4.2.5 Static Electricity (Physics Only)

In GCSE Physics, static electricity refers to the buildup of electric charge on the surface of materials. Unlike current electricity, which involves a continuous flow of electrons, static electricity remains "static" (still) until it can jump or flow to another object.

1. Static Charge and Friction

When certain insulating materials are rubbed against each other, they become electrically charged. This is known as charging by friction.

1. The Process: Negatively charged electrons are rubbed off one material and onto the other.
2. The Result: * The material that gains electrons becomes negatively charged.
3. The material that loses electrons is left with an equal positive charge.

Exam Tip: Remember, protons never move during charging by friction! Only electrons (which are on the outer shells of atoms) are transferred.

2. Attraction and Repulsion

Objects with an electric charge exert a non-contact force on other charged objects. The rules of electrostatic force are:

1. Opposite charges attract (Positive attracts Negative).
2. Like charges repel (Positive repels Positive, or Negative repels Negative).

If a charged object is brought near an uncharged (neutral) insulator, it can still be attracted due to induced charge, where the electrons in the neutral object shift slightly away or toward the charged object.

3. Electric Fields

A charged object creates an electric field around itself. An electric field is a region where a second charged object will experience a non-contact force.

Field Characteristics:

1. Direction: Field lines always point away from positive charges and towards negative charges.
2. Strength: The closer the lines are together, the stronger the field. As you move further from the object, the field strength decreases.

Sparking and Dielectric Breakdown:

A spark occurs when there is a high potential difference between a charged object and the earth (or another object).

1. A strong electric field causes the surrounding air particles to become ionised (split into positive ions and electrons).
2. Ionised air becomes a conductor, allowing a current to flow through it—this is the spark.

4. Mathematical Definition of Charge

While often covered in general electricity, you must apply the charge flow equation to static scenarios:

Q = I * t

Note: In static electricity, we also consider the field strength (E) qualitatively, but the AQA syllabus primarily focuses on the force F increasing as the distance d decreases.

5. Edge Cases & Common Misconceptions

1. Conductors vs. Insulators: Static charge only builds up on insulators (like plastic, glass, or amber). In conductors (like metals), electrons flow away immediately to the earth, preventing a buildup.
2. Earthing: If a charged object is connected to the ground via a conductor, it is "earthed." Electrons will flow up from the ground or down to it to neutralise the object.
3. The "Positive Move" Myth: Students often write that "positive charge moves." In a solid insulator, the positive nuclei are fixed. Only electrons move.

6. Mathematical Example

Question: A synthetic comb develops a static charge of $-0.005C$ after being rubbed through hair. If the discharge (spark) to a metal tap lasts for $0.0002s$ , calculate the average current during the spark.

Solution (FIFA Method):

1. Formula:
2. $I=\frac{Q}{t}$
3. Insert values: $I=\frac{0.005}{0.0002}$
4. Fine-tune (Calculation): I = 25
5. Answer: Current = 25 A