Distribution of Charge – Outcomes Discuss where charges reside on conductors. Demonstrate where charges reside on conductors. Describe point discharge. Demonstrate point discharge. Describe how lightning occurs. Describe how lightning conductors work.

Demonstrate Where Charges Reside Charges tend to reside on the outside of conductors. Charge a hollow conductor. Using a proof plane, touch the inside of the conductor. Hold up the proof plane to an electroscope and note no deflection. Touch the outside of the conductor with the proof plane. Hold up the proof plane to an electroscope and note deflection. Charges reside on the outside of conductors.

Demonstrate Where Charges Reside

Demonstrate Where Charges Reside Charges tend to accumulate at points. Charge a pear-shaped conductor. Using a proof plane, touch the flat side of the conductor. Hold up the proof plane to an electroscope and note minor deflection. Touch the pointed side of the conductor with the proof plane. Hold up the proof plane to an electroscope and note deflection. Charges reside around points on conductors.

Demonstrate Where Charges Reside

Describe Point Discharge a.k.a. “the point effect” If the charges around a point build up enough, they will attract or repel nearby ions and free electrons. As these ions move, they collide with other atoms, knocking off electrons, creating more ions. The unlike ions will partly neutralise the conductor, while the like ions will create an “electric wind” away from the conductor.

Demonstrate Point Discharge Hold a lit candle up to a pointed part of a pointed conductor. Charge the conductor. Note the flickering of the candle flame in response to the electric wind.

Describe How Lightning Occurs In clouds, there is friction between the air and water / ice molecules – this causes a build up of charge (the exact mechanism is unknown). Negative charges sink to the bottom of the cloud and accumulate.

Describe How Lightning Occurs The cloud wants to discharge, but is blocked by an insulator (air). To cross this gap, a serious charge needs to build up. This leads to a serious discharge (a.k.a. lightning).

Describe How Lightning Conductors Work Lightning conductors work in two ways: They provide a conductive path to ground for lightning. This prevents potential damage to buildings etc.

Describe How Lightning Conductors Work The cloud repels electrons in the conductor, making it positively charged. Since the conductor is pointy, it will exhibit point discharge, blowing positive charges upward. These positive charges neutralise the cloud, preventing lightning.