Prism Glow - Electronics - EEES - Chapter 1 - DC Circuits

Chapter 1-1 : Basic Electrical Quantities

The most elemental quantity is electric charge.
Charge is measured in coulombs.
The charge on an electron is negative and is equal to 1.591 X 10^-19 coulomb.
A field of force exists in the vicinity of a charge. This is the electric field.
Charges must move in order to make an energy transfer.
The rate of motion of charge in a circuit is called the current.
The unit of current is called the ampere; one ampere exists when charge flows past a point in a circuit at the rate of one coulomb per second.

Expressed quantitatively, i = dq/dt

where

is in amperes, and dq/dt

is the change in coulombs over the change in time.

A whiteboard discussion of the concepts of coulomb and ampere may help to clarify the points a little bit.

Whiteboard with Coulomb and Ampere Illustrations

Click to Enlarge

Going the other direction, q = integral i dt

where

is in coulombs and is the summation of the product of

, amperes, and

, the change in time.

Charge is the quantity that flows through a circuit whereas current is a rate, that being, the time rate flow of charge.
Current is a vector quantity; it has both magnitude and direction.
The potential difference between two points in a circuit is the work or energy involved in transferring a unit of positive charge, one coulomb, from one point to the other.
Potential difference is measured in volts.
Work is defined as moving a charge through a potential difference.

A whiteboard discussion of the concept of volt may help to clarify the point a little bit.

Click to Enlarge

The energy,

, associated with moving a charge

through a potential difference of

volts is,

. This is the definition of joules or watt-secs.

A source of electrical energy that generates a potential difference is called an electromotive force.
A circuit carrying current also generates another type of field of force. That is the magnetic field. Its motions, caused by a changing current in the circuit, may also cause induced voltages in other components.
Power is the rate of energy transfer. It is the time derivative of the energy.

Power,

, may be represented by p = dw/dt = e(dq/dt) = ei

. This is watts or joules/sec.

Where current and voltage are functions of time then it is possible to represent the total energy transfer as W = integral p dt (from 0 to t) = integral ei dt (from 0 to t)

W = integral p dt (from 0 to t) = integral ei dt (from 0 to t)

. This is watt-secs or joules.

And when we have

and

constant, and not time dependent instantaneous values, we do not represent them in lower case, but as upper case. W = EIt

. This is joules.

Electronics > EEES > Chapter 1 - DC Circuits > 1-1 Basic Electrical Quantities
Chapter 1-1 : Basic Electrical Quantities The most elemental quantity is electric charge. Charge is measured in coulombs. The charge on an electron is negative and is equal to 1.591 X 10^-19 coulomb. A field of force exists in the vicinity of a charge. This is the electric field. Charges must move in order to make an energy transfer. The rate of motion of charge in a circuit is called the current. The unit of current is called the ampere; one ampere exists when charge flows past a point in a circuit at the rate of one coulomb per second. Expressed quantitatively, where is in amperes, and is the change in coulombs over the change in time. A whiteboard discussion of the concepts of coulomb and ampere may help to clarify the points a little bit. Click to Enlarge Going the other direction, where is in coulombs and is the summation of the product of , amperes, and , the change in time. Charge is the quantity that flows through a circuit whereas current is a rate, that being, the time rate flow of charge. Current is a vector quantity; it has both magnitude and direction. The potential difference between two points in a circuit is the work or energy involved in transferring a unit of positive charge, one coulomb, from one point to the other. Potential difference is measured in volts. Work is defined as moving a charge through a potential difference. A whiteboard discussion of the concept of volt may help to clarify the point a little bit. Click to Enlarge The energy, , associated with moving a charge through a potential difference of volts is, . This is the definition of joules or watt-secs. A source of electrical energy that generates a potential difference is called an electromotive force. A circuit carrying current also generates another type of field of force. That is the magnetic field. Its motions, caused by a changing current in the circuit, may also cause induced voltages in other components. Power is the rate of energy transfer. It is the time derivative of the energy. Power, , may be represented by . This is watts or joules/sec. Where current and voltage are functions of time then it is possible to represent the total energy transfer as . This is watt-secs or joules. And when we have and constant, and not time dependent instantaneous values, we do not represent them in lower case, but as upper case. . This is joules.