Alternating Current Class 12 Physics Chapter 7 One Shot | New NCERT syllabus CBSE | Full chapter

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The text discusses the importance of alternating current (AC) in electrical devices, explaining the differences between AC and direct current (DC). It also covers various AC circuits, phasor diagrams, power calculations, resonant circuits, transformers, and AC generator operation.

Insights

AC is crucial for powering common household devices like fridges and TVs, as it involves current that changes direction periodically.
Different types of AC circuits, including resistive, inductive, capacitive, and series LCR circuits, have distinct characteristics affecting power calculations and phase relationships between voltage and current.
Resonance in circuits, such as series LCR circuits, plays a significant role in maximizing current amplitudes and tuning radio circuits for clear reception, with impedance being critical for achieving resonance.

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Recent questions

What type of current powers most household devices?
Alternating current (AC)

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Summary

00:00

Understanding Alternating Current in Electrical Circuits

The electricity in homes is alternating current (AC).
AC is essential for most electrical devices like fridges, washing machines, TVs, and microwaves.
AC involves current that changes direction periodically.
Direct current (DC) has electrons flowing in one direction, while AC has electrons changing direction.
Batteries provide DC, while AC is needed for devices like fridges and TVs.
AC generators produce AC, and their operation will be discussed later in the video.
Different types of AC circuits include resistive, inductive, capacitive, and series LCR circuits.
In a resistive AC circuit, there is a resistor and an AC source producing a sine wave potential difference.
The average power in a resistive circuit over a complete cycle is half of the product of current squared and resistance.
Root mean squared (RMS) voltage and current are used in AC circuits to represent equivalent DC values for power calculations.

17:37

Understanding Discrete and Continuous Electrical Concepts

Discrete and continuous concepts are explained using the example of stairs and ramps.
Discrete steps are like stairs, while continuous points are like a ramp.
To find the root mean squared (RMS) voltage of discrete values, square each value, find the mean, and take the square root.
For continuous values, integrate the squared values over time and divide by the time period to find the RMS voltage.
Alternating current is discussed, with the average current calculated by integrating over a complete cycle.
Phasor diagrams are introduced to show the relationship between voltage and current in AC circuits.
Phasors are vectors that rotate with angular velocity and denote the magnitude and phase angle of voltage and current.
In a resistive circuit, voltage and current phasors are in the same phase.
In an inductive AC circuit, inductive reactance is introduced as the resistance from the inductor.
The relationship between voltage and current in an inductive circuit shows current lagging behind voltage by a phase angle of π/2.

34:35

Phase Difference in Inductive and Capacitive Circuits

Voltage and current in an inductive circuit are out of phase by π/2.
In an inductive circuit, voltage leads current by π/2, while current lags behind voltage by the same amount.
Confusion often arises between lag and lead in current and voltage in circuits.
In an inductive circuit, the current lags behind the voltage.
The graph of voltage and current in an inductive circuit forms sine curves, with the current lagging behind the voltage.
The average power over a complete cycle in an inductive circuit is zero.
In a capacitive AC circuit, the voltage and current are in phase, with a phase difference of π/2.
In a capacitive circuit, the current leads the voltage by π/2.
The graph of voltage and current in a capacitive circuit forms sine curves, with the current leading the voltage.
The average power over a complete cycle in a capacitive circuit is zero.

52:17

Understanding Omega in LCR Circuit Resonance

The angle Omega is being discussed, with t coming before v in a race scenario.
The concept of aa being first due to a 90-degree difference is explained.
Instructions on rotating anti-clockwise to determine before and after positions are given.
The LCR circuit is introduced, consisting of an inductor, capacitor, and resistor connected in series.
The voltage source in the LCR circuit is detailed as v = vm1 Omega t.
Kirchhoff's law is applied to find the voltage drop across the resistor, inductor, and capacitor.
The expression for current in the LCR circuit is derived.
The impedance of the LCR circuit is discussed, dependent on resistance, capacitive reactance, and inductive reactance.
The impedance diagram of the LCR circuit is explained, along with the significance of the phase difference.
The concept of resonance in systems, illustrated with a pendulum example, is introduced as oscillating at high amplitude at a specific frequency.

01:13:53

"Resonance in Circuits: Key Concepts Explained"

Movement of the swing will continue if the frequency remains the same.
Pulling the string will increase the amplitude of the swing.
Resonance in a series LCR circuit is discussed, where amplitude is highest at resonant frequency.
Impedance should be minimum for maximum current amplitude.
Resonance occurs when the values of capacitive and inductive reactance are equal.
Resonance is crucial in circuits with both inductors and capacitors.
Resonance plays a key role in tuning radio circuits for clear reception.
Sharpness of resonance is determined by the quality factor of the circuit.
The peak voltage of an AC supply can be used to calculate the RMS voltage.
Capacitive reactance and current change inversely with frequency doubling.

01:31:04

AC Circuit Power and Transformers Explained

Current is directly proportional to frequency, meaning if frequency doubles, capacitive reactance halves and current doubles.
The relationship between x and omega is inversely proportional, while omega and frequency are directly proportional.
Power in an AC circuit is calculated using the formula v = vm2.
Average power in an AC circuit is calculated by taking the average of the constant terms and cos2 omega t over one complete cycle.
Different types of circuits have varying power dissipations: resistive circuits have power equal to i s z * k f, inductive circuits have zero power dissipation, and capacitive circuits have power equal to zero.
In LCR circuits, at resonance, the power dissipater is equal to i s z or i s r.
Transformers work based on electromagnetic induction, with primary and secondary coils wrapped around a soft iron core.
The induced emf in the secondary coil is equal to - ns5 / dts-i, while the back emf in the primary coil is equal to - np5 / dt3.
The voltage across the secondary and primary coils is the same, allowing for voltage step-up or step-down by controlling the number of turns in the coils.
Step-up and step-down transformers adjust voltage levels by controlling the number of turns in the secondary and primary coils.

01:49:23

Understanding Transformers and AC Generators in Electricity

Voltage increases when it goes up.
Voltage across the secondary coil is greater than the primary coil.
Current in the secondary coil is less than in the primary coil.
One expression explains the relationships between voltage and current.
Electricity in homes comes from sub-area power stations.
Step-up transformers amplify voltage, step-down transformers reduce it.
Voltage stabilizers are used for appliances to prevent damage from high voltage.
Transformers are not 100% efficient, leading to energy losses.
Ways to minimize energy losses in transformers include reducing flux leakage, resistance, eddy currents, and hysteresis loss.
AC generators produce alternating current through electromagnetic induction.

02:07:02

"Rotating Coil Generates AC Energy"

Explains the process of rotating an energy coil to generate electrical energy in the form of alternating current.
Describes the step-by-step stages of rotating the coil, starting with the plane of the armature being perpendicular to the magnetic field.
Details the changes in induced EMF as the coil is rotated at different angles, with specific values at 0°, 90°, 180°, and 270°.
Emphasizes the importance of understanding the coil rotation and EMF changes for AC generator operation, while also promoting a free learning platform for educational resources.

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