Magnetic Effects of Electric Current

Every magnet has two poles — a north pole (N) and a south pole (S). When you bring two magnets close, like poles repel (N–N or S–S push apart) and unlike poles attract (N–S pull together). These poles are where the magnet's pull is strongest.

A magnetic field is the region around a magnet where its magnetic force can be felt. It is not just an amount — it has both a magnitude (how strong) and a direction, so at every point the field points a definite way. We picture this field using magnetic field lines (also called lines of force).

To map the field, we use a small compass needle. Placed near a magnet, the needle turns and lines up along the field. By moving it step by step and joining the directions it points, we can trace the field lines around the magnet.

Properties of magnetic field lines:

• Outside the magnet, the lines run from the North pole to the South pole; inside the magnet they run from S to N. So each field line is a closed, continuous curve (a complete loop).
• The direction of the field at a point is the direction in which the north pole of a compass points there — this is along the tangent to the field line at that point.
• Field lines are closer together where the field is stronger (crowded near the poles) and farther apart where it is weaker. Their crowding shows the relative strength of the field.
• Two field lines never cross each other. If they did, a compass placed at the crossing would have to point in two directions at once — which is impossible.

Oersted's discovery. When a compass is placed near a wire carrying current, its needle deflects. Switch the current off and the needle swings back. This simple observation proved something big: an electric current produces a magnetic field around it. Reverse the direction of the current and the needle deflects the other way, which shows the field also has a direction.

Shape of the field. Around a straight current-carrying conductor, the magnetic field lines are concentric circles (circles with a common centre) drawn around the wire. These circles lie in planes perpendicular to the wire. If you sprinkle iron filings on a card through which the wire passes, they settle into these ring-shaped patterns.

Direction — the right-hand thumb rule. This rule (also called Maxwell's corkscrew rule) tells you which way the circular field lines point:

• Imagine holding the wire in your right hand.
• Point your thumb in the direction of the current.
• Your curled fingers then show the direction of the magnetic field lines circling the wire.

Because the lines are closed loops, they never begin or end and they never cross one another. If you flip the current's direction, the field circles simply reverse their sense.

Strength of the field. The field is stronger when the current is larger — a bigger current gives more closely packed field lines. The field is also strongest close to the wire and gets weaker as you move away: the circles are drawn closer together near the wire and spread farther apart with distance. So at a fixed point, more current means a stronger field, and at a fixed current, farther away means a weaker field.

When current flows through a straight wire, its magnetic field lines are circles around the wire. Now bend that wire into a circular loop. Every point on the loop still has these circular field lines. Close to the wire the circles are small; as you move towards the centre of the loop the arcs grow larger, and right at the centre the field is almost a straight line, perpendicular to the plane of the loop (pointing straight through the middle of the ring).

You can find the direction of this field with the right-hand thumb rule: hold the wire in your right hand with the thumb pointing along the current; your curled fingers then show the direction of the field lines. Apply it at any part of the loop.

The field at the centre becomes stronger in three ways:

• a larger current through the loop;
• a smaller loop (a tighter ring concentrates the field);
• more turns of wire. Each turn carries the same current in the same direction, so their fields add up — a coil of n turns gives about n times the field of a single loop.

A solenoid is a long coil made of many circular turns of insulated copper wire wound closely together. Inside a current-carrying solenoid the field lines are parallel straight lines, which means the field is uniform — the same strength and the same direction at every point inside. The overall pattern is exactly like that of a bar magnet: one end acts as a fixed North pole and the other as a fixed South pole (fixed for a given current direction). The field lines run from the N end to the S end outside the solenoid and continue through the inside to form closed loops — like all field lines, they never cross.

This strong, uniform inside field is used to make an electromagnet. Place a piece of soft iron inside the solenoid and switch on the current: the whole arrangement becomes a powerful magnet. Soft iron is chosen because it is a magnet only while the current flows — switch the current off and it loses almost all its magnetism. That on/off control is what makes electromagnets so useful.

A current-carrying conductor makes its own magnetic field around it. If you now place that conductor inside an external magnetic field, the two fields interact and the conductor experiences a force. This force can push the wire and make it move. This simple idea is the working principle behind electric motors.

The size of this force is not the same in every position. It is largest (maximum) when the current flows at right angles (90°) to the magnetic field, that is, when the current is perpendicular to the field. If the current flows along (parallel to) the field, the force is zero.

To find the direction of the force, we use Fleming's left-hand rule. Stretch the thumb, the forefinger and the middle finger of your left hand so that all three are mutually perpendicular (at right angles to one another). Then:

• Forefinger points in the direction of the magnetic Field.
• Centre (middle) finger points in the direction of the Current.
• Thumb points in the direction of the force (the direction the conductor moves, or thrust).

An easy way to remember it: Forefinger–Field, Central–Current, Thumb–Thrust. Notice we use the left hand here for the force on a conductor (the motor rule); the right hand is kept for a different rule about induced current.

The direction of the force depends on both the current and the field. If you reverse the current, the force reverses (the wire moves the opposite way). If you reverse the magnetic field, the force also reverses. But if you reverse both at the same time, the force stays in its original direction.

An electric motor is a device that changes electrical energy into mechanical (rotational) energy. It works on a simple idea you already know: a current-carrying conductor placed in a magnetic field experiences a force.

Inside a simple motor, a rectangular coil (called the armature) is placed between the poles of a magnet. When current flows, the two opposite sides of the coil carry current in opposite directions. So, by Fleming's left-hand rule, these two sides feel forces in opposite directions — one side is pushed up and the other is pushed down. This turning effect makes the coil rotate.

Fleming's left-hand rule tells us the direction of this force. Stretch the thumb, forefinger and middle finger of your left hand so that they are mutually perpendicular (at right angles to one another). Then:

• the forefinger points along the magnetic field (from N to S),
• the middle (central) finger points along the current,
• the thumb points along the force (the motion of the conductor).

The key part that keeps the motor spinning is the split-ring commutator. After every half rotation the coil reaches a position where it would otherwise stop or turn back. At exactly this moment the split ring reverses the direction of current through the coil. Because the current reverses, the side that was being pushed up is now pushed down (and vice versa), so the coil keeps turning continuously in the same direction.

The main parts of an electric motor are:

• Coil (armature): the rectangular loop of insulated wire that rotates.
• Magnet: provides the magnetic field between its poles.
• Split ring (commutator): two halves of a ring that reverse the current after each half turn.
• Brushes: two carbon (conducting) brushes that press against the split ring and connect the coil to the battery or supply.

Electric motors are used in many everyday appliances such as electric fans, water pumps, mixers and grinders, washing machines and refrigerators.

Electromagnetic induction was discovered by Michael Faraday. The big idea is simple: a changing magnetic field through a coil produces (induces) an electric current in that coil. You do not connect any cell or battery — the movement itself creates the current.

Take a coil connected to a galvanometer (a device that shows the presence and direction of a small current). Now bring a bar magnet near the coil and watch the needle:

• Push the magnet towards the coil — the galvanometer needle deflects to one side, showing an induced current.
• Pull the magnet away — the needle deflects to the opposite side, so the induced current reverses direction.
• Hold the magnet still (near or inside the coil) — the needle reads zero. No change of field means no induced current.

So a current is induced only when there is relative motion between the magnet and the coil — that is, only when the magnetic field through the coil is changing. It does not matter whether the magnet moves or the coil moves; what matters is the change. Moving them faster gives a bigger deflection.

The direction of this induced current is found by Fleming's Right-Hand Rule. Stretch the thumb, forefinger and middle finger of your right hand so that all three are mutually perpendicular (at right angles to one another). Then:

• Forefinger points along the direction of the magnetic field (N to S).
• Thumb points along the direction of motion of the conductor.
• Middle finger then gives the direction of the induced current.

Notice that induction is basically the reverse of the motor effect. In a motor we supply current and get motion (force), and we use Fleming's Left-hand rule. In induction we supply motion and get current, and we use Fleming's Right-hand rule. This effect is exactly what a generator uses to produce electricity.

An electric generator does the opposite job of an electric motor. A motor takes electrical energy and gives motion; a generator takes motion and gives electricity. In short, it converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction — whenever the magnetic field passing through a coil changes, a current is induced in that coil.

Inside the generator a coil, called the armature, is rotated in the magnetic field between the poles of a magnet. As the coil keeps turning, the amount of magnetic field passing through it changes continuously, so an induced current flows in the coil. The direction of this induced current is found using Fleming's right-hand rule: stretch the thumb, forefinger and middle finger of your right hand so they are at right angles to one another — the forefinger points along the magnetic field, the thumb points along the direction of motion of the conductor, and then the middle finger shows the direction of the induced current. (Remember: right hand for the generator, left hand for the motor.)

In an AC generator, the two ends of the coil are joined to two separate slip rings, and each brush keeps touching its own ring all the time. As the coil turns, its two sides swap their direction of motion in the field after every half turn, so the induced current reverses its direction after every half rotation; the slip rings simply pass this changing current out to the circuit without altering it. A current that keeps changing its direction like this is called alternating current (AC).

In a DC generator, the slip rings are replaced by a split-ring commutator — the same device used in an electric motor. The commutator swaps the connections at just the right moment, so the brushes always send out current flowing in the same direction. This one-way output is called direct current (DC).

The two kinds of current differ in a simple way:

• AC (alternating current): changes its direction periodically. In India the mains supply is AC with a frequency of 50 Hz, meaning it goes through 50 back-and-forth cycles every second.
• DC (direct current): flows in only one direction and does not reverse.
• A major advantage of AC is that it can be transmitted over long distances far more easily than DC, which is why power stations supply AC to our homes.

Electricity reaches your home through a mains supply of three wires. Each wire has its own job and its own insulation colour so you can tell them apart:

• Live wire — red or brown insulation. It carries the incoming supply and is at high potential.
• Neutral wire — black or blue insulation. It completes the circuit and is at (nearly) zero potential.
• Earth wire — green insulation. It is a safety wire connected to a metal plate buried in the ground.

Inside the house the supply usually splits into two separate circuits: one for high-power appliances such as heaters and geysers, and another for lights and fans. In each circuit, every appliance is connected in parallel across the live and neutral wires. Because of this, each appliance gets the full supply voltage, can be switched on or off independently, and the failure of one appliance does not stop the others.

The earth wire is a safety measure. It is connected to the metal case of an appliance and also to the earth. If the live wire accidentally touches the metal case, the leakage current flows straight down the low-resistance earth wire into the ground instead of through your body — so you do not get a shock.

A fuse is a safety device made of a thin wire with a low melting point. It is always placed in the live wire, in series with the circuit. If the current rises above a safe value, the fuse wire heats up, melts and breaks the circuit — cutting off the supply before the wiring can overheat or catch fire.

Two situations make the current dangerously high, and in both the fuse melts to protect the circuit — which is why a fuse of the correct rating should never be bypassed:

• Overloading — too many appliances are run from the same circuit, so together they draw more current than the wires can safely carry.
• Short-circuiting — the live and neutral wires touch each other directly (often when the insulation is damaged), giving an extremely large current in an instant.