Final answer:
The causes of brush discharges in static electricity include high voltage, low humidity, and sharp objects; these factors individually contribute to the build-up and release of static charge, and therefore, all of the above are correct.
Step-by-step explanation:
Static electricity is a phenomenon where an unbalance of electric charges within or on the surface of a material occurs. The causes of propagating brush discharges in static electricity are multifaceted. Brush discharges may resemble tiny lightning bolts and can be observed under certain circumstances where static electricity is involved. The factors influencing such discharges include:
- High voltage: This is necessary to overcome the air's natural insulation; higher voltages can jump across gaps, leading to a visible discharge.
- Low humidity: Dry air has less conductivity, allowing for higher static charge buildup, leading to stronger discharges.
- Sharp objects: These create points of high electric field strength, encouraging a breakdown of the air's insulative properties, and therefore, discharge.
All the above factors contribute to the occurrence and characteristics of static discharges. So the mention correct option in final answer to the question, 'What are some causes of propagating brush in static discharges?' is number 4, All of the above. High voltage, low humidity, and sharp objects all play roles in the propagation and intensity of static electricity discharges.