Comprehensive WBBSE Class 10 Physical Science Notes Chapter 6 Current Electricity can help students make connections between concepts.
Current Electricity Class 10 WBBSE Notes
Electric current: Electric current is generally means a continuous flow of electrons, ions or any electrically charged particles through a medium.
Electric current is the rate of flow of charge through any cross-section of a canductor.
\(\left(I = \frac{Q}{t}\right)\)(I = electric current, Q = total charge flowing, t = time)
Units of electric unit
SI unit : Ampere (A): One ampere of electric current is defined as the one coulomb of charge flowing through any cross section of a conductor in one second.
Nature of electric current: Although electric current has both magnitude and direction, yet it is a scalar quantity, as it does not obey vector laws.
Different types of electric currents are:
Steady direct current: An electric current is called a steady direct current if its magnitude and direction do not change with time.
Variable direct current : An electric current is called variable direct current if its magnitude changes with time but direction remains unchanged.
Alternating current : An electric current is called alternating current if its magnitude changes with time and direction also changes periodically.
Current denisity: Current density of a conductor is the amount of current passing per unit area of the conductor held perpendicular to the flow of charges.
Drift velocity: It is the average velocity with which free electrons in a conductor get drifted in a direction opposite to the direction of the applied electric field.
Mobility of the charge carrier: The ratio of the drift velocity of the electrons or charge carriers and the strength of the applied electric field is called the mobility of the charge carrier.
Canductor: The substances through which electric charge flows easily are known as conductors.
Non-conductor: The substances which do not allow electric charge to pass through them are called non-conductors or insulators.
Electric cell : A device in which electric energy is obtained from the chemical energy is known as electric cell.
Negative electrode: The metal rod in which there is excess of negative charge is called negative electrode of cell.
Positive electrode: The metal rod in which there is excess of positive charge is called positive electrode.
Open circuit: When the electrodes of a cell are not connected by a conductor externally, the cell is said to be in open circuit.
Close circuit: When the electrodes are connected internally with a conductor, the cell is said to be in closed circuit.
Electromotive force (e.m.f): The potential difference between the electrodes of a cell in open circuit is called e.m.f.
Potential difference : It is the electrical condition of a point in an electric field or on a current carrying conductor that indicates whether electrons will flow from it or it from another connected point.
Electrical charge: It is the physical property of a matter that causes it to experience a force when placed near another matter.
When ebonite rod is rubbed with flannel, ebonite rod is negatively charged and at the same time flannel acquires an equal amount of positive change. When a glass rod is rubbed with silk, glass rod is positively charged and the same time the silk acquires an equal amount of negative charge.
Electrostatic series: There is a list of substances called electrostatic series, which roughly shows that the two bodies when selected for rubbing will be charged in such a way that the body appearing earlier in the list is positively charged and that coming later in the list is negatively charged. The series is :
Silk, Human body, Cotton, Wood, Sealing wax, Amber, Resin, Sulphur, Rubber, Ebonite.
Basic properties of electric charge :
- Charges are of two types – positive and negative.
- Like charges repeal each other and unlike charges attract each other.
- The positive and negative charges tend to cancel each other.
Coulomb’s Law: The force between two point charges at rest is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them. The force acts along the line joining the two charges and its value depends on the nature of the intervening medium.
\(F \infty \frac{q1 q}{r^2} or } F = K \frac{q1 q2}{r^2}\)(K = electrostatic force constant or Coulomb’s constant and depends upon the system of units and the medium intervening the charges)
Validity for coulomb’s law :
- It holds basically for the point charges
- It is valid over distance as small as of the order of 10-15 m to several kilometers.
- It depends on the nature of intervening medium.
Dielectric constant : The dielectric constant or relative permittivity of a medium is the ratio of abslute permittivity of the medium to the absolute permittivity of free space.
Dielectric constant depends on the temperature of the medium.
Dielectric constant of water is about 80, so force between two given charges a given distance apart placed in water is 80 times less than when placed in vacuum or air.
Unit of charge:
SI unit of charge is coulomb (c): One coulomb is such a point charge which when placed in vacuum are metre apart from another similar point charge of equal strength repel one another with a force of 9 × 109 N
CGS unit of charge is electrostatic unit (esu): One electrostatic unit of charge is such a point charge which when placed in vacuum one centimetre apart from another similar point charge of equal strength repel one another with a force of one dyne.
1 coulomb = 3 × 109 esu charge
Electric potential: Electric potential at a point in the electric field is defined as the work done per unit charge in moving a unit positive test charge from infinity to that point against the electrostatic force of the field irrespective of the path followed.
Unit of electric potential :
SI unit : Volt (c) : The potential at point is one volt if one joule of work is done in moving one coulomb of charge from infinity to that point in an electric field.
CGS unit: Siat Volt : Potential at a point is one stat volt if one erg of work is done in moving one stat coulomb of charge from infinity to that point in the electric field.
1 stat volt = 300 volt
Potential difference between two points: Potential difference between any two points in an electric field is the work done per unit charge is moving a unit positve charge from one point to the other against the electrostatic force of the field irrespective of the path followed.
Unit of potential difference :
- SI unit : Volt
- CGS unit : Stat volt
Ohm’s Law (1826) : The temperature and other physical condition remaining constant, the current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor.
V ∝ I or, V = RI
Graphical representation of Ohm’s Law: The variation of V and I for two conductors A and B is shown in fig(a). These are straight line having constant
slope = \(\frac{V}{I}\) = R
So, higher the slope means the higher resistance of the conductor. Thus we find B has higher resistance R than that of A.
In the same way fig(b) shows variation of I and V for these two conductors A and B.
Here slope \(\frac{1}{V}\) = \(\frac{1}{R}\), shows that larger the slope, smaller is the value of the resistance of the conductor.
Ohmic conductors : Ohm’s law is found to be valid for a wide range of current and potential in all the metallic conductors and also in some other materials. These conductors are called ohmic conductors.
Non-Ohmic conductors: When the Ohm’s law is found not to be applied for conduction of electricity through vacuum tube, semiconductors and also to electric discharge through gases at low pressure. These are called non-ohmic conductors.
Resistance : The resistance of a conductor is the opposition offered by it to the flow of electric current through it
R = \(\frac{V}{I}\)
(R = resistance of a conductor
V = Potential difference
I = current flowing through a conductor)
Resistivity or specific resistance : The resistivity or specific resistance of the material of a conductor is numerically equal to the resistance between the opposite faces of a unit cube when current flows normally to the faces.
- Unit of Resistivity :
- Ohm-centimetre (Ω-cm)
- Ohm-metre (Ω-m)
Resistance of a given conductor depends on the following factors:
- Length of the conductor: The resistance of a given conductor is directly proportional to its length. R ∝ l
- Area of the conductor: The resistance of a given conductor is inversely proportional to its area of cross-section :
- [R ∝ \(\frac{1}{A}\)]
- Nature of the meterial and temperature of the material of the conductor: We have resistivity,
r = \(\frac{R A}{l}\)
Difference between Resistance and Resistivity :
| Resistance | Resistivity |
| i. Resistance of a conductor depends on the nature of the material of a conductor and also on its dimensions. | i. Resistivity of a conductor depends on the nature of the material of a conductor but independent of its dimension. |
| ii. Resistance is the property of a body. | ii. Resistivity is the property of the material of the body. |
| iii. Ohm-meter is an instrument for measuring resistance. | iii. Oi.m-meter is the unit of resistivity. |
| iv. Dimensional formula of resistance (R) = [ML2T-3A-2] | iv. Dimensional formula of resistivity (ρ) [ρ] =[ML3T-3A-2] |
Combination resistance: In different electri al circuit more than one resistance are connected together. This is known as resistance.
Series combination : A number of resistances are said to be in series combination if they are connected one after the other in such a way that the same current flows through all the resistance when same potential difference is applied across the combination.
r = r1 + r2 + r3
Parallel combination : A number of resistance are said to be connected in parallel if one end of all the resistances are joined together and the other ends joined to another point such that the potential difference across each resistance is equal to the applied pr ential difference across the combination.
Internal resistance of a cell : Internal resistance offered by the electrolyte and electrodes of a cell when electric current flows through the cell.
Joule’s Laws :
First Law : The amount of heat produced in a conductor in a given interval of time is proportional to the square of the current passed
H ∝ P2 (when R and t are kept constant)
Second Law : The amount of heat produced by a given current in a given time is proportional of the resistance of the conductor.
H ∝ R (when I and t are kept constant)
Third Law : The amount of heat produced in a given conductor by a given current is proportional to the time for which the current passes.
H ∝ t (when I and R are kept constant)
Combining the three laws, we have
H ∝ I2 R t
(when I, R and t vary)
or, H = \(\frac{I^2 R t}{J}\)
(J = mechanical equivalent of heat
= 4.2 joul/calorie.)
If I be in ampere, R in Ohm, t in second and H in calorie, then
H = \(\frac{I^2 R t}{4 \cdot 2}\)
or, H = 0.24 I2 R t calorie
Fuse : For the safety of the electrical gadgets one thin wire made of an alloy of lead (75 %) and tin (25 %) and which has high resistance and low melting is kept in an insulator box of china clay. The thin coil is called fuse wire. The wire is kept in series with the main circuit of the household electrical appliances.
Electric iron: In this a thin but long wire of nichrome is kept coiled and sandwitched betwen two sheets of mica. This is called the heating element of the iron. The element is kept inside two iron plates and the heating element is kept insulated from the external body of the iron. On passing current through the element it gets heated and the iron in turn becomes hot.
Electric heater: In this, a thin and long wire of nichrome, is kept coiled in the spiral grooves of a porcelain block. This block is kept in metallic case and there is arrangement for making electrical connection with two ends of the coil.
Electric bulb : A thin but long coiled coil (filament) of tungsten wire is introduced in evacualed glass blub. Sometimes the bulb is filed with some inert gases. Electric contact to the filament is made by two comparatively thick wires. The resistance of the filament wire is such that it may glow being strongly heated by the electric current.
If a bulb be marked as ‘220 V-60 W’ then we mean that the lamp should be used at a potential difference of 220 Volts and on doing so it will glow fully and electrical energy will be spent at the rate of 60 joules per second or a power of 60 W} will be required.
It should be remembered that although same current through the filament of the bulb and the line wires, the resistance of the filament wire being much larges, the heat (I2 R) produced in it is much larger and hence glows brightly. It may be further noted that they resistance of the bulb marked 200 V-100 W is less than another marked 200 V-60 W. As a result more current passes through the coils of the former bulb and more heat (I2 R) is produced in it thus glows more brightly.
Units of electrical power and energy :
Watt : The SI unit of electrical power is watt. A watt is the power expended when one ampere of current flows under a potential difference of one volt.
so placed that the current carrying wire lies between the palm and the needle and the palm always facing the wire and the fingers pointing in the direction of the current, then the outstretched thumb will give the direction of deflection of the north pole of the needle.
Magnetic field: The magnetic field at any point in the field is numerically equal to the force experienced by a unit charge moving with a unit velocity perpendicular to the direction of the magnetic field at that point.
Fleming’s left hand rule : If the first finger, the middle finger and the thumb of the left hand be stretched mutually perpendicular to each other in such a way that the first-finger points in the direction of the magnetic field and the middle-finger points in the direction of electric current, then the thumb gives the direction of the force acting on the conductor.
Ampere’s circuital Law : It states that the line integral of magnetic field around any closed path in vacuum is equal to absolute permeability times the total current enclosed by the path.
Moving coll galvanometer : A moving coil galvanometer is an instrument for detection and measurement of small electric current.
Ammeter : An ammeter is a low resistance galvanometer, which is used to measure current in a circuit.
Voltmeter : A voltmeter is a high resistance galvanometer used to measure the potential difference between two points of an electrical circuit.
Diamagnetic substances : Diamagnetic substances are those in which the individual atoms or molecules or ions do not possess any net magnetic moment of their own.
Paramagnetic substances: Paramagnetic substances are those in which each individual atom or molecule or ion has a net non-zero magnetic moment of its own and placing such a material in an external magnetic field, it tries to align the individual dipole moments in the direction of the magnetic field.
Barlow’s wheel: Action of the magnet on a current and how rotational motion can be produced due to this effect can be shown by the experiment on Barlow’s wheel.
Electric motor: The device or machine which converts the electrical energy into mechanical energy is known as Electric Motor.
Solenoid : If many turns an insulated wire would around a cylinder the resulting coil is known as solenoid.
An ammeter has a low resistance and as it is connected in series in a circuit, so it reads slightly less than the actual current.
While giving reading a small current always passes through a voltmeter, so it cannot be used for determining the emf of a cell. For which a potentiometer is used, where at balanced condition no current passes through the cell.
The resistance of an ideal ammeter is zero.
The resistance of an ideal voltmeter is infinity.
The ammeter is a low resistance instrument and connected in series in an electrical circuit.
The voltmeter is a high resistance instrument and connected in parallel across the circuit.
Ferromagnetic substances: Ferromagnetic substances are those in which each individual atom or molecule or ion has a non-zero magnetic moment, same as in a paramagnetic substance.
Magnetic flux: The magnetic flux through any surface held in a magnetic field is measured by the total number of magnetic lines of force crossing the surface.
Faradav’s laws of electromagnetic induction :
First Law : Whenever there is a change in the magnetic flux linked with a coil, an emf is induced in it and its emf lasts as long as the change in the magnetic flux continues.
Second Law : The magnitude of the emf induced in the coil is proportional to the rate of change of magnetic flux linked with the coil.
Lenz’s Law : The induced current will be in such a direction that it opposes the cause which produces it.
Faraday’s law as modified by Lenz’s law: The emf induced in a coil is given by the negative of the rate of change of magnetic flux linking with its turns.
Fleming’s right hand rule: If the first finger, central finger and thumb of the right hand be stretched in the mutually perpendicular directions such that the first finger points along the direction of the field and thumb be along the direction of motion of the conductor, then the central finger would give the direction of induced current. It is also called dynamo rule.
Eddy currents : Eddy currents, first discovered by Foucault in the year 1895, are the currents induced in the body of a conductor when the amount of magnetic flux linked with the conductor changes. After the name of its discoverer, it is also called Foucault current.
Alternating current : An electric current, magnitude of which changes with time and direction reverses periodically is called alternating current (a.c.).
I = I0 sin ω t or I = I0 cos ω t
where I is the instantaneous value of current i.e. the magnitude of the current at any instant of time t, I0 is the peak value or maximum value or amplidue of a.c. and ω is the angular frequency of a.c.
The terms used for alternating current hold equally for alternating e.m.f and may be represented as
E = E0 sin ω t or E = E0 cos ω t
where E and E0 are the instantaneous and maximum or peak value of the alternating emf respectively.
Advantages and disadvantages of a c. over d.c
Advantages of a.c. over d.c. :
- Generation, transmission, distribution of a.c. is more economical and convenient.
- A.C. can be controlled more effectively and easily with very little loss of power by using inductors.
- A.C. can be transmitted with very little loss of power using transformer.
- A.C. can be easily converted in d.c. by using rectifiers.
- Using transformers a.c. voltages can be stepped up or stepped down to any desired value.
Disadvantages of a.c. over d.c.
- A.C is more dangerous than d.c.
- A.C. cannot be used in electrolysis.
- Calibrations of A.C. meters for small measurements are difficult as the markings are not equidistant.
- Conduction of a.c. through metal wires suffer skin effect.
A.C. generator or Dynamo: An a.c. generator or dynamo is an device used to convert mechanical energy into electrical energy.
An a.c. generator works on the principle of electromagnetic induction i.e. when a coil is rotated in uniform magnetic field, an emf is induced in the coil.
The main components of a.c. generator are:
- Armature : It is a coil having a large number of turns of insulated copper wire wound on a soft iron core.
- Field magnet : A strong permanent magnet with cylindrical pole pieces is used as a field magnet. The uniform magnetic field produced by it is perpendicular to the axis of rotation of the coil.
- Slip rings : Two terminals of the armature coil are connected to two slip rings made of brass. These rings rotate with the coil.
- Brushes : There are two carbon brushes touching two slip rings and remain fixed while slip rings rotate along with the coil. The brushes are connected to the output through load.
Following Faraday’s laws of electromagnetic induction, the induced emf produced in the coil given by
E = E0 sin ω t
D.C. generator or Dynamo: A d.c. generator is similar, in many ways, to an a.c generator. In the internal circuit of a d.c. generator, current is produced in the same way as in the internal circuit of an a.c. generator but the current in the external circuit of a d.c. generator, is unlike that of an a.c. generator, direct and not alternating. The alternating current is converted into direct current by a commutator which is nothing but a pair of semicircular ring. These are called split rings.
Barlow’s wheel : The action of magnet on current and there by rotation of the conductor itself, is demonstrated by Barlow’s wheel.
Factors governing the speed of Rotation :
- Rotational speed increases with current and vice versa.
- Rotational speed increases with intensity of magnetic field and vice versa.
- If alternating current (ac) is used instead of direct current (dc), the wheel wire try to reverse it’s direction with the change in direction of current resulting no rotation.
Factors governing the direcion of Rotation :
The direction of rotation will reverse if direction of current is reverse keeping direction of magnetic field intact.
The direction of rotation will reverse if direction of magnetic field is reversed keeping direction of current intact.
The direction of rotation will remain unchanged if both direction of magnetic field and direction of current are reversed.
Electrical Switch: An electrical switch in any device used to interrupt the flow of electrons in a circuit.
Types of Switch :
Toggle switch : Toggle switches are actuated by a lever angled in one of two or more positions. The common light switch used in household wiring is an example of toggle switch.
Push button switch : Push button switches are two-position devices actuated with a button that is pressed and released most push button switches have an internal spring mechanism returning the button to its ‘out’ or ‘unpressed’ position for momentary operation.
Selector switch : Selector switches are actuated with a rotary knob or lever of some sort to select one of two or more positions.
Joystick swich : A joystick to actuated by a lever free to move in more than one axis of motion.
Lever actuator limit switch : These limit switches closely resemble rugged toggle or selector hand switches fitted with a lever pushed by the machine part.
Proximity switch : Proximity switch sense the approach of a metallic machine part either by a magnetic or high-frequency.
Speed switch : These switches sense the rotary speed of a shaft either by a centrifugal weight mechanism mounted on the shaft or by some kind of non-contact detection of shaft motion such as optically or magnetic.
Pressure switch : Gas or liquid
pressure can be used to actuate a switch mechanism if that pressure is applied to a piston, diaphram, or bellows with converts pressure to mechanical force.
Socket : Socket may refer to
In machanics
- Socket wrench : A type of wrench that uses separate, removable sockets to fit different sizes of nuts and bolts.
- Socket head screw: A screw (or bolt) with a cylindrical head containing a socket into which the hexagonal ends of an Allen wrench will fit.
- Socket termination : A termination used at the ends of wire rope.
In biology :
- Eye socket: A region in the skull where the eyes are positioned.
- Tooth socket: A cavity cantaining a tooth, in those bones that bear teeth.
- Dry socket: A painful opening as a result of the blood not clotting after a tooth is pulled.0
- Ball and socket joinf.
In computer networking :
- Networking socket : an end-point in a communication across a network or the internet.
- Unix domain socket : an end point in local inter-process communication.
Electrical and electronic connectors :
- AC power plugs and sockets : Electrical devices used to connect to a power source onto which another device can be plugged or screwd in.
- Antenna socket, a female antenna connector for television cable.
- CPU socket : The connector on a computer’s motherboard for the CPU.
- Jack (connector) : One of several types of electronic connectors.
- Lamp socket : A connector into which a light bulb screws.
Live wire, neutral wire: An electric current is a flow of electric charge. Most mains powered appliances need three wires to work safely. They are live, neutral and earth.
Only two of the wire are used when the appliance works properly. These are the live (brown) and the neutral (blue) wires. The live wire carries current to the appliance at a high voltage. The neutral wire completes the circuit and carries current away from the appliance. The third wire called the earth wire (green/yellow) is a sefely wire and connects the metal case of the appliance to the earth. This stops a fault making the case of the appliance live.
Earthing: If a fault occurs where the live wire connects to the case the earth wire allows a large current to flow through the live and earth wires. This overheats the fuse which melts and breaks the circuit appliance such as hairdryers are said to be ‘double insulated’ and there’s no need for an earth wire because the case is made of a non conducting plastic. If a faulty live wire touches the inside of the plastic case there’s little risk as the case is an insulator.
Main switch (Isolator): The 100 A rated main switch is a switch disconnector which can be used as an Isolator, and can be used for residential and light commercial applications.
The operating switch can be locked in the ON or OFF position, using a device lock. The main switch has a positive contact status indicator, i.e. when green window is visible this indicates that there is a 4 mm contact gap.
Diagram showing Domestic wiring : Generally the live wire is red, neutral wire is black and earth wire is green in domestic electrical wiring. All appliances are kept in parallel through a switch so as to ensure 220 volt across all appliances. Each appliance should be provided with separate switch.
