METL 2441 Cathodic Protection Lecture1

Introduction Corrosion and cathodic protection are both electrochemical phenomena. Electrical instruments are used extensively in corrosion evaluation, testing, and control. Knowledge of various electrical terms, laws, and circuits is essential to anyone working with corrosion and cathodic protection.

Electrical Terms

Electricity Electricity is the flow of electrons through a conducting material, such as a metal wire. The flow of electricity is much like the flow of fluid in a pipe. Pressure is needed to force fluid to flow through a pipe.

Electricity There must be a pressure difference between one point in a pipe and another point if fluid is to flow between the two points. The same applies to electron flow; a pressure difference is needed between two points in a wire or other conducting material.

Electricity In electricity, that pressure difference is called emf, voltage, potential, potential difference, and is sometimes referred to as electrical pressure.

Electrons Electrons are atomic particles that carry a negative charge and serves as a glue that helps to hold matter together, much like mortar in a brick wall. Electron flow through a conducting material produces what is known as electricity.

Voltage Voltage, also called electromotive force is an expression of the potential difference in charge between two points in an electrical field. Voltage is the energy that puts electrical charges in motion and is measured in volts, millivolts, and microvolts. Units of volts, millivolts, and microvolts are all used in corrosion work.

Relationship of Voltage Units 1,000 Volts = 1 kilovolt 1.000 Volts = 1000 millivolts 0.100 Volts = 100 millivolts 0.010 Volt = 10 millivolts 0.001 Volt = 1 millivolt 0.000001 Volt = 1 microvolt

Common Voltage Symbols emf = electromotive force - any voltage unit E or e = voltage across a source of electrical energy (e.g. battery, corrosion potential) V or v = voltage across a sink of electrical energy (e.g. resistor)

Current or Amperage The flow of charges along a conducting path and is measured in amperes. Frequently abbreviated as amps, milliamps, or microamps. Units of amps, milliamps, and microamps are all used in corrosion work. Ampere = the common unit of current. A flow rate of charge of 1 coulomb per second is the unit of charge carried by 6.24 x 1018 electron charges.

The Relationship of Amperage Units 1,000 amperes = 1 kiloampere 1.000 ampere = 1000 milliamperes 0.100 ampere = 100 milliamperes 0.010 ampere = 10 milliamperes 0.001 ampere = 1 milliampere 0.000001 ampere = 1 microampere

Common Symbols for Current I = any unit of current such as: amperage unit A = Amperes or Amps mA = milliamperes or milliamps A = microamperes or microamps

Coulomb A unit used to measure electric charge. One coulomb is equal to the quantity of  charge that passes a point in an electric  circuit in one second when a current of one  ampere is flowing through a circuit. One coulomb is equal to 6.24 x 1018 electric charges.

Resistance As related to electricity, resistance is the opposition to the flow of electric charge moving through a material.

Resistivity The property of a conductor that relates its resistance to its geometry. The unit of resistivity is the ohm-cm or ohm-m.

Ohm The ohm (Ω) is the standard unit of measurement for resistance.

Electrical Load An electrical component or portion of a circuit that consumes electric power.

Voltage Drop The amount of voltage loss that occurs through all or part of a circuit due to resistance or impedance.

Electric Circuit The path followed by an electric current

Electrical Laws

Ohm’s Law A relationship between the ratio of voltage and an unvarying electrical current to the resistance of a circuit. The law states that a voltage of 1 volt will create a current of 1 ampere in a circuit having a resistance of 1 ohm.

Ohm’s Law Ohm’s Law can be expressed as follows: E or V = Voltage (electromotive force) I = Current (amperes) R = Resistance (Ohm’s) E or V = IR I = E⁄R R = E⁄I E/d

Power Power is the rate of doing work or otherwise expending some form of physical energy. Power is measured in watts. In electrical calculations the unit of power is the watt (W)

Power Equations Equations for power: P = EI P = I2R where: P = Power in watts R = Resistance in Ohms E = Voltage in volts I = Current in amperes

Kirchhoff’s Voltage Law States that the sum of applied voltages around any closed loop of a circuit is equal to the sum of the voltage drops across the resistances in that loop.

Kirchhoff’s Current Law States that as much current flows away from a point as flows toward it. The law is especially useful when analyzing parallel circuits and for tracing current flow in complex piping networks.

Series Circuit Current flow is the same at every point in an individual, uninterrupted and continuous path from the source of voltage through the various loads and back to the source.

Series Circuit Acronyms ET = Total voltage across a circuit RT = Total resistance in a circuit IT = Total current in a circuit

Series Circuit Rules Current is the same level at every point in the circuit. The sum of individual voltage drops (ET) must equal the total applied voltage or voltages. (an example of Kirchhoff's Voltage Law) The total resistance (RT) of a series circuit equals the sum of the individual resistances.

Series-Parallel Circuit A series-parallel circuit combines the elements of both a series circuit and a parallel circuit. Very complex circuits can be reduced to a circuit consisting of series-parallel elements. Important in the design of ICCP systems. (ICCP groundbed anode cables represents a series circuit, while the groundbed itself is a parallel circuit)

Parallel-Series Circuit The series-parallel circuit has one or more resistors installed in series with a parallel circuit.

Direct Current (DC) Direct current flows in only one direction. Pure direct current is produced by a battery and appears as a straight line when viewed on an oscilloscope.

Alternating Current (AC) Alternating current, such as that which we have in our homes and buildings, reverses direction on a cyclic basis. Alternating current most commonly reverse direction 100 or 120 times a second. A full cycle is completed in one second. The word hertz (hz) is used to represent a cycle. AC is known as 50 hz or 60 hz current.

Transformers Transformers may be used to increase or decrease voltage or to isolate an incoming voltage source from the outgoing voltage. The transformer has a laminated iron core. There are two sets of windings on the core, the primary and the secondary. The primary winding is connected to the voltage source. The secondary winding is connected to the unit to which voltage is to be supplied.

Impedance Impedance is the total opposition that a circuit presents to alternating current. Impedance is similar to resistance in a direct current circuit. Impedance is equal to a complex ratio of AC voltage to AC current.

Impedance Impedance depends on the frequency and wave shape of the current. Impedance is measured in ohms, as is DC resistance.

Examples Explain, step by step, how to calculate the amount of current (I) that will go through the resistor in this circuit: Resistor current = 0.02553 amps, or 25.53 milliamps (mA). What is the value of this resistor, in ohms (Ω)?   Resistor value = 2700 Ω, or 2.7 kΩ.

Examples Electricity always takes the path of least resistance. Explain how this proverb relates to the following circuit, where electric current from the battery encounters two alternate paths, one being less resistive than the other:   The 250 Ω resistor will experience a current of 40 mA, while the 800 Ω resistor will experience a current of 12.5 mA.

Examples Three resistors receive the same amount of current (4 amps) from a single source. Calculate the amount of voltage “dropped” by each resistor, as well as the amount of power dissipated by each resistor: E1 Ω = 4 volts E2 Ω = 8 volts E3 Ω = 12 volts P1 Ω = 16 watts P2 Ω = 32 watts P3 Ω = 48 watts

Examples Three resistors receive the same amount of voltage (24 volts) from a single source. Calculate the amount of current “drawn” by each resistor, as well as the amount of power dissipated by each resistor:   I1 Ω = 24 amps I2 Ω = 12 amps I3 Ω = 8 amps P1 Ω = 576 watts P2 Ω = 288 watts P3 Ω = 192 watts