Class notes Physics

Physics- Theory


Alternating Current:-


1. Value of current changes every moment and its direction changes twice in
a cycle
2. Value of voltage changes every moment and its polarity changes twice in a
cycle.
3. Vrms:- The constant voltage which generates the same energy consumption
as done by AC voltage in a given time period equal to AC. Vrms=Vm/√2
4. Irms:-The constant current which generates the same energy consumption
as done by AC voltage in a given time period equal to AC. Irms=Im/√2
5. XL:- Inductive reactance:- Resistance offered by an inductor in an AC
circuit. It is directly proportional to inductance and frequency.
6. Xc:- Capacitive Reactance:- Opposition offered by a capacitance in an AC
circuit.
7. Z:- Impedance:- Opposition in an LCR circuit.
8. Resonance:- Xc=XL, ຜo=1/√LC, R=Z
9. Quality Factor is the measure of sharpness of resonance(𝛚0/2△𝛚).
10. More the value of Quality Factor, bandwidth is less, more L, more
selective the circuit will be, more sharp the resonance.
11.While tuning a circuit, we vary the capacitance of the circuit such that the
frequency of the circuit is equal to the frequency of the radio signal picked
up.
12. Resonance is only possible when both L and C are present, i.e, in an
RLC or LC circuit. This is because the voltage across the inductor and
capacitor, being out of phase, cancel each other out. Resonance is not
possible in an RL or RC circuit.
13. Current lags voltage by 𝜋/2 in an inductive circuit. Current leads
voltage by 𝜋/2 in a capacitive circuit.
14. In a resistive circuit, power factor(cos ɸ) is 1.
15. In a purely inductive or capacitive circuit, the power factor is 0.
16. In a LCR circuit, power factor ranges between 0 to 1(excluding 0 and 1).
17. In a LCR circuit at resonance, the power factor is 1.

, 18. Principle of Transformer:- Transformers work on the principle of
mutual inductance, i.e, change in flux through one coil induces emf in the
neighbouring coil.
19. Transformer contains two coils- primary coil having Np turns and
secondary coil having Ns turns. Usually the primary coil is the input coil
while the secondary coil is the output coil. As an alternating current passes
through the primary coil, it generates a magnetic flux which induces emf in
the secondary coil. The value of the induced emf depends on the number
of turns in the secondary coil.
20. When Np>Ns, then the transformer is called a step-down transformer.
The current increases in a step-down transformer.
21. When Np<Ns, then the transformer is called a step-up transformer. The
current decreases in a step-up transformer.
22. Long distance transmission and distribution of electrical energy is done
using transformers. The voltage is stepped up at the main power station,
such that current decreases and the power loss(I2R) loss is reduced. When
the electrical energy reaches the sub-stations, it is stepped down. It is
stepped down further again at distributing substations and utility poles
before the 240V supply reaches our home.



EMI:-


1. Magnetic Flux:- Magnetic Field lines passing through a given area.
2. Electromagnetic Induction:- Induction of EMF/Current in a loop due to
change in magnetic flux.
3. Faraday’s Laws of Electromagnetic Induction:-
● Induced emf/current stays in the loop as long as there is change in
magnetic flux, i.e, Magnetic flux should change every moment and
not be constant/uniform.
● The magnitude of induced emf is equal to the rate of change of
magnetic flux through the loop.
4. Lenz’s Law:- The polarity of induced emf in a circuit generates a current
such that it opposes the source of magnetic flux through it.
5. Electromagnetic Inertia: Property of any loop to oppose any change in
magnetic flux passing through it.