1.0 Static Electricity
Electricity begins with the study of Electric Charges. All matter is composed of atoms containing protons (positive) and electrons (negative). When two different insulators are rubbed together, electrons are transferred from one to the other, making them "electrified" or charged. This is known as Static Electricity.
Fundamental Laws of Electrostatics
- Like charges (e.g., + and +) repel each other.
- Unlike charges (e.g., + and -) attract each other.
- Conservation of Charge: Charge can neither be created nor destroyed; it can only be transferred.
1.1 Methods of Charging
A body can be charged in three main ways:
- Friction: Rubbing two bodies (e.g., a glass rod rubbed with silk).
- Conduction: Charging by physical contact with a charged body.
- Induction: Charging a neutral body by bringing a charged body near it without touching. The near end develops an opposite charge.
1.2 The Gold Leaf Electroscope (GLE)
The GLE is a sensitive instrument used to detect the presence and nature of an electric charge. It consists of a brass rod passing through a cork in a glass jar, with two thin gold leaves attached at the bottom.
- Detection: If a charged body touches the disc, the leaves diverge.
- Testing Nature: If a body with a similar charge is brought near a charged GLE, divergence increases. If it has an opposite charge, divergence decreases.
Quantization of Charge
$$Q = n \times e$$
Where $Q$ = Total charge (Coulombs), $n$ = integer, $e$ = charge of an electron ($1.6 \times 10^{-19}\,C$).
A lightning conductor is a metal rod with sharp spikes installed on top of buildings. It provides a low-resistance path for the atmospheric electricity (lightning) to flow directly into the ground, protecting the structure from fire or collapse.
A polythene piece rubbed with wool is found to have a negative charge of $3 \times 10^{-7}\,C$. Estimate the number of electrons transferred.
Solution:
1. Given: $Q = 3 \times 10^{-7}\,C$, $e = 1.6 \times 10^{-19}\,C$.
2. Formula: $n = Q / e$.
3. Calculation: $n = (3 \times 10^{-7}) / (1.6 \times 10^{-19}) \approx \mathbf{1.87 \times 10^{12}}$ electrons.
Final Answer: Approximately $1.87 \times 10^{12}$ electrons were transferred from wool to polythene.
Why do you sometimes get a tiny "shock" when touching a metal doorknob after walking across a carpet? It’s because you’ve built up static charge through friction, and the metal knob allows that charge to suddenly discharge through your hand!
2.0 Current Electricity
While static electricity deals with charges at rest, Current Electricity deals with charges in motion. When an electric potential difference is applied across a conductor (like using a battery), electrons flow from the negative terminal to the positive terminal, creating an electric current.
Key Terms
- Electric Current ($I$): The rate of flow of charge. $I = Q / t$. The S.I. unit is Ampere (A).
- Potential Difference ($V$): The work done in moving a unit positive charge from one point to another. $V = W / Q$. The S.I. unit is Volt (V).
- Resistance ($R$): The obstruction offered by a conductor to the flow of current. The S.I. unit is Ohm ($\Omega$).
2.1 Ohm's Law
Ohm's Law states: "The current flowing through a conductor is directly proportional to the potential difference across its ends, provided physical conditions (like temperature) remain constant."
Mathematical Expression
$$V = I \times R$$
A graph of $V$ vs $I$ for an Ohmic conductor is always a straight line passing through the origin.
2.2 Factors Affecting Resistance
The resistance of a wire depends on four main factors:
- Length ($l$): $R \propto l$. (Longer wires have more resistance).
- Area of Cross-section ($A$): $R \propto 1/A$. (Thicker wires have less resistance).
- Material: Different materials have different Resistivity ($\rho$).
- Temperature: Resistance of metals increases with temperature.
1. Ammeter: Measures current. Always connected in Series. It has very low resistance.
2. Voltmeter: Measures potential difference. Always connected in Parallel. It has very high resistance.
A heater is connected to a 220V supply and draws a current of 5A. Calculate its resistance.
Solution:
1. Given: $V = 220\,V, I = 5\,A$.
2. Formula: $R = V / I$.
3. Calculation: $R = 220 / 5 = \mathbf{44\,\Omega}$.
Final Answer: The resistance of the heater is $44\,\Omega$.
Pure water is actually an insulator! It only becomes a conductor when impurities like salts or minerals are dissolved in it, providing ions to carry the electric charge. This is why you should never touch switches with wet hands—tap water is rarely "pure."
3.0 Magnetism
Magnetism is a property by which a substance exerts an attractive or repulsive force on other substances. A Magnet is a material that produces a magnetic field and has two poles: North (N) and South (S). In ICSE 9, we focus on the properties of magnets and the invisible fields they create.
Properties of Magnets
- Attractive Property: Magnets attract ferromagnetic substances like Iron, Cobalt, and Nickel.
- Directive Property: A freely suspended magnet always comes to rest in the Geographic North-South direction.
- Law of Poles: Like poles repel; unlike poles attract.
- Monopoles do not exist: If you break a magnet in half, each piece becomes a complete magnet with its own N and S poles.
3.1 Magnetic Field Lines
A magnetic field is the space around a magnet where its influence can be detected. We represent this field using Magnetic Field Lines. These are imaginary curves that show the direction a North Pole would move.
Characteristics of Field Lines
- They form closed continuous loops.
- Outside the magnet, they move from North to South.
- Inside the magnet, they move from South to North.
- They never intersect each other (because at a single point, the field cannot have two different directions).
- Closer lines indicate a stronger magnetic field.
3.2 Earth's Magnetism
The Earth itself behaves like a giant bar magnet. This is due to the Dynamo Effect—the movement of molten iron and nickel in the Earth's outer core.
- Magnetic Declination: The angle between the geographic meridian and the magnetic meridian.
- Neutral Points: Points in the magnetic field of a magnet where the Earth's magnetic field is exactly cancelled by the magnet's field. At these points, a magnetic needle shows no specific direction.
When a piece of soft iron is placed near a strong magnet, it acquires magnetic properties. This is called Magnetic Induction. Remember: Induction precedes attraction. The magnet first induces an opposite pole in the iron, and then attracts it.
Why do magnetic field lines not intersect each other?
Solution:
1. If two field lines were to intersect, it would mean that at the point of intersection, the magnetic field has two different directions.
2. A compass needle placed at that point would have to point in two directions at once, which is physically impossible.
Final Answer: Therefore, magnetic field lines can never cross.
The Earth's magnetic poles aren't fixed! They drift by several kilometers every year. In fact, every few hundred thousand years, the Earth's magnetic field completely flips—North becomes South and South becomes North!