Revision Notes

Electric current produces magnetic field

When an electric current is passed through a conductor, then a magnetic field is produced around the conductor, i.e., the conductor behaves like a magnet, as long as the current flows through it.

Magnetic Field and Field Lines

Magnetic field

The space surrounding a magnet in which its influence in the form of magnetic force can be detected, is called magnetic field.

Magnetic field lines

The curved paths along which iron filings arrange themselves due to the force acting on them in the magnetic field of a bar magnet are called magnetic field lines.

The direction of the magnetic field at any point is obtained by drawing a tangent to the field line at that point.

Properties of Magnetic Field Lines

• A magnetic field line is directed from the North Pole to the South Pole outside the magnet.
• A magnetic field line is a closed and continuous curve.
• The magnetic field lines are closer where the magnetic field is strong and farther apart where the magnetic field is weak.
• The magnetic field lines never intersect each other.
• Parallel and equidistant field lines represent a uniform magnetic field.

Magnetic Field due to a Current-carrying Conductor

Magnetic Field due to a Current through a Straight Conductor

The magnetic field lines around a straight conductor carrying a current are concentric circles.

The direction of a magnetic field is given by the Right-Hand Thumb Rule.

Strength of Magnetic field

The magnitude of the magnetic field due to a straight current-carrying conductor at a given point is

• Directly proportional to the current flowing through the conductor
• Inversely proportional to the distance of that point from the conductor

Magnetic Field due to a Current-carrying Circular Coil

• The magnetic field lines near the coil are nearly circular or concentric.
• The magnetic field at the centre of the coil is maximum and almost uniform.
• Looking at the face of a coil, if the current around it is in the clockwise direction, then it faces the South Pole. If the current around it is in the anticlockwise direction, then it faces the North Pole. This is called the Clock rule.
• The magnitude of a magnetic field at the centre of the coil is
  • Directly proportional to the current flowing through it
  • Inversely proportional to the radius of the coil
  • Directly proportional to the number of turns of the coil

Magnetic Field due to a Current-carrying Solenoid

An insulated copper wire wound on some cylindrical cardboard or plastic tube, such that its length is greater than its diameter and it behaves like a magnet when a current is made to flow through it, is called a solenoid.

• The pattern of the magnetic field lines around a current-carrying solenoid is similar to that produced by a bar magnet as shown in the figure below.
• The magnetic field inside a solenoid is uniform.
• In accordance with the Clock rule, the end of the solenoid at which the current flows in the anticlockwise direction behaves as a North Pole, while the end at which the current flows in the clockwise direction behaves as a South Pole.
• The magnitude of the magnetic field inside the solenoid is directly proportional to the
  • Current flowing through it
  • Number of turns per unit length of the solenoid

Force on a Current-carrying Conductor in a Magnetic Field

• A current-carrying conductor when placed in a magnetic field experiences a force.
• The direction of the force gets reversed when the direction of the current is reversed or when the direction of the magnetic field is reversed.
• The force acting on a conductor is found to be maximum when the current and magnetic field are at right angles to each other.
• When the conductor is placed parallel to the magnetic field, no force acts on it.
• Fleming’s Left-Hand Rule gives the direction of the magnetic force acting on the conductor.

Electric Motor

• An electric motor is a device that converts electrical energy to mechanical energy, and it works on the principle of force experienced by a current-carrying conductor in a magnetic field.
• An electric motor consists of a rectangular coil of insulated copper wire. The coil is placed between the two poles of a magnet.
• The current in the coil enters from the source battery through the conducting brush X and flows back to the battery through brush Y. Current in the arms AB and CD is in opposite directions.
• On applying Fleming’s left-hand rule, we find that the force acting on the arm AB pushes it downwards while the force acting on the arm CD pushes it upwards. Thus, the coil and the axle O rotate in the anti-clockwise direction.
• After half rotation, the current in the coil gets reversed with the help of a commutator. This reverses the direction of the force acting on the two arms AB and CD. However, these arms have reversed positions after that half rotation.
• Thus, the coil and the axle rotate in the same direction, i.e. anti-clockwise.

Electromagnetic Induction

• The phenomenon due to which a changing magnetic field within a conductor or closed coil induces electric current in the conductor or a coil is called electromagnetic induction.
• The change in magnetic field in a coil may be due to the
  • Relative motion between the coil and the magnet placed near it.
  • Relative motion between the coil and a current-carrying conductor placed near it
  • Change of current in the conductor placed near the coil
• Fleming’s Right-Hand Rule is used to find the direction of induced current.

Lenz’s law

It states “In all cases of electromagnetic induction, the direction of induced current is such that it always opposes the cause (the motion of the conductor) which produces it.”

Electric Generator

• Electric generators are based on the principle of electromagnetic induction and they convert mechanical energy into electrical energy.
• An electric generator consists of a rotating rectangular coil placed between the two poles of a permanent magnet.

• The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.
• Outer ends of the two brushes B1 and B2 are connected to the galvanometer to show the flow of current in the given external circuit.
• When the axle is rotated, arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say that the coil ABCD is rotated clockwise.
• By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus, an induced current flows in the direction ABCD. If there are more turns in the coil, the current generated in each turn adds up to give a large current through the coil.
• After half rotation, CD and AB reverse direction, and thus, CD starts moving up and AB starts moving down. As a result, the directions of the induced currents in both the arms change.
• Such a current which changes direction after equal intervals of time is called an alternating current. Thus, this device is also called an AC generator.
• To get a direct current, a split-ring type commutator must be used. The generator is then called a DC generator.
• The difference between direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

Domestic Electric Circuits

• In our homes, we receive electric power through a main supply called the mains. We receive an AC electric power of 220 V with a frequency of 50 Hz.
• One of the wires in the electricity wiring of houses has a red insulation and is called the live wire. The other, of black insulation is called the neutral wire. The third is the earth wire which has green insulation and is connected to a metallic plate deep inside the Earth.
• The earth wire in wiring is used as a safety measure to ensure that any leakage of current in the metallic body does not give the user a severe shock.
• A fuse is an important safety device used to protect circuits and appliances from short-circuiting (which occurs when a live wire and a neutral wire come in contact) or overloading (which occurs when an electric circuit draws more current than the permitted value).