Electrical-marine-wiring and electrical-theory

Electrical-marine-wiring - Understand voltage, current and resistance

It is important to understand the nature of voltage, current and resistance. They are fundamental parts of design, and faultfinding simple electrical circuits. These are applied using Ohms Law and you must be able to understand and apply this Law as it is essential to making calculations.

After completing this lesson unit you will:

1. Understand the definition of voltage and current

2. Understand the definition of resistance

3. Understand the basic theory of diodes and semi-conductors

4. Understand Ohms Law and how make calculations

5. Understand the definition of Power

What is Voltage?

Voltage is the unit of potential difference, electrical force, or "pressure" that will cause a current to flow in a circuit. Voltage is named after Alessandro Volta (1754-1827)

A more formal definition is given as follows:

"The volt is the measure of the work required to move an electric charge. When 0.7376 foot pound of work is required to move 5.25 x 10^18 electrons between two points, each with its own charge, the potential difference is 1 volt. Potential difference is measured in Volts (V or E)."

Another definition is 1 volt is the amount of EMF required to move 1 ampere of current through a resistance of 1 ohm

Quantity Symbol Decimal 1 millivolt 1 mV 1/1000 V 1 volt 1V or 1 volt 1 volt 1 kilovolt 1 kV 1000 volts

Using the water analogy, Voltage is the force that pushes the piston and the water (electrons) forward out of a cylinder or water pipe.

WHAT IS CURRENT?

Current is the movement, motion or rate of flow of an electrical charge, or the flow of free electrons through a conductor or circuit. One ampere of current is one coulomb (6.25 x 10^18 6 million million million electrons) move past a given point per second. To make current flow an Electromotive Force (EMF) is required

Quantity Symbol Decimal 1 milliampere = 1 mA = 1/1000 A 1 ampere 1A = 1 amp = 1 ampere 1 kiloampere = 1 kA = 1000 amperes

Using the water analogy it is the actual water flow moving through the tube (wire) when the piston is pushed.

What is Conventional Current and Electron Flow?

Conventional Current Flow states that electrons flow from positive to negative and Benjamin Franklin theorized this. This theory states that an invisible fluid known as electricity tended to flow through a wire from the positive to the negative. This theory became the accepted convention and was known as "conventional current" in electrical theory, mathematics, textbooks and electrical equipment for the following 100 years.

The Electron Flow theory states that electrons flow from negative to positive. Scientists discovered that electrons actually flow from negative to positive. Because electrons are negatively charged, it follows that they are attracted by positively charged bodies and repelled by negatively charged bodies. Electron flow is the correct theory, and the conventional flow theory is still the major one used. Either theory can be used as long as the orientations are correct. Conventional flow will be used in the following modules.

What is the difference between AC and DC?

AC is Alternating Current and one that reverses direction at regular intervals. Alternating Current is used at frequencies of 60 Hertz in the USA (50 Hz elsewhere) and takes the form of a sine wave. It reverses direction every .0083 seconds. This will be covered in greater detail in the AC Module.

DC is Direct Current and one that flows in the same direction, either continuously, intermittently or pulsating.

WHAT IS RESISTANCE?

Resistance is the opposition or restriction to the flow of electrons in a DC conductor or circuit. It resists flow! All materials and parts of a circuit resist the flow of electrons and create or cause resistance. The movement of electrons through a circuit with resistance (or a resistor) will cause some of the electrical energy to convert to heat energy. The symbol used is the Greek symbol Omega and the unit of measurement is the Ohm. The term impedance is also used with respect to resistance in AC circuits and this is used commonly used with in electronics theory.

Quantity Symbol Decimal 1 ohm 1 1 ohm 1 kilohm 1kOhm 1000 ohms 1 megohm 1MegOhm 1,000,000 ohms

There are 4 main factors that affect the value of resistance in a conductor:

1. Resistivity of the material the conductor is made from, ie copper, silver etc

2. Length of the conductor

3. Cross-Sectional Area of conductor, i.e wire size

4. Temperature of conductor

As the quantity of electron flow is dependent on how readily particular atoms give up their electrons and accept new electrons, materials that allow this are termed conductors. Copper, Silver and Aluminum are the best conductors. Materials that do not give up electrons easily or which restrict the flow, are called insulators. PVC, rubber, glass and porcelain are all good insulators. Insulators are used to surround or cover conductors such as an insulated wire

Length The longer the conductor is the greater the resistance. Resistance is increased or decreased in proportion to the overall conductor length.

Cross-Sectional Area As the cross-sectional area of a conductor increases, the resistance decreases, and vice versa. For example, if the area of a conductor is doubled, the resistance is cut in half.

Temperature When the temperature of a conductor increases, the resistance increases. The temperature factor is not as predictable as the other factors, but it must be considered when dealing with electrical circuits.

The general symbols used in circuit diagrams or prints is the open rectangle or a zigzag line. It is easier to use the rectangle when making your own diagrams

WHAT IS A CONDUCTOR?

A conductor is the material that is used to carry and distribute electrical current to the various electrical equipment and devices such as lights, pumps etc. As discussed in the basic electrical introduction, Silver, Copper and Gold are the best conductors and for the purposes of most electrical equipment, copper is the principal one used in cables.

As an insulator has a high resistance and stops the flow of electrons these are used to cover or insulate conductive cables, or cover and protect electrical connections or other conductive components. In higher voltage systems insulators also have a safety function to protect against contact.

WHAT IS A SEMICONDUCTOR?

A semiconductor is a material that changes resistance by the application of a voltage to it, or by changing the temperature. The best known is Silicon and Germanium. These are used with other materials, in a process called doping, to produce semiconductor devices. Diodes, Transistors and Silicon Controlled Rectifiers (SCR) are such devices and the diode will be the most common device used in many electrical systems such as alternators and battery chargers. Other device types will be covered in the electronics module.

BASIC DIODES

Within this module diodes and rectifiers are important devices in battery charging. When the battery polarity allows current flow through the diode, the diode is said to be forward-biased. When the battery polarity is reversed and the diode blocks current, the diode is said to be reverse-biased. A diode is a switch or non-return valve. It is "closed" when forward-biased and "open" when reverse-biased.

The direction of the diode symbol arrow points against the direction of electron flow. This is because the diode symbol uses conventional flow notation in schematics. This shows current as a flow of charge from the positive (+) side of the voltage source to the negative (-). This convention is used for all semiconductor symbols possessing arrows.

Check valves are pressure-operated devices. They open and allow flow if the pressure across them is of the correct "polarity" to open the gate. In the analogy shown below, greater fluid pressure is on the right than on the left. If the pressure is of the opposite "polarity," the pressure difference across the check valve will close and hold the gate so that no flow will occurs.

Like check valves, diodes are essentially "pressure-" operated (voltage-operated) devices. The essential difference between forward-bias and reverse-bias is the polarity of the voltage dropped across the diode.

WHAT IS OHMS LAW?

The most fundamental relationship between the three primary electrical characteristics of current, voltage and resistance is Ohms Law. A German mathematician, George Simon Ohm, formulated this relationship in the 19th century. Ohms Law states that current is directly proportional to voltage and inversely proportional to resistance. If you know 2 of the 3 values the 3rd can be calculated.

Watts Divided by Volts = Amperes (amps)

Watts Divided by Amperes (amps) = Volts

Volts Divided by Amperes (amps) = Ohms

Volts Times Amperes (amps) = Watts

Examples 1. A circuit has a current of 4 amperes and resistance of 10 ohms. What is the voltage?

If E = I x R, E (volts) = 4 amps x 10 ohms, E = 40 Volts

2. A circuit has a voltage of 120 volts and resistance of 60 ohms. What is the current?

If I = E/R, I (amps) = 120 volts / 60 ohms, I = 2 amps

Ohms Law Triangle

1. Draw your triangle.

2. Draw a line horizontally through the center and write in a V for voltage in the top remaining triangle

3. Draw a line vertically though the centre lower half and on the left-hand section write in an I for current, and on the right-hand section write an R for resistance

To use the triangle, place your finger over the unknown value to view the calculation required.

Example If the known resistance of 10 ohms, and the voltage is 13.8 volts across the resistance and we need to know the current, place your finger over the I in the triangle leaving V over R.

V / R = 13.8 / 10 = 1.38 Amps

Useful Points to Remember

1. If the resistance in a circuit is increased the conductance will be decreased. There is an inverse relationship between resistance and conductance.

2. If the potential difference is increased across a given material in a circuit, that is the voltage is increased, the current flow through the material will also increase.

3. The potential difference between any two points in a conductor is directly proportional to the current flowing through it.

WHAT IS POWER?

The unit of electrical power is the watt (W). It is named after James Watt (1736 -1819). The definition is that one watt of power equals the work done in one second by one volt of potential difference in moving one coulomb of charge. It is the power expended when 1 ampere of direct current flows through 1 ohm of resistance

All resistive materials that conduct a current generate heat.

The Power Formula

The following formula uses Ohms Law parameters to calculate power. In some cases nameplate data may express the power in watts and the current value is required

P = V x I

P = V Squared/R

P = I Squared x R

With these equations it is possible to derive other formula to calculate the voltage, current, resistance and power from any two known parameters.