CBSE Class 10 Science Chapter 13 Magnetic Effects of Electric Current

Class Notes of Ch 13 : Magnetic Effects of Electric Current

Class 10th Science

Magnetic Effects of Electric Current

      Topics:

• Introduction
• Magnetic Field around a Bar Magnet
• Magnetic Field in a Straight Current Carrying Conductor
• Magnetic Field in a Circular Carrying Conductor
• Magnetic Field due to Current in a Solenoid
• Force acting on a Current carrying Conductor
• Electric Motor
• Electromagnetic Induction
• Faraday's Experiment-1
• Faraday's Experiment-2
• Faraday's Experiment-3
• Faraday's Conclusions
• Electric Generator
• AC Generator
• DC Generator
• Domestic Electric Circuits

Introduction

We use many appliances at home, like the mixers, grinders, fans etc which draw electricity and convert them to motor movement i.e. mechanical energy. Also, we know of turbines, windmills, generators etc which move a mechanical part to generate electricity. These are possible because when electric current flows through a wire, it produces a magnetic effect around it. So in this chapter, we will study about these interesting facts -  'Magnetic effects of Electric Current'.

Magnetic Field around a Bar Magnet

• A magnet always exerts a influence around the region surrounding it. This region is called the Magnetic Field.
• Magnetic Field has both direction and quantity.
• The fields always emerge out of the North pole and always merge into the South pole
• Inside the magnet, the field is from the south pole to the north pole, ie merge into the south pole and emerge out of the north pole.

• The strength of the magnetic field is determined by the closeness of the field lines.
• If the lines are closer and crowded, it means that the strength of the magnetic field is high and exerts a strong force on a magnet which is brought in its proximity.
• If the lines are farer and less crowded, it means that the strength of the magnetic field is relatively low and exerts a weaker force on a magnet which is brought in its proximity.
• When a magnetic compass is brought in the proximity of a bar magnet, it deflects and always points in the north-south direction.
• When iron filing are brought in the vicinity of a bar magnet, they arrange themselves along the field lines.
• The magnetic field lines are such that they never cross each other. If they did cross at a certain point, it means that at that point, the compass needle would point towards two directions, which is logically incorrect.

Magnetic Field in a Straight Current Carrying Conductor

• There are current carrying conductors in opposite directions. In both cases, the current carrying conductor is intercepted by a cardboard placed at right angles to the current carrying conductor.
• There are some iron fillings sprinkled on the conductor.
• When current flows through the conductor, the iron filing arrange themselves along the magnetic field.
• We can see that the magnetic field in both cases is in opposite directions as is the current.
• The magnetic field produced by a current-carrying straight wire depends inversely on the distance from it and directly on the current passing through it.
• From this we see that the current carrying conductor produces a magnetic field around it. The direction of this magnetic field is given by Right Hand Thumb Rule.

Right Hand Thumb Rule

Suppose that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of flow of current. Then, your fingers which wrap around the conductor indicate the direction of magnetic field lines. 

Magnetic Field in a Circular Carrying Conductor

• At every point of current-carrying circular conductor, the magnetic field is in the form of concentric circles as represented above.
• The circles are concentric in nature with increasing diameters as the move farther from the current carrying wire.
• At the centre of the circular loop, the circles appear like straight lines.
• The magnetic field produced by a current-carrying straight wire depends inversely on the distance from it and directly on the current passing through it.

The Right Hand Thumb Rule is applicable here at every point of the current carrying conductor.

Magnetic Field due to Current in a Solenoid

• A coil with many circular close turns of insulated copper wire (like a cylinder as shown above) is a solenoid.
• One end of such a solenoid behaves like the north pole and the other as a south pole.
• Therefore magnetic field due to current in the solenoid is similar to a bar magnet. The fields always emerge out of the North pole and always merge into the South pole
• The field inside the solenoid is uniform.
• The strong magnetic field inside the solenoid is so strong that it can be used to magnetize a piece of soft iron when it is placed inside the coil. The magnet formed like this is called a Electromagnet.

Force Acting on a Current Carrying Conductor

• An electric current flowing through a conductor produces a magnetic field. This field will exert a force on a magnet placed in the proximity of the conductor.
• This means that the magnet also will exert an equal and opposite force on the current-carrying conductor.
• The direction of this force is given by Left Hand Thumb Rule- Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular to each other (as shown in the figure). If the middle finger points in the direction of the current in the conductor, the forefinger points in the direction of the magnetic field and the thumb points in the direction of the force acting on the conductor.

• Devices that use this applications is electric motor, electric generator, loudspeakers, microphones and measuring instruments.

Electric Motor

Electric motor is a device that converts electrical energy to mechanical energy.

Parts of a Electric Motor

• Insulated Copper wire: A rectangular coil of wire ABCD
• Magnet Poles: A magnet as placed above ie North Pole and South Pole. This creates a magnetic field as shown above.
• Split Rings: Two disjoint C-shaped rings P and Q. It acts as a commutator (which can reverse the direction of current)
• Axle: The split rings are placed on the axle which can rotate freely.
• Brushes: The outside of the split rings are connected to conducting brushes X and Y.
• Source Battery: To source current.

Working

• When the current begins to flow, current flows through brush X, then A to B, B to C, C to D and then to brush Y and into the battery.
• Now applying Fleming's Left Hand Rule to wire AB, Current is along AB, Magnetic Field is as shown (North-> South), the motion of the wire is downwards.
• Now applying Fleming's Left Hand Rule to wire CD, Current is along CD, Magnetic Field is as shown (North-> South), the motion of the wire is upwards.
• The rectangular coil begins to move in the anti-clockwise direction
• Note that during anti-clockwise motion, the split rings and axle also move, whereas the brushes don't move.
• After half a rotation, Wire CD and Split ring Q moves to the left.  Wire AB and Split ring P moves to right. Brushes X and Y donot move.
• Now applying Fleming's Left Hand Rule to wire CD, Current is along DC. (Battery -> Split ring Q -> DC , Magnetic Field is as shown (North-> South), the motion of the wire is downwards.
• Now applying Fleming's Left Hand Rule to wire AB, Current is along BA. (Battery -> Split ring Q -> DC --> CB -> BA --> Split ring P) , Magnetic Field is as shown (North-> South), the motion of the wire is upwards.
• So, again the coil rotates in the anti-clockwise direction.
• The reversal of current in the coil results in the continous rotation of the coil. The reversal of current is achieved by the commutator rings.

Electromagnetic Induction

This process by which a changing magnetic filed in conductor induces a current in another conductor is called electromagnetic induction. The scientist Michael Faraday did many experiments in this field.

Faraday's Experiment-1

• Take a wire, a bar magnet and a galvanometer
• Move the magnet towards the coil of wire.
• The galvanometer moves to indicate a current in the wire.
• When the direction of the magnet is reversed, the current reverses (indicated by the galvanometer needle swaying in the opposite direction)
• When the speed of movement of the magnet changes, the galvanometer deflects faster.
• Conclusion : Moving a magnet towards a coil induces a current in the coil whose direction and magnitude is given by the galvanometer.

Faraday's Experiment-2

• Take a wire, a current carrying conductor/wire and a galvanometer
• Move the current carrying conductor towards the coil of wire. (Magnet is replaced by current carrying conductor)
• The galvanometer moves to indicate a current in the wire.
• Conclusion: Current is induced in a coil when a current carrying conductor is brought in its vicinity.

Faraday Experiment-3

• Take a wire, a current carrying conductor/wire and a galvanometer
• Donot move the current carrying conductor. Keep it stationary
• The galvanometer moves to indicate a current in the wire.
• In general, the current carrying conductor/magnet is called primary coil. The conductor in which the current is induced is called the secondary coil
• Conclusion: Relative motion between the current carrying conductor and the wire is not mandatory for inducing current in the wire.

Faraday's Conclusions

• As the current in the primary coil changes, the magnetic field associated with changes.
• Then the magnetic field associated with the secondary coil also changes. And this causes the current
• This process by which a changing magnetic filed in conductor induces a current in another conductor is called electromagnetic induction.
• The direction of the current is given by Flemings Right Hand Rule : Stretch the thumb, forefinger and middle finger of your right hand such that they are mutually perpendicular to each other (as shown in the figure). If the thumb points in the direction of motion of the conductor, the forefinger points in the direction of the magnetic field and then the middle finger points in the direction of the induced current.

Electric Generator

A electric generator is a circuit that converts mechanical energy into electrical energy.

Examples: Electric Generators , Turbines (for generation of hydroelectricity) etc, Windmills (for generation of electricity etc).

AC Generator

Parts of an AC Electric Generator

• Insulated Copper wire: A rectangular rotating coil of wire ABCD
• Magnet Poles: A magnet as placed above ie North Pole and South Pole. This creates a magnetic field as shown above. The rectangular coil is placed between these magnets
• Split Rings: Two disjoint C-shaped rings R1 and R2 are internally attached to the Axle.. Ends of the coil are connected to R1 and R2. The inner portion of these rins are made of non-conducting material
• Axle: The split rings are placed on the axle which is made to rotate freely from an external source.
• Brushes: The outside of the split rings are connected to conducting brushes B1 and B2. B1 and B2 is kept pressed on R1 and R2 respectively.
• Galvanometer : To measure current. The outer ends of the brushes are connected to the galvanometer to measure the current

Working

• The axle is rotated such that it moves in the clockwise directions that is AB moves up and CD moves down.
• According to Fleming's Right Hand rule, the induced current is setup in the coil along B1-> AB -> BC -> CD -> B2. This means that the external current flows from B2 to B1.
• After half a rotation, arm CD starts moves up and AB moves down.
• According to Fleming's Right Hand rule, the induced current is setup in the coil along B2-> AB -> BC -> CD -> B1. This means that the external current flows from B1 to B2.
• Thus after every half rotation of the coil, the current changes direction. This is called an AC current.
• AC current(Alternating current) : Changes its direction after equal intervals of time. It is easier to transmit this current over long distances due to lesser loses and hence this is the current that is supplied to our houses from the electricity department.

DC Generator

• The arrangement is the same as DC Motor except that the source battery is replaced with a galvanometer.
• The working is also the same.
• The brushes X and Y are fixed.
• Commutators (split rings) P and Q are used to get unidirectional flow of current. This is DC current
• DC current(Direct current): Does not change direction with time. Eg: Current from a simple battery/cell.

Domestic Electric Circuits

• We receive power in our house through a main supply, commonly called mains.
• It is supplied through overhead cables or underground cables.
• There are 3 types of wires in domestic circuits : Earth Wire, Live Wire, Neutral Wire.
  • Earth wire: It is generally green in color. It is usually connected to a metal plate placed in the earth near the house as a safety measure to ground gadgets that have a metallic body. (refrigerator, toaster). In case of charges leaking on to the metallic body, the charges gets grounded and thus prevent shocks.
  • Live wire: Positive wire generally red in color
  • Neutral wire: Negative wire generally black in color
• The potential difference (or voltage) supplied in our country is 220V
• When they come into our houses, they pass through a circuit called a Fuse. Whenever there is a high voltage, voltage fluctuation, overloading, short circuit the fuse melts and prevents the high voltage reaching the electric appliance. This saves the electrical gadget.
• Then through the metre board in the house, these wires pass on to different electric gadgets
• Generally 2 types of electric circuits are used at homes
  • 15A : Appliances which have higher power ratings. (geysers, refrigerators, ACs)
  • 5A: Appliances which have lower power ratings. (TV, bulbs, fans).

SCIENCE Revision Notes

Chapter:01  Chemical Reaction & Equation
Chapter:02  Acid Base & Salt
Chapter:03  Metals & Non Metals
Chapter:04  Carbon & its Components
Chapter:05  Periodic Classification of Elements
Chapter:06  Life Processes
Chapter:07  Control & Coordinates
Chapter:08  How do Organisms Reproduce
Chapter:09  Heridity & Evolution
Chapter:10  Light Reflection Refraction
Chapter:11  The Human Eye & the Colourful World
Chapter:12  Electricity
Chapter:13  Magnetic Effect of Electric Current
Chapter:14  Source Of Energy
Chapter:15  Our Environment
Chapter:16  Management of Natural Resource

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