Question

If the switch is open but near the point of contact where it would close the circuit, a spark can arc across the switch. Is a spark more likely while the switch is being closed to connect the circuit, or while the switch is being opened to disconnect the circuit? Why?

Answer 1

Answer:

A spark is more likely to be generated when switch is being opened to disconnect the circuit as compared to when switch is being closed to make the circuit.

Explanation:

In high voltage applications, a spark is generated when the switch is closed to make the circuit and and also when switch is opened to break the circuit. This phenomenon happens when the voltage potential surpasses the breakdown voltage of the air.

A spark is more likely to be generated when switch is being opened to disconnect the circuit as compared to when switch is being closed to make the circuit.

This happens due to the fact that there is no back emf induced when the switch is closed. When we open the switch, the inductor resists the change in current and tries to maintain the continuous current and because of this huge amount of voltage across the gap appears.

A fly-back diode is usually used to protect the devices from back emf.

Answer 2

Answer:

Explanation:

According to Faraday's law, the suddenly collapsing magnetic flux will induce an emf in the circuit, the direction of which will be such as to maintain the current in the circuit to oppose the change in magnetic flux. The magnitude of this emf will be proportional to the rate of change in magnetic flux, so when you flick the switch quickly, the magnetic flux changes quickly resulting in a larger emf. This in turn creates a large electric field gradient across the contacts of the switch. If this is greater than the ionisation potential of the air, then the molecules of the air in the vicinity of the switch will partially ionise, allowing them to conduct electricity which is manifested as a 'spark' across the switch. Conversely, when you flick the switch gently (slowly), resulting potential gradient across the switch becomes smaller.

Furthermore, if the circuit is largely 'inductive' (such as an incandescent lamp, heater element, or electric motor) then it is more likely to produce a more visible spark when the circuit is opened, because the 'inductive' element(s) in the circuit are storing energy in the form of magnetic field(s), which results in higher induced emf when the field collapses.