Basic Concepts of DC Circuits

Introduction An electric circuit is an interconnection of electrical elements. An electric circuit is an interconnection of electrical elements.

Systems of Units

Charge The most basic quantity in an electric circuit is the electric charge. The most basic quantity in an electric circuit is the electric charge. Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C). Charge is an electrical property of the atomic particles of which matter consists, measured in coulombs (C). The charge e on an electron is x C. The charge e on an electron is x C.

Charge (cont.)

Current A unique feature of electric charge or electricity is that it is mobile; it can be transferred where it can be converted to another form of energy. A unique feature of electric charge or electricity is that it is mobile; it can be transferred where it can be converted to another form of energy. When a conducting wire is connected to a battery, the charges are compelled to move; positive charges in one direction and negative charges in the opposite direction. When a conducting wire is connected to a battery, the charges are compelled to move; positive charges in one direction and negative charges in the opposite direction.

Current (cont.) This motion of charges is what creates an electrical current. This motion of charges is what creates an electrical current. Electric current is the time rate of change of charge, measured in amperes (A). Electric current is the time rate of change of charge, measured in amperes (A). 1 ampere = 1 coulomb / second 1 ampere = 1 coulomb / second It is conventional to take the current flow as the movement of positive charges, although current is actually due to negatively charged electrons. It is conventional to take the current flow as the movement of positive charges, although current is actually due to negatively charged electrons.

Current (cont.) A direct current (dc) is a current that remains constant with time. A direct current (dc) is a current that remains constant with time. An alternating current (ac) is a current that varies sinusoidally with time. An alternating current (ac) is a current that varies sinusoidally with time.

The Relationship Mathematically, the relationship between current i, charge q, and time t is Mathematically, the relationship between current i, charge q, and time t is The charge transferred between time t 0 and t is found by integrating both sides; The charge transferred between time t 0 and t is found by integrating both sides;

Voltage To move an electron in a particular direction requires some work or energy transfer. This work is performed by an external electromotive force (emf), typically a battery. To move an electron in a particular direction requires some work or energy transfer. This work is performed by an external electromotive force (emf), typically a battery. This emf is also known as potential difference or voltage. This emf is also known as potential difference or voltage.

Voltage (cont.) The voltage between two points a and b is the energy (or work) needed to move a unit charge from a to b. The voltage between two points a and b is the energy (or work) needed to move a unit charge from a to b. where w is energy in joules (J) and q is charge (C). The voltage is measured in volts (V). where w is energy in joules (J) and q is charge (C). The voltage is measured in volts (V). 1 volt = 1 joule / coulomb = 1 newton*meter/ coulomb 1 volt = 1 joule / coulomb = 1 newton*meter/ coulomb

Voltage Polarity The plus (+) and minus (-) signs are used to define reference direction or voltage polarity. The plus (+) and minus (-) signs are used to define reference direction or voltage polarity. v ab = -v ba v ab = -v ba The polarity can be interpreted in two ways: The polarity can be interpreted in two ways:

Power Although current and voltage are the two basic variables, they are not sufficient by themselves. Although current and voltage are the two basic variables, they are not sufficient by themselves. For practical purposes, we need to know how much power a device can handle and how much energy is consumed over a period of time. For practical purposes, we need to know how much power a device can handle and how much energy is consumed over a period of time.

Power (cont.) To relate power and energy to voltage and current, we recall that: To relate power and energy to voltage and current, we recall that: power is the time rate of expending or absorbing energy, measured in watts (W). power is the time rate of expending or absorbing energy, measured in watts (W). We write this relationship as We write this relationship as p is power in watts (W), w is energy in joules (J) and t is time in seconds (s). p is power in watts (W), w is energy in joules (J) and t is time in seconds (s).

Power (cont.) The power p is a time-varying quantity and is called instantaneous power. The power p is a time-varying quantity and is called instantaneous power. p > 0, power is absorbed p > 0, power is absorbed p < 0, power is supplied p < 0, power is supplied power absorbed = - power supplied power absorbed = - power supplied

Power (cont.) The law of conservation of energy must be obeyed in any circuit. For with this reason, the sum of power in a circuit must be zero. The law of conservation of energy must be obeyed in any circuit. For with this reason, the sum of power in a circuit must be zero.

Energy Energy is the capacity to do work, measured in joules (J). Energy is the capacity to do work, measured in joules (J). The energy absorbed or supplied by an element from time t 0 to t is The energy absorbed or supplied by an element from time t 0 to t is

Circuit Elements An element is a basic building block of a circuit. An element is a basic building block of a circuit. There are two types of elements: active and passive. There are two types of elements: active and passive.

Circuit Elements (cont.) Active elements are capable of generating energy Active elements are capable of generating energy Generators Generators Batteries Batteries Amplifiers Amplifiers Passive elements cannot create energy Passive elements cannot create energy Resistors Resistors Capacitors Capacitors Inductors Inductors

Circuit Elements (cont.) The most important active elements are voltage and current sources because they deliver power to the circuit. The most important active elements are voltage and current sources because they deliver power to the circuit. There are two types of sources: independent and dependent. There are two types of sources: independent and dependent.

Circuit Elements (cont.) Independent sources provide a specified voltage or current that is completely independent of other circuit variables. Independent sources provide a specified voltage or current that is completely independent of other circuit variables. Dependent sources have their source quantity controlled by another voltage or current. Dependent sources have their source quantity controlled by another voltage or current.

Circuit Elements (cont.) There are 4 possible types of dependent sources: voltage controlled voltage source, current controlled voltage source, voltage controlled current source, and current controlled current source. There are 4 possible types of dependent sources: voltage controlled voltage source, current controlled voltage source, voltage controlled current source, and current controlled current source.