Learning Objectives • Apply SI (International System of Units) units and engineering notation for standard electrical quantities. • Apply unit conversion factors when solving engineering problems. • Describe the concepts of voltage potential and current.
Given a speed of 60 miles per hour (mph), convert to kilometers per hour: convert answer for part (a) to meters per second: Example Problem 1 Some common unit conversions are found in Appendix A.
Engineering prefixes In the SI system, common multiple powers of 10 are denoted using engineering prefixes.
Engineering notation It is common practice in engineering to avoid using exponential notation if a suitable engineering prefix exists. For example: 15 10-5 sec 150 s General guideline: use closest prefix so that you have at least one NON-ZERO number to the left of the decimal place 0.15 msec 150 sec not common engineering practice common engineering practice not common engineering practice common engineering practice
Express the following using engineering notation: 10 104 volts 0.1 10-3 watts 250 10-7 seconds Example Problem 2 100 * 103 Volts or 100 kilo-Volts (kV) 100 * 10-6 Watts or 100 microWatts (W) 25 * 10-6 seconds or 25 microseconds (s)
Significant Digits Keep all digits in calculator while performing computations. Include at least 3 significant digits in all answers. Try to keep at least one digit to the right of the decimal point.
Voltage & Current • The term voltage is encountered practically every day. • We are aware that most outlets in our homes are 120 volts. • Although current may be a less familiar term, we know what happens when we place too many appliances on the same outlet; the circuit breaker opens due to the excessive current that results.
Atomic theory Electrons have a negative charge (-). Electrons orbit the nucleus at distinct orbital radiuses known as shells. The outermost shell is called the valence shell.
Charge • Materials accumulate charge by the transfer of electrons. • The amount of charge is denoted by the letter Q and the unit of measurement is called a Coulomb (C). • 1 Coulomb of charge is the total charge associated with 6.24 * 1018 electrons. • Therefore, the charge on 1 electron (Qe) is:
Voltage Defining the positive ion • If we separate the 29th electron from the rest of the atomic structure of copper by a dashed line as shown, we create regions that have a net positive and negative charge as shown in the figure to the right. • This positive region created by separating the free electron from the basic atomic structure is called a positive ion. • In general, every source of voltage is established by simply creating a separation of positive and negative charges.
What is Voltage? Work is required to separate positive and negative charges. These separated charges have potential energy. The voltage (or potential difference) (V) between two points is defined as one volt if it requires one joule (J) of energy (W) to move one coulomb of charge (Q) from one point to another. Where: V = volts W = joules (J) Q = coulombs (C)
Voltage • Since it would be inconsequential to talk about the voltage established by the separation of a single electron, a package of electrons called a coulomb (C) of charge is defined as follows: • One coulomb (C) of charge is the total charge associated with 6.242 x1018 electrons. • If a total of 1 joule (J) of energy is used to move the negative charge of 1 coulomb (C), there is a difference of 1 volt (V) between the two points.
If 600J of energy are required to move 9.36x1019 electrons from one point to another, what is the potential difference (voltage) between the two points? Example Problem 3 Recall: Q = 1 C = 6.24*1018 electrons Solve for Q Put in equation
Voltage • Since the potential energy associated with a body is defined by its position, the term potential is often applied to define voltage levels. • For example, the difference in potential is 4 V between two points of a circuit, or the potential difference between a point and ground is 12 V, and so on…
Determine the energy expended moving a charge of 50 μC through a potential difference of 6 V. Example Problem 4
Voltage Sources • In general, direct current (dc) voltage sources can be divided into three basic types: • Batteries (chemical action or solar energy). • Generators (electromechanical). • Power supplies (rectification—a conversion process to be described in your electronics courses). Symbol For DC Voltage Source
What is Current? The rate of flow of charge is known as electric current. The measure of current, an ampere (I) is defined as a rate of flow of one coulomb of charge per second. Where: I = amperes (A) Q = coulombs (C) t = time (s)
If 840 coulombs of charge pass through the imaginary plane of below during a time interval of 2 minutes, what is the current? Example Problem 5
Direction Of Current Initially it was believed that current was the flow of positive charges. This is called conventional current direction. The actual flow of charge is by electrons (negative charge) called electron flow direction. We will use conventional current (figure (a) below).
CurrentSafety Considerations • It is important to realize that even small levels of current through the human body can cause serious, dangerous side effects. • Experimental results reveal that the human body begins to react to currents of only a few milliamperes (mA). • Although most individuals can withstand currents up to perhaps 10 mA for very short periods of time without serious side effects, any current over 10 mA should be considered dangerous and potentially lethal.
DC Terminal Characteristics • A dc voltage source will provide ideally a fixed terminal voltage, even though the current demand from the electrical system may vary. • Figure shows terminal characteristic of: • Ideal voltage source • Ideal current source
Current • In summary; the applied voltage (or potential difference) in an electrical/electronics system is the “pressure” to set the system in motion, and the current is the reaction to that pressure.