0195136047.pdf

12 CIRCUIT CONCEPTS

vab Load iab

a

b

+

−

vab Source iba

a

i

b

+

−

Figure 1.1.3Load and source conventions.

EXAMPLE 1.1.4

A typical 12-V automobile battery, storing about 5 megajoules (MJ) of energy, is connected to a

4-A headlight system.

(a) Find the power delivered to the headlight system.

(b) Calculate the energy consumed in 1 hour of operation.

(c) Express the auto-battery capacity in ampere-hours (Ah) and compute how long the

headlight system can be operated before the battery is completely discharged.

Solution

(a) Power delivered:P=VI= 124 =48W.

(b) AssumingVandIremain constant, the energy consumed in 1 hour will equal

W= 48 ( 60 × 60 )= 172. 8 × 103 J= 172 .8kJ

(c) 1 Ah= (1C/s)(3600 s) =3600C. For the battery in question, 5× 106 J/12 V =

0. 417 × 106 C. Thus the auto-battery capacity is 0. 417 × 106 / 3600 ∼=116 Ah. Without

completely discharging the battery, the headlight system can be operated for 116/4= 29

hours.

Sources and Loads

A source–load combination is represented in Figure 1.1.4. Anodeis a point at which two or

more components or devices are connected together. A part of a circuit containing only one

component, source, or device between two nodes is known as abranch. A voltageriseindicates

an electric source, with the charge being raised to a higher potential, whereas a voltagedrop

indicates a load, with a charge going to a lower potential. The voltageacrossthe source is the

same as the voltage across the load in Figure 1.1.4. The current delivered by the source goes

throughthe load. Ideally, with no losses, the power(p=vi)delivered by the source is consumed

by the load.

When current flows out of the positive terminal of an electric source, it implies that non-

electric energy has been transformed into electric energy. Examples include mechanical energy

transformed into electric energy as in the case of a generator source, chemical energy changed