Electrical Engineering All Engineering Career

The resistance of a conductor varies inversely with its cross-sectional area.

According to the formula for resistance (\(R\)):

\[ R = \dfrac{{\rho \cdot L}}{{A}} \]

Where:
- \( \rho \) (rho) is the resistivity of the material,
- \( L \) is the length of the conductor, and
- \( A \) is the cross-sectional area of the conductor.

As you can see from the formula, resistance (\(R\)) is inversely proportional to the cross-sectional area (\(A\)). Therefore, as the cross-sectional area increases, the resistance decreases, and vice versa.

Electric pressure is also called voltage. Voltage is the difference in electric potential between two points, which determines the flow of electric current between those points. It is often referred to as electric pressure because it is analogous to pressure in a fluid system - just as pressure drives the flow of fluid in a pipe, voltage drives the flow of electric current in a circuit.

In gases, the flow of current is due to electrons, positive ions, and negative ions.

When a gas is subjected to a high enough electric field, electrons can be stripped from the atoms or molecules in the gas, creating positively charged ions and free electrons. These free electrons and positive ions can then move through the gas, constituting an electric current. Additionally, in some cases, negative ions may also be present and contribute to the current flow. Therefore, the flow of current in gases involves a combination of electrons, positive ions, and sometimes negative ions.

Conductance is the reciprocal of resistance. 

Conductance (\(G\)) is a measure of how easily a material conducts electricity, while resistance (\(R\)) is a measure of how much a material opposes the flow of electricity. They are inversely related, meaning that as resistance increases, conductance decreases, and vice versa. Mathematically, conductance (\(G\)) is defined as the reciprocal of resistance (\(R\)):

\[ G = \dfrac{1}{R} \]

Therefore, conductance is the reciprocal of resistance.

The best conductor of electricity among the options provided is silver.

While copper is also an excellent conductor of electricity and is commonly used in electrical wiring, silver has even higher conductivity. However, silver is less commonly used than copper in practical applications due to its high cost and other factors such as tarnishing.

Among the options provided, aluminum does not have a negative temperature coefficient.

Materials with a negative temperature coefficient exhibit a decrease in resistance as temperature increases. Aluminium typically has a positive temperature coefficient, meaning its resistance increases with temperature. 

The resistance of a few meters of wire conductor in a closed electrical circuit is typically considered to be low.

While it's not exactly zero, the resistance of a short length of wire in a closed circuit is usually negligible compared to other components in the circuit, especially if the wire is made of a good conductor like copper. This is why we often say that wires offer very little resistance to the flow of current in a circuit.

When a 40W bulb is replaced by a 100W bulb in series with a room heater, the total resistance in the circuit decreases because the 100W bulb has a lower resistance compared to the 40W bulb. This happens because the 100W bulb draws more current due to its higher power rating.

With a decrease in total resistance in the circuit, the total current flowing through the circuit will increase. As a result, the heater output will increase because it will receive more current.

So, the correct answer is: increase.

One commercial unit of energy is equal to one kilowatt-hour (kWh).

This unit is commonly used by utility companies to measure electricity consumption. It represents the amount of energy consumed by a load of one kilowatt (1000 watts) over a period of one hour.

To find the energy consumed by the electrical lamp, we can use the formula:

\[ \text{Energy (kWh)} = \dfrac{\text{Power (W)} \times \text{Time (hours)}}{1000} \]

Given:
Power (W) = 100 W
Time (hours) = 30 minutes = 0.5 hours
Voltage (V) = 220 V

First, let's find the energy consumed in one hour (kWh):\[ \text{Energy (kWh)} = \dfrac{100 \, \text{W} \times 0.5 \, \text{hours}}{1000} = 0.05 \, \text{kWh} \]

So, the energy consumed in 30 minutes is 0.05 kWh.