1. The curve representing Ohm's law in linear.
2. The condition in Ohm's Law is that the temperature should remain constant.
3. The Ohm's Law can be applied with certain reservations to electrolytes.
4. The presence of an electric current is made known by the effects produced.
5. An electric current can neither be seen nor touched.
6. Three important effects produced by the presence of a current heating, magnetic and electric shock.
7. Thermistor has negative coefficient of resistance.
8. International ohm is defined in terms of resistance of a column of mercury.
9. Resistors commonly used in power circuits are wire wound resistors.
10. When current flows through a heater coil it glows but the supply wiring does not glow because the resistance of heater coil is more than that of supply wires.
11. If the voltage applied across an electric press is reduced by 50 percent, the power consumed by the press will be reduced by is 25 percent.
12. In a parallel circuit the potential difference across the resistance is always constant.
13. In a series circuit the current is constant.
14. Voltage applied across a circuit acts as a force.
15. The resistance of carbon filament in carbon-filament lamps increases when its temperature is decreased.
16. The heating effect of current has an undesirable side effect in a vacuum cleaner.
17. When current flows in a conductor, the heat is produced because of inter atomic collision.
18. The temperature coefficient of a conductor is defined as the increase in resistance per ohm pwer degree centigrade.
19. The value of Joule's mechanical equivalent of heat,is equal of 4.2 Joules per calorie.
20. It was experimentally found by James Precott Joule that the heat produced in a current carrying conductor is proportional to the square of current.
21. The resistance of a conductor increases when its temperature is increased.
22. The specific resistance, depends upon the nature of the material of the conductor only.
23. Resistance of a conductor increases when its length increases.
24. The resistance of conductor is the hindrance by which the conductor opposes the flow of the current.
25. The minimum requirements to cause the flow of current are a voltage source and a conductor
Some Important Characteristics of Capacitors
1.The current through capacitor is zero, if the voltage across it is not changing with time.
2. A capacitor is sort of open circuit to direct current.
3. The capacitor never dissipates energy but only stores it.
4. A capacitor resits an abrupt change in voltage across it.
5. A finite amount of energy can be stored in a capacitor even if the current through capacitor is zero, such as when the voltage across it is constant.
6. It is impossible to change the voltage across a capacitor by a finite amount in zero time, for this requires infinite current through the capacitor.
Electrostatics Question and Answer
1. Relative permitivity of vacuum is unity.
2. In the electric field, the potential is the work done in joules to bring positive charge of one coulomb from infinity to that point.
3. The unit of field intensity is newtons/coulomb.
4. Coulomb's law for the force between electric charges most closely resemble with Newton's law of gravitation.
5. Mica medium has highest value of dielectric strength.
6. The maximum value of potential gradient in cable occurs in conductor.
7.A region around a stationary electric charge has electric field.
8. Inside a hollow spherical conductor electric field is zero.
9. The effect of the dielectric is to reduce the working voltage.
10. Electrolytic capacitor is the most commonly used type but it has two disadvantages, namely low insulation resistance and suitable for Direct current only has high capacitance and low insulation resistance.
11. In a radio a gang condenser is a type of air capacitor.
12. A sphere of one metre redius can attain a maximum potential of three kilo volt.
13.The power dissipated in a pure capacitor is zero.
14. In a capacitor the eletric charge is stored in dielectric.
15. One farad is one coulomb per volt.
16. If a dielectri is placed in an electric field, the field strength decreases.
17. If the medium of a parallel plate capacitor consists of mica and air, the capacitance is increased by increasing the area of plates.
18. A capacitor with capacitance is charged through a resistance. The time constant of the charging
circuit is given by RC.
Properties of Electric Lines of Forces
A line of force is supposed to start or emanate, from a unit with a positive charge. A line of force does not form closed loop unlike a magnetic line of force. The line of force is always normal to the surface of the body at the point from where they originate or terminate. No two lines of force can across each other. An electric line of force in the same direction repel each other and those in opposite direction attract each other. There will be neutral point in this case between the two sphere. An electric line of force has a tendecy to take an easy path.
Key Facts and Tutorials On Electrical and Distribution Lines
1. The economical section of a feeder can be obtain by applying Kelvin's law.
2. There should be be no break in the neutral, which is usually earthed at the supply end, of a two wire Alternating Current distribution system because excess current protection would be affected.
3. In actual practice the potential at the two feeding points is unequal. To calculate the voltage drop, the difference in potential is converted into ampere metres and the moments of the lower feeding point start from this initial value.
4. Uniformly loaded distribution fed at equal potential from both ends is treated like the distributor because the voltage drop is exactly halved.
5. Voltage drop in a uniformly loaded distributor fed at once end is calculated by assuming the whole of the load concentrated at middle point.
6. For Alternating current distribution the power factor of the load has to be taken into consideration and the calculations become cumbersome. The approximate method which gives results with in plus or minus five percent of the actual voltage drop involves calculating the centre of gravity of the load, resistance and reactance per double run and average power factor.
7. To determine the distribution of load at the point of minimum potential, the moments in ampere metres about of the feeding points must be equal to the moment in ampere metres about the other feeding point.
8.At the point of minimal potential in a distributor fed from both ends, the load at the point is supplied from right and left hand feeding points.
9. With point loads in a distributor fed at both ends, in order to determine the maximum voltages drop it is necessary to know the point of minimum voltage.
10. For a three wire Direct Current distributor fed at one end, if the total voltage drop in the neutral is positive it is added to the positive drop and deducted from the negative drop.
Limitation of high transmission voltage
a. Increased cost of line support
For high transmission voltage, the insulation required between the conductors and the earthed tower is more. This increases the cost of line supports.
b. High towers:
For high transmission voltage, the clearance between conductors and groundo should be more. Therefore, higher lower is required.
c. Longer cross arms:
FOr higher transmission voltage, distance between the conductors should be more. Therefore, longer arms are required.
All About Magnetism and Electromagnetism How it works Tutorials
Magnet
A magnet is a substance that attracts pieces of iron. The phenomenon by which this attraction takes place is called magnetism.
Magnets are two types
Natural Magnets
The natural magnets are those iron ores which are obtained from mine and have the property of attracting iron pieces naturally.
Artificial magnets
The artificial magnets are those which are created by artificial means. An artificial magnet can be further divided into two types are temporary magnets and permanent magnets.
Temporary magnets
A temporary magnet is that in which magnetism remains temporarily. If a wire is wound on a soft iron piece and direct current is passed through the wire, then soft iron piece becomes a temporary magnet. It is because the iron piece will retain magnetism so long as the current is flowing.
A Permanent magnets are made from steel which is in general harder than soft iron apart from steel, alloys like cobalt steel, tungsten steel, etc. are also used as permanent magnets. These are used in Direct current machines to create magnetic flux, electrical instruments, moving coil loudspeaker etc.
Properties of Magnet
1. A magnet always attracts iron and its alloys
2. The magnet has two poles and when it is freely suspended it comes to rest pointing north and south directions. The end which points towards north is known as North-pole and the other which points toward south is known as South-Pole. The attracting power of the magnet is concentrated around two points one each at end ends.
3. Like poles repel and unlike poles attact each other.
4. If a magnet is broken into pieces, each pieces becomes and independent magnet.
5. A magnet can import its properties to any magnetic material.
Magnetic effect of electric current
When and electric current flows through a conductor, a magnetic field is set up all along the length of the conductor. In this connection following are the important points worth noting.
1. The magnetic lines of force are circular in a plane perpendicular to the current.
2. The field near the conductor is stronger and becomes weaker as we go away from the conductor.
3. The magnetic field becomes stronger if current is increased and vice-versa.
4. The direction of the field is reversed it current is reversed.
Facts of Magnetism and Electromagnetism
1. A magnet is a piece of iron or other magnetic material which can attract small pieces of thesematerial towards it.
2. A freely suspended magnet always rests in north south direction.
3. A natural magnet is called lodestone.
4. A magnet is able to attract nickel, cobalt and steel.
5. Externally, magnetic line of force travels north to south.
6. A material commonly used for shielding or screening magnetism is soft iron.
7. Magnetism is the property of certain materials of attracting small iron pieces toward them.
8. The magnetism that remains in a magnet after the magentising force has been removed is called its residual.
9. Iron becomes magnetized by induction when it is near to one end of a magnet.
10. Magnetic lines of force are called flux.
