12. Magnetic Effects of Electric Current

Magnetic field : The area around a magnet, in which the magnetic force can be detected, is called magnetic field.
Magnetic field is a vector quantity because magnetic field has both direction and magnitude.
The unit of magnetic field strength is oersted.
Magnetic field lines - The imaginary lines representing the magnetic field around a magnet are called magnetic field lines.
Properties of magnetic field lines.
(i) The magnetic field lines emerge from north pole and merge at the south pole
(ii) The direction of the magnetic field lines inside the magnet is from it
s south pole to the north pole.
(iii) The magnetic field lines are a closed curve.
(iv) Two magnetic field lines do not intersect each other.
If two magnetic field lines intersect each other, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
(v) Where the magnetic field lines are relatively closer, the magnetic field is stronger. Since the magnetic field lines are denser near the poles, a magnet is more powerful near the poles

Magnetic field due to a current carrying conductor.
Electric current carrying conductors can be of two types:
(i) straight conductor
(ii) circular loop conductor.
Magnetic Field due to a Current through a Straight Conductor
The magnetic field lines around a current carrying straight conductor are in the form of concentric circular rings around a conductor.

Magnetic Field due to a Current through a Circular Loop
When electric current is passed through a circular coil of wire, a magnetic field is produced in the form of concentric circles near the loop, the direction of which can be given by the right-hand thumb rule. At every point of current carrying circular loop, the concentric circles representing the magnetic field around it becomes larger and larger as we move away from the wire. At the center of the loop, the magnetic field appears as a straight line.
This magnetic field is proportional to the number of turns and the current flowing in the wire of the circular loop.

Right-hand thumb rule :
Maxwell's right-hand thumb rule is used to find the direction of the magnetic field associated with a current-carrying conductor.
According to this rule, when a current-carrying conductor is holding the right hand in such a way that the thumb is towards the direction of the current, then the direction of the bent fingers expresses the direction of the magnetic field.
It is also called Maxwell's cork screw rule.

Maxwell's clockwise screw rule - According to this rule, when a screw is rotated clockwise in such a way that the tip of the screw moves in the direction of the electric current, then the direction of rotation of the screw expresses the direction of the magnetic field.
This rule is used to determine the direction of magnetic field around a straight current carrying conductor.
Magnetic Field due to a Current in a Solenoid
Solenoid : A coil of many circular turns of insulated wire wrapped closely in the shape of a cylinder is called a solenoid.

When electric current is passed through a solenoid, the solenoid behaves like a magnet. One end of the solenoid behaves as a magnetic north pole, while the other behaves as the south pole.
The magnetic field lines inside the solenoid are in the form of parallel lines.
This indicates that the magnetic field is the same at all points inside the solenoid. That means there is a uniform magnetic field inside the solenoid.
The strong magnetic field produced inside the solenoid can be used to make a magnet by placing a magnetic material, such as soft iron, inside the solenoid. The magnet so formed is called an electromagnet.
An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire.
Force on a Current Carrying Conductor in Magnetic Field
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. The direction of force acting on a current-carrying wire placed in a magnetic field is perpendicular to the direction of current, and the direction of magnetic field. The direction of this force can be determined by Fleming's left-hand rule.
The force acting on a current carrying conductor in a magnetic field is due to interaction between magnetic field due to current carrying conductor and external magnetic field in which the conductor is placed.
The maximum force is exerted on a current-carrying conductor only When the direction of current is at right angles to the direction of the magnetic field
No force acts on a current-carrying conductor when it is parallel to the magnetic field.
The direction of force on a current-carrying conductor placed in a magnetic field can be reversed by reversing the direction of magnetic field or the direction of current flowing in the conductor.

Fleming's Left Hand Rule :
According to Fleming's left hand rule, If the forefinger , thumb and middle finger of left hand are stretched mutually perpendicular to each other , such that the forefinger points the direction of magnetic field and middle finger points the direction of current, then the thumb points the direction of the force applied on the conductor.

Home Ectrical Circuit : We receive supply of electrical power in our homes through mains wires. These main wires come to our homes either through the help of electric poles or underground cables. One of the wires in this supply, which usually has a red insulating cover, is called the positive wire. The other wire which has a black insulating cover, is called neutral wire negative wire. In our country there is a potential difference of 220 V between these two wires.
These wires pass into an electricity meter through a main fuse in the house.These are connected to the line wires of the house through the main switch.Through the main switch they are connected to the line wires in the house. Generally two separate circuits are used in houses. One circuit is for 15 A current rating which is used for high power electrical appliances like geysers, air conditioners/coolers etc. The second circuit is for 5 A current rating which is used for bulbs, fans etc.

Earth Wire : The earth wire usually has a green insulating cover. It is connected to a metal plate deep in the earth near the house. This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc.
This ensures that in case of any leakage of electric current in the metallic cover of the appliance, the potential of that appliance becomes equal to the ground potential, as a result the person using this appliance remains safe from severe electric shock.
Electric fuse - The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unnecessary high electric current.
Short-circuiting : When the insulation of the wires is damaged or there is a fault in the appliance, then live wire and the neutral wire come into direct contact. In such a situation, the current in the circuit suddenly increases . This is called short-circuiting.
Overloading - When too many appliances are connected to a single socket, the electric circuit draws more current than the permitted value. This is called Overloading
Overloading can also occur due to an accidental increase in the supply voltage.

Mention the special feature regarding shape of magnetic field lines.
Magnetic field lines are continuous closed curve.
What type of core is used to make an electromagnet?
Soft iron core is used in making an electromagnet.
What does the thumb indicated in Fleming’s left hand rule?
Thumb indicate the direction of force on conductor/motion of the conductor.
State the frequency of the power supply generated in India.
50 Hz
Name two safety measures commonly used in electric circuits and appliances.
Proper earthing and using a fuse in the electric circuit.
Mention the angle between a current carrying conductor and magnetic field for which the force experienced by this current carrying conductor placed in magnetic field is largest?
The angle between the current and magnetic field is 90∘.
Mention the voltage and frequency of current that we receive at our house.
In our house we get AC of voltage 220 V and frequency 50 Hz.
Name the type of electric current generated by the most of the power stations in our country.
Alternating current.
State the effect on the strength of magnetic field produced at a point near a straight conductor if the electric current flowing through it increases.
The strength of the magnetic field increases.
State the observation made by Oersted on the basis of his experiment with current carrying conductors.
Every current carrying conductor has a magnetic field around it.
What is the direction of magnetic field lines inside a bar magnet ?
From South pole to the North pole.
At what place of the magnet are the magnetic field lines denser ?
Near the poles of the magnet.
What does the direction of thumb indicate in the right-hand thumb rule?
Thumb points the direction of current in the conductor.
Out of three wires live, neutral and earth, which one goes through ON/OFF switch ?
Live wire
What does the degree of closeness of magnetic field lines near the poles signify ?
The degree of closeness of magnetic field lines represents magnetic strength.
What are magnetic field lines ?
The imaginary lines representing the magnetic field around a magnet are called magnetic field lines.
What is meant by a ‘magnetic field’?
The area around a magnet, in which the magnetic force can be detected, is called magnetic field.
State the direction of the magnetic field inside the bar magnet.
The direction of the magnetic field lines inside the bar magnet is from its south pole to the north pole.
When is the force experienced by a current carrying conductor placed in a magnetic field the maximum?
A current carrying conductor experience maximum force in a magnetic field when the direction of current is at right angles to the direction of the magnetic field.
What is a solenoid?
A coil of many circular turns of insulated wire wrapped closely in the shape of a cylinder is called a solenoid.
Name the type of current: (a) used in household supply (b) given by a cell.
The type of current used in household supply is alternating current (AC). The type of current given by a cell is direct current (DC).
List two methods of producing magnetic fields.
Current carrying wire produces magnetic field.
Permanent magnets produce magnetic fields around them.
What does crowding of magnetic field lines indicate ?
Crowding of magnetic field lines indicates that magnetic field in that region is stronger.
What is the pattern of field lines inside a solenoid? What do they indicate?
The magnetic field lines inside the solenoid are in the form of parallel lines.
This indicates that the magnetic field is the same at all points inside the solenoid.
If field lines of a magnetic field are crossed at a point, what does it indicate?
If two magnetic field lines intersect each other, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
How is the magnetic field produced in a solenoid used?
The strong magnetic field produced inside the solenoid can be used to make a magnet by placing a magnetic material, such as soft iron, inside the solenoid. The magnet so formed is called an electromagnet.
Mention the colour code used for live, neutral and earth wire.
Live wire – Red
Neutral wire – Black
Earth wire – Green
What precaution should be taken to avoid the overloading of domestic electric circuits?
Electric fuse should be connected to the circuit
Too many appliances should not be connected in a single socket.
Too many appliances should not be used at the same time
Name any three factors on which the magnitude of the magnetic field due to solenoid depends.
(i) Number of turns in the solenoid
(ii) Area of cross section of the coil
(iii) Strength of current in solenoid
What is short-circuiting in an electric supply?
When the insulation of the wires is damaged or there is a fault in the appliance, then live wire and the neutral wire come into direct contact. In such a situation, the current in the circuit suddenly increases . This is called short-circuiting.
Why are magnetic field lines closed curves?
The magnetic field lines emerge from north pole and merge at the south pole and inside the magnet the direction of field lines is from south pole to north pole, hence the magnetic field lines are a closed curve.
What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Or
What is the advantage of the third wire of earth connection in domestic appliances?
Earth wire is used as a safety measure in devices that have a metal body.
In case of any electric fault in domestic appliances, current may comes in appliance body.The third wire called earth wire transfer this current to the earth. As a result the potential of appliance becomes equal to the ground potential and user remains safe from electric shock.
What is the role of fuse, used in series with any electrical appliance?
The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unnecessary high electric current.
Differentiate overloading and short- circuiting.
Overloading occurs when an electric circuit draws more current than the permitted value and short circuiting occurs when neutral and live wire come in direct contact.
What is the usual current rating of the fuse wire in the line to feed :
(i) lights and fans
5 A
(ii) Appliances of 2 kW or more power such as geysers, air conditioners
15 A.
Draw a diagram to represent a uniform magnetic field in a given region.
---------------￫---------
s ----------------￫--------- N
---------------￫---------
Uniform magnetic field is represented by equidistant parallel lines.
Two magnets are lying side by side as shown below. Draw magnetic field line between poles P and Q.
Draw the patterns of magnetic field fines due to a bar magnet.
Identify the poles of the magnet in the given figure.

A1 is North Pole, B1 is South Pole because magnetic field lines goes from north pole to south pole of the magnet.
When does an electric short circuit occur?
Electric short circuit occurs when the insulation of the wires is damaged or there is a fault in the appliance, then live wire and the neutral wire come into direct contact. In such a situation, the current in the circuit suddenly increase and short-circuiting occurs.
What is an electromagnet? What does it consist of?
The strong magnetic field produced inside the solenoid can be used to make a magnet by placing a magnetic material, such as soft iron, inside the solenoid. The magnet so formed is called an electromagnet. An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire.
State the rule used to determine the direction of magnetic field produced around a straight conductor carrying current.
Maxwell's right-hand thumb rule
According to this rule, when a current-carrying conductor is holding the right hand in such a way that the thumb is towards the direction of the current, then the direction of the bent fingers expresses the direction of the magnetic field.
It is also called Maxwell's cork screw rule.
An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Given :
Power of the oven = 2 kW = 2000W, Voltage = 220 V.
P = V × I
I = P/V
I = 2000 /220 = 9.09 A
The current drawn by the electric oven is 9.09 A, which is higher than the circuit’s safe limit. As a result, the fuse melts and the circuit is broken
Differences Between a Bar Magnet and an Electromagnet
1. The bar magnet is a permanent magnet while An electromagnet is a temporary magnet
2. A permanent magnet produces a comparatively weak force of attraction while An electromagnet can produce very strong magnetic
force.
3. The strength of a permanent magnet cannot be changed while The strength of an electromagnet can be changed by changing the number of turns in its coil or by changing the current passing through it.
Explain with the help of a labelled diagram the distribution of a magnetic field due to a current through a circular loop.

When electric current is passed through a circular coil of wire, a magnetic field is produced in the form of concentric circles near the loop, the direction of which can be given by the right-hand thumb rule. At every point of current carrying circular loop, the concentric circles representing the magnetic field around it becomes larger and larger as we move away from the wire. At the center of the loop, the magnetic field appears as a straight line.
Mention important properties of magnetic field lines
(i) The magnetic field lines emerge from north pole and merge at the south pole
(ii) The direction of the magnetic field lines inside the magnet is from its south pole to the north pole.
(iii) The magnetic field lines are a closed curve.
(iv) Two magnetic field lines do not intersect each other.
Mention the shape of the magnetic field lines around a current carrying straight conductor.
The magnetic field lines around a current carrying straight conductor are in the form of concentric circular rings around a conductor.
State Fleming’s left hand rule.
Or State the rule to determine the direction of a force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular.
According to Fleming's left hand rule, If the forefinger , thumb and middle finger of left hand are stretched mutually perpendicular to each other , such that the forefinger points the direction of magnetic field and middle finger points the direction of current, then the thumb points the direction of the force applied on the conductor.
Draw the pattern of field lines due to a solenoid carrying electric current. Mark the north and south poles in the diagram.
Draw a sketch of the pattern of field lines due to a current flowing through a straight conductor.
Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop:
In the following diagram the current is flowing clockwise. If we are applying right hand thumb rule to the left side of the loop then the direction of magnetic field lines inside the loop are going into the table while outside the loop they are coming out of the table.
An alpha particle (+ve charged particle) enters a magnetic field at right angle to it as shown in figure. Explain with the help of a relevant rule, the direction of force acting on the alpha particle.

Force on a-particle will be in the upward direction as per Fleming’s left hand rule.
Refer to the image below and state how the magnetic field pattern indicates regions where the magnetic field is stronger outside the magnet? What happens to the magnetic field when the current in the circuit is reversed?

The magnetic field strength is more in the region where zfield lines are crowded. This means the field strength is maximum near the poles and it reduces as we go away from the poles.
When the magnetic field when the current in the circuit is reversed, the direction of the magnetic field is also reversed.