Physics (0625) | Electric circuits

Study Notes: Electric Circuits (IGCSE Physics 0625)

Hello future electrician! This chapter is all about how electricity behaves when we route it through different paths and components. Understanding circuits is crucial because almost every modern device, from your phone to traffic lights, relies on these fundamental concepts.
Don't worry if this seems tricky at first—we'll use analogies to make the flow of current and voltage easy to visualize!

Key Terms Review (Core & Supplement)

• e.m.f. (Electromotive Force): The electrical work done by a source (like a battery) in moving a unit charge around a complete circuit. It’s measured in volts (V).
• Potential Difference (p.d.) or Voltage (V): The work done per unit charge moving between two points in a circuit (i.e., across a specific component). It’s also measured in volts (V).
• Current (I): The rate of flow of charge. Measured in amperes (A).
• Resistance (R): The opposition to the flow of current. Measured in ohms (\(\Omega\)).

1. Circuit Diagrams and Components (4.3.1)

You need to be able to draw, interpret, and understand the function of common circuit components. Think of these symbols as the alphabet of electronics!

Common Circuit Symbols and Functions

(Note: You should be familiar with the visual symbols provided in the syllabus chart. Below are the names and functions.)

• Cell / Battery (Source): Provides the e.m.f. to the circuit. A battery is just two or more cells joined together.
• Fixed Resistor: Limits the current flow. Its resistance value is constant.
• Variable Resistor: Allows the resistance to be changed, which controls the current or voltage elsewhere in the circuit.
• Heater / Lamp (Filament): Converts electrical energy into thermal energy and light.
• Ammeter (A): Measures current. Must be placed in series with the component being measured.
• Voltmeter (V): Measures potential difference (voltage). Must be placed in parallel across the component being measured.
• Fuse: A safety device. Contains a thin wire that melts and breaks the circuit if the current becomes too high (excess current protection).
• Switch: Used to connect or disconnect the circuit.
• Motor: Converts electrical energy into kinetic energy (motion).
• Electric Bell: Converts electrical energy into sound energy.

Special Components (Sensing and Control)

• Thermistor (NTC - Negative Temperature Coefficient): A resistor whose resistance changes with temperature.
  Function: As temperature increases, its resistance decreases. Used in temperature sensors (e.g., electronic thermometers, fire alarms).
• LDR (Light-Dependent Resistor): A resistor whose resistance changes with light intensity.
  Function: As light intensity increases, its resistance decreases. Used in light sensors (e.g., automatic night lights, burglar alarms).

Extended Content: Diodes and LEDs (Supplement)

Diodes are like one-way streets for current.

• Diode: Allows current to flow easily in one direction (forward bias) but has very high resistance and blocks current flow in the reverse direction (reverse bias). Used for rectification (turning a.c. into d.c.).
• Light-Emitting Diode (LED): A diode that emits light when current flows through it in the correct (forward) direction. They are highly efficient light sources.

2. Series Circuits (4.3.2)

In a series circuit, all components are connected end-to-end, forming a single path for the current to flow.

Analogy: Imagine a single, one-lane highway. Every car must travel on this one path.

Rules for Series Circuits

1. Current (\(I\))

The current is the same at every point in the circuit. If you measure the current just after the battery or just before the next resistor, the value is identical.

$$I_{\text{total}} = I_1 = I_2 = I_3$$

2. Potential Difference (\(V\)) (Supplement)

The total potential difference (voltage) supplied by the source is shared among the components.

$$V_{\text{source}} = V_1 + V_2 + V_3$$

Think: The battery provides 6V of "energy." If Resistor 1 uses 4V, Resistor 2 must use the remaining 2V.

3. Total Resistance (\(R\))

The total resistance is the sum of the individual resistances.

$$R_{\text{total}} = R_1 + R_2 + R_3$$

Don't forget: You can also calculate the combined e.m.f. of cells connected in series by simply adding their individual e.m.f. values (e.g., two 1.5 V cells in series give 3.0 V).

3. Parallel Circuits (4.3.2)

In a parallel circuit, components are connected across the same points, creating multiple paths for the current to flow.

Analogy: Imagine a multi-lane highway. Cars split up to take different toll booths and then rejoin the main road later.

Rules for Parallel Circuits

1. Potential Difference (\(V\)) (Supplement)

The potential difference across each parallel branch is the same and equal to the source voltage.

$$V_{\text{source}} = V_1 = V_2 = V_3$$

Think: Every component is connected directly to the positive and negative terminals of the battery, so they all feel the full force (voltage).

2. Current (\(I\))

The total current leaving the source splits up to flow through the different branches and then combines again when the branches meet. This is the Junction Rule (Supplement):

$$I_{\text{total}} = I_1 + I_2 + I_3$$

Common Mistake: Remember, current takes the path of least resistance. A smaller resistance branch will carry a larger current than a higher resistance branch. (Core fact: current from the source is larger than the current in each branch.)

3. Total Resistance (\(R\))

Adding more resistors in parallel actually decreases the total resistance of the circuit. (Core fact: combined resistance is less than that of either resistor by itself.)

Why? Because adding another path provides more routes for the charge to flow, making it easier for the current to move overall.

To calculate the total resistance \(R_p\) of two resistors (\(R_1\) and \(R_2\)) in parallel (Supplement calculation):

$$ \frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2} $$

Advantages of Parallel Wiring (Core)

Lamps in household lighting circuits are almost always connected in parallel because:

1. If one lamp breaks or blows, the current can still flow through the other parallel branches, so the other lamps remain lit. (In series, if one breaks, the entire circuit fails.)
2. Each lamp receives the full mains voltage (e.g., 230 V), meaning they all shine at their brightest intended level.

4. Potential Dividers (4.3.3)

A potential divider is simply a series circuit used to provide a fraction of the source voltage. It allows us to "tap into" a smaller, usable voltage from a larger supply.

The Principle of Potential Division (Core)

In any series circuit, the voltage (p.d.) across a resistor is directly proportional to its resistance, provided the current is constant.

Think: If Resistor A is twice as big as Resistor B, it will use up twice as much voltage.

The Potential Divider Equation (Supplement)

For two resistors \(R_1\) and \(R_2\) connected in series to a source, the voltages \(V_1\) (across \(R_1\)) and \(V_2\) (across \(R_2\)) relate to the resistances:

$$ \frac{V_1}{V_2} = \frac{R_1}{R_2} $$

This rule is used to find voltages in simple series circuits.

Action of a Variable Potential Divider (Potentiometer) (Supplement)

A variable resistor connected as a potential divider is often called a potentiometer.

A potentiometer has three terminals. The supply voltage is connected across the outer two terminals, and the output voltage is taken between one outer terminal and the central 'wiper' terminal.

Action: By moving the wiper, you change the ratio of the resistance in the circuit. This allows you to smoothly vary the output voltage from zero up to the full supply voltage.

• Real-World Example: Volume controls on older stereo systems use potentiometers. Adjusting the knob changes the resistance ratio, thereby adjusting the voltage supplied to the speaker, which changes the volume.