Electric circuits

Time to play with LEGOs! Just kidding- Circuits are wayy cooler 😂

You'll see that electrical circuits can include many different components. The way these components are connected is shown using circuit diagrams.

These diagrams use standard symbols to represent each type of component. Circuit diagrams help in both designing circuits and understanding how they work.

Circuit diagrams and components

Component	Function/Description
Cells	Provide a source of direct current (DC) through a chemical reaction.
Batteries	Made of multiple cells connected together to provide a higher voltage.
Power Supplies	Convert input voltage into a regulated output voltage for powering electronic systems.
Generators	Convert mechanical energy into electrical energy using electromagnetic induction.
Potential Dividers	Use resistors to divide voltage into smaller parts, allowing voltage control and signal reduction.
Switches	Open or close a circuit to control the flow of current.
Resistors (Fixed & Variable)	Fixed resistors offer constant resistance; variable resistors allow resistance to be adjusted.
Heaters	Convert electrical energy into heat, used in heating elements.
Thermistors (NTC)	Resistors that decrease in resistance as temperature increases.
Light-Dependent Resistors (LDRs)	Resistors that decrease in resistance as light intensity increases.
Lamps	Convert electrical energy into light, used for illumination.
Motors	Convert electrical energy into mechanical energy (rotational motion).
Ammeters	Measure electric current in a circuit; connected in series.
Voltmeters	Measure voltage across a component; connected in parallel.
Magnetising Coils	Produce a magnetic field when electric current flows through them.
Transformers	Transfer electrical energy between coils using electromagnetic induction; used to change voltage levels.
Fuses	Safety devices that break the circuit if current is too high, protecting against overloads.
Relays	Electromechanical switches operated by an electric current, used for controlling high-power circuits.

Series and Parallel circuits

There are 2 types of circuits:

Series circuits: In series circuit, the current remains the same and voltage differs across each component.

For instance, in a circuit with 9V power source, the sum of potential difference across all components should be equal to 9V.

The total potential difference across the components in a series circuit is equal to the sum of the individual potential differences across each component.

Parallel circuits: In a parallel circuit voltage (potential difference) stays the same and current is divided between components.

Ok, time to have some fun. In true IGCSE Pro fashion, we built a fun interactive learning module to help you better understand series and parallel circuits.

Covering this topic is very important since questions related to these are very easy to score.

Go ahead, give it a shot!

🔗 Series Circuit

💡

Circuit Analysis

Per Bulb

Current

Bulbs

🔀 Parallel Circuit

💡

Circuit Analysis

Per Bulb

Total Current

Bulbs

🧠 What You're Learning

Click the power buttons to see how electricity flows differently in series vs parallel circuits!

⚡ First Power On!

🔬 Circuit Scientist

🔧 Circuit Modifier

🎓 Circuit Master

Did you get the quiz at the end correct? If you didn't don't fret, give it a shot again and make sure to read every single line within the interactive learning module.

Calculating Potential Difference and Current in Circuits

To calculate potential difference, and current in both series and parallel circuits, we use the resistance along with the formula: V = I × R

Where,

V - Voltage in Volts
I - Current in Amps
R - Resistance in Ohms (Ω)

This formula represents Ohms law, as you'll see below!

Ohm's Law Calculator

Click a variable to solve for it

Voltage (V)

Current (A)

Resistance (Ω)

Enter any two values, then click the variable you want to solve for.
V = Voltage, I = Current, R = Resistance

Calculating the total resistance in series and parallel circuits

For series circuits, we simply add the resistance of each component.

It is a slightly different story for parallel circuits. Let's take a look at the section below.

Parallel Resistance Formula

Parallel Circuit Resistance Formula

Total Resistance in Parallel Circuit

$$\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \cdots + \frac{1}{R_n}$$

Formula Explanation:

R_total = Total equivalent resistance of the parallel circuit
R₁, R₂, R₃ = Individual resistances of each component
n = Total number of resistors in parallel

Key Points:

The total resistance in a parallel circuit is always less than the smallest individual resistance
Adding more resistors in parallel decreases the total resistance
Current divides among the parallel branches, but voltage remains the same across each branch

Example for Two Resistors

$$R_{total} = \frac{R_1 \times R_2}{R_1 + R_2}$$

Example for Three Resistors (Solving for R_total)

$$R_{total} = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}}$$

Usually, we prefer connecting components such as light bulbs in parallel instead of series.

That is because of one very simple reason which is that if one light bulb fails, the others will work.

But if one light bulb fails in series, all stop working.

Additionally, all bulbs will get the same amount of voltage in a parallel circuit and current will be divided.

Junctions

A junction is something which divides the current and then joins it back, see the diagram below.

Basically the current splits when entering a junction and then joins back when leaving the junction.

The total current going inside junction must be equal to the current leaving the junction.

Action and uses of Circuit Components

A variable potential divider adjusts the output voltage by changing the position of an adjustable resistor or potentiometer within the circuit.

Essentially it is a resistor whose resistance you can change (a.k.a. a variable resistor).

Equation for a potential divider:

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

The equation for a potential divider states that the ratio of the resistances is equal to the ratio of the voltages across them.

LDR (Light Dependent Resistor)

Use: LDRs are used to detect and measure light levels in various applications like automatic lighting systems and light meters.
How it works: LDRs have a resistance that decreases as the intensity of light falling on them increases. This property allows them to sense and respond to changes in light levels by adjusting the electrical current passing through them.

LED (Light-Emitting Diode)

Use: LEDs are used for illumination, indicators, and displays in a wide range of electronic devices and lighting applications.
How it works: LEDs are semiconductor diodes that emit light when forward biased. When a forward voltage is applied, electrons and holes recombine within the LED’s semiconductor material, releasing energy in the form of light. The emitted light’s color depends on the specific semiconductor materials used in the LED.

💡

LEDs are a major part of the energy efficiency movement worldwide, and are rapidly replacing CFLs and other less efficient sources of light.

Thermistor (NTC):

Use: NTC thermistors are employed for temperature measurement, control, and compensation in devices like thermostats and temperature sensors.
How it works: NTC thermistors contain semiconducting metallic oxides that exhibit a significant decrease in resistance as temperature rises. This characteristic enables them to generate electrical signals corresponding to temperature variations, allowing for accurate temperature monitoring and control.

Relays

Use: Relays are electrically operated switches that control high-power circuits with low-power control signals. They are essential components in automation systems and industrial control applications.
How it works: Relays consist of an electromagnetic coil that, when energized, creates a magnetic field, causing the relay’s contacts to open or close. This mechanism allows them to control the flow of current in circuits, enabling the switching of high-power loads with the help of low-power control signals.

Semiconductor Diode (Forward Bias, Reverse Bias, and Rectifier):

Use: Semiconductor diodes are used for rectification, signal detection, and switching in electronic circuits.
How it works:

Forward Bias: Applying a positive voltage to the anode and negative voltage to the cathode of a diode allows current flow through the diode, making it conductive.
Reverse Bias: Applying a positive voltage to the cathode and negative voltage to the anode of a diode blocks current flow, acting as an insulator.
Rectifier: Diodes are commonly used as rectifiers to convert alternating current (AC) to direct current (DC). When connected in the forward bias configuration, diodes allow current flow during positive half-cycles of the AC signal while blocking it during negative half-cycles. This process rectifies the AC signal, resulting in a pulsating DC waveform. Additional filtering components can be used to smooth the pulsations and obtain a steady DC output.

Alternative to Practical practice

With the electric circuit construction kit below, you can form your own series and parallel circuits and use the voltmeter, and/or ammeter tools to get the most out of this activity.

This is the end of this guide. Thank you for using IGCSE Pro! If you found value from our notes, be sure to join our mailing list and spread the word!