If bends in a wire doesn't affect its resistance, then why does a wounded coiled wire show sparks instead of plain wire?

While bending a wire itself does not significantly affect its resistance, the situation becomes more complex when dealing with wound coils, such as those found in transformers, motors, or electromagnets. Coils are not just bent wires; their geometry and winding method affect their electromagnetic properties, particularly self-inductance and mutual inductance, leading to phenomena such as sparking that do not occur with ordinary straight wires.

Reasons for Sparking in Wound Coils

Inductive Effects

• Self-inductance: When current flows through a coil, it generates a magnetic field around the coil. If the current changes suddenly (e.g., when switching the circuit on or off), the magnetic field changes, inducing an electromotive force (EMF) known as self-inductance. This sudden change can lead to very high voltage spikes, resulting in sparking.

• Mutual Inductance: In multi-turn coils, the change in current in one turn affects the current in adjacent turns, known as mutual inductance. Sudden changes in current can lead to voltage spikes, causing sparking.

Capacitive Effects

Turn-to-turn Capacitance: Due to the capacitance between turns in a coil, sudden changes in current can lead to voltage spikes, potentially resulting in sparking.

Switching Transients

• Sparking on Disconnection: When disconnecting the power supply to a coil, the self-induced EMF causes the stored magnetic energy to try to maintain the current, leading to high voltages across the switch, which can result in arcing or sparking.

• Sparking on Connection: When connecting the power supply to a coil, the establishment of current can also cause instantaneous high voltages, leading to sparking.

Differences Between Ordinary Wires and Coils

• Geometric Structure: Ordinary wires are typically straight or slightly bent, while coils are tightly wound, leading to higher self-inductance and mutual inductance in coils.

• Electromagnetic Effects: Changes in current in coils produce significant changes in the magnetic field, whereas changes in current in ordinary wires produce minimal magnetic field changes, resulting in less noticeable electromagnetic effects.

• Energy Storage: Coils can store substantial amounts of magnetic energy, and the release of this energy during sudden changes in current can lead to high voltage spikes, resulting in sparking.

Preventing Sparking

To avoid sparking in coils, several measures can be taken:

• Using Flyback Diodes: When disconnecting the power supply to a coil, a flyback diode can provide a path for the current in the coil, absorbing the self-induced EMF and reducing the occurrence of sparking.

• Using Damping Resistors: In some cases, a damping resistor can be connected in series with the coil to reduce the rate of current change, thus reducing the self-induced EMF.

• Using Soft Switching Techniques: By controlling the rate of current change, soft switching techniques can reduce voltage spikes, thereby minimizing sparking.

Summary

Coils, due to their unique geometric structure and electromagnetic properties, are more prone to sparking when the current changes suddenly compared to ordinary wires. This is because of the voltage spikes caused by the self-inductance and mutual inductance effects in coils. Through proper design and technical approaches, the occurrence of sparking can be effectively reduced or eliminated.