Electricity Revision Notes

DC Circuit Basics and Polarity

Direct current (DC) involves a flow of electrical charge in one constant direction. Sources like batteries provide DC power. A battery has a positive (+) terminal and a negative (-) terminal. By convention, the 'conventional current' is said to flow from the positive terminal, through the circuit, and back to the negative terminal. This is the direction positive charges would move. It's important to remember that electrons, which are negatively charged particles, actually move in the opposite direction, from negative to positive. However, for circuit analysis and drawing, we primarily use the conventional current direction. This consistent directionality is a defining characteristic of DC circuits, contrasting with alternating current (AC) where the current direction periodically reverses.

Common Circuit Symbols

Circuit diagrams use standard symbols to represent components. Key symbols include:

• Cell: Two parallel lines, one long (positive) and one short (negative).
• Battery: Multiple cells connected in series.
• Wire: A straight line.
• Connected Wires: A dot at the junction of crossing lines.
• Lamp (or bulb): A circle with a cross inside.
• Resistor: A rectangle or a zig-zag line.
• Ammeter: A circle with an 'A' inside (measures current, placed in series).
• Voltmeter: A circle with a 'V' inside (measures potential difference, placed in parallel).
• Switch: A break in the circuit, allowing current to be turned on or off.

Understanding these symbols is fundamental to interpreting and constructing any electrical circuit diagram.

Drawing Circuit Diagrams

When drawing a circuit diagram, follow these guidelines to ensure clarity and accuracy:

1. Always use the standardised symbols for each component.
2. Draw wires as straight lines, usually at right angles when changing direction, to create neat and easy-to-read diagrams.
3. Ensure all connections are clear. Use a dot to indicate where wires are purposefully joined.
4. Place components in positions that logically represent their function within the circuit. For instance, an ammeter must be in series with the component whose current it is measuring, and a voltmeter must be in parallel across the component whose potential difference it is measuring.
5. Clearly indicate the positive and negative terminals of sources like cells or batteries. The longer line in the cell symbol always represents the positive terminal.

The Fundamental Nature of Electric Charge

Electric charge is a basic property of matter. It is not something that can be created or destroyed, but rather transferred. All matter, from the smallest particles to the largest objects, possesses this property. It is the foundation for all electrical phenomena and interactions observed in the universe. Understanding charge is key to comprehending electricity and magnetism.

Types of Electric Charge: Positive and Negative

There are two distinct types of electric charge: positive and negative. These labels were assigned arbitrarily in the past, but they are now universally recognised. Protons, which are subatomic particles found in the nucleus of an atom, carry a positive charge. Electrons, which are even smaller subatomic particles that orbit the nucleus, carry a negative charge. These opposing charges are crucial for the stability and behaviour of atoms and molecules.

Neutrality and Charge Imbalance

Most objects in our everyday experience appear to be electrically neutral. This is because they contain an equal number of positive charges (protons) and negative charges (electrons), whose effects cancel each other out. However, if an object gains or loses electrons, this balance is disrupted. Gaining electrons results in an excess of negative charge, making the object negatively charged. Losing electrons results in a deficit of negative charge (or an excess of positive charge), making the object positively charged.

Magnetic Field Definition and Characteristics

A magnetic field is an area around a magnet where magnetic force can be experienced. This force attracts or repels other magnets or magnetic materials. The two main characteristics of a magnetic field are its strength and its direction. Understanding these characteristics helps to predict how magnets will interact with other magnetic objects or fields.

Magnetic Field Strength

The strength of a magnetic field is not uniform; it varies from point to point. It is strongest at the poles (the ends) of a magnet and weakens significantly as the distance from the magnet increases. This means that an object will experience a greater magnetic force when it is closer to the magnet's poles and a weaker force when it is further away. The density of magnetic field lines in a diagram indicates the relative strength, with closer lines showing a stronger field.

Magnetic Field Direction

The direction of a magnetic field at any point is conventionally defined as the direction that the North pole of a small, free-moving compass needle would point if placed at that point. Outside a bar magnet, magnetic field lines are drawn to show that they emerge from the North pole and curve around to enter the South pole. Arrows are used on these lines to indicate this direction. Inside the magnet, the field lines are considered to run from the South pole to the North pole, completing continuous loops. These field lines never intersect.

Visualising Magnetic Fields

Magnetic fields can be visualised using either iron filings or a compass. When iron filings are sprinkled around a magnet, they align themselves along the magnetic field lines. A small plotting compass, when moved around a magnet, will change its orientation, with its North pole pointing in the direction of the magnetic field at each position. By marking the direction indicated by the compass at various points and joining them up, the invisible magnetic field lines can be mapped out.

What are Linear and Non-Linear Components?

A circuit component is linear if its resistance remains constant regardless of the voltage across it or the current flowing through it. This means that current is directly proportional to voltage. An I-V graph for a linear component will show a straight line passing through the origin. Examples include standard resistors at a constant temperature. A circuit component is non-linear if its resistance changes with the applied voltage or current. This means that current is not directly proportional to voltage. An I-V graph for a non-linear component will show a curved line. Examples include filament lamps and diodes.

Interpreting I-V Graphs for Linear Components

For a linear component like a fixed resistor, the I-V graph is a straight line through the origin. This signifies a constant resistance. According to Ohm's Law (V=IR), if R is constant, then I is directly proportional to V. The gradient of this straight line is equal to 1/R. A steeper line indicates lower resistance, while a less steep line indicates higher resistance. The key characteristic is the consistent gradient, demonstrating constant resistance.

Interpreting I-V Graphs for Non-Linear Components

For non-linear components, the I-V graph is a curve. This curvature indicates that the resistance of the component is changing as the voltage or current changes. For example, a filament lamp's I-V graph will curve towards the voltage axis (x-axis) at higher voltages, showing that its resistance increases as it gets hotter. A diode's I-V graph shows current flowing significantly only after a certain 'turn-on' voltage in one direction and very little current in the other direction, demonstrating a highly variable resistance.

Practical Investigation (PAG P6)

To investigate the I-V characteristics of a component, you typically set up a circuit with a variable power supply (or a power supply with a variable resistor in series), an ammeter in series with the component, and a voltmeter in parallel across the component. The independent variable is the voltage (V), which is varied, and the dependent variable is the current (I) measured. Control variables include temperature (for some components like resistors) and the specific component being tested. Results are plotted on an I-V graph to determine if the component is linear or non-linear based on the graph's shape.