Question

The cable of an electric fan becomes so worn that the live wire makes electrical contact with the metal case. The case is earthed. The plug to the fan contains a 5 A fuse There is a current of 4 A when the fan works normally. What will happen in this case?
(A) The current will run to earth and the fuse will not be affected
(B) The metal case will become very hot
(C) The metal case will become live and dangerous
(D) The fuse will melt and switch off the circuit

Answer 1

Hint: When a voltage is connected across a wire, an electric field is produced in the wire. Metal wire is a conductor. Some electrons around the metal atoms are free to move from atom to atom. This causes a difference in energy across the component, which is known as an electrical potential difference. In parallel circuits, the electric potential difference across each resistor $(\Delta \mathrm{V})$ is the same. The current in a resistor follows Ohm's law: $I=\Delta V / R .$ since the $\Delta V$ is the same for each resistor, the current will be smallest where the resistance is greatest.

Complete step by step answer
We know that if the electric potential difference between two locations is 1 volt, then one Coulomb of charge will gain 1 joule of potential energy when moved between those two locations. Because electric potential difference is expressed in units of volts, it is sometimes referred to as the voltage. Voltmeters are used to measure the potential difference between two points. There is a misconception about potential and voltage. Many of us think that both are the same. But voltage is not exactly potential; it is the measure of the electric potential difference between two points.
When the live wire (high potential) is in contact with the earthed metal case (zero potential) and since there is a potential difference, current flows. As there is little to no resistance in the completed path, the current flow is very large (larger than $4 \mathrm{A}$ melting the fuse). The fuse melts due to high current surge and switches off the circuit.

Hence, the correct option is option D.

Note: We know that electrical potential difference is the difference in the amount of potential energy a particle has due to its position between two locations in an electric field. This important concept provides the basis for understanding electric circuits. The voltage between two points is a short name for the electrical force that would drive an electric current between those points. Specifically, voltage is equal to energy per unit charge. The unit of potential difference generated between two points is called the Volt and is generally defined as being the potential difference dropped across a fixed resistance of one ohm with a current of one ampere flowing through it.