Static Charge and Simple Circuits
Static Electricity
Charge is a fundamental property of all matter. In most objects, the number of positive and negative charges is equal, so the object has zero net charge and appears 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 only builds up on insulators because electrons cannot flow away through the material. On a conductor, any excess charge would immediately spread out or flow to earth. If enough static charge builds up, it can discharge as a spark — this is what causes lightning, where charge jumps between clouds and the ground.
Static electricity is caused by the transfer of electrons between insulating materials. Like charges repel; opposite charges attract. Only electrons move — protons stay in the nucleus.
Current, Charge and Time
Current is the rate of flow of charge through a conductor. In metals, it is free electrons that carry the charge. For current to flow, two conditions must be met: there must be a source of potential difference (voltage), and the circuit must be a closed loop with no gaps.
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). One ampere means one coulomb of charge passes a point in the circuit every second. In a single closed loop (series circuit), the current is the same at every point — charge is not used up as it flows through components.
Current is measured with an ammeter, connected in series. Potential difference is measured with a voltmeter, connected in parallel across the component being tested.
Resistance and Ohm’s Law
Resistance opposes the flow of current. The greater the resistance, the smaller the current for a given voltage. The relationship between potential difference, current and resistance is:
$$V = I \times R$$
Where \(V\) is potential difference (V), \(I\) is current (A), and \(R\) is resistance (\(\Omega\)). This can be rearranged to find current (\(I = \frac{V}{R}\)) or resistance (\(R = \frac{V}{I}\)). Strictly speaking, Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant. This only applies to ohmic conductors — components whose resistance does not change with current.
In a series circuit, all components are connected in a single loop. Current is the same everywhere. Potential difference is shared between components — the sum of voltages across all components equals the supply voltage. Total resistance = sum of individual resistances: \(R_{\text{total}} = R_1 + R_2 + R_3\).
In a parallel circuit, components are on separate branches. Potential difference is the same across each branch. Current splits between branches — more current flows through the branch with lower resistance. Adding resistors in parallel decreases the total resistance because there are more paths for current to flow. This is why household circuits use parallel wiring: each appliance gets the full mains voltage, and one device failing does not break the others.
I-V Characteristics
An I-V characteristic graph shows how current varies with potential difference for a component. You should know the shapes for three key components:
• Resistor (ohmic conductor): a straight line through the origin. The gradient is constant, meaning resistance does not change. This confirms the component obeys Ohm’s law.
• Filament lamp: a curve that starts steep and flattens at higher voltages. As more current flows, the filament gets hotter, increasing its resistance. The curve is symmetrical in both directions (forward and reverse voltage).
• Diode: current only flows in one direction (the forward direction). In the reverse direction, resistance is so high that effectively no current flows. There is a threshold voltage (about 0.6 V for silicon) below which even forward current is negligible.
An ohmic conductor has a straight-line I-V graph (constant resistance). A filament lamp’s resistance increases with temperature (curved graph). A diode only conducts in one direction.
Some components have resistance that changes with environmental conditions:
• A light-dependent resistor (LDR) has high resistance in the dark and low resistance in bright light. It is used in circuits that need to respond to light levels, such as automatic streetlights and burglar alarms.
• An NTC thermistor (negative temperature coefficient) has high resistance when cold and low resistance when hot. It is used in temperature-sensing circuits such as thermostats, fire alarms, and car engine sensors.
Both LDRs and thermistors are used in potential divider circuits to produce a variable voltage output that depends on light or temperature.