Static Electricity and Simple Circuits

Static Electricity

Charge is a fundamental property of all matter. In most objects, the number of positive charges equals the number of negative charges, so the object is electrically neutral. When two insulating materials are rubbed together, friction causes electrons to transfer from one surface to the other, creating static electricity.

The material that gains electrons becomes negatively charged. The material that loses electrons becomes positively charged. It is always the electrons that move — protons are locked inside the nucleus and cannot transfer between objects. For example, when a polythene rod is rubbed with a cloth, electrons move from the cloth to the rod: the rod becomes negative and the cloth becomes positive.

Static charge builds up only on insulators because electrons cannot flow away through the material. On a conductor, any excess charge spreads out immediately. If enough charge accumulates, it can discharge as a spark — this is what causes lightning.

Electric Fields

A charged object creates an electric field in the space around it. Electric field lines show the direction a positive charge would move if placed in the field — they point away from positive charges and towards negative charges. Where field lines are closer together, the field is stronger.

Two charged objects exert forces on each other through their electric fields — like charges repel and unlike charges attract. This force acts even without the objects touching, which is why a charged balloon can attract small pieces of paper without contact.

Current, Charge and Potential Difference

Current is the rate of flow of charge through a conductor. For current to flow, there must be a source of potential difference and a complete (closed) circuit. The equation linking charge, current and time is:

$$Q = I \times t$$

Where \(Q\) is charge (coulombs, C), \(I\) is current (amperes, A), and \(t\) is time (seconds, s). In a series circuit, current is the same at every point because charge is not used up. Current is measured with an ammeter in series; potential difference is measured with a voltmeter in parallel across the component.

Resistance and Ohm’s Law

Resistance opposes the flow of current. The greater the resistance, the smaller the current for a given potential difference. The relationship is:

$$V = I \times R$$

Where \(V\) is potential difference (V), \(I\) is current (A), and \(R\) is resistance (Ω). Ohm’s Law states that current is directly proportional to potential difference, provided temperature remains constant. This applies only to ohmic conductors — components whose resistance stays constant.

I-V Characteristics

An I-V characteristic graph shows how current varies with potential difference for a component. You need to know the shape for three key components (PAG P6):

• Fixed resistor (ohmic conductor): a straight line through the origin. Gradient is constant, so resistance is constant. Obeys Ohm’s Law.
• Filament lamp: an S-shaped curve that starts steep (low resistance when cool) and flattens as PD increases (higher resistance as the filament heats up). As current increases, the filament temperature rises, so resistance increases. The curve is symmetrical in forward and reverse directions.
• Diode: current only flows in one direction (the forward direction). In the reverse direction, resistance is very high and virtually no current flows. There is a threshold voltage (roughly 0.6 V for a silicon diode) below which even forward current is negligible.