Magnetic Effects of Electric Current

Magnetic field

The region surrounding a magnet in which its force can be detected. It has both magnitude and direction; its direction at a point is the direction in which the north pole of a compass needle placed there would move.

Why does a compass needle get deflected when brought near a bar magnet?

A compass needle is itself a small bar magnet. Near another magnet it is in that magnet's field, and since like poles repel and unlike poles attract, the needle turns.

Magnetic field lines

Lines representing a magnetic field, along which iron filings align. By convention they emerge from the north pole and merge at the south pole, running south to north inside the magnet, so they are closed curves; they are crowded together where the field is stronger.

Why do two magnetic field lines never cross each other?

If they did, a compass needle placed at the point of intersection would have to point in two directions at once, which is not possible.

What is the pattern of the magnetic field around a straight current-carrying wire, and how does its strength vary?

The field lines are concentric circles centred on the wire. At a given point the field grows stronger as the current increases and weaker as the distance from the wire increases, and reversing the current reverses the direction of the field.

Right-hand thumb rule

Imagine holding a current-carrying straight conductor in your right hand with the thumb pointing along the current; the way your fingers curl round it then shows the direction of the magnetic field lines. It is also called Maxwell's corkscrew rule.

How is the right-hand thumb rule used to find the field around a horizontal power line carrying current from east to west?

Point the thumb along the current, from east to west; the curl of the fingers gives the field. Seen from the east end, the field circles clockwise in a plane at right angles to the wire; seen from the west end, it circles anticlockwise, at points below and above the wire alike.

What is the magnetic field like inside a current-carrying circular loop, and what does its strength depend on?

Inside the loop, every part of the wire contributes field in one common direction, and at the centre the circles are so large that they appear as straight lines. The field depends directly on the current, and a coil of n turns produces n times the field of a single turn.

Solenoid

A long cylindrical coil made of many closely wound circular turns of insulated copper wire. With current flowing, its field resembles a bar magnet's, one end acting as a north pole and the other as a south pole; inside, the field lines are parallel straight lines, so the field is uniform.

Electromagnet

A magnet formed when a piece of magnetic material such as soft iron is placed inside a current-carrying solenoid and magnetised by its strong internal field. It consists of a soft iron core wrapped with a coil of insulated copper wire.

What decides the direction of the force on a current-carrying conductor in a magnetic field, and when is the force largest?

The force reverses if either the current or the field is reversed, so its direction depends on both. It is largest when the current is at right angles to the magnetic field, and the force is then perpendicular to both.

Fleming's left-hand rule

Stretch the thumb, forefinger and middle finger of the left hand so they are mutually perpendicular. If the first finger points along the magnetic field and the second finger along the current, the thumb points in the direction of motion, or force, on the conductor.

How do you find the direction of the force on an electron moving at right angles to a magnetic field?

Conventional current flows opposite to the motion of electrons, so take the current as opposite to the electron's direction of motion. Then apply Fleming's left-hand rule: first finger along the field, second finger along that current, and the thumb gives the force.

Live wire

The mains supply wire, usually with red insulation, also called the positive wire. The other supply wire, the neutral or negative wire, has black insulation, and in India the voltage across the two is 220 V.

Earth wire

The safety wire with green insulation, joined to a metal plate buried deep in the ground beside the house and to the metallic bodies of appliances such as irons, toasters and refrigerators. It provides a low-resistance path that keeps any leaked current at the earth's potential, so the user does not get a severe shock.

What is a short circuit, and why is it dangerous?

It happens when the live and neutral wires touch each other directly, as when insulation is damaged or an appliance is faulty. The current in the circuit then increases abruptly, which can damage the appliance and the circuit.

What can cause overloading of a domestic electric circuit?

Direct contact between the live and neutral wires, an accidental hike in the supply voltage, or connecting too many appliances to a single socket. In each case the current rises beyond what the circuit is meant to carry.

Electric fuse

A safety device connected in series with an appliance, containing a wire of suitable melting point. If an unduly high current flows, Joule heating melts the wire and breaks the circuit, protecting the appliance and the circuit from damage.

How is the role of a fuse different from that of the green earth wire in a domestic circuit?

A fuse protects the circuit and appliances: it melts and stops an unduly high current, as in a short circuit or overload. The earth wire protects the user: it carries current leaking to an appliance's metal body safely to earth, so touching the appliance does not give a severe shock.

How should the rating of a fuse be chosen for an appliance?

Just above the current the appliance normally draws. A 1 kW electric iron on 220 V draws about 4.54 A, so a 5 A fuse is used; a fuse rated far higher would not melt when the current rose unduly, leaving the appliance unprotected.

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