• How is work done in an electric circuit?

Voltage, applied across a circuit, causes the free electrons to move towards the positive terminal of the source and as a result, current is flowing through the circuit. Therefore, moving the electrons in the circuit indicates that the work is being done.

• What the formula V=IR defines?

Resistance is defined from the formula V=IR and any type of conducting device can obey this formula whether or not it obeys ohm’s law. Basically, ohm’s law states that V is proportional to I, i.e., R is independent of V.

• Why does room light turn on immediately the switch is closed?

As soon as the switch is closed, the electric field is established immediately in the whole circuit. As a result, free electrons begin drifting everywhere in the wire at once. Hence, room light turns on at once as the switch is closed.

• It is noticed that a 100 watt bulb is brighter than a 60 watt bulb. Explain why?

100 watt bulb releases 100 joules of energy/second while 60 watt bulb releases 60 joules of energy/second. Hence, a 100 watt bulb is brighter than a 60 watt bulb.

• Explain the basic difference between an overload and a short circuit.

An overload occurs when the circuit is carrying more than the rated current while a short circuit indicates excessively high current (more than 10 times the rated current) through the circuit.

• Ohm’s law is true for all conductors. Yes or no?

No. It is true only for metallic conductors given that other physical conditions unchanged.

• Why is matter electrical in nature?

Every matter is composed of atoms and atom contains charged particles i.e., proton and electrons. Therefore, it can be said that matter is electrical in nature.

• Is a filament lamp obeys ohm’s law?

No. Because the resistance of the filament changes with the changes in the temperature of the filament and temperature of the filament changes with the changes in the voltage applied across the lamp, which violates the ohm’s law.

• Why the insulators have negative temperature co-efficient of resistance?

Insulators have negative temperature co-efficient i.e., resistance of insulators decreases with the increases in temperature. Rise in temperature in insulators causes to produce more free electrons (due to breaking some bonds) which in turn cause to increase in the drift movement through the insulator. Hence, resistance of the insulator decreases.

• What is the importance of power rating of a machine?

The power rating of a machine should not be exceeded while the machine is in operation, otherwise due to the excessive heat the machine may be damaged.

• Name the alloys that have high resistance but almost zero temperature co-efficient of resistance are and why?

The rise of temperature of a substance causes two effects-

1. It causes the molecules to vibrate more rapidly, thus obstructing the movement of free electrons through the substance.
2. It may break some bonds, releasing free electrons.

Zero temperature co-efficient of resistance means there is negligible increases in resistance with increase in temperature. Example of such high resistance alloys are Eureka, Manganin, Constantan etc. due to the fact that the above said effects almost cancel each other.

• Reason behind positive charges and negative charges attract each other?

Positive charge indicates shortage of electrons while negative charge means excess of electrons. Therefore, positive charges attract negative charges to maintain minimum electric potential i.e., zero potential.

• Inspite of having large number of free electrons in metals, why there is no current establishment in the absence of electric field?

In the absence of electric field, free electrons in the metal move in all direction in irregular manner within the metal. At this moment, they repeatedly conflict with the positive ions in the metal which causes their direction to change. Hence, net movement in any particular direction is zero. So, there is no current establishment.

However, an external electric field causes the free electrons start drifting towards the particular direction with a small velocity.

• Which alloys are used to make standard resistance and why?

As the alloys constantan and magnanin have high resistivity and negligibly small temperature co-efficient of resistance, they are used to make standard resistance.

• Conductors have positive temperature co-efficient of resistance. What does it mean?

It means that the resistance of conductors increases with the increase in temperature. Rise in temperature of a metal conductor causes to increase the molecular vibration which in turn hampers the movement of free electrons through the conductor. As a result, resistance of the conductor increases.

• What is the function of an electric circuit?

An electric circuit converts the electrical energy into some other forms of energy. For example, electrical energy is converted into mechanical energy by introducing a motor with an electric circuit..

• Describe the fundamental difference between e.m.f. and p.d.?

The e.m.f. of a cell is 5V means that the cell supplies energy of 5 joules to each coulomb of charge.

The p.d. between two points A and B of a circuit is 5V means that each coulomb of charge will lose energy of 5 joules in moving from A to B (given that point A is at higher potential).

• What is the net charge on a wire while it is carrying current?

Zero. Drift of free electrons towards a definite direction causes flowing of current in a wire. But, at any circumstances, number of electrons in the wire is equal to the number of protons. Hence, net charge on the wire is zero while it s carrying current.

• Is volt can be called a unit of energy?

Yes, because volt is the unit of electric potential. And,

Electric Potential (V) = Work done/Q= W/Q

Thus, the unit of electric potential can be described as Jules/C or volt.

• Inspite of very high thermal speeds of free electrons, why they are fail to escape from the surface of a conductor?

In a conductor, free electrons can travel from one atom to another within the conductor due to the attraction of the positive ions, which always pulling them back. As a consequence, free electron encounters forces that prevent it to escape from the surface of the conductor.