Electricity Chapter Notes | Year 9 Science IGCSE (Cambridge) - Class 9 PDF Download

Parallel and Series Circuits

Circuit Flow

• Series Circuits: In a series circuit, all components are connected in a single line, creating one path for the current to flow. This means that the current is the same throughout the circuit because there is only one route for it to take. When the current flows out of one component, it goes directly into the next one. If a switch in a series circuit is turned off, all devices, like lamps and buzzers, stop working at the same time because the current flow is interrupted.

• Parallel Circuits: Parallel circuits have multiple paths, or branches, for the current to flow through. The term "parallel" comes from how circuit diagrams show branches as parallel lines. In a parallel circuit, the current from the power source splits at a branch point. For identical components, the current divides equally among the branches. After passing through the branches, the currents merge where the branches reconnect, combining to match the total current provided by the power source. The rule for parallel circuits is that the current from the power source equals the total of the currents in all branches. For example, if a circuit has branches carrying 1.0 A, 2.0 A, and 0.5 A, the total current from the power source is 3.5 A (1.0 A + 2.0 A + 0.5 A). If the total current and all but one branch current are known, the missing branch current can be found by subtracting the known currents from the total.

Advantages of Parallel Circuits

• Devices can work independently; if one device fails, the others keep functioning.
• Voltage is consistent across each branch, ensuring reliable performance.
• Current in each branch can be adjusted to meet specific requirements.

Current and Voltage in Parallel Circuits

Dynamic Circuits

What is Voltage?

• Voltage is a measure of the electrical energy in a circuit, and it is expressed in volts (V).
• Voltage is related to the energy provided by a cell, battery, or power supply, but it is not the same as the energy itself.
• Most single cells deliver a voltage of 1.5 V, while batteries, which are made up of multiple cells connected in series, usually provide higher voltages such as 6 V, 9 V, or 12 V.
• The mains voltage supplied by power stations typically ranges from 220 to 240 V in many countries, such as those in Europe. However, some countries, like the United States and Canada, use lower mains voltages of 110 to 120 V.
• The energy source in a circuit, whether it is a cell, battery, or mains supply, is referred to as the supply.
• Voltage is also connected to the energy that components transform, such as lamps that convert electrical energy into light and heat energy.
• Components have a voltage rating that indicates the maximum safe voltage they can handle. For example, school experiment lamps are often rated for 3 V or 6 V.

Measuring Voltage

Voltage Measurement with a Voltmeter

Energy Measurement

• Voltage is measured using a voltmeter, which can be either digital or analog. The voltmeter has a specific symbol in circuit diagrams.
• Unlike an ammeter that measures current by being connected in series, a voltmeter is connected in parallel to a component to measure the energy difference across it.
• For example, in a circuit with a lamp, the ammeter is placed in series with the lamp, while the voltmeter is connected across the lamp's terminals.

Voltage in a Series Circuit

• In a series circuit, the total voltage supplied by the cell, battery, or power supply is equal to the sum of the voltages across each component. This is in line with the principle of energy conservation.
• When identical components, like three lamps, are connected in series, each lamp receives the same voltage, and their combined voltage equals the battery's voltage.
• If the components are different, such as a lamp and a buzzer, the component that consumes more energy, like the lamp, will have a higher voltage across it. However, the total voltage remains equal to the supply voltage.
• In a series circuit, while the current remains constant, the voltage varies across different components.

Adding More Components in a Series Circuit

• When more components are added to a series circuit, the supply voltage is divided among the components, leading to a reduction in the voltage across each one.
• Adding components also increases the total resistance of the circuit, making it more difficult for the power supply to push electrons through, which results in a decrease in current.
• For instance, a series circuit with multiple lamps will have a smaller current compared to one with fewer lamps, even when using the same battery.
• Adding cells in series increases the supply voltage. For example, two 1.5 V cells in series provide 3 V, while eight cells can provide 12 V.
• Increasing the number of cells in a series circuit raises the supply voltage. However, the current remains constant unless the total resistance of the circuit is reduced.

Voltage in a Parallel Circuit

In a Parallel Circuit

Electrical Dynamics

• Voltage in Parallel Circuits: In a parallel circuit, each branch is connected directly across the supply terminals. This means that the voltage across each branch is equal to the supply voltage. For example, if there is a 1.5 V cell, every lamp in the parallel branches receives 1.5 V, regardless of the path taken.
• Consistent Voltage Across Components: Even when different components, such as a lamp and a buzzer, are used in the branches, the voltage across each branch remains equal to the supply voltage. For instance, with a 9 V battery, each component receives 9 V.
• Current Variation: While the voltage is the same across all branches and matches the supply voltage, the current can vary in each branch due to the presence of multiple paths.

Adding More Components in a Parallel Circuit

• Increased Current Paths: Adding more branches in a parallel circuit creates additional paths for current to flow. This makes it easier for the current to move, resulting in an increase in the total current through the cell.
• Same Voltage Across Branches: Each branch in a parallel circuit maintains the same voltage, which is equal to the supply voltage.
• Impact on Branch Current: When more components are added to a single branch, the resistance of that branch increases. This leads to a reduction in the current flowing through that specific branch.
• Adding Cells in Parallel: Incorporating more cells in parallel keeps the supply voltage the same while enhancing the total current capacity. This results in:
  • An increase in the voltage across each branch.
  • An increase in the total current through the cell.
  • An increase in the current flowing through each branch.

Resistance

• Definition of Resistance: Resistance refers to how easily electrons can pass through a material. It is a measure of the opposition to the flow of electric current.
• Conductors vs. Insulators: Conductors, such as copper, have low resistance, allowing electrons to flow through them easily. In contrast, insulators, like most plastics, have high resistance, which hinders the flow of electrons.
• Measurement of Resistance: Resistance is measured in ohms, represented by the Greek letter omega (Ω). A lower resistance value indicates that a material allows electric current to pass through it more readily.
• Examples of Resistance: A 100-meter length of copper wire typically has a resistance ranging from 0.5 to 1.0 ohms due to its conductive properties. On the other hand, a 1-centimeter length of certain plastics can have a resistance exceeding 1,000,000,000,000 ohms, showcasing their insulating capabilities.
• Impact of Resistance on Current: Resistance plays a crucial role in slowing down the flow of electrons, which in turn reduces the current in a circuit. Copper wires are designed with low resistance to minimize their impact on current flow, ensuring efficient conduction of electricity. Insulators, like the plastic coating on copper wires, have such high resistance that they typically prevent current from flowing through them.
• High Resistance Conductors: Some conductors, such as lamp filaments, are intentionally made with high resistance. This design allows them to convert electrical energy into heat and light effectively. The higher the resistance in a circuit, the lower the current that flows through it.

Ohm's Law

Ohm's Law is a fundamental principle in electricity that describes the relationship between voltage, current, and resistance in a circuit. It states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance of the conductor. The law is usually expressed with the formula:

• V = I × R

• Where:
• V is the voltage in volts (V),
• I is the current in amperes (A), and
• R is the resistance in ohms (Ω).

Practical Examples:

• If a circuit has a voltage of 12 volts and a resistance of 4 ohms, the current can be calculated as follows:
• I = V / R = 12 V / 4 Ω = 3 A

Applications:

• Ohm's Law is used in various applications, including electrical engineering, circuit design, and troubleshooting electrical issues.

Introduction to Ohm's Law

Electrical Circuit

• Ohm's Law describes the relationship between voltage, current, and resistance in electrical components.
• It was discovered by Georg Simon Ohm.
• The law states that Voltage (V) = Current (I) × Resistance (R).
• In this formula: V is measured in volts (V), I in amps (A), and R in ohms (Ω).
• Ohm's Law is primarily applicable to resistive components like resistors and wires.

Resistors

• A resistor is designed to have a specific and known level of resistance.
• Resistors typically feature coloured bands that indicate their resistance value in ohms.
• The symbol used for a resistor in circuit diagrams is a rectangle, often accompanied by the resistance value or the letter R.
• When the precise resistance value is not critical or needs to be determined, the letter R is used in the diagram.

Practical Circuits

• Practical circuits consist of various standard components, each with its own symbol, including:
  • Cell : Provides electrical energy to the circuit.
  • Switch : Controls the flow of current by opening or closing the circuit.
  • Resistor : Restricts current flow by offering specific resistance.
  • Variable Resistor : Enables adjustable resistance to manage current or voltage, used in applications like dimmers and volume controls.
  • Ammeter : Measures the current flowing through the circuit.
  • Voltmeter : Measures the voltage across a component.
  • Lamp : Converts electrical energy into light and heat.
  • Buzzer : Emits sound when current passes through it.
• Circuit diagrams employ standardized symbols to accurately depict these components.
• Constructing practical circuits involves correctly connecting these components to achieve the intended functionality.

Variable Resistors

• A variable resistor is a component that allows for the adjustment of resistance within a circuit.
• These resistors are commonly used in applications such as dimmer switches and volume control knobs.

Variable Resistors

Adjustable Glow

• Variable resistors are components used to change the resistance in a circuit. They are important for controlling current or voltage in various applications.
• Examples of Variable Resistors:
  • Dimmer Switches: Used in lamps to adjust brightness levels.
  • Volume Controls: Found in music players to adjust sound loudness.
• Circuit Symbol: The symbol for a variable resistor resembles that of a fixed resistor but includes an arrow to indicate adjustable resistance.
• Fixed Resistor: Refers to resistors with a constant resistance, as discussed earlier.
• Effect on Lamp Brightness:
  • Increasing the resistance of a variable resistor reduces the current, making the lamp dimmer.
  • Decreasing the resistance allows more current to flow, making the lamp brighter.

Everyday Circuits

• Electric circuits play a crucial role in various everyday devices and systems, such as:
• Homes
• Schools
• Cars
• Bicycles
• Phones
• Music systems
• Watches
• Computers
• Scientists are constantly innovating and creating new circuits to enhance the functionality and convenience of everyday life.
• Example: Refrigerator Light Circuit
• Explanation: A switch linked to the refrigerator door controls the light inside. When the door opens, the switch closes, turning on the light. Conversely, when the door closes, the switch opens, turning off the light.
• Example: Adjustable Buzzer Circuit
• Explanation: Some devices feature a buzzer that emits a warning sound for incorrect actions, with adjustable volume using a variable resistor.
• Example: Car Headlamp and Buzzer Circuit
• Explanation: In specific cars, a buzzer alerts the driver when the door is opened with the headlamps on. The headlamps are connected in parallel, allowing them to be controlled by a single switch (S1). The buzzer is in the same circuit branch, regulated by a second switch (S2) linked to the car door. When the door opens, S2 closes, and when it closes, S2 opens. The buzzer activates only when both S1 (headlamps on) and S2 (door open) are closed, enabling current flow through the buzzer.
• Example: Battery Voltage Indicator Circuit
• Explanation: Devices powered by 12 V batteries, like cars or boats, may include a battery voltage indicator to monitor voltage levels. A voltmeter is connected in parallel to the battery, with a switch in the same branch to control voltage measurement. Closing the switch completes the circuit, allowing the voltmeter to display the battery's voltage.
• Example: Voltage Reduction for Low-Voltage Lamps
• Explanation: A lamp with a low voltage rating (e.g., 6 V) can be safely used with a higher voltage battery (e.g., 12 V) by employing a voltage regulator and resistors to prevent damage.
• Two resistors of equal resistance are connected in series with the battery, dividing the voltage equally (e.g., 6 V per resistor). The 6 V lamp is connected in parallel with one resistor, receiving the appropriate 6 V supply to operate safely, ensuring proper functionality without risk of damage.

An Investigation that 'Failed' - but Won a Nobel Prize

The Nobel Prize in Physics 2010 was awarded for the discovery of the accelerating expansion of the Universe through observations of distant supernovae. This discovery was unexpected and challenged previous notions about the Universe's expansion. The two teams that made this discovery faced various challenges and setbacks, which could be seen as 'failures' in the traditional sense. However, their perseverance and the eventual success of their observations led to a groundbreaking revelation in astrophysics. The award highlights the importance of unexpected findings in scientific research and the value of persistence in the face of difficulties.

The Michelson-Morley Experiment

Cosmic Reflections

In 1887, scientists knew that the Earth orbits the Sun at about 29.78 km/s (roughly 107,000 km/h ) and that light travels at an incredibly high speed. Albert Michelson, an American scientist, theorized that light would appear to move faster when it was going against the Earth's motion and slower when it was traveling in the same direction. To explain this, he used an analogy of throwing an object from a moving car: when you throw something backward from a moving car, it seems to move away faster than when you throw it forward. However, this is just an analogy, and throwing objects from cars is not safe.

Michelson teamed up with a colleague named Morley to test this theory through an experiment. They used a light ray and mirrors to measure the speed of light in different directions relative to the Earth's motion. Despite conducting the experiment multiple times, the results did not support Michelson’s prediction. Instead, they found that the speed of light was constant in all directions, regardless of how the Earth was moving.

• These results were revolutionary, providing the first evidence that the speed of light is constant in air. Michelson was awarded a Nobel Prize in Physics for this groundbreaking work.
• This experiment highlights how significant scientific discoveries can arise from unexpected results and the importance of drawing meaningful conclusions from experimental data.