Electricity Revision Notes

Ohm's Law: V = I × R

Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in an electrical circuit. Voltage, measured in volts (V), is the electrical potential difference that pushes charges around a circuit. Current, measured in amperes (A), is the flow of electrical charge past a point per unit time. Resistance, measured in ohms (Ω), is how much a component opposes the flow of current. You must be able to recall and apply this formula to calculate any of the three quantities if the other two are known. For example, if V = 12 V and I = 2 A, then R = V/I = 12 V / 2 A = 6 Ω. This equation is fundamental to understanding electrical circuits.

Ohmic vs. Non-Ohmic Resistors

An ohmic resistor is a component whose resistance remains constant regardless of the current flowing through it or the voltage across it, provided its temperature remains constant. The current-voltage graph for an ohmic resistor is a straight line passing through the origin. This constant resistance is a characteristic property of the material and its dimensions. Most metal wires behave as ohmic resistors. In contrast, a non-ohmic resistor is a component whose resistance changes as the current (or voltage) across it changes. A common example is a filament lamp. As current increases, the filament heats up, causing its resistance to increase. This leads to a non-linear current-voltage graph for a non-ohmic component.

Factors Affecting Resistance in Non-Ohmic Resistors

For certain resistors, the value of resistance changes as the current changes. Filament lamps are a prime example: as the current through them increases, their temperature rises significantly. This increased temperature causes the internal resistance of the filament material to increase. This means that if you double the voltage across a filament lamp, the current will increase, but it won't necessarily double, because the resistance itself has gone up. Diodes and thermistors are other examples of non-ohmic components where resistance varies with temperature or applied voltage, leading to non-linear characteristics.

PAG practical: Investigating I-V Characteristics

A common PAG involves investigating the current-voltage characteristics of different components. You would set up a circuit with a variable power supply, an ammeter in series to measure current, and a voltmeter in parallel to measure voltage across the component (e.g., a resistor, filament lamp, or diode). By varying the voltage and recording corresponding current readings, you can plot an I-V graph. For an ohmic resistor at constant temperature, you would expect a straight line through the origin. For a filament lamp, you would observe a curve, showing that resistance increases as current increases due to heating. This experiment helps to visually understand the difference between ohmic and non-ohmic behaviour.

Production of Static Electricity by Rubbing

Static electricity arises from a charge imbalance within or on a material. It is commonly produced by rubbing two different insulating materials together, a process known as triboelectric charging. During rubbing, electrons are transferred from one material to the other. The material that gains electrons becomes negatively charged, while the material that loses electrons becomes positively charged. For instance, rubbing a plastic rod with a cloth often results in the rod gaining electrons and becoming negatively charged, with the cloth becoming positively charged. The charges remain static on the surfaces unless there is a path for them to move, such as through a conductor or an electrostatic discharge.

Forces Between Charged Objects

Objects with static electric charges exert forces on each other even when they are not in physical contact. The fundamental rules for these electrostatic forces are: like charges repel, and opposite charges attract. For example, two positively charged objects will push each other away, and two negatively charged objects will also push each other away. However, a positively charged object and a negatively charged object will pull towards each other. These forces are due to the electric fields created by the charges. A charged object can also attract an uncharged object by causing a temporary separation of charges within the uncharged object through induction.

Sparking: Release of Static Charge

A spark is a sudden and visible discharge of static electricity. It occurs when a significant amount of static charge accumulates on an object, creating a strong electric field between that object and another object (either charged or earthed). If this electric field becomes strong enough, it can ionise the air molecules in the gap between the objects. This ionised air then becomes a conductor, providing a low-resistance path for the accumulated charge to jump across. The rapid flow of charge results in a flash of light, often accompanied by a crackling sound and a small amount of heat. This phenomenon is why you might get a static shock when touching a metal object after walking across a carpet, or in extreme cases, it causes lightning.

Evidence of Electrostatic Forces

There is clear evidence that charged objects exert forces. For example, if you rub a balloon on your hair, the balloon becomes charged. The charged balloon can then pick up small pieces of paper, showing attraction. If you have two balloons charged in the same way (e.g., both rubbed on hair), and you bring them close, they will push each other away, demonstrating repulsion. Similarly, a charged plastic comb can deflect a thin stream of water from a tap without touching the water, as the charged comb attracts the polar water molecules. These observations confirm that electrostatic forces act at a distance without direct contact.

Series Circuits: Key Features

In a series circuit, all components are connected one after another, forming a single path for the current. This means the electric current is the same at every point in the circuit. If one component fails or is removed, the entire circuit breaks, and current stops flowing to all other components. This is because there is only one pathway for the electricity. The total resistance of a series circuit is found by adding the individual resistances of each component (R_total = R1 + R2 + ...). Also, the total voltage supplied by the power source is divided among the components in the circuit.

Parallel Circuits: Key Features

In a parallel circuit, components are connected across the same two points, creating multiple independent pathways or branches for the current. This arrangement means that the voltage across each component in parallel is the same as the voltage of the power source. The total current supplied by the source splits among these branches, with the sum of the currents in each branch equalling the total current. If one component in a parallel branch fails, the other branches remain operational, and current continues to flow through them. This makes parallel circuits more reliable for everyday use, such as in household wiring.

Current and Voltage Distribution

In series circuits, the current is constant throughout for all components, but the voltage is divided, meaning each component only receives a fraction of the total supply voltage (unless it's the only component). In contrast, in parallel circuits, the voltage across each parallel component is the same as the supply voltage. However, the total current from the source is shared among the different branches. Current takes the path of least resistance, so more current will flow through branches with lower resistance.

Impact on Circuit Operation

The way components are connected significantly impacts how a circuit behaves. In series, adding more components (like bulbs) increases total resistance, reducing the total current and making the existing bulbs dimmer. Removing a component breaks the entire circuit. In parallel, adding more components (like bulbs) provides more pathways for current, which decreases the overall resistance of the circuit and increases the total current drawn from the supply. If one component fails in parallel, the others can continue to function, which is critical for many applications.

The Basis of Static Charge

All uncharged objects contain an equal number of positive protons and negative electrons, making them electrically neutral. Static electricity arises when there is an imbalance of these charges on an object's surface. This imbalance is primarily due to the movement of electrons, which are much lighter and more mobile than protons. Protons are fixed within the atomic nucleus and do not transfer during friction. Therefore, any change in an object's overall charge is due to the gain or loss of electrons.

How Electrons Transfer During Friction

When two different insulating materials are rubbed together, the friction can provide the necessary energy to dislodge electrons from the atoms of one material and transfer them to the atoms of the other material. This process is known as triboelectric charging. For example, when a plastic ruler is rubbed with a wool cloth, electrons are transferred from the wool to the plastic ruler. The ruler gains electrons and becomes negatively charged, while the wool loses electrons and becomes positively charged. The ease with which a material gains or loses electrons depends on its electron affinity and the type of material it is rubbed against.

Resulting Charges and Electrostatic Forces

The object that gains electrons acquires an overall negative charge, as it now has more electrons than protons. The object that loses electrons acquires an overall positive charge, as it now has fewer electrons than protons (meaning the positive charge of its protons is no majority). Once charged, these objects will exert electrostatic forces on other charged or uncharged objects. Objects with opposite charges (positive and negative) will attract each other, while objects with like charges (positive-positive or negative-negative) will repel each other. This explains various static phenomena like hair standing on end or attracted dust particles.