Static Electricity and Electric Fields

Static Electricity and Electric Fields

Static electricity is caused by the transfer of electrons between objects. Unlike current electricity, the charges in static electricity do not flow in a circuit — they build up on the surface of insulators.

How Static Charge Builds Up

When two insulating materials are rubbed together, electrons are transferred from one surface to the other. This is called charging by friction.

• The material that gains electrons becomes negatively charged
• The material that loses electrons becomes positively charged
• Only electrons move — protons are fixed in the nucleus and do not transfer

Example: When a polythene rod is rubbed with a cloth, electrons transfer from the cloth to the rod. The rod becomes negatively charged; the cloth becomes positively charged.

Important: The charges are not created — they are separated. The total charge before and after rubbing is the same. This is conservation of charge.

Forces Between Charged Objects

• Like charges repel (positive repels positive; negative repels negative)
• Unlike charges attract (positive attracts negative)

A charged object can also attract a neutral object. This is because the charge on the object induces (rearranges) the charges in the neutral object, pulling opposite charges to the near side. This is why a charged balloon sticks to a wall.

Detecting Charge — The Gold Leaf Electroscope

A gold leaf electroscope can detect static charge:

1. When a charged object touches the metal cap, charge spreads to the gold leaf and the metal plate

2. Both the leaf and plate gain the same charge

3. Like charges repel, so the gold leaf rises

4. The greater the charge, the more the leaf rises

Dangers of Static Electricity

Static charge can be dangerous in certain situations:

• Fuelling aircraft — Fuel flowing through pipes can build up static charge. A spark could ignite the fuel vapour. Solution: The aircraft is earthed with a bonding wire before refuelling, providing a path for charges to flow safely to the ground.
• Lightning — Charge builds up in clouds due to friction between ice particles. When the potential difference between the cloud and the ground is large enough, a massive spark (lightning) discharges the cloud.
• Operating theatres — Sparks from static could ignite flammable anaesthetic gases. Staff wear antistatic clothing and shoes.

Uses of Static Electricity

Electrostatic paint sprayers:

• Paint droplets are given the same charge as they leave the nozzle
• Like charges repel, so the droplets spread out evenly
• The object being painted is given the opposite charge (or earthed)
• The droplets are attracted to the object, giving an even coat
• Less paint is wasted because it wraps around the object, even painting the back

Electrostatic precipitators (in chimneys):

• Smoke particles pass charged metal plates
• The particles gain a charge and are attracted to oppositely charged collecting plates
• This removes harmful particles from waste gases before they enter the atmosphere

Photocopiers and laser printers use static charge to attract toner powder to specific areas of a drum, which is then transferred to paper.

Electric Fields

A charged object creates an electric field around it. Any other charged object placed in this field will experience a force.

• Electric field lines point away from positive charges and towards negative charges
• The closer together the field lines, the stronger the field
• The field is strongest close to the charged object and gets weaker with distance

Between two oppositely charged parallel plates, the electric field is uniform — the field lines are parallel and equally spaced. The field strength depends on the voltage across the plates and the distance between them.

A spark occurs when the electric field strength is strong enough to ionise the air molecules, creating a conducting path for charge to flow.

Exam Tips

• Always say that ELECTRONS are transferred, never that "charge" or "protons" are transferred
• When explaining attraction of a neutral object, describe the induced charge separation
• For electric field diagrams, draw arrows from positive to negative, and ensure they do not cross