Electricity Revision Notes

Defining Current (I)

Current (I) is the rate of flow of electrical charge through a circuit. It represents how many electrons pass a point in the circuit per second. The standard unit for current is the ampere (A). An ammeter is used to measure current and must be connected in series with the component through which the current is being measured. If there is a break in a series circuit, current cannot flow.

Defining Potential Difference (V)

Potential difference (V), or voltage, is the energy transferred per unit charge between two points in a circuit. It's the 'electrical push' that drives the current. The standard unit for potential difference is the volt (V). A voltmeter is used to measure potential difference and must be connected in parallel across the component for which the voltage is being measured. A higher potential difference means more energy is supplied to, or used by, the charge.

Defining Resistance (R)

Resistance (R) is a measure of how much a component opposes the flow of electrical current. Components with higher resistance make it harder for current to flow. The standard unit for resistance is the ohm (Ω). Resistors are components specifically designed to have a certain resistance. Changing the resistance in a circuit affects the current flowing through it for a given potential difference. For example, a longer wire of the same material and cross-sectional area will have higher resistance.

Relationship between I, V, and R

The current (I) flowing through a component is directly proportional to the potential difference (V) across it and inversely proportional to its resistance (R). This means if you increase the potential difference, the current increases (assuming constant resistance). If you increase the resistance, the current decreases (assuming constant potential difference). This fundamental relationship is key to understanding how circuits behave and is explored in practical investigations like PAG P6 where you might measure how current changes with potential difference for a resistor, lamp, or diode.

Series Circuits: Resistors and Net Resistance

In a series circuit, components are connected one after another, forming a single continuous loop. When resistors are connected in series, the current must pass through each resistor sequentially. Each additional resistor in a series circuit adds further opposition to the flow of charge (current). Think of it like adding more obstacles to a single path. Because the current has to overcome the resistance of every component, the total resistance of the circuit increases. This increased net resistance leads to a smaller overall current for a given voltage across the entire circuit. The total resistance is the sum of individual resistances: R_total = R1 + R2 + R3...

Parallel Circuits: Resistors and Net Resistance

In a parallel circuit, components are connected across the same two points, creating multiple alternative pathways for the current. When resistors are joined in parallel, the current has options to split and flow through different branches. Adding more resistors in parallel provides additional routes for the current to take. This is similar to opening more lanes on a congested highway; traffic can flow more easily. By providing more paths, the overall ability for current to flow is enhanced, meaning the total or net resistance of the circuit decreases. The net resistance in a parallel circuit is always less than the resistance of the smallest individual resistor in the parallel combination. The formula for total resistance in parallel is more complex, but qualitatively, it always results in a lower overall resistance.

Analogy for Resistance

An excellent analogy for understanding resistance in circuits involves water flow through pipes. If resistors are connected in series, it's like joining several narrow pipes end-to-end to create a longer, more restrictive pathway for water. This increases the total 'resistance' to water flow. If resistors are connected in parallel, it's like connecting several pipes side-by-side between two points. This creates multiple channels for water to flow, effectively widening the overall pathway and making it easier for water to pass through. This reduces the total 'resistance' to water flow, even if each individual pipe is narrow. This analogy helps to visualise why series connections increase resistance and parallel connections decrease it.

Experiment: Measuring Resistance in Series and Parallel (PAG P7)

An experiment to investigate resistance in series and parallel involves using resistors, an ammeter, a voltmeter, a power supply, and connecting wires. For series, connect two resistors end-to-end, then measure the total current and voltage across the combination. For parallel, connect the two resistors across the same two points, then measure the total current and voltage. Use Ohm's Law (R = V/I) to calculate the total resistance for each configuration. You should observe that the total resistance for the series arrangement is higher than the individual resistances, while for the parallel arrangement, the total resistance is lower than the smallest individual resistance. Ensure safe current limits are respected and components do not overheat.

Ohmic Resistors: Constant Resistance

An ohmic resistor is a component where the resistance remains constant, provided its temperature does not significantly change. This means that the current flowing through it is directly proportional to the voltage applied across it. A graph of current against voltage for an ohmic resistor will be a straight line passing through the origin. Fixed resistors and most metallic wires at a stable temperature are good examples of ohmic resistors. The key characteristic is that the ratio V/I (voltage applied / current flowing) is always the same, representing the constant resistance (R).

Non-Ohmic Resistors: Changing Resistance

Unlike ohmic resistors, the resistance of non-ohmic components does not remain constant. Their resistance can change significantly as the current, voltage, or temperature varies. This means that the relationship between current and voltage is not linear. When sketching a current-voltage graph for a non-ohmic component, it will not be a straight line. The value of resistance (R = V/I) will therefore change depending on the operating conditions. Understanding these changes is important for designing and analysing circuits with these components.

Filament Lamps: An Example of Changing Resistance

A common example of a non-ohmic component is a filament lamp. When a small current flows through the lamp, the filament is cool and its resistance is relatively low. As the current increases, the filament heats up considerably. The increased temperature causes the atoms within the filament material to vibrate more vigorously, which makes it harder for electrons to flow, thus increasing the resistance. Therefore, as the current (and power) through a filament lamp increases, its resistance increases. A graph of current against voltage for a filament lamp would show a curve flattening off at higher currents/voltages, indicating increasing resistance.

Thermistors and Diodes: Other Non-Ohmic Devices

Thermistors are resistors whose resistance changes significantly with temperature. Most common thermistors (NTC thermistors) have a resistance that decreases as their temperature increases. Diodes are semiconductor devices that only allow current to flow easily in one direction (forward bias) and have a very high resistance in the opposite direction (reverse bias). In the forward direction, their resistance also changes significantly with voltage, typically showing a very low resistance once a certain threshold voltage is met. Both thermistors and diodes are examples where resistance is not constant but depends on external conditions or the direction of current flow.

Understanding Electric Current

Electric current is defined as the rate of flow of electric charge. It is measured in Amperes (A). In a circuit, charged particles (often electrons in metals) move from a higher potential energy point to a lower potential energy point. The direction of conventional current is defined as the direction of flow of positive charge, from the positive terminal to the negative terminal of the power supply, even though electrons (negative charges) actually move in the opposite direction.

Current in a Single Closed Loop (Series Circuits)

For any single closed loop in an electrical circuit, the current is the same at every point. This is a direct consequence of the principle of conservation of charge. Charge cannot build up or be used up at any point in the circuit. Therefore, the rate at which charge passes through any cross-section of the wire or component must be constant. If you measure the current just before a light bulb, through the light bulb, and just after the light bulb, all three measurements will yield the exact same value, provided they are all part of the same series loop.

Analogy: Water Flow in a Pipe

A useful analogy for understanding current in a series circuit is water flowing through a single pipe. Imagine a pump pushing water through a closed loop of pipe. The amount of water flowing past any point in that pipe per second must be the same throughout the entire loop. If the pipe narrows, the water speeds up, but the volume of water passing per second remains constant. Similarly, in a series circuit, current (flow of charge) remains constant, even if components like resistors slow down the charge flow locally.