Electrical Engineering All Engineering Career

Ohms law establishes a relationship between current and voltage.
It states that the current in a conductor is proportional to the applied voltage.

All metal exhibit conduction ability, while non-metals do not.

Thus, D. Paper is an insulating material.

With the rise in temperature, the resistance of semi-conductors typically decreases.

Carbon has a negative temperature coefficient of resistance.

The filament of an electric bulb is typically made of tungsten.

With rise in temperature, the resistance of pure metals typically increases.

The ratio of the mass of a proton to that of an electron is approximately 1840.

When the length of a wire conductor is doubled, its resistance increases. The resistance is directly proportional to the length of the conductor according to the formula:

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

Where:
- \( R \) is the resistance
- \( \rho \) is the resistivity of the material
- \( L \) is the length of the conductor
- \( A \) is the cross-sectional area of the conductor

Since the length is doubled, and assuming the cross-sectional area remains constant, the resistance will also double.

If the initial resistance is 0.2 ohms, doubling it will result in a resistance of \( 0.2 \times 2 = 0.4 \) ohms.

So, the resistance becomes 0.4 ohms when the wire conductor's length is doubled.

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points. However, there are certain cases where Ohm's Law may not be directly applicable:

1. Vacuum tubes: Ohm's Law may not directly apply to vacuum tubes, as their behavior is often nonlinear and depends on factors such as temperature and biasing.

2. High voltage circuits: Ohm's Law may not accurately describe the behavior of circuits operating at very high voltages, where other factors such as insulation breakdown and corona discharge become significant.

3. Circuits with low current densities: In circuits with very low current densities, such as in superconductors or very high resistance materials, Ohm's Law may not provide an accurate description of the circuit behavior.

So, the correct answer is vacuum tubes.

To find the total resistance of resistors connected in parallel, you can use the formula:

\[ \dfrac{1}{R_{\text{total}}} = \dfrac{1}{R_1} + \dfrac{1}{R_2} + \dfrac{1}{R_3} \]

Given resistances \( R_1 = 10 \) ohms, \( R_2 = 15 \) ohms, and \( R_3 = 30 \) ohms, substitute the values into the formula:

\[ \dfrac{1}{R_{\text{total}}} = \dfrac{1}{10} + \dfrac{1}{15} + \dfrac{1}{30} \]

\[ \dfrac{1}{R_{\text{total}}} = \dfrac{3}{30} + \dfrac{2}{30} + \dfrac{1}{30} \]

\[ \dfrac{1}{R_{\text{total}}} = \dfrac{6}{30} \]

\[ R_{\text{total}} = \dfrac{30}{6} \]

\[ R_{\text{total}} = 5 \text{ ohms} \]

So, the total resistance of the combination is 5 ohms.