Distinguish between intrinsic and extrinsic semiconductors, and solve a problem involving current, charge, and electrons.

Given: Current $I = 3\, \text{A}$, Time $t = 20\, \text{s}$.
The formula for charge is $Q = I \times t$.

The charge that has passed the point is $\boxed{\text{60 C}}$.

ii) Calculate the number of electrons that have passed the point.

The elementary charge $e = 1.6 \times 10^{-19}\, \text{C}$.
The formula is $Q = n \times e$, where $n$ is the number of electrons.

The number of electrons that have passed the point is $\boxed{3.75 \times 10^{20}}$.

2. Three 40 $\Omega$ bulbs are connected to a 240V power source through an ammeter as shown in the figure below.

2. a) What property of the ammeter enables it to be connected as shown?

An ammeter has a very low internal resistance, which allows it to be connected in series in a circuit without significantly affecting the total resistance or the current flow.

2. b) Give two advantages of connecting bulbs in parallel in our homes.

• Each appliance receives the full supply voltage, allowing them to operate at their intended power.
• If one appliance fails or is switched off, the others continue to operate independently.

2. c) Calculate;

i) The combined resistance of the above circuit.

The two parallel bulbs each have a resistance of $R_1 = 40\, \Omega$ and $R_2 = 40\, \Omega$.
The formula for two parallel resistors is $R_p = \frac{R_1 \times R_2}{R_1 + R_2}$.

The parallel combination ($R_p$) is in series with the third bulb ($R_3 = 40\, \Omega$).
The total resistance $R_{total} = R_p + R_3$.

The combined resistance of the circuit is $\boxed{60\, \Omega}$.

ii) The current flowing through the ammeter.

Given: Voltage $V = 240\, \text{V}$, Total resistance $R_{total} = 60\, \Omega$.
The formula for current is $I = \frac{V}{R_{total}}$.

The current flowing through the ammeter is $\boxed{4\, \text{A}}$.

iii) The power consumed when all the bulbs are in use.

Given: Voltage $V = 240\, \text{V}$, Total current $I = 4\, \text{A}$.
The formula for power is $P = V \times I$.

The power consumed when all the bulbs are in use is $\boxed{960\, \text{W}}$.

3. a) Explain why the specific latent heat of vaporization of a substance is much greater than its specific latent heat of fusion.

The specific latent heat of vaporization is much greater than the specific latent heat of fusion because more energy is required to overcome the intermolecular forces and completely separate the molecules in a liquid to form a gas. In vaporization, molecules must break free from the liquid's surface and move far apart, doing work against atmospheric pressure and intermolecular attraction. In fusion, molecules in a solid only need enough energy to break free from their fixed positions and move past each other in the liquid state, where intermolecular forces are still significant.

3. b) i) Give two differences between boiling and evaporation.

• Boiling occurs at a specific temperature (boiling point) throughout the entire liquid, forming bubbles, while evaporation occurs at any temperature from the surface of the liquid.
• Boiling is a rapid process, whereas evaporation is a slow and gradual process.

3. b) ii) State one way by which evaporation can be increased.

One way evaporation can be increased is by increasing the surface area of the liquid.

4. a) i) Define the principal focus and the focal length of a convex lens.

• The principal focus (F) of a convex lens is the point on the principal axis where rays of light parallel to the principal axis converge after passing through the lens.
• The focal length (f) of a convex lens is the distance between the optical center of the lens and its principal focus.

4. a) ii) Explain what is meant by a real image.

A real image is an image formed when actual light rays converge at a point after reflection or refraction. Real images can be projected onto a screen.

4. a) iii) Draw a labeled diagram to show how such an image is produced by a convex lens.

I cannot draw diagrams directly. However, to produce a real magnified image with a convex lens, the object must be placed between the principal focus (F) and twice the focal length (2F) from the optical center of the lens.

Here's how the diagram would be drawn:

1. Draw a principal axis and a convex lens at its center.
2. Mark the principal focus (F) and twice the focal length (2F) on both sides of the lens.
3. Place an object (e.g., an arrow) upright between F and 2F on one side.
4. Draw two principal rays from the top of the object:
   • Ray 1: Parallel to the principal axis, refracts through the lens and passes through F on the other side.
   • Ray 2: Passes through the optical center of the lens without deviation.
5. The point where these two refracted rays intersect on the other side of the lens forms the top of the image.
6. Draw the image as an inverted arrow from the principal axis to this intersection point. The image will be real, inverted, and magnified, located beyond 2F on the opposite side of the lens.

4. b) A ray of light travels at a speed of $3 \times 10^8$ m/s in air and enters a glass of refractive index 1.5.

Calculate the speed of light in the glass.

Given: Speed of light in air $c = 3 \times 10^8\, \text{m/s}$, Refractive index of glass $n = 1.5$.
The formula for refractive index is $n = \frac{c}{v}$, where $v$ is the speed of light in the medium.

The speed of light in the glass is $\boxed{2 \times 10^8\, \text{m/s}}$.