Electricity Class 10 Notes

Electric current is the rate of flow of charge through a conductor, and it is defined as the charge flowing past a point per unit time. Charge is measured in coulomb and time in second, so the unit of current, the ampere, is one coulomb per second.There are two directions to keep straight. Electrons, which carry the charge, move from the negative terminal to the positive terminal of the cell, while the conventional direction of the current is taken to be from the positive terminal to the negative terminal, opposite to the electron flow. Current is measured with an ammeter, which is connected in series so that the whole current passes through it.

• Electric current I equals the charge Q flowing in time t, so I = Q divided by t.
• The SI unit of current is the ampere, and 1 ampere means 1 coulomb of charge flowing in 1 second.
• Charge is measured in coulomb, and 1 ampere is therefore equal to 1 coulomb per second.
• Electrons flow from negative to positive, but the conventional current is taken as positive to negative.
• An ammeter has very low resistance and is connected in series with the circuit.
• A current is measured only when the circuit is complete, so an open key or a removed cell gives no reading.

Potential difference between two points of a conductor is the work done in moving a unit charge from one point to the other. It is defined as the work done in joule divided by the charge in coulomb, and its unit is the volt, which is the same as joule per coulomb.A voltmeter is always connected in parallel across the two points between which the potential difference is to be measured. For resistance work, current flows through the conductor, so energy is used up, but for a potential difference no current need flow at all.

• Potential difference V equals the work W done in moving charge Q, so V = W divided by Q.
• The unit of potential difference is the volt, and 1 volt equals 1 joule per coulomb.
• A voltmeter is connected in parallel across the points of measurement.
• The SI unit of charge is the coulomb, and that of current is the ampere.
• A cell or battery is a source of potential difference that drives charge around a closed circuit.
• An electric circuit is complete only when the key is closed, so that a potential difference can drive a current through it.

Ohm's law states that at constant temperature the potential difference across the ends of a conductor is directly proportional to the current flowing through it. Written out, this gives V proportional to I, so V equals IR for a constant R, and the resistance is R equals V divided by I.Resistance is the opposition offered by a conductor to the flow of current, and it is measured in ohm. A V-I graph for an ohmic conductor is a straight line passing through the origin, and the slope of that line is the resistance, so a steeper line means a larger resistance.

• At constant temperature, V is directly proportional to I, so V equals IR.
• Resistance R equals V divided by I, and its unit is the ohm, written as the symbol for ohm.
• The V-I graph of an ohmic conductor is a straight line through the origin.
• The slope of the V-I graph is the resistance, so a steeper line is a larger resistance.
• The condition of constant temperature is essential, since R changes with temperature.
• Resistance is a property of a conductor and does not depend on the current or the potential difference across it.

Resistance depends not only on the material of the conductor but also on its length and on its cross-sectional area. The property that brings these together is the resistivity of the material, written as the Greek letter rho, so that R equals rho multiplied by l divided by A.Resistivity is a property of the material and is defined as the resistance offered by a unit cube of that material, so it depends on the nature of the material and on the temperature but not on its dimensions. Metals such as copper and aluminium have very low resistivity, while alloys such as nichrome and manganin have high resistivity, which is why heating elements and fuse wires are made of alloys and not of pure metals.

• R equals rho multiplied by l divided by A, where l is the length and A is the cross-sectional area.
• Resistance is directly proportional to length, so a longer wire of the same material has more resistance.
• Resistance is inversely proportional to cross-sectional area, so a thicker wire has less resistance.
• Resistivity depends on the nature of the material and on the temperature, and not on the dimensions.
• Silver and copper are the best conductors, while rubber and glass are among the worst and are used as insulators.
• An alloy such as nichrome has much higher resistivity than its constituent metals, which is why it is used in heating elements and in fuse wires.

Resistors are joined in series when the same current has to pass through each of them one after the other, and in parallel when each resistor is to receive the full potential difference. The total resistance of a series combination is the simple sum of the individual resistances.In a parallel combination the reciprocals of the individual resistances add up, so the equivalent resistance is always less than the smallest resistance in the group. Both arrangements are used in daily life, since a series combination reduces the current while a parallel combination gives a low equivalent resistance to draw a large current.

• In series, the same current flows through every resistor and the potential difference divides in proportion to the resistances.
• The equivalent resistance in series is R1 plus R2 plus R3 and so on, and it is greater than the largest single resistance.
• In parallel, the same potential difference exists across every resistor and the current divides among the branches.
• The equivalent resistance in parallel is given by one over R equal to one over R1 plus one over R2 and so on.
• The equivalent resistance in parallel is always less than the smallest individual resistance in the group.
• For two equal resistors, the series value is double one resistor and the parallel value is half of it, and both are asked often.

When an electric current passes through a resistor, some of the electrical energy is converted into heat, and this is known as the heating effect of electric current. The amount of heat produced in t seconds is H equals I squared multiplied by R multiplied by t, which is Joule's law of heating.Joule's law states that the heat produced in a conductor is directly proportional to the square of the current, to the resistance of the conductor and to the time for which the current flows. Since the current enters as a square, doubling the current produces four times the heat, which is why even a small increase in current can burn the wiring of a house.

• H equals I squared multiplied by R multiplied by t, with I in ampere, R in ohm and t in second.
• The heat produced is directly proportional to the square of the current, the resistance and the time.
• Applications in the home: the electric heater, the electric iron, the toaster and the filament lamp.
• A filament lamp glows because the filament is heated to a white heat by the current passing through it.
• A fuse wire is of a material with high resistivity and a low melting point, so it melts and breaks the circuit when the current is too large.
• Because H contains I squared, a slight overloading of a circuit produces a very large amount of heat.

Electric power is the rate at which electrical energy is consumed or supplied by a device, and it is measured in watt. A watt is a joule per second, so a 100 watt bulb converts 100 joule of electrical energy into heat and light every second.Power can be written in three equivalent forms, and the one to use is decided by what the question gives. If the current and the potential difference are known use P equals VI, if the current and the resistance are known use P equals I squared R, and if the potential difference and the resistance are known use P equals V squared divided by R.

• P equals V multiplied by I, and this is the form to use when both V and I are given.
• P equals I squared multiplied by R, used when the current and the resistance are given.
• P equals V squared divided by R, used when the potential difference and the resistance are given.
• The SI unit of power is the watt, and 1 watt is 1 joule per second.
• 1 kilowatt equals 1000 watts, and 1 horsepower is about 746 watts.
• A commercial unit of electrical energy is the kilowatt-hour, and 1 kilowatt-hour is called 1 unit, so a 100 watt bulb burning for 10 hours uses 1 unit.

Domestic supply comes as alternating current through three wires, the live wire, the neutral wire and the earth wire, brought in through a three-pin plug. The live wire is at a high potential difference above the earth, the neutral wire completes the circuit at almost zero potential difference, and the earth wire is joined to a metal plate in the ground.Two faults spoil a domestic circuit. Overloading happens when too many appliances are run on one circuit or when the total power exceeds what the wiring can carry, and a short circuit happens when live and neutral wires touch directly. A fuse of thin wire kept in the live wire melts and breaks the circuit in either case, and the earth wire provides a low-resistance path so that the fuse blows before the user is electrocuted.

• Live wire: carries current to the appliance at high potential difference and is connected through the fuse.
• Neutral wire: completes the circuit by taking the current back to the supply at nearly zero potential difference.
• Earth wire: connected to a metal plate in the ground, and it does not carry current in normal working.
• Overloading: too many appliances on one circuit, and the heat produced melts the fuse and disconnects the supply.
• Short circuit: the live and neutral wires touch, the current rises sharply and the fuse wire melts instantly.
• Earthing: the earth wire gives a low-resistance path for the current so that the fuse blows first and the person is not electrocuted.