Chapter 12

Electric Current And

Related Parameters

Engineers understand the importance of

electricity and electrical power and the role

they play in our everyday lives.

As future engineers you should know what is

meant by: voltage, electric current, and

the difference between direct and

alternating current.

You should also know the various sources of

electricity and understand how electricity is

generated

The ampere is defined as that constant current which, if maintained in

two straight parallel conductors of infinite length, of negligible circular

cross section, and placed 1 meter apart in a vacuum, would produce between

these conductors a force equal to 2×10-7 Newton per meter of length.

In SI units, the unit of charge is the coulomb (C).

One coulomb is defined as the amount of charge that passes a point in a

wire in 1 second when a current of 1 ampere is flowing through the wire.

The electric force exerted by one point charge on another is proportional

to the magnitude of each charge and is inversely proportional to the square

of the distance between the point charges.

Moreover, the electric force is attractive if the charges have opposite

signs, and it is repulsive if the charges have the same sign.

The electric force between two point charges is given by Coulomb’s law

Voltage represents the amount of work required to move charge between two

points, and

Current represents the amount of charge that is moving between the two

points per unit time.

Electromotive force (emf) represents the electric potential difference

between an area with an excess of free electrons (negative charge) and an

area with an electron deficit (positive charge).

The most common sources of electricity are chemical reaction, light, and

magnetism.

Batteries

In all batteries, electricity is produced by the chemical reaction that takes

place within the battery.

Voltage

To increase the voltage

output, batteries are

often placed in a series

arrangement

Batteries connected in a

parallel arrangement,

produce the same voltage,

but more current.

Power Plants

Electricity that is consumed at homes, schools, malls, and by various industries is

generated in a power plant.

Water is used in all steam power-generating plants to produce electricity. A simple

schematic of a power plant is shown in Figure 12.

Direct current (DC) is the flow of electric charge that occurs in one

direction, as shown in Figure 12.4(a). Direct current is typically produced

by batteries

Direct current was not economically feasible to transform because of the

high voltages needed for long-distance transmission

Direct Current and Alternating Current

Alternating current (AC) is the flow of electric charge that periodically

reverses. The magnitude of the current starts from zero, increases to a

maximum value, and then decreases to zero. This flow pattern is repeated

in a cyclic manner.

The time interval between the peak value of the current on two successive

cycles is called the period, and the number of cycles per second is called

the frequency. The peak (maximum) value of the alternating current in

either direction is called the amplitude.

Alternating current is created by generators at power plants.

The current drawn by various electrical devices at your home is alternating

current.

The alternating current in domestic and commercial power use is 50 cycles

per second (hertz).

Direct Current and Alternating Current

Kirchhoff’s Current Law

The law states that at any given time, the sum of the currents entering a node

must be equal to the sum of the current leaving the node

Electrical Circuits and Components

An electrical circuit refers to the combination of various electrical

components that are connected together. Examples of electrical components

include wires (conductors), switches, outlets, resistors, and capacitors.

Circuit Connections:

Electrical components can be connected in either a series or a parallel arrangement.

Series Circuit:

Similar to a constant flow of water in a series of pipes of varying size, the

electric current flowing through a series of elements in an electric circuit is

the same (constant). For a circuit that has elements in a series arrangement,

the following is true:

• The voltage drop across each element can be determined using Ohm’s law.

• The sum of the voltage drop across each element is equal to the total

voltage supplied to the circuit.

• The total resistance is the sum of resistance in the circuit.

one current and v = v1+v2+v3

Parallel Circuit

Consider the circuit shown in Figure below. The resistive elements in the given

circuit are connected in a parallel arrangement. For this situation, the electric

current is divided among each branch.

One volt, I = I1 + I2 + I3

Capacitors