Electricity

• (a) Resistance ($R$): Apply Ohm's Law ($R = \frac{V}{I}$):

$$R = \frac{230\text{ V}}{0.5\text{ A}} = \mathbf{460\text{ }\Omega}$$

• (b) Electric Charge ($Q$):
Convert time to seconds: $t = 5\text{ minutes} = 5 \times 60 = 300\text{ seconds}$.

$$Q = I \times t = 0.5\text{ A} \times 300\text{ s} = \mathbf{150\text{ Coulombs}}$$

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• The Live wire carries high electrical potential ($220\text{ V}$), while the Neutral wire is at zero potential ($0\text{ V}$).
• If the fuse is connected in the Live wire and melts due to excessive current, it immediately disconnects the appliance from the high-voltage supply.
• If the fuse were mistakenly installed in the Neutral wire, even if the fuse blew, the appliance would remain live at $220\text{ V}$.
• Anyone touching the appliance would complete a circuit to the ground and receive a fatal electric shock.

1. Longer: The earth pin is longer so that when plugging in, it enters the socket FIRST, ensuring the appliance is safely grounded before the live and neutral pins make electrical contact. When unplugging, it disconnects LAST.
   2. Thicker: The earth pin is thicker so that it cannot accidentally be forced into the live or neutral holes of the wall socket, preventing dangerous short-circuits.

• Ohm's Law: The electric current ($I$) flowing through a conductor is directly proportional to the potential difference ($V$) applied across its ends, provided physical conditions (such as temperature and mechanical strain) remain strictly constant.
• Mathematical Formula:

$$\mathbf{V = IR}$$

• In a graph of Potential Difference ($V$, vertical axis) against Current ($I$, horizontal axis), the slope represents Electrical Resistance ($R = \frac{\Delta V}{\Delta I}$).

• An Electric Fuse is an electrical safety device consisting of a short piece of thin wire that melts and breaks the circuit whenever current exceeds a predetermined safe limit.
• Two Essential Properties:
1. Low Melting Point: It melts easily ($\approx 250^{\circ}\text{C}$) before copper connecting wires catch fire.
2. High Electrical Resistance: It generates rapid Joule heating ($H = I^2Rt$) when excessive current surges through it.

1. Discharging Clouds (Action of Points): As a charged cloud hovers over a building, it induces an opposite charge on the sharp metal spikes of the conductor. The high electric field at the sharp tips ionizes the air, spraying a fountain of opposite ions that neutralize the cloud charge peacefully.
   2. Safe Grounding: If a direct lightning strike still hits, the thick copper strip provides a path of exceptionally low electrical resistance, channeling millions of Joules of current safely into the moist earth without damaging the building structure.

• Ammeter:
1. Measures Electric Current ($I$).
2. Always connected in SERIES with the component.
3. Has an extremely low internal resistance (ideally zero) so it does not reduce the circuit current.
• Voltmeter:
1. Measures Potential Difference ($V$).
2. Always connected in PARALLEL across the component.
3. Has an extremely high internal resistance (ideally infinite) so it draws virtually no current from the circuit.

• Short-Circuiting: Occurs when the naked Live wire comes into direct physical contact with the Neutral wire (due to damaged insulation or appliance fault). The resistance drops to almost zero, causing a massive surge of current and severe sparks/fires.
• Overloading: Occurs when too many high-power appliances (e.g., air conditioners, geysers, heaters) are connected simultaneously to a single socket/circuit. The total current drawn exceeds the rated capacity of the supply wires, causing overheating.