Electrical Circuit Direct Current lecture, Lecture notes of Circuit Theory

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Electrical Circuits 1 (DC)

DC ??? AC???

• Direct current: the flow of electric charge is only

in one direction

Current

Q.1 The charge flowing through the imaginary surface of Fig. 1 is 0.16 C every 64 ms. Determine the current in amperes. Q.2 Determine the time required for 4*10^16 electrons to pass through the imaginary surface of Fig. 1 if the current is 5 mA.

Voltage

The flow of charge described in the previous section is established by an external “pressure” derived from the energy that a mass has by virtue of its position. A potential difference of 1 volt (V) exists between two points if 1 joule (J) of energy is exchanged in moving 1 coulomb (C) of charge between the two points. FIG. 3 Defining the unit of measurement for voltage.

Basic electrical circuit

Fig 5. Basic electrical circuit

Conductors

Conductors are those materials that permit a generous flow of electrons with very little external force (voltage) applied. TABLE 1: Relative conductivity of various materials.

Semiconductors

Semiconductors are a specific group of elements that exhibit characteristics between those of insulators and conductors. Although silicon (Si) is the most extensively employed material, germanium (Ge) and gallium arsenide (GaAs) are also used in many important devices. Semiconductor materials typically have four electrons in the outermost valence ring.

Ammeters, voltmeters…..DMM!

FIG. 6 Digital multimeter

Resistance

The flow of charge through any material encounters an opposing force similar in many respects to mechanical friction. This opposition, due to the collisions between electrons and between electrons and other atoms in the material, which converts electrical energy into another form of energy such as heat, is called the resistance of the material. The unit of measurement of resistance is the ohm (Ω), for which the symbol is FIG. 6 Resistor symbols

Factors affecting resistance

**_1. Material

2. Length
3. Cross-sectional area
4. Temperature_** How would change in l , A , ρ & temperature affect the resistance of the copper wire?

Temperature effects

Conductors (^) Semiconductors Insulators As with semiconductors, an increase in temperature will result in a decrease in the resistance of an insulator. The result is a negative temperature coefficient.

Effect of temperature on resistance of copper The temperature of -234.5°C is called the inferred absolute temperature T of copper.

Conductance

By finding the reciprocal of the resistance of a material, we have a measure of how well the material will conduct electricity. The quantity is called conductance, has the symbol G, and is measured in Siemens (S)

OHM’S Law

Note: Temperature must be constant Q.4 Determine the current resulting from the application of a 9-V battery across a network with a resistance of 2.2 Ω. Q.5 Calculate the resistance of a 60-W bulb if a current of 500 mA results from an applied voltage of 120 V.