Chapter – 12

Magnetic Effects of Electric Current

In this post we have given the detailed notes of class 10 Science Chapter 12 (Magnetic Effects of Electric Current) in English. These notes are useful for the students who are going to appear in class 10 board exams.

Chapter 12: Magnetic Effects of Electric Current

Magnetic Poles 🧲

• The end of a freely suspended magnet that points towards the north is called the north-seeking pole, or north pole.
• The other end, which points towards the south, is called the south-seeking pole, or south pole.

Magnetic Field: The region/area around a magnet in which the effect (force) of the magnet can be detected is called the magnetic field.

Magnetic Field Lines: A number of lines drawn around a magnet, which appear to emerge from the north pole of the magnet and merge into the south pole, are called magnetic field lines.

Properties of Magnetic Field Lines:

• Magnetic field lines emerge from the north pole and merge at the south pole.
• Inside the magnet, the direction of the field lines is from the south pole to the north pole.
• Magnetic field lines are closed curves.
• The degree of closeness of the field lines shows the relative strength of the magnetic field — magnetic field is stronger where the field lines are closer together.
• Two magnetic field lines never intersect each other.

Magnetic Field Due to a Current-Carrying Conductor:

• An electric current flowing through a metallic conductor produces a magnetic field around it.
• When a current-carrying conductor is brought near and parallel to a compass needle, the direction of current flow reverses the direction of deflection of the compass needle.
• If the current in the conductor is increased, the deflection of the compass needle also increases.
• As the current in the conductor increases, the magnitude of the magnetic field produced at a given point also increases.
• When a compass is moved away from a current-carrying conductor, the deflection of the needle decreases.
• As the magnitude of current in the wire increases, the magnitude of the magnetic field produced at a given point also increases.
• The magnetic field produced by a current-carrying conductor decreases as the distance from the conductor increases.
• As we move away from a straight current-carrying conductor, the size of the concentric circles representing the magnetic field around it becomes larger.

Right-Hand Thumb Rule

Imagine that you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current, then the direction in which the fingers wrap around the conductor gives the direction of the magnetic field lines around the conductor. This rule is called the right-hand thumb rule. It is also known as Maxwell’s corkscrew rule.

Magnetic Field Due to a Current-Carrying Circular Loop

The magnetic field produced by a current-carrying conductor at a point depends inversely on its distance from the conductor. Similarly, in a current-carrying circular loop, the size of the concentric circles representing the magnetic field around each point of the loop keeps increasing as we move away from the wire.

Properties of Magnetic Field Due to a Current-Carrying Circular Loop

• At the centre of the circular loop, the arcs of these large circles appear as straight lines.
• The magnetic field lines produced by every point on the current-carrying wire appear as a straight line at the centre of the loop.
• All the magnetic field lines inside the loop point in the same direction.
• The magnetic field produced by a current-carrying conductor at a given point is directly proportional to the current flowing through it.
• If a coil has n turns, the magnetic field produced is n times as strong as that produced by a single turn, because the current in each turn flows in the same direction, so the fields due to individual turns simply add up.

Solenoid 🌀

A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.

Magnetic Field Due to Current in a Solenoid: When current flows through a solenoid, one end behaves like the north pole of a magnet, while the other end behaves like the south pole.

Properties of Field Lines Inside and Around a Solenoid:

• The magnetic field lines inside the solenoid are in the form of parallel straight lines.
• This indicates that the magnetic field is the same at all points inside the solenoid, i.e., the field inside a solenoid is uniform.
• Because the field lines inside a solenoid are parallel straight lines, this property is used in making electromagnets.
• A strong magnetic field is produced inside a solenoid.

Electromagnet 🧲

The strong magnetic field produced inside a solenoid can be used to magnetise a magnetic material, such as soft iron, when it is placed inside the solenoid. The magnet formed in this way is called an electromagnet.

Properties of an Electromagnet:

• The magnetic field produced by an electromagnet is generally stronger.
• The strength of the magnetic field can be controlled by varying factors such as the number of turns in the solenoid and the amount of current flowing through it.
• The polarity of the magnetic field produced by the solenoid can be reversed by reversing the direction of current flow.

Difference Between Electromagnet and Permanent Magnet

Force on a Current-Carrying Conductor in a Magnetic Field:

Arrange a strong horseshoe magnet such that a rod is placed between its two poles with the magnetic field pointing upward. To do this, the north pole of the horseshoe magnet is placed vertically below an aluminium rod, and the south pole vertically above it. When an electric current is passed through the aluminium rod from end B to end A, it is observed that the rod gets displaced. It is also observed that when the direction of the current is reversed, the direction of displacement of the rod also reverses.

Conclusions:

• A magnetic field exerts a force on a current-carrying conductor placed within it.
• When placed in a magnetic field, a force acts on a current-carrying aluminium rod.
• Reversing the direction of current in the conductor reverses the direction of the force.
• Reversing the direction of the applied magnetic field also reverses the direction of the force on the current-carrying rod.
• The direction of the force acting on the conductor depends on both the direction of the current and the direction of the magnetic field.

Force on a Conductor Depends On:

• The direction of the current, and
• The direction of the magnetic field.

Fleming’s Left-Hand Rule 👈

According to this rule, stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of current in the conductor, then the thumb points in the direction of motion of the conductor, or the direction of the force acting on it. This rule is known as Fleming’s left-hand rule.

Devices Based on This Rule: Electric motors, electric generators, loudspeakers, microphones and electric measuring instruments are some devices that make use of current-carrying conductors and magnetic fields.

MRI: MRI stands for Magnetic Resonance Imaging. It is a special technique in which a magnetic field is used inside the body to obtain images of different parts of the body. These images are analysed to diagnose diseases.

Two Parts of the Human Body Where Magnetic Fields Are Generated:

• The human brain.
• The human heart.

Electric Motor ⚙️

An electric motor is a rotating device that converts electrical energy into mechanical energy. It is used in electric fans, refrigerators, electric mixers, washing machines, computers, MP3 players, etc.

Principle of an Electric Motor: An electric motor works on the principle of the magnetic effect of electric current. When current is passed through a coil wound on an iron core placed in a magnetic field, the coil experiences a force, causing the armature of the motor to rotate within the magnetic field. The direction of rotation of the coil is given by Fleming’s left-hand rule. This is the underlying principle of an electric motor.

Role of the Split Ring in an Electric Motor: The split ring in an electric motor acts as a commutator. A commutator is a device that reverses the direction of flow of current in a circuit.

Commutator: A device that reverses the direction of current flow in a circuit is called a commutator.

Features of Commercial Motors: A commercial motor is a powerful motor. It is powerful because of the following features:

• Electromagnets are used in place of permanent magnets.
• A large number of turns of the current-carrying coil are used.
• The coil is wound on a soft iron core. The soft iron core on which the coil is wound, together with the coil, is called the armature. This increases the power of the motor.

Electromagnetic Induction 🔄

The phenomenon by which a changing magnetic field of one conductor induces current in another conductor is called electromagnetic induction. Electromagnetic induction was discovered by Michael Faraday. Faraday’s discovery showed how a moving magnet can be used to generate an electric current.

When a magnet is moved towards a coil, an electric current is generated in the coil’s circuit, which is indicated by the deflection of a galvanometer’s needle. The relative motion between the coil and the magnet produces an induced potential difference, which causes an induced electric current to flow in the circuit.

Galvanometer 📊

A galvanometer is an instrument used to detect the presence of current in a circuit.

If the current through it is zero, its pointer stays at the zero mark. It may deflect either to the left or to the right of the zero mark, and this deflection depends on the direction of the flow of current.

Different Ways of Inducing Current in a Coil: There are two ways to induce current in a coil:

• By moving the coil through a magnetic field.
• By changing the magnetic field around the coil.

Producing induced current by moving a coil through a magnetic field is more convenient.

Fleming’s Right-Hand Rule

According to this rule, stretch the thumb, forefinger and middle finger of your right hand so that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the thumb points in the direction of motion of the conductor, then the middle finger points in the direction of the induced current. This rule is called Fleming’s right-hand rule.

Electric Generator 🔁

Principle of Electric Generator: In an electric generator, mechanical energy is used to rotate a conductor placed in a magnetic field, which results in the production of an electric current.

An electric generator consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to two rings R₁ and R₂. Two fixed conducting brushes B₁ and B₂ are pressed separately against the rings R₁ and R₂. The two rings R₁ and R₂ are attached to a shaft. The shaft may be rotated mechanically to rotate the coil inside the magnetic field. When the coil rotates in the magnetic field produced by the permanent magnet, it cuts across the magnetic field lines. When coil ABCD is rotated clockwise, by applying Fleming’s right-hand rule, an induced current is set up along the direction AB and CD.

Alternating Current: An electric current that reverses its direction periodically after equal intervals of time is called alternating current (A.C.). The device used to produce electricity in this way is called an A.C. generator.

Direct Current: An electric current that flows in only one direction is called direct current (D.C.).

Difference Between AC and DC

Alternating Current (a.c.):

• It reverses its direction after a fixed interval of time.
• It is generated by an electric generator (AC generator).

Direct Current (d.c.):

• It always flows in one direction only.
• It is generated by a cell or battery.

Advantage of AC: The main advantage of AC is that electric power can be transmitted over long distances without much loss of energy.

Frequency of AC in India:

• The frequency of AC in India is 50 Hz.
• The AC generated in India reverses its direction every 1/100 of a second.

Earth Wire ⚠️

In a domestic electric circuit, apart from the live wire and the neutral wire, there is a third wire with green insulation, called the earth wire.

Advantage of the Earth Wire in a Domestic Circuit: The earth wire is a safety measure that ensures that if there is any leakage of current to the metallic body of an appliance, the person using it does not get a severe electric shock. This wire is connected, at one end, to the domestic circuit, and at the other end, to a metal plate buried deep in the earth.

Key Points to Remember

• A current-carrying conductor produces a magnetic field around it; its direction is given by the right-hand thumb rule. 🧲
• Magnetic field lines are closed curves that never intersect; they are denser where the field is stronger.
• Inside a solenoid, the magnetic field is uniform and parallel to its axis — this property is used to make electromagnets. 🌀
• A current-carrying conductor placed in a magnetic field experiences a force whose direction is given by Fleming’s left-hand rule (used in electric motors). 👈
• Electromagnetic induction (discovered by Michael Faraday) is the production of induced current due to a changing magnetic field; the induced current’s direction is given by Fleming’s right-hand rule.
• An electric motor converts electrical energy into mechanical energy; a split ring/commutator reverses the current direction every half rotation. ⚙️
• An electric generator converts mechanical energy into electrical energy, based on electromagnetic induction. 🔁
• AC reverses direction periodically (50 Hz in India) and is produced by a generator; DC flows in one direction only and is produced by a cell/battery.
• AC can be transmitted over long distances with minimal loss of energy, which is its main advantage over DC.
• The earth wire (green insulation) is a safety device that protects users from electric shocks due to current leakage in appliances. ⚠️

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