Electric Current Test - 23

Fuse wire should melt and disconnect the circuit from mains supply if the current increases beyond a rated value. For this reason, its resistance should be high for more heat generation and melting point should be low for fast melting. So the assertion is correct. Fuse wire is not for small current flow. Fuse wire may be of various current ratings depending on the type of appliance being used and the capacity of the wiring. So, the reason is false.

Resistance of the superconductor falls to zero at critical temperature. This property is very useful for power transmission without any loss. Assertion and reason both are true but reason does not explain the assertion.

Power rating describes the heat dissipation capability of the resistor. If the heat generated is more than the power rating of the resistor, it will burn. So the assertion is false. Resistance value is temperature dependent. So, the reason is true, But reason does not explain the assertion.

Kirchhoff ’s voltage law says that the sum of the voltages around any closed loop is zero. A closed loop starts from a node, traces a path through the circuit and returns to the same node. Since the total work done in moving a charge around this close path the zero, hence the electric field is conservative. So, the assertion is true. Potential difference between two points in a circuit does not depend on the path. This is true for conservative fields. Hence the reason is also true and it explains the assertion.

In the balanced condition of a Wheatstone bridge, there is no current flow through the galvanometer. This is also called null condition. So, the assertion is true. When the potential of point B and D are same then only there is no current flow through the galvanometer. Hence the reason is also true and explains the assertion.

Meter bridge wire is generally made of Constantan. The assertion is true. Temperature coefficient of resistance of Constantan is very low. So, its resistance variation is negligible even after long use. This is the requirement of an ideal meter bridge. So the reason is true and properly explains the assertion.

Potentiometer is used to measure the e.m.f. of a cell. The assertion is true. Voltmeter draws certain amount of current from the cell. So, V = E – iR. The measured value by the voltmeter is less than the actual e.m.f. of the cell. But the potentiometer draws no net current at balance point, hence it measures the actual e.m.f. of the cell. So, the reason is also true and explains the assertion properly.

In a simple electric circuit, electrons flow from the negative - terminal to the positive - terminal. But in general, the positive - terminal of the battery is considered to be the point of higher potential. Therefore, the Assertion is wrong.

Voltmeter is a galvanometer with high resistance. It measures potential drop across any part of an electrical circuit. It is connected in parallel so that it does not draw any current itself (due to high resistance) and does not affect net resistance of the circuit.

The electric power of a heater is more than that of a bulb. As P∝1/R; P∝1R, , the resistance of heater is less than that of the electric R bulb. When a heater connected in parallel to the bulb is switched on, it draws more current due to its lesser resistance, consequently, the current through the bulb decreases and so it becomes dim. When the heater coil becomes sufficiently hot, its resistance becomes more and then it draws a little lesser current. Consequently, the current through the electric bulb recovers. Hence, dimness of the bulb decreases.