Unit-6 | ELECTRICITY & MAGNETISM | Class-10

Overview: Electricity is a form of energy produced either due to the change in number of electrons in a body or due to the flow of electrons through a conductor. Magnetism is the directive and attractive property shown by a magnet. Here, we will discuss current electricity, circuit, effects of current electricity and about transformer.

Electricity:

Electricity is a form of energy produced either due to the change in number of electrons in a body or due to the flow of electrons through a conductor. Those substances that can produce electricity are called the sources of electricity.There are two kinds of electricity:
1. Static electricity: The electricity produced due to the change in number of electrons in a body is called static electricity.

2. Current electricity: The electricity produced due to the flow of electrons through a conductor is called current electricity.
    Mathematically, 
    Current (I) = total charge (Q) / time taken (t)i.e. I = Q/t

    The S.I. unit of current is Ampere. 


    Electric Circuit:

    A complete path formed by connecting an electric load, connecting wires, source and switch in which electricity can flow is called an electric circuit.There are two types of electric circuits:

    1. Open Electric Circuit: This is the type of electric circuit in which the electric load doesn't glow because either the connecting wire is faulty or the switch is turned off. This is why the current cannot flow completely throughout the circuit.


    Open Electric Circuit



    2. Closed Electric Circuit: This is the type of electric circuit in which the electric load glows and current can completely flow throughout the circuit because the connecting wires are good and the switch is also turned on.

    Closed Electric Circuit

    Connection of Electric Loads:

    Connecting two or more than two electric loads might be necessary sometimes. These connections can be made in two ways:

    1. Series combination of resistance: In this combination, the loads are connected in such a way that the positive terminals of the resistance is connected to the negative terminal of another resistance and so on. If the load used is lamp then, the brightness of the lamp increases as the number of lamps is decreased. The total resistance can be measured as: R = R1 + R2 + R3...

    Series combination of resistance


    2. Parallel combination of resistance: In this combination, the loads are connected in such a way that all the positive terminals are joined at a point and all the negative terminals are joined at another point. If the load used is lamp, the brightness of lamp isn't affected by the decrease or increase in the number of lamps. The total resistance can be measured as: 1/R = 1/R1 + 1/R2 + 1/R3....

    Parallel combination of resistance


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    Combination of cells:

    A cell is a source of electric current which converts chemical energy into electric energy. The cells can be used by combining or joining them together in the following ways:
    • Series combination of cells: In this combination, the cells are connected in such a way that the positive terminal of a cell is connected to the negative terminal of another cell. The current flowing in the circuit increases with the increase in the number of cells. The total pd across the external resistance is equal to the sum of the pd of all the resistances i.e. V =  V1 + V2 + V3
    Series combination of cells

    • Parallel combination of cells: In this combination, the cells are connected in such a way that all the positive terminals are connected at one point and all the negative terminals are connected at another point. The current flowing in the circuit doesn't increase with the increase in number of cells. The total pd is equal to the individual pd of each cell. i.e. V =  V1 = V2 = V3 
    Parallel combination of cells


    Electric Power:

    Electric Power is the rate at which the electric work is done, per unit time.
    i.e. Electric Power (P) = electric work (W) / time taken(t)
    or, P = (Q x V) / t  
    or, P = (Q/t) x V
    so, P = I x V

    or, P = I x V 
    or, P = I x IR [ V = I R]
    so, P = I^2 R
    Its S.I. unit is watt (W). 

    Fuse:

    Fuse is a safety device used in electric circuit to prevent short-circuiting or overloading. 
    It is a short metal wire having low melting point made of an alloy of tin and lead.
    Fuses are available of 3A, 5A, 10A, 13A, 15A and 30A.
    Always, the fuses should be used of slightly larger than the maximum current taken by the appliances.

    Electric Power Consumption:

    The power of electricity consumed can be measured as:

    Consumption of electricity (E) = Electric Power (P) x no. of devices (N) x time taken(t)

    Note: The electric power should be written in kw (kilowatt).

    The total cost of electricity consumed due to the use of electrical appliances is;Cost = Electricity consumed x rate in kwh
    1kwh = 3.6 x 10^6 J


    DC and AC:

    Direct Current (DC) is the electricity whose polarity doesn't change with time. Batteries, DC generators are the examples of it. It cannot be used in transformers.

    Alternating Current (AC) is the electricity whose polarity changes with time. Sources are AC generators or dynamic. It can be used in transformers.

    Direct current (DC) and Alternating current (AC)


    Here, polarity means the direction of the flow of current.

    Effects of current electricity:

    i) Heating Effect: If current is supplied to any conductor, it gets heated up and produces heat. This effect is called the heating effect of electric current. It is used in the construction of electric heater, rice cooker, toaster, etc. 

    i) Lightning Effect: If current is supplied to any resistance, and that current is utilized to produce light then, this effect is called the lightning effect of electric current. It is used to construct electric bulbs, lamps and fluorescent lamps.

    • Electric Bulb (filament lamp): A coiled tungsten wire, known as filament is used as the main lightning element in electric bulbs. It is filled with inert gases. It converts 10% of electrical energy to light energy. Its average life span is about 1000 hours.

    • Fluorescent Lamp (Tube light): Fluorescent powder is used as the main lightning element in fluorescent lamp. It is filled with Mercury vapour. It converts 30% of electrical energy to light energy. Its average life span is about 3000 hours. 

    iii) Magnetic Effect: The state in which a conductor shows magnetism when electric current is passed through it, it is called magnetic effect. 


    Electromagnet: 

    An electromagnet is a type of magnet which possess magnetism by application of electricity. 

    The strength of an electromagnet depends on:

    a) number of turns of wire
    b) amount of current supplied in the circuit
    c) purity of soft iron core [ increasing the purity of soft iron core also increases the strength of an electromagnet. ]


    Uses of electromagnet are:

    a) Electromagnets are used in electrical devices such as electric bell, an electric train, electric motor, etc. 
    b) It is used to remove pieces of iron from wound.
    c) It is used in MRI machines to take photos of the view of human's internal organs.


    Electric bell: 


    Electric bell is the application of electromagnet which converts electrical energy into sound energy. 


    Inter-relation between electricity and magnetism:

    Both electricity and magnetism can be produced with the help of each other. According to Oersted's experiment, it is known that a current produces a magnetic field. And, according to Michael Faraday, magnetism produces current through electromagnetic induction.


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    Magnetic Flux:

    Magnetic flux is the measurement of total magnetic field which passes through a given area.

    Electromagnetic Induction:

    Electromagnetic Induction is the production of electromotive force across an electrical conductor due to the change in magnetic field.

    Faraday's laws of electromagnetic Induction:

    1. Whenever there is a change in magnetic flux, an electric current (and pd) is induced in the coil.2. The induced pd lasts as long as there is change in magnetic flux.3. The magnitude of the induced current (and pd) is directly proportional to the rate of change in magnetic flux.


    iv) Chemical Effect: When electric current is passed through a substance, it brings about a chemical reaction in that substance. This effect of electric current is called its chemical effect.


    Electrolysis of water:

    Water decomposes into its constituent components when it passes through electricity. This is called electrolysis of water.


    The electrode connected to the positive terminal of cell is called anode. The electrode connected to the negative terminal of cell is called cathode.

    At anode:
    4OH  →  2H2O + O2 (gas)

    At cathode:
    4H  → 2H2 (gas)


    Electroplating:

    Electroplating is the process of depositing one metal over another by electrolysis. Substances to which electroplating is to be done should be connected as anode, and to which it should be done is connected as cathode. One of the solutions of anode should be used as electrolyte.



    CuSO4 ionizes in water as:
    CuSO4 Cu++ + SO4--

    At cathode, Cu++ +2e-  → Cu
    At anode, Cu   → Cu++ +2e-

    Hence, the copper from anode gets deposited to the cathode and the iron nail is coated with the copper layer.


    Application of electroplating:

    • It is used in purification of metal.
    • It is used in electrotyping.



    Fleming's right hand rule (Dynamo rule):

    Fleming's right hand rule states that, "The thumb, forefinger and the central finger of the right are stretched so that they are mutually perpendicular to each other. If forefinger pointing the direction of magnetic field, the thumb pointing the direction of motion of the conductor, them the central finger points the direction of induced current."

    Electric generator or dynamo:

    A generator or dynamo is a machine that converts mechanical energy into electrical energy thus, is used to produce electricity.

    Generator:

    A generator is a machine which converts mechanical energy into electrical energy.A generator uses electromagnet to produce electricity through electromagnetic induction.

    Motor effect:


    When current carrying conductor is kept in magnetic field, it starts to move which is called motor effect.Machines based on motor effect convert electrical energy into mechanical energy. These machines are called electric motors.

    Fleming's left hand rule (Motor rule):

    Fleming's left hand rule states that, "if the first three fingers of our left hand are stretched so that they are mutually perpendicular to each other with the forefinger pointing the direction of magnetic field, middle finger pointing the direction of current, then the motion of conductor is given by the thumb. "

    Transformer:

    Transformer is a device which changes low AC voltage to a high AC voltage or high AC voltage to low AC voltage.
    Secondary voltage (Vs) / Primary Voltage (Vp) = No. of turns in secondary coil (Ns) / No. of turns in primary coil (Np)



    There are two types of transformers:

    • Step up transformer: The transformer which converts low AC voltage to high AC voltage is called step up transformer. It is used in power stations to transmit current to long distances.

    • Step down transformer: The transformer which converts high AC voltage to low AC voltage is called step down transformer. It is used in the household devices.



    #Numerical Problems:


    1. Calculate the capacity of fuse needed to be put in a heater of 880 W and 220 V?

    Solution:

    We have, Power(P) = 880 W

    Voltage (V) = 220 V

    We have,

    P = I x V

    or, 880 = I x 220

    or, I = 880/220

    So, I = 4A

    Therefore, the current used by the heater is 4A and a fuse of 5A is needed to be put into the heater.

    2. Find the amount of the electric bill of a hostel for 30 days, if 20 bulbs of 100 W and 6 fans of 75 W were used for 5 hours a day at the rate of Rs 7 per unit.

    Solution:

    Power of the bulbs used (P) = 100 W = 100/1000kw  = 0.1 kw

    Electrical energy consumed by bulbs in a day (E)= P x N x t= 0.1 x 20 x 5= 10 kwh

    Electricity consumed in one month by bulbs = 10 x 30= 300 kwh

    Power of the fans used (P) = 75 W = 75/1000 kw = 0.075 kw

    Electrical energy consumed by fans in a day = P x N x t= 0.075 x 6 x 5= 2.25 kwh

    Electricity consumed in one month by fans = 2.25 x 30 = 67.5 kwh

    Now, total energy consumed in that month = 300 + 67.5= 367.5 kwh

    Amount of electric bill = energy consumed x rate = 367.5 x 7 = Rs. 1837.5

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    Practice Yourself

    1. In a step down transformer, the number of turns in secondary coil is 2 times the magnitude of output voltage for the input 440 V.

    2. A step down transformer has 440 V primary voltage and 1000 turns of primary coil. How many turns of secondary coil will be needed to produce 220 volts from that transformer?

    3. In a hostel, an electric bulb of 100W for 12 hours and an electric heater of 750W are used for 4 hours everyday. If the cost of one unit of electrical energy is Rs 7, find the cost of using the bulb and heater for 20 days.





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