7.1 Electricity
Electricity is a form of energy generated by the movement or accumulation of electric charges (protons and electrons).
Electrostatics
Electrostatics is the study of static electric charges (charges at rest). Static charges are produced by friction between two different insulating materials, causing electrons to transfer from one object to another.
- Objects that gain electrons become negatively charged.
- Objects that lose electrons become positively charged.
- Electrostatic Laws: Like charges repel (+ and +, - and -); Opposite charges attract (+ and -).
Electroscope
An electroscope is an instrument used to detect the presence and type of electrostatic charge on an object. When a charged object touches the electroscope's metal cap, charge transfers down the rod, causing the thin gold leaf to diverge due to electrostatic repulsion.
Electrostatic Phenomena in Daily Life
- Lightning: Caused by massive electrostatic discharge between clouds or between clouds and the ground. Lightning conductors direct this electrical discharge safely into the earth.
- Comb & Paper: Rubbing a plastic comb against dry hair transfers electrons to the comb, allowing it to attract neutral pieces of paper.
7.2 Flow of Electric Current in Series and Parallel Circuits
Key Electrical Quantities and Equations
- Electric Current ($I$): The rate of flow of electric charges (electrons) through a conductor. Measured in Amperes (A) using an Ammeter connected in series.
- Voltage / Potential Difference ($V$): The potential difference between two points that drives electric charges through a conductor. Measured in Volts (V) using a Voltmeter connected in parallel across a component.
- Resistance ($R$): The opposition offered by a conductor to the flow of electric current. Measured in Ohms ($\Omega$).
- Ohm's Law: Electric current ($I$) flowing through a conductor is directly proportional to potential difference ($V$) across it, provided temperature remains constant.
$$\text{Formula: } V = I \times R \quad \text{or} \quad R = \frac{V}{I}$$
Series vs. Parallel Circuits
| Parameter |
Series Circuit |
Parallel Circuit |
| Current ($I$) |
Same current flows through all components.
$I = I_1 = I_2 = I_3$ |
Current splits across branches.
$I = I_1 + I_2 + I_3$ |
| Voltage ($V$) |
Total voltage is shared across components.
$V = V_1 + V_2 + V_3$ |
Voltage across each branch is equal.
$V = V_1 = V_2 = V_3$ |
| Resistance ($R$) |
Total resistance increases with more resistors.
$R = R_1 + R_2 + R_3$ |
Effective resistance decreases with more branches.
$\frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}$ |
| Impact of Component Failure |
If one bulb blows, the entire circuit breaks and all bulbs turn off. |
If one bulb blows, remaining branches continue operating independently. |
7.3 Magnetism and Electromagnetism
Properties of Magnets
- Has two poles: North (N) and South (S).
- Like poles repel; Opposite poles attract.
- Magnetic field lines point from the North pole to the South pole.
- Magnetic field strength is strongest at the poles (indicated by dense field lines).
Electromagnetism
An electromagnet is a temporary magnet created when an electric current flows through a coil of insulated wire wound around a soft iron core.
- Right-Hand Grip Rule: Determines the direction of the magnetic field around a current-carrying straight wire or solenoid (Thumb points to current direction; curled fingers show magnetic field direction).
- Factors Increasing Electromagnet Strength:
- Increasing the magnitude of electric current ($I$).
- Increasing the number of turns in the coil.
- Inserting a soft iron core inside the coil.
- Applications: Electric bells, electromagnetic cranes in scrap yards, circuit breakers, relays, and Maglev trains.