Magnetic Effects of Electric Current Notes - Field Lines, Solenoid, Motor, Generator and Domestic Circuits

Electricity and Magnetism are linked to each other. This can be demonstrated by the following experiment:

• Place a straight thick copper wire between points X and Y in an electric circuit. The wire is kept perpendicular to the plane of the paper.
• Horizontally place a small compass near the copper wire.
• When the current is passed, the compass needle is deflected, indicating that the electric current through the copper wire produces a magnetic effect.

• An electric current-carrying wire behaves like a magnet.

Hans Christian Oersted (1820) accidentally discovered the deflection of a compass needle in the presence of an electric current. The unit of magnetic field strength is named oersted.

MAGNETIC FIELD AND FIELD LINES

• A compass needle is a small bar magnet. Its ends point towards the north and south directions. It gets deflected when brought near a bar magnet.
• The end pointing towards the north is called the north seeking (north pole). The other end pointing towards the south is called the south seeking (south pole). Like poles of magnets repel, and unlike poles attract each other.

Experiments to Obtain Magnetic Field & Field Lines

Experiment 1:

• Fix a white paper on a drawing board and place a bar magnet in its center.
• Sprinkle some iron filings uniformly around the bar magnet.
• Tap the board gently. The iron filings near the magnet align along the field lines.

Reason: The magnet exerts its influence in the surrounding region, causing the iron filings to experience a force and arrange in a pattern.

The area around a magnet that has magnetic force is called a magnetic field. The lines along which the iron filings align themselves represent magnetic field lines.

Experiment 2:

• Place a bar magnet on a white paper fixed on a drawing board. Mark the boundary of the magnet.
• Place a compass near the north pole of the magnet. The south pole of the needle points towards the north pole of the magnet. Mark the position of the two ends of the needle.
• Move the needle to a new position such that its south pole occupies the position previously occupied by its north pole. Proceed step by step until you reach the south pole of the magnet.
• Join the points marked on the paper by a smooth curve. This curve represents a field line.
• Repeat the procedure to draw multiple lines. These lines are called magnetic field lines.
• The deflection in the compass needle increases as the needle is moved towards the poles.

• A magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken as the direction in which the north pole of the compass needle moves. Thus, the field lines emerge from the north pole and merge at the south pole. Inside the magnet, the direction of field lines is from south to north, making magnetic field lines closed curves.
• The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger (i.e., greater force acting on the pole of another magnet) where the field lines are crowded.
• Two field lines do not cross each other because the compass needle cannot point towards two directions at the point of intersection.

MAGNETIC FIELD DUE TO A CURRENT-CARRYING CONDUCTOR

Experiment to Find the Direction of the Field

• Take a long straight copper wire, 2 or 3 cells of 1.5 V each, and a plug key. Connect them in series.
• Place the copper wire parallel to and over a compass needle.
• If the current flows from north to south, the north pole of the compass needle moves towards the east.
• If the current flows from south to north, the needle moves in the opposite direction (towards west).
• It means that the direction of the magnetic field produced by the electric current is also reversed.

Magnetic Field due to a Current through a Straight Conductor

• Insert a straight thick copper wire through the center, normal to the plane of a rectangular cardboard.
• Connect the copper wire vertically between points X and Y, in series with a battery (12 V), a variable resistance (or rheostat), an ammeter (0–5 A), and a plug key.
• Sprinkle some iron filings uniformly on the cardboard.
• Close the key and gently tap the cardboard a few times. The iron filings align as a pattern of concentric circles, representing magnetic field lines around the copper wire.
• Place a compass at a point (say P) over a circle. The direction of the north pole of the compass needle gives the direction of the field lines at P.
• The direction of magnetic field lines is reversed if the direction of current through the copper wire is reversed.
• If the current in the copper wire is varied, the deflection in the needle also changes. If the current is increased, the deflection also increases. Thus, the magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
• If the compass is placed at a farther point (say Q) from the conducting wire, the deflection in the needle decreases. Thus, the magnetic field produced by the current in the conductor decreases as the distance increases (inversely proportional). The concentric circles representing the magnetic field around a current-carrying straight wire become larger as we move away from it.

Right-Hand Thumb Rule

• It is an easy way to find the direction of the magnetic field associated with a current-carrying conductor.
• Imagine a current-carrying straight conductor is held in the right hand such that the thumb points towards the direction of the current. Then the fingers will wrap around the conductor in the direction of the magnetic field lines. This is known as the right-hand thumb rule (also called Maxwell’s corkscrew rule).

Problem:

• A current through a horizontal power line flows in the east to west direction. What is the direction of the magnetic field at a point directly below it and at a point directly above it?

Solution:

• Applying the right-hand thumb rule, the magnetic field (at any point below or above the wire) turns clockwise in a plane perpendicular to the wire when viewed from the east end, and anti-clockwise when viewed from the west end.

Magnetic Field due to a Current through a Circular Loop

• Suppose a straight wire is bent to form a circular loop and a current is passed through it. At every point of the circular loop, the concentric circles around it become larger as the distance from the wire increases.
• At the center of the circular loop, the arcs of these big circles appear as straight lines. Every point on the wire gives rise to a magnetic field appearing as straight lines at the center of the loop. By applying the right-hand rule, every section of the wire contributes to the magnetic field lines in the same direction within the loop.
• The magnetic field produced by a current-carrying wire at a given point depends directly on the current passing through it. So, for a circular coil with n turns, the field produced is n times as large as that produced by a single turn. This is because the current in each circular turn has the same direction, and the field due to each turn adds up.

• Experiment: Take a rectangular cardboard having two holes. Insert a circular coil with a large number of turns through them, normal to the plane of the cardboard. Connect the ends of the coil in series with a battery, a key, and a rheostat. Sprinkle iron filings uniformly on the cardboard. Plug the key and tap the cardboard a few times. Concentric rings of iron filings emerge on the pattern. At the center, it appears as a straight line.

Magnetic Field due to a Current in a Solenoid

• A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
• The pattern of the magnetic field lines around a current-carrying solenoid looks similar to the pattern of the field around a bar magnet.
• One end of the solenoid behaves as a magnetic north pole and the other as the south pole. The field lines inside the solenoid are in the form of parallel straight lines, indicating that the magnetic field is uniform at all points inside the solenoid.
• If a magnetic material (e.g., soft iron) is placed inside a current-carrying solenoid, it becomes magnetized. The magnet so formed is called an electromagnet.

FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

• An electric current through a conductor produces a magnetic field. This field exerts a force on a magnet placed near the conductor.
• French scientist Andre Marie Ampere (1775–1836) suggested that the magnet also exerts an equal and opposite force on the current-carrying conductor. This can be demonstrated through an activity.
• The displacement of the rod (AB) suggests that:
  • A force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field.
  • Direction of force is also reversed when the direction of current through the conductor is reversed.

• Change the direction of field to vertically downwards by interchanging the two poles of the magnet. The direction of force acting on the current-carrying rod gets reversed. It shows that the direction of the force on the conductor depends upon the direction of current and the direction of magnetic field.
• Displacement of the rod is largest (or magnitude of the force is highest) when the direction of current is at right angles to the direction of the magnetic field. Here, direction of the force can be found through Fleming’s left-hand rule. According to this, stretch the thumb, forefinger, and middle finger of the left hand such that they are mutually perpendicular. First finger points in the direction of magnetic field, second finger points in the direction of current, and the thumb points in the direction of motion or the force acting on the conductor.

• Electric motors, electric generators, loudspeakers, microphones, speakers, and measuring instruments are devices that use current-carrying conductors and magnetic fields.

Problem:

• An electron enters a magnetic field at right angles to it, as shown below. The direction of force acting on the electron will be:
  (a) to the right.          (b) to the left.
  (c) out of the page.     (d) into the page.

Solution:

• Answer is (d). The direction of force is perpendicular to the direction of magnetic field and current as given by Fleming’s left-hand rule. The direction of current is taken opposite to the direction of motion of electrons. The force is therefore directed into the page.

ELECTRIC MOTOR

• It is a rotating device that converts electrical energy to mechanical energy.
• It is used in electric fans, refrigerators, mixers, washing machines, computers, MP3 players, etc.
• An electric motor consists of a rectangular coil ABCD of insulated copper wire. It 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 & Q of a split ring. Inner sides of halves are insulated and attached to an axle. The external conducting edges of P & Q touch two conducting stationary brushes X & Y.

• Current from the source battery enters the coil ABCD through conducting brush X and flows back to the battery through brush Y. The current in arm AB (from A to B) and arm CD (from C to D) is in opposite direction.
• On applying Fleming’s left-hand rule, the force acting on arm AB pushes it down while the force acting on arm CD pushes it up. Thus, the coil and axle rotate anti-clockwise. At half rotation, Q makes contact with the brush X and P with brush Y. So, the current in the coil gets reversed and flows along the path DCBA.
• A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.
• The reversal of current also reverses the direction of force acting on the arms AB and CD. Thus, the arm AB is pushed up and the arm CD is pushed down. So, the coil and axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation. Thus, the coil continues to rotate continuously.
• The commercial motors use:
  • An electromagnet in place of the field.
  • Large number of turns in the solenoid.
  • A soft iron core on which the coil is wrapped. Together with the solenoid, they are called an armature. This enhances strength of the field.

ELECTROMAGNETIC INDUCTION

• Consider a conductor moving inside a magnetic field or a magnetic field changing around a fixed conductor. This was first studied by English experimental physicist Michael Faraday (1831). He discovered how a moving magnet can be used to generate electric currents. This effect can be observed by the following activity.

Experiment:

• Take a coil of wire AB having a large number of turns.
• Connect the ends of the coil to a galvanometer.
• Take a strong bar magnet and move its north pole towards the end B of the coil.
• The needle of the galvanometer shows a momentary deflection to the right. It indicates the presence of a current in the coil AB. The deflection becomes zero the moment the motion of the magnet stops.
• Now withdraw the north pole of the magnet away from the coil. So the galvanometer is deflected toward the left, showing that the current is set up in the opposite direction.
• Place the magnet stationary near the coil. Keep its north pole towards the end B of the coil. The galvanometer needle deflects toward the right when the coil is moved towards the magnet. Similarly, the needle moves to the left when the coil is moved away.

• When the coil is stationary with respect to the magnet, the deflection of the galvanometer drops to zero.
• If the south pole of the magnet is moved towards the end B, the deflections in the galvanometer are opposite to the previous case. When both the coil and magnet are stationary, there is no deflection in the galvanometer.
• Thus, this activity shows that the motion of a magnet with respect to a coil produces an induced potential difference, which sets up an induced electric current in the circuit.

• Galvanometer: An instrument to detect the presence of current in a circuit. The pointer remains at zero for zero current. It deflects to the left or right depending on the direction of the current.

Experiment using current-carrying coil:

• Take two coils of copper wire having many turns (say 50 and 100 turns). Insert them over a non-conducting cylindrical roll.
• Connect coil-1 (larger number of turns) in series with a battery and a plug key. Connect coil-2 with a galvanometer.
• Plug in the key. The galvanometer needle instantly jumps to one side and just as quickly returns to zero. It indicates a momentary current in coil-2.
• Disconnect coil-1 from the battery. The needle momentarily moves to the opposite side. It means that the current flows in the opposite direction in coil-2.
• As soon as the current in coil-1 reaches either a steady value or zero, the galvanometer in coil-2 shows no deflection.

• We conclude that a potential difference is induced in coil-2 whenever the electric current through coil-1 is changing (starting or stopping). Coil-1 is called the primary coil and coil-2 is called the secondary coil. As the current in the first coil changes, the magnetic field associated with it also changes. Thus, the magnetic field lines around the secondary coil also change. Hence, the change in magnetic field lines associated with the secondary coil is the cause of the induced electric current in it. This process, by which a changing magnetic field in a conductor induces a current in another conductor, is called electromagnetic induction.
• In a coil, current can be induced either by moving it in a magnetic field or by changing the magnetic field around it. It is convenient to move the coil in a magnetic field.
• The induced current is highest when the direction of motion of the coil is at right angles to the magnetic field.
• The direction of the induced current can be found by Fleming’s right-hand rule. Stretch the thumb, forefinger, and middle finger of the right hand perpendicular to each other.
  • Forefinger indicates the direction of the magnetic field.
  • Thumb shows the direction of motion of the conductor.
  • Middle finger shows the direction of the induced current.

ELECTRIC GENERATOR

• The principle of electromagnetic induction can be applied to produce large currents for use in homes and industry.
• In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to yield electricity.

• 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 the two rings R₁ and R₂. Their inner sides are insulated.
• The two conducting stationary brushes B₁ and B₂ are kept pressed separately on the rings R₁ and R₂, respectively.
• 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 brushes are connected to a galvanometer.
• When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet.
• 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 a larger number of turns in the coil, the current generated in each turn adds up to give a large current through the coil. Thus, the current in the external circuit flows from B₂ to B₁.
• After half a rotation, arm CD starts moving up and AB moves down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B₁ to B₂. Thus, after every half rotation, the polarity of the current in the respective arms changes. Such a current, which changes (reverses) direction after equal intervals of time, is called an alternating current (AC). This device is called an AC generator.
• The current which always flows in one direction is called direct current (DC). To get a DC, a split-ring type commutator is used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. Thus, a unidirectional current is produced. This device is called a DC generator.
• Most power stations constructed these days produce AC. In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz.

Advantage of AC over DC:

Electric power can be transmitted over long distances without much loss of energy.

DOMESTIC ELECTRIC CIRCUITS

• In homes, electric power is supplied through a main supply (mains), supported through overhead electric poles or by underground cables. One wire has red insulation cover (live wire or positive) and the other wire has black insulation (neutral wire or negative). In our country, the potential difference between the two is 220 V.
• At the meter-board, these wires pass into an electricity meter through a main fuse. Through the main switch, they are connected to the line wires. These wires supply electricity to separate circuits in the house.
• Often, two separate circuits are used: one of 15 A current rating for appliances with higher power ratings such as geysers, air coolers, etc., and the other of 5 A current rating for bulbs, fans, etc.
• The earth wire (green insulation) is usually connected to a metal plate deep in the earth near the house. This is a safety measure, especially for metallic appliances (e.g., electric press, toaster, table fan, refrigerator, etc.). The metallic body is connected to the earth wire, which provides a low-resistance conducting path for current. 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.

• In each separate circuit, different appliances can be connected across live and neutral wires. Each appliance has a separate switch to ON/OFF the flow of current. To ensure each appliance has an equal potential difference, they are connected in parallel to each other.
• When the live wire and the neutral wire come into direct contact, it causes overloading. This occurs due to:
  • Damage to the insulation of wires or a fault in the appliance.
  • An accidental hike in the supply voltage.
  • Connection of too many appliances to a single socket.
• Due to overloading, the current in the circuit abruptly increases. This is called short-circuiting. An electric fuse can prevent damage to the circuit and appliance by stopping unduly high electric current. The Joule heating in the fuse melts it to break the electric circuit.