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CBSE Class 12th Physics Notes: Alternating Current (Part ‒ III)

Jan 10, 2017 17:00 IST

    Get CBSE class 12th Physics key notes on alternating current. These notes are continuation of CBSE Class 12th Physics Notes: Alternating Current Part ‒ I and Part – II.

    In part I, we have studied important concepts like Alternating Current, Alternating Voltage, Average Value of Alternating Current and Voltage, Root Mean Square value of Alternating Current, Phasors, Different Types of A.C. circuits, Charging and Discharging of a Capacitor etc. Whereas, in part II, we have studied AC Voltage Applied to a Series LCR Circuit, Resonance in a Series LCR Circuit, Sharpness of Resonance (Quality Factor), Power Dissipation in AC Circuit, Power Factor, Wattless Current, LC Oscillations.

    In this part (or part III) we will study the concepts given below:

    Transformer

    Types of Transformer

    Step-Up Transformer

    Step- Down Transformer

    Applications of Transformer in Long Distance Power Transmission

    Efficiency of Transformer (ƞ)

    Power Losses in a Transformer

    Hysteresis Loss

    Copper Loss

    Eddy Current Loss

    The key notes are given below:

    Transformer

    A Transformer is used to convert low voltage (or high current) to high voltage (or low current) and vice versa. It is based on the principal of electromagnetic induction.

    One of the coils called the primary coil has Np turns. The other coil is called the secondary coil; it has Ns turns. Often the primary coil is the input coil and the secondary coil is the output coil of the transformer.

    When an alternating voltage is applied to the primary, the resulting current produces an alternating magnetic flux which links the secondary and induces an emf in it. The value of this emf depends on the number of turns in the secondary.

    AC Transformer Diagram

    Image Source: NCERT Textbooks

    In a transformer, if all the magnetic flux linked with the secondary then,

    NS/NP = VS/VP =k

    Here, k is called transformation ratio. Now, NP and NS are the number of turns in the primary and the secondary respectively and VP and VS are the rms voltages across the primary and secondary respectively.

    The primary coil of a transformer is connected to an AC source and load is connected to the secondary coil of the transformer.

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    Types of Transformer

    There are two types of transformer and they are:

    •   Step-Up Transformer

    •   Step-Down Transformer

    Step-Up Transformer

    The transformers which are used to convert low voltages into higher ones are called ‘step-up’ transformer. In step-up transformer VS > VP for which NS > NP or k > 1.

    Step- Down Transformer

    The transformers which are used to convert high voltages into lower ones are called ‘step-up’ transformer.

    In step-down transformer VS < VP for which NS < NP or k < 1.

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    Applications of Transformer in Long Distance Power Transmission

    Electric power generated in power stations is transmitted over long distances with the help of transformer.

    To reduce the loss of electrical energy (in the form of heat), a step-up transformer is used at power station to step-up voltage (or to lower the current). At the customer end this voltage is stepped down with the help of step-down transformer.

    If the transformer is ideal then,

    Power drawn from AC source = Power delivered to the load or VPIP = VSIS

    Here, IS and IP are the primary and the secondary rms currents.

    Efficiency of Transformer (ƞ)

    ƞ = [(Power Output) / (Power Input)] × 100 or ƞ = [(VSIS) / (VPIP)] × 100

    An ideal transformer has 100 % efficiency. The efficiency of a normal transformer is less than 1.

    Power Losses in a Transformer

    Power output of a transformer is less than the power input due to unavoidable losses of energy.

    These losses are given below:

    Hysteresis Loss

    In one complete cycle of AC, the core of transformer is taken through a complete cycle of magnetization. In this process, energy is lost in the form of heat. Such type of loss is minimized by using a magnetic material having low hysteresis loss.

    Copper Loss

    Heat is generated as AC flows through the copper coil. Due to which heat is lost to the surroundings. This is called copper loss. This loss can be minimized by using thick wires having low resistivity.

    Eddy Current Loss

    The alternating magnetic flux induces eddy currents in the iron core & causes heating. The loss can be minimized by using by using an insulated laminated core.

    CBSE Class 12th Physics Notes: Alternating Current (Part ‒ I)

    CBSE Class 12th Physics Notes: Alternating Current (Part ‒ II)

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