1.0 Static Electricity: Charges at Rest
Electricity is a branch of Physics that deals with the behavior of electric charges. In its simplest form, electricity exists as "Static Electricity," where charges are produced by friction and remain localized on the surface of an object.
The Nature of Charges
According to the Fundamental Law of Electrostatics:
- Like charges (e.g., positive and positive) repel each other.
- Unlike charges (e.g., positive and negative) attract each other.
- The S.I. unit of electric charge is the Coulomb (C).
1.1 Methods of Charging
A neutral body can be charged in three primary ways:
- Charging by Friction: Two different insulators are rubbed together, causing electrons to transfer from one to the other.
- Charging by Conduction: An uncharged conductor is brought into direct contact with a charged body.
- Charging by Induction: A charged body is brought near (without touching) a neutral conductor, causing a redistribution of charges within it.
Quantization of Charge
$$Q = n \times e$$
Where: $Q$ = Total charge, $n$ = Number of electrons, $e$ = Charge of one electron ($1.6 \times 10^{-19}\,C$).
Lightning is a massive spark of static electricity between clouds or between a cloud and the earth. Lightning Conductors (metal rods with sharp points) are installed on tall buildings to provide a low-resistance path for the charge to flow safely into the ground, protecting the structure from fire or damage.
If a body has a net charge of $1.6 \times 10^{-18}\,C$, how many electrons has it lost or gained?
Solution:
1. Given Charge ($Q$): $1.6 \times 10^{-18}\,C$.
2. Elementary Charge ($e$): $1.6 \times 10^{-19}\,C$.
3. Formula: $n = Q / e$.
4. Calculation: $n = (1.6 \times 10^{-18}) / (1.6 \times 10^{-19}) = 10$.
Final Answer: The body has a deficit (or excess) of 10 electrons.
When you rub a balloon against your hair, it gains electrons from your hair and becomes negatively charged. It can then stick to a wall because it induces an opposite charge on the wall's surface!
2.0 Current Electricity: Charges in Motion
When electric charges flow through a conductor in a definite direction, they constitute an Electric Current. For this flow to occur, there must be a source of energy that creates a "pressure difference" known as Potential Difference.
2.1 Electric Current and Potential
- Electric Current (I): The rate of flow of charge.
Formula: $$I = \frac{Q}{t}$$
S.I. Unit: Ampere (A). Measured using an Ammeter. - Potential Difference (V): The work done in moving a unit positive charge from one point to another.
S.I. Unit: Volt (V). Measured using a Voltmeter.
Ohm's Law
$$V = I \times R$$
At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.
2.2 Resistance and Factors
Resistance (R) is the opposition offered by a conductor to the flow of current. Its S.I. unit is the Ohm ($\Omega$). It depends on:
- 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."
- Temperature: For most metals, resistance increases with temperature.
Electrons flow from the negative terminal to the positive terminal. However, by convention, the Conventional Current is always taken to flow from the positive terminal to the negative terminal. Don't mix these up in circuit diagrams!
An electric heater is connected to a 220V supply. If it draws a current of 5A, calculate its resistance.
Solution:
1. Given Potential ($V$): $220\,V$
2. Given Current ($I$): $5\,A$
3. Formula (Ohm's Law): $R = V / I$
4. Calculation: $R = 220 / 5 = 44\,\Omega$.
Final Answer: The resistance of the heater is $44\,Ohms$.
Distilled water is an insulator, but normal tap water contains dissolved salts and minerals that allow it to conduct electricity. This is why you should never touch electrical switches with wet hands!
3.0 Heating Effect and Safety Components
When an electric current passes through a conductor with high resistance, a part of the electrical energy is converted into heat energy. This is known as the Heating Effect of Current. We use this principle in heaters, but it can be dangerous if not controlled by safety devices.
3.1 Electric Fuse and MCB
- Electric Fuse: A safety device containing a thin wire with a low melting point. If the current exceeds a safe limit, the wire melts and breaks the circuit, preventing fire.
- MCB (Miniature Circuit Breaker): A modern electromagnetic switch that automatically "trips" (turns off) during an overload. Unlike a fuse, it can be reset without replacement.
Joule's Law of Heating
$$H = I^2 \times R \times t$$
Where: $H$ = Heat produced, $I$ = Current, $R$ = Resistance, $t$ = Time.
3.2 Household Wiring: The Three Wires
In a standard household circuit, power is supplied through three distinct wires, each color-coded for safety:
- Live Wire (L): Usually Red/Brown. Carries current at high potential ($220\,V$).
- Neutral Wire (N): Usually Black/Blue. Completes the circuit at zero potential.
- Earth Wire (E): Usually Green/Yellow. Connected to the metal casing of appliances to prevent electric shocks.
Short Circuit: Occurs when live and neutral wires touch directly, leading to massive current flow.
Overloading: Occurs when too many high-power appliances are connected to a single socket, exceeding its capacity.
An appliance of 1.1 kW is operated at 220 V. What should be the minimum rating of the fuse used for this appliance?
Solution:
1. Power ($P$): $1.1\,kW = 1100\,W$.
2. Voltage ($V$): $220\,V$.
3. Formula ($P = VI$): $I = P / V$.
4. Calculation: $I = 1100 / 220 = 5\,A$.
Final Answer: The fuse rating should be slightly more than $5\,A$ (e.g., a $6\,A$ fuse).
The Earth wire is safety's best friend. If the live wire accidentally touches the metal body of your toaster, the earth wire provides a very low resistance path to the ground, causing the fuse to blow and saving you from a fatal shock!
4.0 Magnetism and Electromagnetism
Electricity and Magnetism are closely related. In 1820, Hans Christian Oersted discovered that an electric current flowing through a wire produces a magnetic field around it. This is the foundation of modern technology, from electric bells to massive industrial cranes.
4.1 Magnetic Field around a Straight Conductor
When current flows through a straight wire, the magnetic field lines form concentric circles around the wire. The direction of this field is determined by the Right-Hand Thumb Rule.
Right-Hand Thumb Rule
Imagine holding the current-carrying conductor in your right hand with the thumb pointing in the direction of the current. Your fingers will curl in the direction of the magnetic field lines.
4.2 The Electromagnet
An Electromagnet is a temporary magnet consisting of a coil of insulated copper wire (solenoid) wound around a soft iron core. It only behaves like a magnet as long as current flows through the coil.
- Soft Iron Core: Used because it gains and loses magnetism quickly.
- Factors Increasing Strength:
- Increasing the current flowing through the coil.
- Increasing the number of turns in the coil.
| Feature | Permanent Magnet | Electromagnet |
|---|---|---|
| Nature | Always magnetic. | Temporary; works with current. |
| Strength | Fixed; cannot be changed. | Variable; can be very strong. |
| Polarity | Fixed N and S poles. | Can be reversed by changing current. |
Never use Steel as the core of an electromagnet intended for temporary use. Steel becomes a permanent magnet once it is magnetized, meaning it won't "turn off" when you cut the current. This would make an electric bell ring forever!
List three uses of electromagnets in daily life.
Solution:
1. Electric Bells: To pull the hammer and strike the gong.
2. Maglev Trains: To levitate and propel the train at high speeds.
3. Medical Scanners (MRI): To create powerful magnetic fields for internal imaging.
The Earth itself is a giant electromagnet! Scientists believe that the motion of molten iron and nickel in the Earth’s outer core creates electric currents, which in turn generate our planet's magnetic field, protecting us from harmful solar radiation.