Magnetic Effects of Electric Current

The previous chapter dealt with the heating effect of current. This chapter starts from a second effect that Hans Christian Oersted stumbled on in 1820: a compass needle placed near a wire swings the moment current flows through the wire. Nothing touches the needle. The current has produced a magnetic field in the space around the wire, and the compass, being a tiny bar magnet, turns to line up with that field. Oersted's observation was the first proof that electricity and magnetism are two faces of one phenomenon, and it opened up the whole subject of electromagnetism.

Activity 12.1 is the version the examiner expects you to describe. A thick straight copper wire XY is connected in a circuit with a battery, a plug key and a resistor. A small compass is placed horizontally near the wire. When the key is inserted, the needle deflects. If you reverse the battery connections so that the current flows the other way, the needle deflects the other way. Two conclusions come straight from this: a current produces a magnetic field, and the direction of that field depends on the direction of the current.

Keep in mind the two ways the chapter will now build on this. First, we study what the field of a current looks like for different shapes of conductor (straight wire, circular loop, solenoid). Second, we turn the idea around: if a current makes a magnet feel a force, then by Newton's third law a magnet must make a current-carrying wire feel a force too. That is Section 12.3, and it leads to Fleming's left-hand rule.

A compass needle is a small bar magnet. The end that points north is its north-seeking pole, or north pole; the other end is the south pole. Like poles repel and unlike poles attract. When a compass is brought near a bar magnet, the needle deflects because the magnet exerts a force on the needle's poles.

The region around a magnet in which its force can be detected is called its magnetic field. Activity 12.2 makes the field visible: place a bar magnet on a sheet of paper, sprinkle iron filings around it and tap the board. Every filing becomes a tiny magnet in the field and settles along a curve. The curves are the magnetic field lines. Activity 12.3 draws one such line properly: put a compass near the north pole of the magnet, mark the two ends of the needle, move the compass so its south pole sits where its north pole was, and keep going until you reach the south pole of the magnet. Joining the dots gives one field line; repeating gives the full pattern.

Magnetic field is a quantity with both magnitude and direction. By convention the direction of the field at a point is the direction in which the north pole of a compass needle placed there would move. The board wants these properties of field lines written as separate points:

1. Outside the magnet the field lines emerge from the north pole and merge into the south pole; inside the magnet they run from south pole to north pole. So field lines are closed curves.
2. Where the lines are crowded (near the poles) the field is strong; where they are far apart the field is weak. The deflection of a compass increases as you move it towards a pole.
3. No two field lines ever cross. If they did, a compass at the crossing point would have to point in two directions at once, which is impossible.
4. The tangent at any point on a field line gives the direction of the field at that point.

A uniform magnetic field is one that has the same magnitude and direction at every point in a region. It is drawn as equally spaced parallel straight lines with arrows all pointing the same way. This drawing is an NCERT in-text question and a frequent 1-mark item.

Activity 12.1 showed that a current produces a field. Activity 12.4 finds its direction. Connect a long straight copper wire in series with two or three cells and a plug key. Lay the wire parallel to and directly above a compass needle, pointing north-south. When current flows from north to south through the wire, the north pole of the needle moves towards the east. Now interchange the cell connections so that the current flows from south to north: the needle now swings towards the west.

The examiner's takeaway is the pairing of observation and conclusion. Observation: the needle deflects when current flows. Conclusion: a current-carrying conductor has a magnetic field around it. Observation: reversing the current reverses the deflection. Conclusion: the direction of the magnetic field depends on the direction of the current.

A common lab-based MCQ describes a student who reports that (i) the deflection of the compass reverses when the terminals of the battery are interchanged, and (ii) the deflection decreases when the compass is moved farther from the wire. Both are correct reports. A student who claims the deflection stays unchanged on reversing the current has made a wrong observation; that is the option to reject.

What determines the actual pattern of the field, that is, how the field lines are arranged in space? The pattern depends on the shape of the conductor. The chapter therefore takes three shapes one by one: a straight wire (12.2.1), a circular loop (12.2.3) and a solenoid (12.2.4). Whatever the shape, two facts stay true throughout: the field at a point grows if the current grows, and the field at a point falls as you move away from the wire.

This activity is asked as a 3-mark 'describe an activity' question and the marking scheme rewards the set-up, the observation and the two variations. Pass a long thick copper wire vertically through the centre of a horizontal cardboard sheet. Connect it in series with a 12 V battery, a rheostat, an ammeter (0 to 5 A) and a plug key. Sprinkle iron filings uniformly on the cardboard, close the key so that current flows, and tap the cardboard gently.

Observation: the filings arrange themselves in concentric circles centred on the wire. These circles are the magnetic field lines of a straight current-carrying conductor. To find their direction, place a compass at a point P on one circle; the north pole of the needle points along the field. Reversing the current reverses the direction of the needle, so the direction of the field lines reverses when the current reverses.

Variation 1, change the current: use the rheostat to increase the current, keeping the compass at the same point. The deflection increases. So the magnitude of the field at a given point increases as the current through the wire increases.

Variation 2, change the distance: keep the current fixed and move the compass from P to a farther point Q. The deflection decreases. So the field produced by a given current decreases as the distance from the wire increases. You can see this in the pattern too: the concentric circles get larger and farther apart as you move out from the wire.

Two sentences to memorise for the 2-mark version: the magnetic field at a point due to a straight current-carrying conductor is directly proportional to the current and inversely proportional to the distance of the point from the conductor.

The right-hand thumb rule gives the direction of the field around a straight current-carrying conductor. Imagine holding the conductor in your right hand so that the thumb points in the direction of the current. The curled fingers then wrap around the conductor in the direction of the magnetic field lines. The same rule is also called Maxwell's corkscrew rule: if you drive a corkscrew forward in the direction of the current, the direction in which you turn the handle is the direction of the field.

The way to apply it in the exam is Example 12.1. A horizontal power line carries current from east to west. What is the field at a point directly below and directly above it? Point the right thumb towards the west. The fingers curl so that, viewed from the east end of the wire (looking along the current), the field goes clockwise in a plane perpendicular to the wire, and anticlockwise when viewed from the west end. Working through the curl: directly below the wire the field points towards the south, and directly above the wire it points towards the north. If you want to check, imagine your thumb pointing west and your palm facing down: the fingertips below the wire point south and the knuckles above point north.

A second favourite: a vertical wire carries current from top to bottom. Viewed from above, the field goes clockwise (thumb pointing down and away from you, fingers curl clockwise as seen from above). If the current flows upward, the field is anticlockwise as seen from above. Questions often reverse this: 'to produce a clockwise field seen from above, current must flow from top to bottom'.

The rule works for any small piece of a bent wire as well, which is how the chapter gets the field of a loop and a solenoid in the next two subsections.

Bend the straight wire into a circular loop and pass a current through it. Near the wire, at every point on the loop, the field lines are still small concentric circles. As you move away from the wire the circles grow larger. By the time you reach the centre of the loop the arcs of these large circles are almost straight lines. Apply the right-hand thumb rule to any small section of the loop: every section sends its field the same way through the inside of the loop. So at the centre the field lines are nearly parallel straight lines running perpendicular to the plane of the loop, all in one direction.

Activity 12.6 shows this pattern. Insert a circular coil of many turns through two holes in a cardboard so that the coil is perpendicular to the cardboard, connect it in series with a battery, a key and a rheostat, sprinkle iron filings, plug the key and tap the board. Around each wire the filings form circles; across the middle of the coil they line up as nearly straight parallel lines.

Two facts you must state: (i) the field at the centre of the loop is directly proportional to the current, and (ii) if the coil has n turns, the field is n times that of a single turn, because the current in every turn goes round in the same direction and the fields of the turns simply add. The field at the centre also increases if the radius of the loop is made smaller, because the centre is then closer to the wire.

Direction of the field, an NCERT in-text question: a loop lies flat on a table and the current goes round it clockwise as seen from above. Curl the right-hand fingers with the current; the thumb points downward. So inside the loop the field points downward, into the table, and outside the loop the field lines return upward, out of the table. For an anticlockwise current the field inside is upward. A useful memory aid: a clockwise current seen face-on makes that face a south pole, an anticlockwise current makes it a north pole.

A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder. When current flows through it, the field pattern looks exactly like that of a bar magnet: lines emerge from one end, loop around outside and enter the other end. One end of the solenoid behaves as a north pole and the other as a south pole. You can find which is which with the right-hand thumb rule on one turn, or with the clockwise/anticlockwise trick: looking at an end, if the current goes anticlockwise, that end is a north pole; if clockwise, a south pole.

Inside the solenoid the field lines are parallel straight lines, equally spaced. This tells you the field inside a long solenoid is the same at every point, that is, it is uniform. This one sentence is the basis of the most repeated MCQ in the chapter: the field inside a long straight current-carrying solenoid is the same at all points, not zero, not more at the ends.

The strength of the field of a solenoid increases with (i) the current, (ii) the number of turns per unit length, and (iii) the presence of a soft iron core inside it. Put a rod of soft iron inside the solenoid while current flows and the strong uniform field magnetises it. The magnet so formed is an electromagnet. It is a temporary magnet: switch off the current and soft iron loses almost all its magnetism, which is exactly why soft iron and not steel is used for the core. Uses: electric bells, cranes for lifting scrap iron, loudspeakers, relays and MRI machines in hospitals.

The 2019 board question asks for two distinguishing features between the field of a solenoid and that of a bar magnet, so learn them: (1) the field inside a solenoid is uniform, while the field of a bar magnet is not uniform anywhere; (2) the field of a solenoid can be switched on and off, and its strength and polarity can be changed by changing the current, whereas a bar magnet's field is fixed. A third: the solenoid field lines pass through the hollow interior, whereas in a bar magnet they pass through the solid material from S to N.

A current produces a field that exerts a force on a nearby magnet. Ampere argued that the magnet must exert an equal and opposite force on the current-carrying conductor. Activity 12.7 demonstrates it and is the standard 3-mark or 5-mark 'describe an activity' item. Hang a small aluminium rod AB (about 5 cm) horizontally from a stand using two connecting wires. Place a strong horseshoe magnet so that the rod lies between its poles with the field directed upward: north pole vertically below the rod, south pole vertically above it. Connect the rod in series with a battery, a key and a rheostat and pass current from B to A. The rod is pushed sideways, to the left. Reverse the current and it moves to the right. Interchange the poles of the magnet (field downward) and the direction of displacement reverses again.

Conclusions: a current-carrying conductor placed in a magnetic field experiences a force; the direction of the force reverses if either the current or the field is reversed; the force is perpendicular to both the current and the field. Experiments also show the displacement is largest, that is, the force is maximum, when the current is at right angles to the field. When the current is parallel to the field, the force is zero.

Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand so that they are mutually perpendicular. If the forefinger points along the magnetic field and the middle finger along the current, the thumb gives the direction of the force (motion) on the conductor. Memory aid: F-B-I from thumb to middle finger: Force, B (field), I (current). Devices based on this force: electric motor, loudspeaker, microphone, galvanometer and other measuring instruments.

The examiner also tests moving charges, because a moving charge is a current. Direction of current is the direction of motion of a positive charge and opposite to the motion of an electron. Example 12.2: an electron enters a field at right angles; take the current opposite to the electron's velocity, then apply the rule. For a particle moving freely in a field the force is always perpendicular to its velocity, so it does no work: the particle's speed and kinetic energy stay the same, but the direction of its velocity, and hence its velocity and momentum, keep changing. A charge at rest, or moving parallel to the field, feels no force.

Electric power reaches a house through the mains, by overhead lines or underground cables. It comes as two wires: the live wire (red insulation) and the neutral wire (black insulation). In India the potential difference between them is 220 V, supplied as alternating current at 50 Hz. At the meter board the two wires pass through a main fuse into the electricity meter and then through the main switch to the line wires of the house.

Inside the house there are usually two separate circuits: a 15 A circuit for high-power appliances such as geysers, air coolers and heaters, and a 5 A circuit for bulbs, fans and similar loads. In each circuit every appliance is connected between the live and neutral wires, in parallel with the others, so that each gets the full 220 V and each has its own switch. The switch and the fuse are always placed in the live wire, so that when the switch is off or the fuse blows, the appliance is disconnected from the live line and is safe to touch.

The third wire is the earth wire (green insulation), connected to a metal plate buried deep in the earth near the house. The metallic body of appliances like an electric iron, toaster, table fan or refrigerator is connected to it. If insulation fails and current leaks to the body, the earth wire gives it a low-resistance path to the ground, keeping the body at earth potential; the leakage current flows to earth instead of through the person, and the large current usually blows the fuse as well.

An electric fuse is a short piece of wire of a metal or alloy with a suitable low melting point, connected in series in the live wire. If the current exceeds its rating, Joule heating melts the fuse and breaks the circuit. Two situations cause an unduly large current. Short-circuiting: the live and neutral wires come into direct contact (damaged insulation or a faulty appliance); the resistance of the path becomes very small and the current rises abruptly to a huge value. Overloading: the current exceeds the safe limit because too many appliances are connected to a single socket, or because of an accidental hike in supply voltage, or because a short circuit has occurred. Precautions: never connect too many high-power appliances to one socket, use a fuse (or MCB) of the correct rating, keep insulation in good condition, and use separate circuits for high-power appliances.

Direct current, alternating current, frequency of AC and the advantage of AC over DC were part of the old Electric Generator section of this chapter. The 2023 CBSE rationalisation removed the electric generator topic altogether, along with the electric motor and electromagnetic induction, and this DC/AC discussion went with it. It is not on the current syllabus and should not be expected as a stand-alone question in present-day board papers, even though it was asked often as a 1-mark or 2-mark item in papers before the rationalisation (hence the older years attached to this section). The rationalised NCERT text keeps only one line on the idea, in the closing summary of the chapter: our homes receive AC power of 220 V at a frequency of 50 Hz. That one line is still worth knowing, because Section 12.4 (Domestic Electric Circuits), which is on the syllabus, depends on it.

For background only: direct current (DC) flows in one direction only; its magnitude may vary but its direction does not. Sources: a dry cell, a car battery, a solar cell, a DC generator. Alternating current (AC) reverses its direction periodically; the mains supply in our homes is AC, produced by an AC generator at the power station.

The frequency of AC is the number of complete cycles per second. In India it is 50 Hz, which means 50 cycles in one second, so one cycle takes 1/50 s = 0.02 s. The current reverses direction twice in every cycle, once after each half-cycle, so it changes direction 100 times in one second, that is, once every 1/100 s = 0.01 s. Read the question carefully: 'how many cycles' is 50, 'how many times does it change direction' is 100.

The advantage once quoted for AC over DC was that it can be transmitted over long distances from the generating station to the consumer without much loss of electrical energy, because its voltage can be stepped up for transmission and stepped down for use. Keep this only as background; do not spend revision time on the deleted electric-generator material itself.