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Electrical Engineering
Electrical engineering is a field of engineering that deals with the study and application of electricity, electronics and electromagnetism.
23,056 Questions
What is the resistance of that draws a current of 25 amperes from a 1.5volts dry cell?
1. Theoretical Answer
R=1.5/25=0.6 Ohms
2. Practical Answer: Dry cells will not be able to deliver 25A as there are two limtation
a) Internal Resistance that limits the max current.round couple of Ohms. Max current that it can supply is about 1-2 A.
b) Capacity of Dry cells which are typical 700maHr. Even if cell could supply 25A it will only be for 1.7 minutes and then cell will become dead.
c) as the cells get drained there is increase in internal resistance which will further reduce the current.
What is the exact resistance value for ground or earth?
There isn't one. Soil type, water content, etc. cause this to vary.
Elastance is the reciprocal of capacitance so what is the reciprocal of inductance?
electric reluctance
What are Conduction Current Density and Displacement Current Density?
We know specifically the definition of current density as current flow through a surface of unit area.
Conduction current is the current through the conductors in the presence of electrostatic fields and displacement current is the current flow due to time varying electric fields
What happens when a charged capacitor is connected across an uncharged one?
When a charged capacitor is connected to an uncharged capacitor charge flows from the charged to the uncharged capacitor until there is no net force on the charge carriers. This means that both capacitors are at the same potential, because if they were not, there would be an electric fieldand hence a force on the charge carriers of magnitudeand the charge carriers would move to equalize the potentials and reduce the electric field and hence the force, eventually to zero.
Of course, as charge flows from one capacitor to the other, the total charge is conserved.
In the diagram above, when switch is closed chraged flows from the top capacitor to the bottom. When charge has stopped flowing,
Also, because the potentials are equal(1)
Suppose then that the capacitor at top left is initially charged to potential ofand that it has capacitanceand is then connected to a capacitance
Initiallystores a charge
Then (3)
From (1) (4)
Sub (4) into (1) to get
Now usefor either capacitor to findUsing it for (2) gives
from A Level Physics Notes
What is relation between power factor and phase sequence?
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power,[1][2] and is a number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf). Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power.
In an electric power system, a load with low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.
Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise power factor. The devices for correction of power factor may be at a central substation, or spread out over a distribution system, or built into power-consuming equipment.
Application of flip flop as a frequency doubler?
One way of using a flip flop as a frequency doubler is to create an oscillator at the desired doubled frequency, and then divide by two with the flip flop, resulting in the original frequency, and then controlling the oscillator with a phase locked loop.
Are potential difference and voltage always the same?
Voltage is simply another word for potential difference.
Overhead cables are made of almunium not of copper explain?
Much of the newer overhead cables is made of aluminum with steel reinforcing it for strenght purposes. Aluminimum is much cheap, and much lighter, thus it is cheaper to use vs. copper for high voltage transmission, and copper has a lower resistance which means more electricity can get through the wire/ cable, which means more electricity can get around the world quicker. most people should use copper becasue it has a lower resistance where aluminuim has a bit of a high resistance not to much but a bit so that is why you use copper not alominuim.
What are time delay relay characteristics?
Time delay protective relays used in the power industry can be time over current or voltage, or definite time over / under voltage / over current.
Definite time type relays use an over / under voltage / current element that picks up when voltage / current reach a specified level. When this picks up, a timer is started. If current / voltage stays above / below the pickup value for long enough to allow the timer to expire, the relay trip contacts are closed.
Time over current elements use an inverse time - current curve to define speed of operation, so the more current is present, the faster the relay will operate.
What is difference between cold load and hot load?
hot load occurs when the power is high and the cold load occurs when yhe power is low
Nikola Tesla invented the poly-phase induction motor in 1883.
Difference between line transformation ratio and transformation phase ratio?
line transformation ratio would be the turns ratio (ie voltage ratio), while the phase transformation ratio is most probably the phase shift introduced by a 3 phase transformer.
What is the conversion for rms power to peak to peak power?
RMS power is Peak-To-Peak power divided by the square root of 2.
This definition, however, only holds true for a non-reactive, or resistive, load, with a power source that is truly sinusoidal.
For a open circuit the VSWR is?
the voltage standing wave ratio is defined (1+p)/(1-p), where p is the the reflection coefficient magnitude. p = 1 for an open circuit, therefore the VSWR will approach infinite.
What is the resistance of a lightbulb using Ohm's law?
By Ohm's law, resistance is voltage divided by current, so the resistance of a light bulb can be measured by observing the voltage across it simultaneously with observing the current through it.
Interestingly, the hot resistance is significantly different that the cold resistance, so measuring resistance with an ohmmeter will not give a meaningful resistance.
This is because the resistance of a light bulb has a positive temperature coefficient. Take a typical 60 W 120V light bulb, for instance... Its cold resistance is about 16 Ohms. Calculate current and power at 120 V and you get 7.5 A and 900 W. The truth is that at 60 W, the bulb pulls 0.5 A and has a resistance of 240 Ohms.
What is the significance of energy meter constant?
the value 'n' of meter constant indicates that consumption of energy is n KWh.
In Ohm law what must be greater to maintain the same current in a conductor with more resistance?
ohms law states V = I*R. If I is to remain the same, and R is increased, the voltage must also increase.
What is the difference between 125V 15A fuse and 230V 15A fuse?
The voltage rating of a fuse (in this case 125V or 230V) defines the maximum circuit voltage that the fuse is safe to be used on. A fuse with a voltage rating of 125V should not be used in a circuit with a voltage greater than 125V. If a 125V fuse were installed in a 230V circuit and an overload current occurred, the fuse could explode.
Why remove resister from circuit to measure resistance?
a meter uses its own supply to measure resistance. you dont have to remove the resistor from the circuit but you must isolate it from the circuit supply to get an accurate reading.
What is excitation in generator and 2 methods each?
when the current is passing through the winding then it is called "Excitation".
Types of excitation
(1)seperately excited generator.
(2)self excited generator.
self generator is classified into 3 types.
1.shunt generator.
2.series generator.
3.compound generator.
compoud generator is again classified into 2 types.
1.short shunt generator.
2.long shunt generator.
At 690V the currents are smaller resulting in smaller rated equipment (smaller cables, transformers, switchboards, equipment ..etc)
at 690V the running current is reduced by40% compared to the 400V system
and the energy losses due to hear are much smaller compared to the 400V system.
many motors are on the border line for a service voltage of 400V but they can easily be accommodated at a service voltage of 690/400V.
refer to IEC 60038 edition 6.2 2002-07.
