The syllabus of CBSE class 12 physics is quite lengthy. There are a total of 15 chapters in NCERT textbooks of class 12 physics (Part 1 and Part 2). Students who are going to sit for the board examinations should know the important topics and derivations from these 15 chapters. Although all the topics are important for CBSE class 12 board examinations, there are some important derivations, topics and questions which have been identified by analyzing some previous year papers of CBSE class 12 Physics.
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Some important topics from alternating current physics for CBSE class 12 boards are:
- The numerical problems and derivations in relation between rms and peak value of current and emf.
- The numerical and derivations in relation to reactance, impedance and average power in series LCR, LR, CR or LR circuit.
- The numerical problems and derivations of resonance in series LCR circuit.
One must know the following derivations:
- AC circuit which contains resistor (no phase difference between emf and current)
- AC circuit which contains inductor (phase difference)
- AC circuit which contains capacitator (phase difference)
- AC circuit which contains LCR in series (all three, along with phasor)
- Power factor
- Power dissipation in LCR in series
- Resonance in LCR (expression for resonant frequency)
- Q-factor and graphs
- Transformers (construction, working, principle)
- Reasons for losses in transformer
Below are given some important notes for class 12 physics alternating currents
Transient current
Transient current can be defined as an electric current which is there for a small finite time, while growing from zero to maximum or decaying from maximum to zero.
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Growth of current in an indicator
Growth of current in an indicator at any instant of time t is given as below:
Where,
= maximum current
L = self-inductance of the indicator
R = resistance of the circuit
Here r/l is the time constant of a l – r circuit
Time constant of a l – r circuit can be defined as the time in which current in the circuit grows to 63.25 of the maximum value of current.
Alternating current
Alternating current can be defined as an electric current whose magnitude changes continuously along with time and changes its direction periodically.
The instant value of alternating current at any instant of time t is given as follows:
The magnitude of alternating current keeps changing with time and its direction gets reversed periodically.
The value of alternating current changes along with time between zero and its peak value. There are two types of terms for finding the value of alternating currents (AC).
Impedance
Impedance (Z) of a circuit is defined as the opposition given by an AC circuit which contains more than one out of three components L, C and R.
Impedance of an AC circuit is given as:
The SI unit is ohm.
Average value of alternating current
The mean or average value of alternating current over half cycle is that value of steady current which sends the same amount of charge through a circuit in the time of half cycle i.e., T/2 is sent by the alternating current in the same circuit at the same time.
Measurement of AC
There are two ways of measuring alternating current:
- Component measurement: which is measuring the component of the current. But component measurement is not possible because component of electric current includes measuring the amount of charge which is passing in current. Hence, it is only theoretical and not practical.
- Effect measurement: which means measuring the effect created by AC. This method is possible because the effect created due to the current can be measured. Thus, effect measurement in the terminology of physics can be defined as:
The magnitude of alternating current can be said to be the value of steady current which does the same work during a period of one cycle.
What is steady current?
It is that type of current which does not change its magnitude or direction with time and is steady throughout all times. Let us understand it better. Let us suppose that a steady current does some amount of work ‘X’ in the time taken by alternating current to complete one cycle. Therefore, the magnitude of steady and alternating current will be the same.
Important topics
The important topics from Alternating current which students should study for boards are L.C.R. circuits, Power Factor, Transformer, Induced EMF, AC circuits and RMS currents.
For 1 mark question and MCQ for class 12 boards, questions related to concepts from these topics, and Phasor and LCR circuits diagram related questions might be asked in the examinations.
