prefixes:

Multiplier	Prefix	Symbol
	exa	E
	peta	P
	tera	T
	giga	G
	mega	M
	kilo	k
	hecto	h
	deka	da
	deci	d
	centi	c
	milli	m
	micro
	nano	n
	pico	p
	femto	f
	atto	a

• Electric charge is expressed in units of coulombs (C).
• Electrons and protons have the smallest electric charge.
  • An electron exhibits a negative charge: C. The charge of an electron is given the symbol: e.
  • A proton exhibits a positive charge: C.
• In the laboratory we'll typically see units of pC, nC, or C since 1 C is quite large (requiring electrons to produce).
• The law of conservation of charge states that charge can neither be created or destroyed. It can only be transferred.

• Current is expressed in amperes (A).
• Electric current is the rate at which charge is flowing.
  • Consists of the flow of electrons.
  • Every electron exhibits the same amount of electric charge.
• In order for current to flow, an electric circuit must have a complete path.

• Elements in an electric circuit are connected with conductors.
• Conductors are composed of material that efficiently transfers electrical energy, e.g., copper, aluminum.
• Insulators are composed of material that does not transfer electrical energy efficiently, e.g., glass.
• A complete circuit provides a path on which electrons (charge) may flow (producing current).
  • Positive charges move in one direction while negative charges move in the other direction.
  • By convention, the direction of current flow is the direction of positive charges – the opposite direction of the flow of electrons.
  • We will use this convention even though we now know that the current flow in conductors is due to electrons (which exhibit a negative charge).
  • This convention is known as the passive sign convention.

We express the relationship between current (i), charge (q) and time (t) as:

Current is expressed in amperes (A). So, 1 A = 1 C/s.

We express the amount charge transferred between time and as:

• This is analogous to when the rate of water flow remains constant.
• By convention, we use the symbol I to represent a constant current.
• We will focus on studying dc current in EE-2050.
• Batteries produce dc currents.

• Electric transmission systems that deliver electricity to your home/office are based on ac currents.

• Voltage is expressed in units of volts (V).
• The voltage, , between two points, a and b, in an electric circuit is the energy needed to move a unit charge from a to b.
• In math speak we define voltage as:

• where w is energy in joules (J) and q is charge in coulombs (C).
• 1 volt = 1 J/C.
• One joule is equivalent to one newton-meter.
• Since energy is conserved, . That is, if there is a voltage drop of some amount from point a to point b in a circuit, it is equivalent to say there is a voltage rise of the same amount from point b to point a in the same circuit.
• In much the same way as we have ac and dc currents, we can have ac and dc voltages. We will represent dc voltages with the symbol: V.

• Recall that an electric circuit is an interconnection of electrical elements.
• Each element is typically connected via wires (conductors).
• There are two types of elements:
  • Passive elements, a.k.a, loads.
  • Active elements, a.k.a., sources.

• An electric load dissipates or stores energy.
• Examples: resistors, inductors, capacitors.

• An electric source provides energy.
• The two most important kinds of electric sources are
  • Voltage sources: laptop battery, electric outlet, automobile's alternator.
  • Current sources: current transformer
• An ideal voltage source will provide a constant voltage regardless of the amount of current required by the circuit.
• An ideal current source will provide a constant current regardless of the voltage required by the circuit.
• Ideal sources are not possible in reality since it would require a voltage source to produce infinite current for a short circuit and a current source to provide infinite voltage for an open circuit.
• A voltage or current source may be an independent or a dependent source.

• Independent sources are typically shown in a circuit diagram using a circular symbol.
  • Independent voltage source symbol:
  • Independent current source symbol:
• A battery symbol is often used designate an independent dc voltage source.

• For example, a voltage source may have an output voltage that is proportional to the current in the circuit.
• Such dependent sources are often found in electronic circuits.
• Dependent sources are typically shown in a circuit diagram using a diamond-shaped symbol.
  • Dependent voltage source symbol:
  • Dependent current source symbol:
• The order of the and the direction of the arrow signify the polarity of the source.
• There are four types of dependent sources:
  1. VCVS – voltage-controlled voltage source.
  2. CCVS – current-controlled voltage source.
  3. VCCS – voltage-controlled current source.
  4. CCCS – current-controlled current source.

• Power is expressed in units of watts (W).
• In math speak we define power as:

• where p is power in watts (W), w is energy in joules (J), and t can be guess with a high degree of accuracy, if you have been paying any attention at all. Hint: it has units of seconds (s).
• Combining a number of the equations above we arrive at the following relationship:

• Essentially this says that the power absorbed or supplied by an element is the product of the voltage across the element and the current through the element.
• If p is positive, power is being absorbed by the element.
• If p is negative, power is being supplied by the element.
• The p in the above equation varies with time and is called instantaneous power.

This requires that: for a circuit with k elements each having power .

The energy absorbed or supplied by a circuit element from time to time t is:

• Energy is expressed in joules (J).
• Electric utility companies express energy in watt-hours (Wh) where 1 Wh = 3,600 J.