Voltage, Current & Resistance

In electronics we are dealing with voltage, current and resistance in circuits.

Voltage

Voltage is the electrical force, that causes current to flow in a circuit. It is measured in VOLTS .

Electrical Current

Current is the movement of electrical charge - the flow of electrons other charged particles through the electronic circuit. The direction of a current is opposite to electrons flow direction. Current is measured in AMPERES (AMPS, A ).

Resistance

Resistance causes an opposition to the flow of electricity in a circuit. It is used to control the amount of voltage and/or amperage in a circuit. It is measured in OHMS.

Electrical symbols

Electronic component are classed into either being Passive devices or Active devices. A Passive Device is one that contributes no power gain to a circuit or system. Examples are Resistors, Light Bulb, Electrical Heaters. Active Devices are components that are capable of generating voltages or currents. Examples are Batteries and other Electrical Curent & Voltage Sources.

By using schematics symbols we can represent real-life devices.

Resistance -This is a resistance, measured in units ohms ohms, . Most often it will be a resistor.

This is a source of emf (electromotive force) or voltage source, with a voltage of , measured in units of volts, V. The most common source you will see will be a battery. However, batteries are really not resistance-free. We can model this case by putting a 'resistor' in the circuit which has the same resistance as the batterys would have.

This is a current source, with a current of , measured in units of amperes , A. Current source is ideal model of electrical power source. The internal current source resistance is infinity. We can model real life battery by putting a 'resistor' in parallel with curent source.

Ohm’s law

Ohm's law is the main basic electrical law and defines the resistance of a device to the flow of electrons.

There are three different notations of Ohm’s law

1. Unknown current

2. Unknown voltage

3. Unknown resistance

(Most people can remember a picture easier than a mathematical formula. By knowing any two values you can figure out the third. Simply put your finger over the portion of the symbol you are trying to figure out and you have your formula)

1.

2.

3.

Circuit analysis Tutorial AKNM Circuit Magic- circuit analysis software

Kirchhoff's Current Law (KCL)

KCL states that the algebraic sum of the currents in all the branches which converge in a common node is equal to zero

Iin = Iout

Kirchhoff's Voltage Law

Kirchhoff's Voltage Law states that the algebraic sum of the voltages between successive nodes in a closed path in the network is equal to zero.

E = IR

Solution using Kirchhoff’s Voltage and current laws

Steps to solve circuit by Kirchhoff’s Laws.

1. Construct circuit with circuit magic schematics editor.

Circuit sample from circuit magic.

2. Construct loops. (See “creating loop” section in user guide) Number of loops (and number of Kirhhoff’s Voltage laws equations) can be determined using following formula. Loop can not include branches with current sources. Due current sources resistance equal infinity.

Loop Number = Branch Number –(Nodes Number –1) – Current sources Number

3. Select Analyze->Solve by Kirhhoff’s laws menu item

4. In dialog box press OK button. if no warning shown. 5. Read solution.

Solution example from circuit magic.

Writing Kirchhoff current law for 3-1 nodes

(Note number of Kirchhoff current laws equations equal Nodes Number –1)

(Node 1)J1+I3+I4+I7=0

(Node 2)-J1+I2-I4=0

Wrining Kirchoff voltage law for 5-1-(3-1) loops

(Loop1) I3·R3-I7·R5=-E2

(Loop2) I2·R2-I7·R5+I4·R4=E1-E2

Linear equations

I3+I4+I7=-2

I2-I4=2

10I3-10I7=-10

11I2+10I4-10I7=-7

Equations solution

I1=2

I2=0,692

I3=-0,846

I4=-1,308

I7=0,154

Resistors in Series & Resistors in Parallel

A series circuit is one with all the loads in a row. Like links in a chain. There is only one path for the electricity to flow.

A parallel circuit is one that has two or more paths for the electricity to flow. In other words, the loads are parallel to each other.

Kennelly's Star-Delta Transformation

 

Kennelly's Delta - Star Transformation

Alternating Current (AC), Voltage, sinusoidal Waveform,

Alternating current (AC) unlike Direct current (DC) flows first in one direction then in the opposite direction. Current amperage The is function of time. Direct current (DC) amperage is constant.

The most common AC waveform is a sine (or sinusoidal) waveform. See figure 1.

Figure 1.

Instantaneous Current and Voltage

where  I = instantaneous current in amperesIp = the maximum, or peak, current in amperes  f - Frequency. The frequency is the number of cycles that is completed each second. Unit of measure: hertz (Hz)   t - time   Other notation

= the angular displacement in degrees or radians

The circuits illustrating all folowing electric circuits laws and circuit analysis techniques are included into Circuit Magic installation Visit main page to learn about Circuit Magic. (Symbolic electrical circuits simulation software to analyse circuits using Kirchhoff's Laws & other methods.)

AC Electrical Devices & schematics symblos

Electronic component are classed into either being Passive devices or Active devices.

By using schematics symbols we can represent real-life devices.

Resistance -This is a resistance, measured in units ohms, . Most often it will be a resistor.

Inductor - A schematics symbol inserted into an electrical circuit to introduce a specified inductance.. Most often it will be a coil.

Inductance-The property of a circuit which impedes a change in current. Inductors are the usual source of inductance. Inductance is measured in henrys. In electronic circuits, the usual measure of inductance is henrys (H), milihenrys (mH) or microhenrys (µH), 1, 1e-3 or 1e-6 henrys respectively.

or Capacitor - A schematics symbol inserted into an electrical circuit to introduce a specified capacitance.

Definition: The property of a circuit which impedes a change in voltage. Capacitors are the usual source of capacitance. Capacitance is measured in farads in honor of Michael Faraday. In electronic circuits, the usual measure of capacitance is microfarads (µF) or picofarads (pF), 1e-6 or 1e-12 farads respectively.

This is a AC voltage source, with a voltage of , measured in units of volts, V. The most common AC Voltage Source you will see will be a home unit. .

A device inserted into an electrical circuit to introduce a specified alternating current (AC). An AC current has a magnitude, phase and frequency.

Applying AC voltage to capacitor, coil, resistor(Java applet)

A circuit having pure resistance would have the alternating current through it and the voltage across it rising and failing together. This is illustrated in applet below,which shows the sine waves for current and voltage in a purely resistive circuit having an ac source. The current and voltage do not have the same amplitude, but they are in phase

In the case of a circuit having inductance, the opposing force of the counter emf would be enough to keep the current from remaining in phase with the applied voltage. You learned that in a dc circuit containing pure inductance the current took time to rise to maximum even though the full applied voltage was immediately at maximum. Applet below shows the wave forms for a purely inductive ac circuit in steps of quarter-cycles(90).

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