Physics [ 2 Marks Questions ]

The component of current (Irms cos Φ) which is in phase with the voltage is called active component. The power consumed by this current = Vrms Irms cos Φ. So that it is also known as ‘Wattful’ current.

RLC circuits have many applications like filter circuits, oscillators and voltage multipliers, etc. An important use of series RLC resonant circuits is in the tuning circuits of radio and TV systems. The signals from many broadcasting stations at different frequencies are available in the air. To receive the signal of a particular station, tuning is done.

A dc ammeter cannot read ac because the average value of ac is zero over a complete cycle.

An inductor L is a closely wound helical coil. The steady DC current flowing through L produces uniform magnetic field around it and the magnetic flux linked remains constant. Therefore there is no self-induction and self-induced emf (back emf). Since the inductor behaves like a resistor, DC flows through an inductor. The AC flowing through L produces a time-varying magnetic field which in turn induces self- induced emf (back emf). This back emf, according to Lenz’s law, opposes any change in the current. Since AC varies both in magnitude and direction, its flow is opposed in L. For an ideal inductor of zero ohmic resistance, the back emf is equal and opposite to the applied emf. Therefore L blocks AC.

Three-phase system has many advantages over single-phase system, which is as follows:

1. For a given dimension of the generator, a three-phase machine produces higher power output than a single-phase machine.
2. For the same capacity, the three-phase alternator is smaller in size when compared to a single-phase alternator.
3. Three-phase transmission system is cheaper. A relatively thinner wire is sufficient for transmission of three-phase power.

The unit of self-inductance is henry. One henry is defined as the self-inductance of a coil in which a change in the current of one ampere per second produces an opposing emf of one volt.

When eddy currents flow in the conductor, a large amount of energy is dissipated in the form of heat. The energy loss due to the flow of eddy current is inevitable but it can be reduced to a greater extent with suitable measures. The design of transformer core and electric motor armature is crucial in order to minimise the eddy current loss.
To reduce these losses, the core of the transformer is made up of thin laminas insulated from one another while for electric motor the winding is made up of a group of wires insulated from one another. The insulation used does not allow huge eddy currents to flow and hence losses are minimized.