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

LearnoHub - Class 11, 12・98 minutes read

The text discusses the importance of alternating current (AC) in homes for powering electrical devices and details various aspects of AC circuits, including resistive, inductive, and capacitive circuits. It also explores phasor diagrams, LCR circuits, resonance, transformers, and the functioning of AC generators.

Insights

  • Alternating current (AC) is crucial for powering most household electrical devices like fridges, TVs, and microwaves, as it involves periodic changes in current direction, unlike direct current (DC).
  • Understanding root mean squared (RMS) voltage and current in AC circuits is essential for calculating power accurately, with resistive circuits producing sine wave potential differences denoted by specific symbols.
  • Phasor diagrams play a vital role in representing the relationship between voltage and current in different AC circuits, showcasing how current leads voltage by Pa/2 in inductive circuits while voltage and current are in phase in capacitive circuits.

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Recent questions

  • What is alternating current?

    AC is the electricity in homes, essential for devices.

  • How is power calculated in a resistive AC circuit?

    Power in a resistive circuit is calculated using current, resistance, and voltage.

  • What is the significance of root mean squared (RMS) values in AC circuits?

    RMS values are crucial for calculating effective voltage and current in AC circuits.

  • How are phasor diagrams used in AC circuits?

    Phasor diagrams represent the relationship between voltage and current in AC circuits.

  • What is resonance in an AC circuit?

    Resonance occurs when a circuit oscillates at maximum amplitude at a specific frequency.

Related videos

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 changing direction periodically, unlike direct current (DC) where electrons flow in one direction.
  • Batteries provide DC, while AC alternates between positive and negative cycles, resulting in an average current of zero.
  • AC is generated by devices like AC generators, which 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, the source produces a sine wave potential difference, denoted by a specific symbol.
  • Instant power in a resistive circuit is calculated as the product of current, resistance, and voltage.
  • Average power in a resistive circuit is half the product of current squared and resistance.
  • Root mean squared (RMS) voltage and current are used in AC circuits to represent effective values, essential for calculating power in AC circuits.

17:40

Understanding RMS, Phasor Diagrams, and Inductive Reactance.

  • Understanding the difference between continuous and discrete steps, with stairs representing discrete steps and ramps indicating continuous movement.
  • Explaining the process of finding the root mean squared (RMS) value for discrete voltage values by squaring each value, finding the mean, and taking the square root.
  • Describing the calculation of RMS for continuous voltage values by integrating over time and then finding the root mean square.
  • Introducing the concept of instantaneous continuous values for voltage, represented by a sine function graph.
  • Detailing the process of calculating average current over a complete cycle by integrating current over time and dividing by the total time period.
  • Discussing the use of phasor diagrams to represent the relationship between voltage and current in AC circuits.
  • Defining phasors as rotating vectors denoting sinusoidally varying quantities.
  • Explaining the graphical representation of voltage and current in a resistive circuit using phasor diagrams.
  • Illustrating the phasor diagram for an inductive AC circuit, showcasing the relationship between voltage and current in an inductive circuit.
  • Introducing inductive reactance as the resistance exhibited by an inductor in a circuit, denoted by Xl and measured in Ohms.

34:40

Understanding Phase Difference in AC Circuits

  • Voltage and current are out of phase by Pa/2.
  • Current leads voltage by Pa/2 in an inductive circuit.
  • Confusion often arises between lag and lead in circuits.
  • In an inductive circuit, current lags behind voltage.
  • Graphs of voltage and current in inductive circuits form sine curves.
  • Current reaches peaks before voltage in inductive circuits.
  • Average power over a complete cycle is zero in an inductor.
  • Capacitive AC circuits involve capacitors without resistance or inductors.
  • Capacitive reactance in a pure capacitive AC circuit is 1/omega.
  • Voltage and current in capacitive circuits are in phase, with current leading by Pa/2.

52:25

Understanding LCR Circuits and Resonance

  • The order of letters in the alphabet determines their position, with "aa" coming before "v."
  • A 90-degree phase difference indicates that one signal is ahead of the other.
  • Rotating anti-clockwise signifies moving forward in a series.
  • In an LCR circuit, the components include an inductor (L), a capacitor (C), and a resistor (R) connected in series.
  • The voltage source in the circuit is represented by v = vm1 Omega t.
  • Kirchhoff's law is applied to determine voltage drops across components like resistors and inductors.
  • The expression for current in the circuit is i = Isin(omega t - Phi).
  • The impedance in an LCR circuit is represented by z = sqrt(r^2 + (xl - xc)^2).
  • The phase difference in the circuit depends on the values of capacitive and inductive reactances.
  • Resonance occurs in a system when it oscillates at a higher amplitude at a specific frequency.

01:14:02

Resonance in AC circuits: Key concepts explained

  • Resonance occurs when the amplitude of a swinging string increases with frequency.
  • Resonance in an AC circuit is discussed, particularly in a series LCR circuit.
  • At resonant frequency, the amplitude of the current is at its maximum.
  • The impedance of the circuit is crucial, with the amplitude of the current being the peak value.
  • The value of impedance must be minimum for maximum current amplitude.
  • Resonance requires both an inductor and a capacitor in the circuit.
  • Tuning a radio involves adjusting the capacitor value to match the resonant frequency.
  • Sharpness of resonance is defined by the quality factor, inversely proportional to resistance.
  • The peak voltage and current of an AC supply can be calculated using RMS values.
  • Capacitive reactance and current change with frequency, inversely and directly proportional, respectively.

01:31:08

AC Circuits: Frequency, Power, and Transformers

  • Current is directly related to frequency, with a proportional relationship.
  • Doubling the frequency results in halving the capacitive reactance and doubling the current.
  • The relationship between x, omega, and frequency is inversely proportional.
  • Calculating power in an AC circuit involves considering inductors, capacitors, and resistors.
  • Power in an AC circuit can be calculated using various formulas and expressions.
  • Different types of circuits, such as resistive, inductive, capacitive, and LCR circuits, have specific power dissipations.
  • Resonance in LCR circuits leads to the dissipation of maximum power.
  • Alternating current has various applications, such as in transformers for voltage conversion.
  • Transformers work on the principle of mutual induction, inducing EMF in primary and secondary coils.
  • Step-up and step-down transformers control voltage by adjusting the number of turns in the primary and secondary coils.

01:49:28

Transformers and AC Generators: Voltage and Current

  • Voltage across secondary coil is greater when voltage across primary coil is off
  • Current in secondary coil is less than in primary coil
  • One expression can explain the relationship between primary and secondary coil currents
  • Electricity in homes comes from sub area power stations
  • Step up transformer amplifies voltage, step down transformer reduces it
  • Voltage stabilizers are used to manage high voltage fluctuations
  • Energy losses in transformers can be minimized through various methods
  • Eddy currents and hysteresis loss contribute to energy loss in transformers
  • AC generators produce alternating current through induced EMF
  • AC generators function by changing magnetic flux through coil rotation

02:07:10

Coiling in AC: EMF Changes with Rotation

  • Explains the process of coiling in alternating current, detailing the changes in EMF as the coil rotates.
  • Stage One involves the coil's armature being perpendicular to the magnetic field, with the area vector and magnetic field aligned.
  • In Stage Two, rotating the coil 90° results in the area vector being perpendicular to the magnetic field, leading to maximum induced EMF.
  • Stage Three sees the coil rotated another 90°, changing the angle between the area vector and magnetic field to 180°, resulting in zero induced EMF.
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