Basic notes in electrical

                                             

Good Conductors: They permit easy Detachment of outer most electrons in their orbit when an energy is applied.

Current (A): Continuous flow of electrons constitutes an electric current.  When a Good conductor (copper wire) is placed in a circuit, electrons (the chargers) are made to detach and move in a definite direction.  Current is the rate of flow of charges i.e., no. of electrons moved in a given time (A=Q/t). Potential energy and closed circuit are the two requirements of electric current.

                                           1 A= 1 Coulomb/1 second.

Why heat is produced in a current carrying conductor: It is due to the collision of Electrons with other atoms while moving when energy is given.

Electric Potential Energy :

                The Natural direction of a positive charge is from Higher to lower potential. If we want to move the positive charge towards the positive terminal we need external work. By moving the positive charge towards positive terminal we are increasing its potential energy doing some work. (It is similarly like lifting a mass away from earth (gravity) to increase its potential energy-mgh ). This is what happens in electrochemical batteries .

   Battery converts chemical to electrical energy. chemical reactions inside battery forces the positive charges to move towards the positive terminal thereby increasing the potential energy of the battery. When this battery is connected externally through a wire, charges flows in its natural direction giving the work out.

       So electrical potential is determined by the location of charges. It expresses amount of energy per unit of charge at a particular location.

Higher electric potential : When  a charge possess relatively large quantity of potential energy at a given location, then that location is said to be high electric potential.

Lower electric potential : When a charge possess relatively small quantity of potential energy at a given location , then that location is said to be low electric potential.

Ø  When a positive charge moves from higher to lower potential area it loses its potential energy, which converts as work in the circuit.

 

Potential Difference or voltage: It is the difference in electric potential between final and initial location when work is done (or gained) upon a charge to change its potential energy.

                                               

                                                    ONE VOLT = 1 joule/1 coulomb .

If Potential difference between two location is 1 volt , then  1 coulomb of charge will gain 1 J of energy when moved between these two locations by external work.  So work done is stored as energy. when the charge moves from higher to lower potential we gets that 1 J of stored energy as output work .

·         In 12V battery, every charge of one coulomb moving in the external circuit will give us 12J of electric potential energy which we use it in various forms.

 In other words, Potential difference is the External force which required to make the electrons detach and move in a definite direction . This External force is the potential energy of the charge which is used  to detach the electrons. Potential difference is expressed in volts hence it is referred as Voltage.

 

When we see our Generator voltmeter and ammeter in MSB, Voltage will be constant say at 450V. Ampere changes according to the load.  Ampere is nothing but amount of charges per unit time(C/s), eg. If we needs 900J/s of Electric potential energy to run a load we need 2 C of charge per second =2 A . (2*450 = 900) .

 

Direction of conventional current:  From higher(+ve) to lower(-ve) potential .

Electrons  (negative charges) moves towards positive potential. But the direction of current is given by the movement of positive charges hence the Direction of current is opposite to the electrons direction.

Ø  Electrons moves in a Zid-Zag manner inside a wire. After each collision electrons change their directions. so the distance travelled by the electrons(chargers) with respect to time is called DRIFT VELOCITY (vd). Drift velocity is very small. i.e., chargers moves at very slow rate

Ø  Current has nothing do with speed of chargers move but no. of(amount) chargers move.

 

How does a bulb glows immediately when we switch on?

Once the circuit is closed , potential difference is obtained . this electric signal reaches ALL the electrons(chargers) in a fraction of second and starts moving immediately. The electrons in the wire nearby light moves first, making the light glow.

Ø  Movement of chargers is slow but starts immediately.

Ø  Rate of flow of chargers is same throughout the circuit.

 

Comparison of Electric current with water line: Consider a water pipe which connects Top tank and ground floor of our house.

Now consider Water flowing inside the pipe as Charge carriers and Head of the pipeline as Potential difference.

Ø  Without Head there will be no flow of water, like that without potential difference there will be no movement of chargers hence no flow of current.  

Ø  Head do not control the quantity of water, like that potential difference just gives energy but doesn’t decides the amount of chargers.

Ø   Switching on light bulb circuit is like opening a tap in a pipeline. After opening the tap the water near the tap comes out first. We don’t wait for the water from the entry point(tank) to come first. Like that in electric current ,  we don’t wait for chargers near the positive terminal to move first. Glow of bulb is due to movement of chargers near to the bulb .  All the chargers move at same time and at the same speed. How long a charge can move is given by the potential energy.

 

 

Voltage drop:

   Potential difference between the point B and C is the voltage drop caused by the load(bulb). Energy of chargers is given out in making the light glow. So there will be loss in potential energy.

 

Difference between electrolytic and electrical current:

 

Electrolytic(eg: inside batteries) : Both Negative and positive chargers move to form the current.

 

Electrical (eg: In a copper wire):  Only negative chargers (electrons) move to form the current.

 

Ohms Law:

    The ratio of potential difference (V) between two points on a conductor to the current (I) flowing between them, is constant, provided the temperature of the conductor does not change.

            V/I = constant      or   V/I= R

 

Resistance:  The property of the substance which opposes the movement of electrons.

Effect of increase in resistance in a circuit:

  Increase in resistance means it is blocking the flow of current more than before. So current passing across the circuit decreases. To maintain the same current it needs more energy to counter act the resistance which can be done by increasing the potential difference across it.

 

Power:

  It is the rate at which electrical energy is supplied to a circuit or consumed by a load

  Power= Work done on a charge/time   = Energy consumed by load/time

Apparent power(S):

    Power given to the circuit . VA     S doesn’t depend on load

Real power (P):

    Useful power consumed by the load to convert into useful work.   P varies with load

Reactive power (Q):

  Power consumed by passive components .VAR. it transfers no energy to the load hence called “wattles power”.    

 

Apparent power = vector sum of Real power and reactive power.

Power factor cos Φ:

It is the Ratio of real power to apparent power.  P.F is unity when voltage and current are in phase. P.F is ZERO when voltage lags or leads current by 90°.  This phase difference is due to the inductive and capacitive substance in the circuit.

 

Inductor: releases energy as magnetic field.

Capacitor: Stores energy as electric field.

 

 

Direct current:

    Continuous flow of charges in one direction. E.g Battery.   Electrons move in the same direction.

 Voltage and current doesn’t changes with time.

 

 

Alternating current: 

Direction of current flow alternates. In a wire electrons move back n forth (changes it direction).

 Here Voltage and current varies with time forming a sine waveform.

 

           Erms=   , Eo is the peak0 value  Erms is the Effective value of A.C.

 

 

 

What is meant by phase in AC?

Two or more AC voltages or currents that are out of step with each other. Their peaks and zero points do not match up at the same points in time. This is meant by phase.

 

What does three-phase current mean?

Three current sinusoidal waveforms equal in magnitude but with a phase shift of 120 degrees.

 

What is the meaning of power factor?

Power factor is the cosine of the angle between voltage and current waveform.

 

 

What is the benefit of improving power factor?

Power factor close to 1 means for the same real power in kW, the load current is less and hence the I²R losses are less. The system efficiency is higher as the losses are lower. Improved voltage regulation is achieved.

 

Why are motor ratings given in KW and that of alternator and transformer given in KVA?

kW is the output mechanical power of a motor and is expressed in kW.

kVA is the net (apparent) power input to the transformer. This input power is the output + losses.

kW = kVA x system power factor

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 TRANSFORMER

                It converts low alternating voltage into high alternating voltage and vice versa. Transfers electric power from one circuit to another.

 Priniciple :

Electromagnetic induction

 Working:

 The a.c input is applied across the primary coil. The continuously varying current in the primary coil produces a varying magnetic flux in the primary coil which pass through the magnetic core and produces varying magnetic flux in the secondary. Hence, an induced emf is produced across secondary.

 

where  k - transformer ratio ,     E - voltage, N - no. of turns and I - current.

Step up  -  E_s>E_p    implies  I_s<I_p         Ns> Np  .     K>1  

Step down- E_s<E_P  implies  I_s>I_p        Ns<Np         K<1           

 Efficiency =EsIs/EpIp ,   For an ideal transformer input power=output power 

 

Losses in transformer

(1)    Hysteresis loss – Due repeated magnetization and demagnetization of the iron core.

(2)    Copper loss-   Some energy lost in the form of heat.

(3)    Eddy current loss- Eddy currents are formed in a iron core whenever there is a change in magnetic flux.

(4)    Flux loss- Flux produced in the primary is not completely linked with the secondary coil due to leakage.

 

Why transformer doesn’t work on D.C?

 For inducing of emf  in secondary coil we need varying  magnetic field.  Varying magnetic field is produced by varying current in primary coil. When d.c is supplied to primary coil we don’t get varying magnetic flux. Hence emf won’t be induced.

 

Why is 440 V used for motors and 110/220 V used for lighting?

Motor load currents are large.  Motors are 3-phase loads. A higher 440V voltage means lesser current for the same power and hence losses and size of cable wires is lower. Lighting is single phase loads and its load currents are small, hence lower voltage means less insulation in the cable wires.

 

Single phase wiring in ships.

The secondary winding of step down transformer is connected in DELTA.  No neutral here. Eg. in 450-110 V transformer. In secondary side Voltage between two phase is 110V. So for lightings we use phase-phase connection. We connect R and Y to the load to form a single phase circuit. Loads are shared evenly to RY RB YB phases(3 branches of single phase).