***Useful Information for Apprentices***

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Ohm's Law

To make a current flow through a resistance there must be a voltage across that resistance. Ohm's Law shows the relationship between the voltage (V), current (I) and resistance (R). It can be written in three ways:

V= I x R or I = V / R or R = V/I

Where :-

V = voltage in volts (V)
I = current in amps (A)
R = resistance in ohms ( ) or :-

V = voltage in volts (V)
I = current in milliamps ( mA)
R = resistance in kilohms (k )
For most electronic circuits the amp is too large and the ohm is too small, so we often measure current in milliamps (mA) and resistance in kilohms (k ). 1 mA = 0.001 A and 1 k = 1000 .
The Ohm's Law equations work if you use V, A and , or if you use V, mA and k . You must not mix these sets of units in the equations so you may need to convert between mA and A or k and .
You can use the VIR triangle to help you remember the three versions of Ohm's Law. Write down V, I and R in a triangle ,

* To calculate voltage, V: put your finger over V, this leaves you with I R, so the equation is ( V = I × R ) * To calculate current, I: put your finger over I, this leaves you with V over R, so the equation is ( I = V/R ) * To calculate resistance, R: put your finger over R, this leaves you with V over I, so the equation is ( R = V/I )


Ohm's Law
Use this method to guide you through calculations: 1) Write down the Values, converting units if necessary. 2) Select the Equation you need (use the VIR triangle). 3) Put the Numbers into the equation and calculate the answer.
It should be Very Easy Now! 3 V is applied across a 6 resistor, what is the current ? * Values: V = 3 V, I = ?, R = 6 * Equation: I = V/R Numbers: Current, I = 3/6 = 0.5 A
* A lamp connected to a 6 V battery passes a current of 60 mA, what is the lamp's resistance? * Values: V = 6 V, I = 60 mA, R = ? * Equation: R = V/I * Numbers: Resistance, R = 6/60 = 0.1 k = 100 * using mA for current means the calculation gives the resistance in k )
* A 1.2 k resistor passes a current of 0.2 A, what is the voltage across it? Values: V = ?, I = 0.2 A, R = 1.2 k = 1200 (1.2 k is converted to 1200 because A and k must not be used together) * Equation: V = I × R * Numbers: V = 0.2 × 1200 = 240 V

 

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Insulation Résistance :- Regs , table 61 – p/158 ,

The Insulation Résistance Test is also known as a ( Megger Test ) it Objective is to Measure the Total Résistance between Two-Points Separated by Insulation , the Test , therefore , Determines how Effective the Insulation is in the Flow of Electrical Current , the Voltage is Typically around 500V-1000V d.c. Hence , the Current is Very Low , because the Current is Low , this Test is Useful for Checking the Quality of the Insulation not Only when a Product is First Manufactured , but also Over-Time as the Product is Used ,

 

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It's Resistivity, not Resistance :-

The problem with using resistance as a measurement is that it depends not only on the material out of which the wire is made, but also the geometry of the wire. If we were to increase the length of wire , for example, the measured resistance would increase. Also, if we were to decrease the diameter of the wire, the measured resistance would increase. We want to define a property that describes a material's ability to transmit electrical current that is independent of the geometrical factors.

In the case of the wire, resistivity is defined as the resistance in the wire, multiplied by the cross-sectional area of the wire, divided by the length of the wire. The units associated with resistivity are thus ohm.m (ohm - meters).

Resistivity is a fundamental parameter of the material making up the wire that describes how easily the wire can transmit an electrical current. High values of resistivity imply that the material making up the wire is very resistant to the flow of electricity. Low values of resistivity imply that the material making up the wire transmits electrical current very easily.

 

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The Resistance of a Wire

Aim: To find out what happens to the resistance of a wire when you change its length.
PLANNING

Resistance is something that opposes or slows something or an object down, in electricity it means the same the current in the wire is slowed down by the atoms that make up the wire. So what happens if we change the length?

the flow of electrons through a wire. The electrons have to get past all the atoms that are constantly moving around the wire, to reach the end of the wire. The atoms slow the electrons down considerably, this is known as "resistance."

The scientific theory is that if you increase the length of the wire the resistance will rise and if you decrease the length the resistance will fall

The Variables The resistance of the wire can be affected/changed, by varying many variables, these include :-

* Length of Wire * Width/Thickness of wire * Type of Wire * Temperature

The length of wire affects the resistance because there are more atoms present in the wire and using scientific knowledge I know that the resistance should increase because there are more obstacles for the electrons to pass.

The width/thickness of wire can change the resistance because if the width of the wire increases the resistance decreases because there is more space for the electrons to pass the atoms. It won't be as compact as usual.

The type of wire can affect the resistance, because each type of wire contains different amount of atoms and if you use different types of wires .

The temperature is a major factor of affecting the resistance as the resistance will decrease if the temperature becomes to hot, because the temperature varies.

* There is a relationship between the voltage and the current because if you increase the voltage, you also increase the current another relationship is between the current and the resistance if you increase the resistance the current decreases.

Ohm's law is only true if the temperature remains constant, because the atoms in the wire start to vibrate as they become warmer, which causes more movement and even more resistance.

 

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Cable has Solid Cores ( Conductors’ ) and thus Doesn’t Bend Easily ( i.e. it isn’t a Flexible Cord ) it is Used in Places where it won’t be Moved Once it is Installed ,

 

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What is the Basic Unit of Electrical Power : > Power , Expressed in Watts = Voltage , in Volts , Times Current , in Amperes , P = E / I → Watts = Volts * Amperes .


Q) How fast does each one make the Electrical Utility Meter on the Side of your House Spin ? The 100 Watt Bulb
A) The Device with the Highest Wattage Spins it the Fastest ,

Q) what is the Word Used to Describe how Fast Electrical Energy is Used ? Power
A) the Watt is the Unit Used to Measure the Rate of Energy Use ,

Q) Which of the Following Two Quantities should be Multiplied together to find Power ? Voltage and Current
Q) Which Two Electrical Units Multiplied together give the Unit “ Watts “ ? Volt and Amperes

Q) a Resistor in a Circuit becomes Very Hot and Starts to Burn , this is Because the Resistor is Dissipating to Much ? Power
Q) if a Current of 2 Amperes flows through a 50 Ohm Resistor , what is the Voltage Across the Resistor ? 50Ω x 2 Amperes = 100 Volts
Q) how is the Current in a D.C. Circuit Calculated when the Voltage and Résistance are Known ? A) Current Equals Voltage Divided by Résistance ,
Q) how is the Résistance in a D.C. Circuit Calculated when the Voltage and Current are Known ? A) Résistance Equals Voltage Divided by Current ,
Q) how is the Voltage in a D.C. Circuit Calculated when the Current are Résistance are Known ? A) Voltage Equals Current Multiplied by Résistance ,

Q) if a 12 Volt Battery Supplies 0.25Ampere to a Circuit , what is the Circuit’s Résistance ? A) > Ohms Law ( I = E/R ) becomes ( R = E/I ) when Solving for ( R ) Résistance is Voltage Divided by Current , Ohms = Volts / Amperes , 12 Volts ÷ 0.25 Amperes = 48Ohms ,

Q) what Voltage would be Needed to Supply a Current of 20mA to Operate an Electric Lamp which has a Résistance of 25Ohms ? A) > Ohms Law ( I = E/R ) becomes ( E = R*I ) when Solving for ( E ) Voltage is Résistance times Current , Volts = Ohms * Amperes , 25Ω x 0.200Amperes = 5 Volts

Q) if a 3 Volt Battery Supplies 300mA to a Circuit , the Circuit Résistance is ? A) > Ohms Law ( I = E/R ) becomes ( R = E/I when Solving for ( R ) Résistance is Voltage Divided by Current , ( Ohms = Volts / Amperes ) 3V ÷ 0.300A = 10Ω ,

Q) Why would a Large Size Resistor be Used instead of a Smaller one of the Same Résistance ? A) > Remember that Power is Voltage times Current , ( P = E*I ) a Resistor Dissipates Power into Heat , a Resistor can Only Dissipate so much Power without Burning Up , i.e. its Power Rating , Larger Resistors can Dissipate more Heat ,

Q) Resistor Wattage Ratings are ?
1) Calculated According to Physical Size ,
2) Expressed in Joules per Second ,
3) Determined by Heat Dissipation Qualities , ***
4) Variable in Step of one Hundred ,

> Materials , Shape , Construction all interact to Determine Heat Dissipation Capabilities ,
A) 1 Might be a Distant Second Best ,
( Choice !! 1 in the French Question Bank includes an Allusion to Tolerance , Obviously False )

 

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Testing :- Three-Phase ,

Line to Line = 400V or alternatively L1-L2, L1-L3, L2-L3 = 400V
Line to Neutral = 230V or alternatively L1-N, L2-N, L3-N = 230V
Line to Earth = 230V or alternatively L1-E, L2-E, L3-E = 230V
Neutral to Earth = 0 V

 

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Electrical Fundamentals :
Q) the particles that orbit around the centre of an atom are ?
A) - Electrons , ◄
Molecules ,
Nucleus ,
Protons ,
- Electrons orbit around the nucleus of an atom ,
Q) an atom which loses or gains one electron is called ?
A) a charged particle or ion , ◄
Balanced ,
An Element ,
A Molecule ,
- Ions are charged particles either positive or negative , atoms that looses an electron becomes a positive ion and an atom that gains an electron becomes a negative ion ,
Q) the conventional theory of current flow says that current flows ?
A) Positive to Negative , ◄
Randomly ,
Negative to Positive ,
None of the above ,
- conventional or hole theory states that current flows from Positive to Negative ,
Q) the force that causes electrons to flow through a conductor is known as ?
A) the Voltage , ◄
The Power ,
The Current ,
The Résistance ,
- Voltage is the force that pushes electrons through a conductor , Voltage is electrical pressure also known as ElectoMotive Force ( EMF )
Q) two identical lamps are connected in Parallel to a 12 Volt source , the voltage across each lamp is ?
A) 12 Volts , ◄
6 Volts ,
4 Volts ,
2 Volts ,
- each branch of a Parallel Circuit receives the source voltage of 12 Volts ,
Q) in a Parallel Circuit which of the following is True ?
A) Circuit Résistance Decreases as Additional Circuits are Added , ◄
Current is equal in all parts of the circuit ,
Only one current path to ground
None of these ,
- by Adding Additional Paths to Ground ( Earth ) Résistance Drops and Current goes Up ,
Q) a Break or Interruption in an electrical circuit is ?
A) an Open , ↔ ( A Break in a Wire is Called an Open ) ◄
a Short ,
a Ground ,
None of the above ,
Q) the sum of voltage drops in a series circuit equals the ?
A) Source Voltage , ◄
Voltage across the largest load ,
Voltage across the smallest load ,
Shunt circuit voltage ,
Q) Electrician A , say circuit protection devices are sensitive to current , Electrician B , say they are sensitive only to voltage , Who is correct ?
A) Electrician A only ◄
Electrician B only ,
Both Electrician A and Electrician B ,
Neither Electrician A nor Electrician B ,
- as current flows through a conductor , it generates heat , when current flow is excessive it melt or opens the protection device ,
Q) the strength of the magnetic field that surrounds a single conductor with current flowing through it ?
A) all of these ◄
Varies directly with the amount of current flowing through the conductor ,
Is usually weak ,
Can be detected using a magnetic compass ,
- magnetic field intensity is weak but varies in size with current strength ,
Q) when the lines of a magnetic field cut across a conductor ?
A) a Voltage is induced into the conductor , ◄
The conductor is permanently induced ,
The conductor is permanently magnetised ,
Magnetism is induced into the conductor ,
- as lines of force cut across the conductor they induce voltage ,

 

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Q) when current stops flowing through an inductor ( coil ) such as in a relay ?
A) a voltage spike is generated , ◄
an A.C. signal is produced ,
a magnetic field is developed ,
all of the above ,
- when current stops flowing through a coil , the magnetic field surrounding the coil collapses causing a voltage spike in the opposite polarity to develop , this opposite polarity voltage is known , CEMF , abbr. counter-electromotive force
Q) the three leads of a bipolar transistor are ?
A) the Base , the Collector , and the Emitter , ◄
Q) in this Relay :- ?
A) Terminal 4 is connected to Terminal 5 until energised , then Terminal 4 is connected to Terminal 3 ◄
Terminal 4 is connected to Terminal 3 until energised , then Terminal 4 is connected to Terminal 5
Terminal 3 is connected to Terminal 5 until energised , then Terminal 3 is connected to Terminal 4
Terminal 1 is connected to Terminal 5 until energised , then Terminal 1 is connected to Terminal 4
- when the control coil is energised , the switch connect terminals 4 and 3 together ,
Q) a device that produces a voltage when put under pressure is ?
A) a Crystal , ◄ ( a battery , a generator , a solar cell )
Piezo crystal under pressure will produce a voltage potential ,
Q) when the lines of a magnetic force cut across a conductor ?
A) a voltage is induced into the conductor , ◄
The conductor is permanently induced ,
The conductor is permanently magnetised ,
Magnetism is induced into the conductor .
- Voltage is induce as the flux cuts through a winding ,
Q) when electrical current is passed through a conductor that is forced into many loops , a magnetic field is created , the strength of the field may be increased by ??
A) Both A and B , ◄
Increasing the turns or coils of the conductor ,
Increasing the amount of the current in the coils .
Neither A nor B ,
- increasing the current or the number of turns will strengthen the field ,

 

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Push button. :- A normally open push button conducts electricity when it is being pressed, otherwise it's an open circuit.
Switch. :- Has an on and an off position. Conducts when it's on and is an open circuit when off.