Electric current and its effects
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Transcript of Electric current and its effects
Topics covered
Electric ChargeElectric CurrentElectric PotentialElectrical ResistanceOhm’s LawSeries and parallel connection of resistorsElectric PowerElectric Energy & its commercial UnitHeating Effects of Electric Current
STATIC ELECTRICITY
TYPES OF ELECTRIC CHARGES
There are two types of Electric Charges:
Positive Charges
Negative Charges
By Convention,
The charge acquired by a glass rod is called Positive Charge
The charge acquired by an ebonite rod is called Negative Charge
Opposite charges (or Unlike charges) attract each other.
Similar charges (or Like charges) repel each other.
SI UNITS OF CHARGE
SI Unit of Charge is Coulomb, denoted by COne Coulomb is that quality of electric charge which exerts a force of
9 * 109 N on an equal charge placed at a distance of 1 metre from it.
Charge possessed by an electron is 1.6 * 10-19 Coulombs
Conductors & InsulatorsSubstances through which electricity can flow are called Conductors.
Ex: Silver, Copper and Aluminium, etc
Those Substances through which electric charges cannot flow, are called Insulators.
Ex: Glass, Ebonite, Rubber, most plastics, Paper, Dry wood, Cotton, Mica, Bakelite, Porcelain, and Dry Air
What makes Substances Conduct Electricity?
The presence of Free Electrons in a substance makes it a conductor.
The electrons present in insulators are strongly held by the nuclei of their atoms.
So no free electrons in an insulator which can move from one atom to another, an insulator does not allow electric charges to flow through it.
Static Electricity ?
Electric Potential
What happens when a positive charge is placed in the electric field due to another charge?
Electric Potential
Electric potential at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point.
Potential Difference
The difference between two points in an electric circuit is defined as the amount of work done in moving a unit charge from one point to the other
point.
Potential difference = Work done / Quantity of Charge moved
Heating Effects of Electric Current
When an electric current is passed through a high resistance wire, like nichrome wire, the resistance
wire becomes very hot and produces heat.
This is called as the Heating effects of Electric current
MAGNETIC EFFECTS OF ELECTRIC CURRENT
Learning ObjectivesMagnetic Field and Magnetic Field LinesMagnetic Field of EarthElectromagnetismMagnetic field patterns produced by current
carrying Conductors of various shapesElectromagnetDifference between Bar Magnet & an
ElectromagnetMagnetism in Human Body
Learning ObjectivesForce on a current carrying conductor placed in a
magnetic fieldKicking Wire ExperimentFleming’s Left Hand RulePrinciple and working of Electric MotorElectromagnetic Induction Fleming’s Right-Hand RuleElectric Generator - AC & DC GeneratorsDomestic Electrical Wiring
Magnetic Effect of Electric Current
“An electric current flowing in a wire produces a magnetic field around it”
Electric Current produces Magnetism
Magnet & Magnetic Field
A magnet is an object which attracts pieces of iron, steel, nickel and cobalt
The space around a magnet in which magnetic force is exerted, is called a
magnetic field
Magnetic Field LinesThe lines drawn in a magnetic field along which a
north magnetic pole would move
The magnetic field lines are also known as magnetic lines of force
The magnetic field lines always begin from the N-pole of magnet and end on the S-pole of the
magnet
DEMONSTRATION ON MAGNETIC FIELD LINES
Properties of Magnetic Field Lines
The magnetic field lines originate from the North pole of a magnet and end at its south pole
The magnetic field lines come closer to one another near the poles of a magnet but they are widely separated at other places
The magnetic field lines do not intersect one another
Magnetic Field of Earth
A freely suspended magnet always points in the north-south direction even in the absence of any other magnetic field.
This suggests that the earth itself behaves as a magnet which causes a freely suspended magnet to point always in a particular direction
The shape of the earths magnetic field resembles that of an bar magnet
The axis of Earth’s Magnetic Field is inclined at an angle of about 150 with the geometrical axis
The Earth’s magnetic Field is due to the Magnetic Effect of Current, which is flowing in the liquid
core at the centre of the earth
Thus, Earth is a huge electromagnet
Why Earth’s Magnetic Field
Questions1. State any two properties of Magnetic Field lines.2. (True/False) The axis of earth’s imaginary magnet and the geographical axis coincide with each other3. Where do the manufacturers use a magnetic strip in the refrigrator? Why is this magnetic strip used?4. Why does a compass needle get deflected when brought near a bar magnet?
ELECTROMAGNETISM
MAGNETIC EFFECTS OF CURRENT
ELECTROMAGNETISMThe magnetic effect of current was discovered by oersted in 1820Oersted found that a wire carrying a current was able to deflect a
compass needleCompass needle is a tiny magnet which can be deflected only by a
magnetic fieldSince, a current carrying wire was able to deflect a compass
needle, it was concluded that a current flowing in a wire always gives rise to a magnetic field around it
ELECTROMAGNETISMThe importance of magnetic effect of electric current lies in the
fact that it gives rise to mechanical forces
The Electric Motor,Electric Generator,Telephone and Radio
All the above devices utilise the magnetic effects of electric current
The magnetic effects of electric current is also called electromagnetism which means electricity produces magnetism
MAGNETIC FIELD PATTERNS
By current-carrying conductors of various shapes
Magnetic Field due to straight current carrying wire
Magnetic Field due to straight current carrying wire
The magnetic field lines around a straight conductor carrying current are concentric circles
whose centres lie on the wire
The magnitude of magnetic field produced by a straight current carrying wire at a given point is:
Directly proportional to the current passing in the wire, and Inversely proportional to the distance of that point from the wire
Important observationsGreater the current in the wire, stronger will be
the magnetic field producedGreater the distance of a point from the wire
current carrying wire, weaker will be the magnetic field produced at that point
As we move away from a current-carrying straight wire, the concentric circles around it representing magnetic field lines, becomes larger and larger indicating the decreasing strength of the magnetic field
Right-Hand Thumb Rule(Maxwell’s Right-Hand Thumb Rule)
Direction of Magnetic Field for a straight current carrying conductor
Imagine that you are holding a current carrying conductor in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire
Maxwell’s Right-Hand thumb rule is also known as Maxwell’s corkscrew rule
Magnetic Field Pattern due to Circular Loop
(Circular Wire)
Important ObservationsIt has been found that the magnetic effect of current increases if
instead of using a straight wire, the wire is converted into a circular loop
Magnetic field lines are circular near the current carrying loop, as we move away the concentric circles representing magnetic field lines become bigger and bigger
At the centre of the circular loop, the magnetic field lines are straight
At the centre of the circular loop, all the magnetic field lines are in the same direction and aid each other, due to which the strength of magnetic field increases
Important Observations
The magnitude of magnetic field produced by a current-carrying circular loop at its centre is:
Directly proportional to the current passing through the circular loop and
inversely proportional to the radius of circular loop
How to increase the strength of the magnetic field in a circular coil ?
The strength of the magnetic field can be increased by taking a circular coil consisting of a number of turns of
insulated copper wire closely wound together.
If there is a circular coil having n turns, the magnetic field produced by this current-carrying circular wire will be n times as large as that produced by a circular loop of
a single turn of wire.
Important ObservationsThe strength of a magnetic field produced by a circular coil carrying current is directly proportional to both, number of turns (n) and current (I); but inversely proportional to its radius (r).
The Strength of the magnetic field produced by a current-carrying circular coil can be increased:
by increasing the number of turns of wire in the coil,by increasing the current flowing through the coil, andby decreasing the radius of the coil
Clock Face RuleA Current-carrying circular wire behave like a thin disc magnet whose one face is a north pole and the other face is a south pole.
According to Clock face Rule
If the current in the circular wire is in the clockwise direction, then that face of the circualar wire will be South pole
If the current around the face of circular wire flows in the anticlockwise direction, then that face of the circular wire will be North pole
Magnetic Field due to a Solenoid
A solenoid is a coil containing a large number of close turns of insulated
copper wire
Magnetic Field due to a Solenoid
The magnetic Field produced by a current carrying Solenoid is similar to the magnetic field produced
by a bar magnet
Important ObservationsThe lines of the magnetic field pass through the Solenoid and
return to the other endThe magnetic field inside the Solenoid are in the form of parallel
straight linesThis indicates that the strength of the magnetic field is same at all
points inside the solenoidIf the strength of the magnetic field is uniform just same in a
region, it is said to be uniform magnetic fieldThus, magnetic field is uniform inside a currnet carrying Solenoid
Important ObservationsThe strength of the magnetic field produced by a current carrying Solenoid depends on:
The number of turns in the Solenoid. Larger the number of turns in the Solenoid, greater will be the magnetism produced
The strength of current in the Solenoid. Larger the current passed through the Solenoid, stronger will be the magnetic field produced
The nature of core material used in the Solenoid. The use of Soft Iron rod as core in a Solenoid produces the strongest magnetism
ElectromagnetAn electric current can be used for making
temporary magnets known as electromagnets.An electromagnet works on the magnetic effect of
current.
Electromagnet
An Electromagnet is a magnet consisting of a long coil of insulated copper wire wrapped around a soft iron core that is magnetised only when electric current is passed
through the coil
The core of the Electromagnet should be Soft iron because soft iron loses all of its magnetism when current
in the coil is switched off
Bar magnet vs ElectromagnetBar magnet is a permanent
magnetA permanent magnet produces a
comparatively weak force of attraction
The strength of a permanent magnet cannot be changed
The polarity of a permanent magnet is fixed and cannot be changed
An electromagnet is a temporary magnet. Its magnetism is only for the duration of current passing through it
An Electromagnet can produce very strong magnetic force
The strength of the electromagnet can be changed by altering the current
The polarity of an electromagnet can be changed by changing the direction of current in its coil
Permanent magnets are usually made of alloys such as:
Carbon steel, Chromium Steel, Cobalt Steel, Tungsten Steel and Alnico
Alnico is an alloy of aluminium, nickel, cobalt and iron
Permanent Magnets
Magnetic Fields in Human Body
Extremely weak electric currents are produced in the human body by the movement of charged particles called
ions. These are called ionic currents.
These create temporary magnetism in the human body. The magnetism produced in the human body is very very
weak compared to the earth’s magnetic field.
Magnetism in Human Body
The two main organs of the human body where the magnetic field produced is quite significant are the
heart and the brain
This magnetism forms the basis of a technique called Magnetic Resonance Imaging (MRI) which is
used to obtain internal parts of our body.
Force on current carrying conductor placed in a Magnetic
Field
Observations from Oersted’s Experiment
A current-carrying wire exerts a force on a compass needle and deflects it from its usual north-south position
A current-carrying wire exerts a mechanical force on a magnet, and if the magnet is free to move, this force can produce a motion in the magnet
A magnet exerts a mechanical force on a current-carrying wire, and if the wire is free to move, this force can produce a motion in the wire
This can be obtained by applying Newton’s Third Law
Why Force being acted upon a Current carrying wire in a magnetic
field ?
Faraday’s Observation in 1821
When a current-carrying conductor is placed in a magnetic field, a mechanical force is exerted on the
conductor which can make the conductor move
This is the Working Principle of a Motor
Important Observations●When a current-carrying conductor is placed in a magnetic field,
a mechanical force is exerted on the conductor which makes it move
●The direction of force acting on a current-carrying conductor placed in a magnetic field is:
i. Perpendicular to the direction of currentii. Perpendicular to the direction of magnetic field
Important point:● Maximum force is exerted on a current-carrying wire only
when it is perpendicular to the direction of magnetic field.
● No force acts on the current-carrying wire when it is parallel to the magnetic field
Important ObservationsThe direction of force on a current-carrying conductor placed in a
magnetic field can be reversed by reversing the direction of current flowing in the conductor
If the direction of current in a conductor and the direction of magnetic field(in which it is
placed), are known, then the direction of force acting on the current-carrying conductor can be found out by using Fleming’s left-hand rule.
Fleming’s Left Hand Rule
Statement
Hold the forefinger, the centre finger and the thumb of your left hand at right angles to one
another. Adjust your hand in such a way that the forefinger
points in the direction of magnetic field and the centre finger points in the direction of current in
the conductor, then the direction in which the thumb points gives the direction of motion of the
conductor
ELECTRIC CURRENT
ALTERNATING CURRENT (AC)DIRECT CURRENT (DC)
Sources of DC & ACDC
BatteriesThermocouplesSolar CellsDynamo
Direct current is also known as Galvanic
CurrentAll the electronic devices
work on Direct Current only
ACAC Generators
All Electrical devices work on AC Supply
We receive AC supply
Alternating Current can be transmitted over long distances without much loss of electrical
energy
ADVANTAGES OF AC OVER DC
The Electric MotorPrinciple, Construction, Working &
Applications
THE ELECTRIC MOTOR
A Motor is a device which converts Electrical energy into Mechanical energy
Principle of a DC Motor
Principle of a MotorAn electric motor utilises the magnetic effect of
current.A motor works on the principle that when a
rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil
which rotates it continuously.
When the coil rotates, the shaft attached to it also rotates.
In this way the electrical energy supplied to the motor is converted into the mechanical energy of
rotation
Construction of a DC Motor
Construction of a Motor1. An electric motor consists of a rectangular coil ABCD of insulated copper wire, which is
mounted between the curved poles of a horseshoe type permanent magnet M in such a way that it can rotate freely between the poles N and S on the shaft.
2. The coils AB and CD of the coil are kept perpendicular to thedirection of magnetic field between the poles of the magnet.
3. A device which reverses the direction of current through acircuit is called a commutator (or split ring).
4. The two ends of the coil are soldered permanently to thetwo half rings X and Y of a commutator.
5. A commutator is a copper ring split into two parts X and Y,these two parts are insulated from one another and mountedon the shaft of the motor.
6. The commutator rings are mountedon the shaft of thecoil and they also rotate when the coil rotates
7. The function of commutator rings is to reverse the direction of current flowing through the coil afterevery half rotation of the coil.
8. To pass in electric current to the coil, we use two carbon strips P and Q known as brushes.9. The battery to supply current to the coil is connected to the two carbon brushes P and Q through a switch.10. The function of carbon brushes is to make contact with the rotating rings of the commutator and through them to supply current to the coil.
Construction of a Motor
Working of a DC Motor1.When an electric current is passed into the rectangular coil, this current
produces a magnetic field around the coil.2.The magnetic field of the horseshoe-type magnet then interacts with the
magnetic field of the current-carrying coil and causes the coil to rotate continuously.
APPLICATIONS OF ELECTRIC MOTOR
Every motor has a shaft or spindle which rotates continuously when current is passed into it.(Into
current carrying conductors)Electric Motor is used in:
Electric FansWashing MachinesRefrigeratorsMixer and GrinderElectric Cars & many other applications
Lets Test Our Memory1.What happens when a current-carrying conductor is placed in a
magnetic field?2.When is the force experienced by a current-carrying conductor placed in
a magnetic field largest?3.In the statement of Fleming’s Left Hand Rule, what do the following
represent?i. Direction of centre fingerii. Direction of forefingeriii. Direction of thumb
4. Name the device which converts electrical energy into mechanical energy?5. What is the role of split rings in an electric motor?6. What is the function of commutator in an electric motor?
ELECTROMAGNETIC INDUCTION
Electricity from Magnetism
ELECTROMAGNETIC INDUCTION
The production of Electricity from Magnetism is called Electromagnetic Induction
Notes➢The current produced by moving a straight wire in a magnetic
field is called Induced Current➢The phenomenon of Electromagnetic Induction was discovered
by a British scientist Michael Faraday and an American scientist Joseph Henry independently in the year 1831
➢The process of Electromagnetic Induction led to the construction of generators for producing electricity at power stations
➢A Galvanometer is an instrument which can detect the presence of electric current in a circuit
When an electric current passes through the Galvanometer, then its pointer deflects either to the left side of zero mark or to the right side of the zero mark, depending on the direction of current.
Observations of Faraday & Henry about EMI
➢A current is induced in a coil when it is moved relative to a fixed magnet➢A current is also induced in a fixed coil when it is moved relative to the
fixed coil➢No current is induced in a coil when the coil and magnet both are
stationary relative to one another➢When the direction of motion of coil is reversed, the direction of current
induced in the coil also gets reversed➢The magnitude of current induced in the coil can be increased :
○by winding the coil on a shaft iron core○by increasing the number of turns in the coil○by increasing the strength of magnet, and○by increasing the speed of rotation of coil (or magnet)
Fleming’s Right-Hand Rule
For the direction of Induced Current
Fleming’s Right-Hand Rule
StatementHold the thumb, the forefinger and the centre finger of your right-hand at right angles to one
another.
Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor
The electrical generator is a machine for producing electric current or electricity.
This electric generator converts mechanical energy into electrical energy
Example
Electric Generator
Dynamo
Principle of Electric Generator
Electric Generator is an application of EMI
GENERATORS
AC GENERATORDC GENERATOR
The difference between DC & AC Generators is in their construction
Domestic Electric CircuitsDomestic Wiring
At the end of Sources of Energy