Concepts to be Covered
In this video, we will revise thirteenth chapter of Science – Magnetic effect of electric current.
In this video, we will cover the following concepts:
- Introduction to magnets and magnetic field
- Magnetic field lines and their properties
- Magnetic field due to a current carrying straight conductor
- Right and thumb rule
- Current carrying circular loop and solenoid
- Force on a current carrying conductor in a magnetic field and Fleming’s left hand rule
- Electric Motor – construction and working
- Electromagnetic Induction and Fleming’s right hand rule
- Electric Generator – construction and working
- Domestic electric circuits
- Previous Years’ Questions
1. Introduction to magnets and magnetic field
Substances which have the property of attracting small pieces of iron, nickel, cobalt, etc., are called magnets. A magnet has two poles – north pole and south pole.Like poles repel, while unlike poles of magnets attract each other.
The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a magnetic field. It has both magnitude and direction.
2. Magnetic field lines and their properties
A magnetic line of force is defined as the path along which free north pole tends to move if kept freely.
Properties of Magnetic Field Lines:
- These are closed curves
- Direction – Inside the magnet – from south pole to North pole
- Outside the magnet – from north pole to south pole.
- No two magnetic lines of force can intersect each other.
- The relative closeness of the lines of force gives a measure of the strength of the magnetic field which is maximum at the poles.
3. Magnetic field due to current carrying straight conductor
If we bring a magnetic needle near a current carrying conductor, it shows deflection. This shows that the current carrying wire is associated with a magnetic field. The magnetic field lines around a straight current carrying conductor are concentric circles as shown in this diagram.
Strength of this magnetic field increases with increase in the current through the conductor. And decreases with increase in the distance with conductor.
4. Right hand thumb rule
The direction of magnetic field associated with a current-carrying conductor can be found by right hand thumb rule.
If we hold a current – carrying conductor in our right hand such that the thumb points in direction of the current, then the direction in which the fingers encircle, gives the direction of magnetic lines.
5. Current carrying circular loop and solenoid
The direction of magnetic field lines around a current carrying circular loop is shown in the diagram. The lines form concentric circle at every point of the loop but would appear to be a straight lines at the centre of the loop. The direction of magnetic field at any point can be found by Right hand thumb rule.
Magnetic Field Lines in a circular current carrying conductor
On similar lines, we now study what is called a solenoid.
A solenoid is a long cylindrical coil of insulated copper wire consisting of large number of circular turns.
A solenoid behaves like a magnet when electric current passes through it. Its magnetic field pattern is shown in the figure.
Magnetic Field Lines in a Solenoid
The field lines inside the solenoid are in the form of parallel straight lines. This indicates the field is uniform inside the solenoid.
Strength of an electromagnet
A soft iron core placed inside a solenoid behaves like a powerful magnet when a current is passed through the solenoid. This device is called an electromagnet.
These are the factors on which the strength of an electromagnet depends:
- Number of turns in the coil: The larger the number of turns in the coil, greater is the strength of the electromagnet
- Strength of the current: the larger the amount of current passed through the solenoid, more powerful is the electromagnet
- Nature of the core material: Some substances produce stronger electromagnet while some do not. For example the core of the magnetic material like soft iron increases the strength of the electromagnet.
Force on a current carrying conductor in a magnetic field and Fleming’s left hand rule
An electric current flowing through a conductor produces a magnetic field, this field exerts a force on a magnet placed near the conductor the direction of this force is found out by Fleming’s left hand rule.
Fleming’s left hand rule –
It states that if the forefinger, thumb and middle finger of left hand are stretched mutually perpendicular and the forefinger points along the direction of external magnetic field, middle finger indicates the direction of current, then thumb points along the direction of force acting on the conductor.
7. Electric Motor
An electric motor is a rotating device used for converting electric energy into mechanical energy.
Let us now understand its construction and working.
- An electric motor consists of a rectangular coil, lets say ABCD of copper wire.
- This coil is placed between the two poles of a magnetic field such that the arms AB and CD are perpendicular to the direction of the magnetic field.
- The ends of the coil are connected to the two halves P and Q of a split ring.
- The inner sides of these halves are attached to an axle.
- The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively.
- Initially, current in the coil ABCD enters brush X and flows back to the battery through brush Y. Thus, the current in arm AB flows from A to B.
- In arm CD, the current flows from C to D, that is, opposite to the direction of current through arm AB.
- On applying Fleming’s left hand rule, we find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards.
- Thus the coil and the axle O, rotate anti-clockwise.
- After half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA.
- The reversal of current also reverses the direction of force acting on the two arms AB and CD.
- Thus now arm AB is now pushed up and the arm CD is now pushed down.
- Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.
8. Electromagnetic Induction
The phenomenon of generation of an electric current in a closed circuit due to changing magnetic field is called electromagnetic induction.
To study this in detail, you can watch the video by clicking the card shown on your screen.
A similar phenomena is also seen when you have two coils placed near each other. When current in one coil is varied, the magnetic field line associated with second coil change and a current is induced in it.
Fleming’s right hand rule –
The direction of induced current in a conductor is given by Fleming’s right hand rule.
It states that if we stretch the thumb, forefinger and the middle finger of right hand at right angle to one another in such a way that the forefinger points in the direction of magnetic field and the thumb points in the direction of motion of conductor then the middle finger will point in the direction of induced current.
9. Electric Generator
It is a device which converts mechanical energy into electrical energy.
Let us now study the construction and working of an AC generator.
AC Generator – Construction
- An electric generator consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet.
- The two ends of the coil are connected to the two rings R1 and R2.
- The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively.
- The two rings R1 and R2 are internally attached to an axle, which may be mechanically rotated from outside to rotate the coil inside the magnetic field.
- Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.
AC Generator – Working
- As the coil rotates, then two rings also rotate with the coil.
- The brushes B1 and B2 keep the contact with the rotating ring when the coil starts rotating with arm AB moving up and CD moving down (clockwise) cutting the magnetic lines.
- According to Fleming’s right hand rule, current is induced in these arms in the direction ABCD.
- After half rotation, arm CD moves down and AB moves up. As a result, the direction of current in each segment changes giving rise to net induced current in the direction DCBA.
- Thus, after every rotation, polarity of current in the respective arm changes, thereby generating an alternating current.
Note – To get a direct current, a split-ring type commutator must be used.
Domestic Electric Circuits
In our homes, we receive supply of electric power through a main supply.
This is a schematic diagram of an domestic electric circuit –
There are mainly three wires in the domestic electric circuits-
- Live wire – usually with red insulation cover.
- Neutral wire – with black insulation.
- Earth wire – has insulation of green colour, is usually connected to a metal plate deep in the earth near the house.It ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.
Two other important terms are overloading and short-circuiting-
- Sometimes, due to defective or damaged wiring, the live and neutral wires come in direct contact, which leads to short circuiting. Due to this, the resistance of the circuit becomes almost zero and an extremely large current flows through the circuit, resulting in overloading.
- It may also occur when a large number of high power appliances are switched on simultaneously or by connecting too many appliances to a single socket.
- Due to an accidental hike in the supply voltage. Fuse wire is placed in series with any electrical appliance in an electric circuit, it saves the appliances from being damaged. If the current exceeds the maximum allowable value, then the fuse wire melts and breaks the circuit.
Previous Years' Questions
Let us now understand this chapter’s weightage.
Generally, 2, 3 or 5 marks questions are asked from this chapter. We advise you to remember both fleming’s right hand rule and left hand rule properly. And also, questions related to solenoid and working and construction of electric motor and electric generator are very important. Make sure you revise them properly before the exam.
Let us now look at all the previous year questions asked from this chapter.
1 Mark Questions
1. When is the force experienced by a current-carrying conductor placed in a magnetic field the largest? [CBSE, 2009]
2. State the rule which gives the direction of magnetic field associated with a current-carrying conductor. [CBSE, 2009]
3. A charged particle enters at right angles into a uniform magnetic field as shown. What should be the nature of charge on the particle of it begin to move in a direction pointing vertically out of the page due to its interaction with the magnetic field? [CBSE 2010]
2 Marks Questions
1. What is meant by the term, ‘magnetic field’? Why does a compass needle show deflection when brought near a bar magnet? [CBSE, 2008]
2. What is a solenoid? Draw the pattern of magnetic field lines of a solenoid through which a steady current flows. What does the pattern of field lines inside the solenoid indicate? [CBSE, 2010]
3. A coil of insulated wire is connected to a galvanometer. What would be seen if a bar magnet with its north pole towards one face of the coil is
- moved quickly towards it,
- moved quickly away from the coil and
- placed near its one face?
Name the phenomena involved. [CBSE, 2010]
5 Marks Questions
1. Explain the meaning of the word ‘electromagnetic’ and ‘induction’ in the term electromagnetic induction. On what factors does the value of induced current produced in a circuit depend? Name and state the rule used for determination of direction of induced current. State one practical application of this phenomenon in everyday life. [Sample Paper 2010]
2. With the help of a labelled circuit diagram wire describe an activity to illustrate the pattern of the magnetic field lines around a straight current carrying long conducting wire .Name the rule that is used to find the direction of magnetic field associated with a current carrying conductor.
Is there a similar magnetic field produced around a thin beam of moving
(a) alpha particles and (b) neutrons? Justify your answer. [Sample Paper 2017]
To download all previous years’ questions, follow the link given in description.
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