Electricity & Magnetism

The current whose direction changes periodically is called alternating current (AC).

The total number of magnetic field lines passing perpendicularly through a given surface is called magnetic flux (symbol: Ø; unit: Weber).

The phenomenon by which a freely suspended current-carrying conductor experiences a force when placed in a magnetic field is called motor effect.

The process of producing current in a closed coil due to the relative motion between a magnet and the coil is called electromagnetic induction.

The phenomenon by which a magnetic field is produced around a current-carrying conductor is called the magnetic effect of electric current.

A dynamo is a simple device that converts the mechanical energy of a rotating wheel into electrical energy, working on Faraday's laws of electromagnetic induction.

A transformer is a device that converts high-voltage AC into low-voltage AC, or low-voltage AC into high-voltage AC.

A transformer that increases the voltage of an AC supply (with more turns in the secondary coil than the primary) is called a step-up transformer.

Negative current means the direction of current has reversed — it is flowing in the opposite direction compared to when the value is positive.

The strong, uniform magnetic field inside a solenoid is used to make an electromagnet, which is applied in electric bells, cranes for lifting magnetic materials, and MRI machines.

The cause of electromagnetic induction is the change in magnetic flux linked with the conductor.

The voltage of alternating current supplied for domestic purposes in Nepal is 230 V on average.

The SI unit of magnetic flux is the Weber (Wb).

Nepal's total estimated hydroelectricity production capacity is 83,000 MW, of which only 42,000 MW is economically feasible.

The laminated core of a transformer is insulated using varnish, shellac, or similar insulating materials between the thin iron plates.

A transformer operates on the principle of mutual induction — a changing current in one coil induces a voltage in a nearby coil.

Transformer A (primary coil has more turns than secondary coil) is used in a mobile phone charger — it is a step-down transformer that reduces the mains voltage to a lower level suitable for charging.

It means the AC completes 50 full cycles every second and changes its direction 100 times per second (twice per cycle — once positive, once negative). The frequency of AC in a power house is controlled by the speed of the generators.

When the bicycle moves, the roller rotates the coil of the dynamo in the magnetic field of a permanent magnet. As the coil rotates, the magnetic flux changes continuously, inducing an electric current in the coil by electromagnetic induction, which is supplied to the bicycle lamp to make it glow.

Moving the wire up and down changes the magnetic flux through the circuit; by Faraday's law, this changing flux induces an EMF and current, causing the galvanometer to deflect.

If the galvanometer is replaced by an AC source and the wire is free, the motor effect will be observed — the current-carrying wire in the magnetic field will experience a force and move.

To increase the effect of Device B: increase the current in the coil, or increase the strength of the magnetic field, or increase the number of turns in the coil.

Application: Generators are used in hydroelectric and thermal power stations to produce large amounts of electrical energy from mechanical energy.

When AC flows through the primary coil, it produces a continuously changing magnetic flux in the soft iron core. This changing flux links with the secondary coil and, by electromagnetic induction, induces a voltage (and hence current) in the secondary coil.

A solar panel produces DC, but a refrigerator runs on AC; since DC cannot directly power an AC appliance, an inverter is needed first.

Without transformers, AC voltage cannot be stepped up for efficient long-distance transmission (high energy loss) or stepped down for safe home use, severely restricting its distribution.

A stronger magnet creates a stronger magnetic field, causing a greater rate of change of magnetic flux when the coil rotates, inducing a larger EMF and more current (Faraday's second law).

Faster riding makes the dynamo coil rotate faster, increasing the rate of change of magnetic flux, which induces a greater EMF and current, making the lamp brighter.

The coil rotates continuously in a generator; slip rings rotate with the coil while brushes remain stationary, allowing alternating current to flow to the external circuit without tangling the wires.

Hydroelectricity uses a renewable, pollution-free source (water) with no fuel cost, whereas thermal electricity burns fossil fuels, produces pollution, and uses non-renewable resources.

Nepal has abundant water resources and favorable mountainous geography providing high water heads, giving an estimated potential of 83,000 MW.

DC does not change its magnitude or polarity, so it cannot produce a changing magnetic field in the core; without changing magnetic flux, no EMF can be induced in the secondary coil.

Lamination reduces eddy currents — unwanted circular currents in the core that waste energy as heat; thin insulated plates break the current paths and reduce these losses.

If the turns were equal, the output voltage would be the same as the input; different turn ratios are needed to step voltage up or down according to requirements.

Step-up transformers raise the generated voltage (e.g., 12 kV to 400 kV) before long-distance transmission, reducing the current to minimise energy loss due to wire resistance.

When the magnet is stopped, there is no relative motion, so no change in magnetic flux occurs through the coil. By Faraday's first law, no EMF is induced, so the galvanometer shows no deflection.

Moving the magnet inward increases the flux through the coil, while moving it outward decreases the flux. Since the direction of change of flux is opposite in both cases, the induced current (and hence galvanometer deflection) is in opposite directions in each case.

If the number of turns is doubled, the induced EMF doubles (by Faraday's second law: induced EMF ∝ rate of change of flux × number of turns), so the deflection of the galvanometer will also double (become larger).

Mechanical energy (of the moving magnet) is converted into electrical energy (induced current in the coil) in this experiment.

The picture shows the motor effect — a current-carrying conductor placed in a magnetic field experiences a mechanical force.

If the direction of current is reversed, the direction of force on the wire also reverses (by Fleming's left-hand rule), so the wire moves in the opposite direction.

Two devices based on motor effect: electric fan and electric mixer/grinder.

When current flows through the wire, it produces its own magnetic field; this field interacts with the external magnetic field of the permanent magnet, and the interaction creates a push/pull force on the wire, causing it to move.

The rectangular coil (armature) is rotated mechanically between the poles of a strong magnet by a turbine. As it spins, the magnetic flux changes continuously, inducing an EMF that rises, falls, and reverses every half rotation — forming a sine-wave AC. Slip rings rotate with the coil while stationary carbon brushes collect the alternating current and deliver it to the external circuit.

In a thermal power plant, fuel (coal, oil, or natural gas) is burned in a boiler to produce heat, which converts water into high-pressure steam. The steam enters a turbine, making it rotate and converting heat energy into mechanical energy. The rotating turbine drives a generator that converts mechanical energy into electrical energy. After passing through the turbine, the steam is cooled in a condenser, converted back to water, and returned to the boiler to repeat the cycle.

The picture shows a step-up transformer (left) and a step-down transformer (right).

A transformer works on the principle of mutual induction: a changing current in the primary coil produces a changing magnetic flux in the core, which induces a voltage in the secondary coil.

Step-up: used in microwave oven; Step-down: used in mobile phone charger, television, radio.

In an ideal transformer, input energy equals output energy (energy is conserved). In practice, small losses occur due to eddy currents and resistance heating, so output energy is slightly less than input energy.

Transformer A is a step-up transformer (raises 12 kV to 400 kV). Transformer B is a step-down transformer (reduces 400 kV to 13 kV).

Transformer A increases the voltage from 12 kV to 400 kV before long-distance transmission, which reduces the current significantly, thereby minimising energy loss due to the resistance of transmission wires.

Transformer C: Vp = 13 kV = 13,000 V, Vs = 240 V, Np = 650, Ns = ?