Section 2 d) Summary

Electrostatic charge is unmoving electric charge, caused by the transfer of electrons when two insulators are rubbed together. Usually, electric charge is allowed to flow in a current, through a conductor. Conductors have free charged particles that allow the current to carry the charge, but insulators do not, so the charge is static; it does not move.

Common conductors include metals, molten or aqueous ionic compounds, and carbon.
Common insulators include plastics such as rubber or acrylic, as well as air, paper and glass.

Like charges repel, and opposites attract. These forces of attraction and repulsion get weaker as distance is increased. This principle allows us to harness charges and use them for our benefit

Applications of electrostatic charge:

Inkjet printers
1. Tiny inkdrops become electrically charged as they are forced out of a small nozzle.
2. They travel past two plates, one positive and one negative. The drop is repelled by the like charge and attracted to the opposite charge, directing its landing spot on the paper.
3. The voltage across the plates is changed to direct each drop to form the image.

Photocopiers
1. A projection of your image is shone onto a positively charged image plate.
2. The light causes charge to leak away from some places.
3. Negatively charged powder (toner) is attracted to the positively charged parts of the plate, then transferred onto positively charged paper.
4. The paper is heated to seal the toner.

Problems and Dangers of electrostatic charge:

Problems

• Screens attract dust as they get charged
• Clothes cling to one another, and the body
• Brushing hair can cause it to become statically charged and stand up

Dangers

• Can build up in clouds and cause lightening, which can be dangerous
• Buildup when fueling machinery could cause a spark, and an explosion
• Touching an object with a large charge can give you a burn, or even kill you

Investigations into electrostatic charge:

Rod and cloth
1. Hold a polythene rod next to running water. Nothing will happen.
2. Rub the rod with a cloth, so friction causes electrons to transfer from the cloth onto the rod, causing both to become charged equally but oppositely.
3. Hold the rod next to the running water again. The stream will be bent, as like charges repel and opposite charges attract.

Gold leaf electroscope
1. Hold the object near the metal disc of a gold-leaf electroscope.
2. If the object is charged, this will induce a charge and cause the gold leaf to rise as it is repelled from the similarly-charged metal plate.

Suspended rods
1. Suspend a rod with a known charge, then hold your object near it.
2. If nothing happens, the object is not charged
3. If there is repulsion, the object has the same charge as the suspended rod.
4. If there is attraction, they have opposite charges.