Electricity Revision Notes

Ohmic Resistors and Ohm's Law

An ohmic resistor follows Ohm's Law, which states that the current (I) through a conductor between two points is directly proportional to the potential difference (V) across the two points, provided the temperature remains constant. The I-V characteristic graph for an ohmic resistor is a straight line passing through the origin. This consistent, linear relationship means its resistance (R = V/I) remains constant regardless of the potential difference applied or the current flowing through it. The gradient of the I-V graph (with I on the y-axis and V on the x-axis) is equal to 1/R, so a steeper gradient means lower resistance.

Non-Ohmic Components: Filament Lamp

A filament lamp is a non-ohmic component, meaning its resistance does not remain constant. Its I-V characteristic graph is a curve that becomes less steep as potential difference increases. This indicates that as the potential difference across the lamp increases, the current also increases, causing the filament to heat up significantly. As the temperature of the metallic filament rises, the resistance of the lamp increases. This is because the vibrating ions in the hotter filament impede the flow of electrons more frequently, reducing current flow for a given potential difference.

Non-Ohmic Components: Diode

A diode is a semiconductor component that allows current to flow predominantly in one direction. Its I-V characteristic graph shows a very distinctive non-linear behaviour. In the 'forward bias' direction (when the potential difference is applied in the correct polarity), very little current flows until a specific 'threshold voltage' (typically around 0.6-0.7 V for silicon diodes) is reached. Beyond this threshold, the resistance of the diode decreases sharply, allowing current to increase significantly with only small increases in potential difference. In the 'reverse bias' direction (when potential difference is applied in the opposite polarity), the diode has extremely high resistance, and virtually no current flows, unless the reverse breakdown voltage is exceeded.

Practical Investigation of I-V Characteristics (PAG P6)

To investigate the I-V characteristics of a component, a circuit is set up with an ammeter in series to measure current and a voltmeter in parallel to measure potential difference. A variable resistor or power supply is used to vary the potential difference across the component. The independent variable is the potential difference, and the dependent variable is the current. Control variables include the temperature of the component (for ohmic resistors) and the type of component being tested. Safety precautions include ensuring connections are secure and avoiding excessive current that could damage components. By plotting the collected data (current on y-axis, voltage on x-axis), the characteristic curve for the component can be determined and analysed.

Resistors in Series: Increased Net Resistance

When resistors are connected in series, they are arranged end-to-end along a single path. The current flows through each resistor sequentially. Each added resistor effectively lengthens the overall resistive path that the current must traverse. This increased 'length' of resistance means more opposition to the flow of charge (electrons). Therefore, adding more resistors in series always increases the net (total) resistance of the circuit. Imagine a longer, narrower pipe; it's harder for water to flow. Similarly, for electricity, adding resistors in series makes it harder for current to flow, increasing the total resistance.

Resistors in Parallel: Decreased Net Resistance

In a parallel connection, resistors are placed across the same two points, providing multiple alternative pathways for the current to flow. When current reaches a junction, it splits, with different portions travelling through each parallel branch. Each additional parallel resistor acts as an extra 'lane' for the current. Even if an added lane has high resistance, it still offers an alternative path which reduces the overall congestion. Providing more pathways means that the total opposition to current flow is reduced. Therefore, adding more resistors in parallel always decreases the net (total) resistance of the circuit because the current has more options to flow, making it easier for charge carriers to move through the circuit.

Qualitative Explanation Summary

The key qualitative difference lies in how current perceives the resistance. In series, current faces all resistances consecutively, adding them up. But in parallel, current can 'choose' paths, effectively bypassing some of the total resistance by having alternatives. This fundamental difference explains why series connections increase net resistance, while parallel connections decrease it. It's about whether you're adding obstacles in line or adding alternative routes.

What is Electric Current (I)?

Electric current (I) is the rate of flow of electric charge. In most circuits, this charge is carried by electrons moving through a conductor. A larger current means more charge is flowing per unit of time. The standard unit for current is the Ampere (A).

What is Potential Difference (V)?

Potential difference (V), also known as voltage, is the energy transferred per unit charge between two points in an electrical circuit. It is the 'driving force' that pushes the current around the circuit. The standard unit for potential difference is the Volt (V).

What is Electrical Resistance (R)?

Electrical resistance (R) is a measure of how much a material or component opposes the flow of electric current. Components with high resistance reduce the current flowing through them for a given potential difference. The standard unit for resistance is the Ohm (Ω).

Relationship between I, V, and R

These three quantities are interconnected. For a given circuit component, the current (I) flowing through it depends on both the potential difference (V) across it and its resistance (R). Specifically, a higher potential difference tends to increase the current, while a higher resistance tends to decrease the current. This relationship is often described by Ohm's Law, V = IR.

Ohmic Resistors

Ohmic resistors are electrical components where the resistance remains constant, provided the temperature and other physical conditions do not change. This type of resistor obeys Ohm's Law, meaning the current flowing through it is directly proportional to the potential difference across it. If you plot current (I) against potential difference (V) for an ohmic resistor, the graph will be a straight line passing through the origin. This consistent resistance makes them predictable in circuits, and they are typically used when a stable resistance is required.

Non-Ohmic Components

Non-ohmic components do not have a constant resistance; their resistance changes as the current or potential difference across them changes. The most common reason for this change is a change in temperature due to the flow of current. As current increases, components can heat up, which affects the movement of charge carriers and thus their resistance. A graph of current against potential difference for a non-ohmic component is curved. Examples include filament lamps and diodes, where resistance varies significantly with operating conditions.

Filament Lamps as Non-Ohmic Conductors

A filament lamp is a classic example of a non-ohmic conductor. When current passes through the thin metal filament, it rapidly heats up. This increase in temperature causes the atoms within the filament material to vibrate more vigorously. These increased vibrations lead to more frequent collisions with the free electrons that are trying to carry the current. Each collision impedes the electron flow, effectively increasing the lamp's electrical resistance. Therefore, as the current increases, the filament gets hotter, and its resistance rises.

Resistance and Temperature

For many materials, especially metals, resistance increases with temperature. In components like filament lamps, the flow of current generates heat (due to the transfer of energy from electrons during collisions). This heat causes the material's internal structure to become more chaotic, hindering the smooth flow of electrons. This explains why the resistance of a filament lamp is low when it's cold but increases significantly once it's glowing brightly due to the high temperature, leading to a non-linear current-voltage relationship.