(a) The pressure-washer pumps water at a high pressure through a small nozzle. 
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic P1.7 — Pressure (Part (a))
• Topic P4.5.5 — The d.c. motor (Part (b))
• Topic P2.2.2 — Melting, boiling and evaporation (Part (c))
(a) 45 N
Use \(F = P \times A = (9.0 \times 10^{6}) \times (5.0 \times 10^{-6})\).
\(F = 45\,\text{N}\).
(b)(i) Arrow drawn upwards at X.
Using the left-hand (motor) rule with current flowing into the page at X between the N pole and S pole, the force on the coil at X acts upwards.
(b)(ii) It reverses the current direction in the coil every half-turn.
The split-ring commutator swaps the contacts every half-rotation, reversing the current in the coil.
This keeps the force on each side of the coil acting in a consistent rotational direction, so the coil continues turning the same way.
(c)(i) Evaporation
The change of liquid water to water vapour at a temperature below boiling point is called evaporation.
(c)(ii) The most energetic molecules escape from the surface of the liquid.
Within the liquid, molecules have a range of kinetic energies.
The most energetic molecules near the surface have enough energy to overcome intermolecular forces and escape as vapour, leaving the droplet to gradually disappear.
Topic codes:
• Topic P2.2.2 — Melting, boiling and evaporation (Part (a))
• Topic P2.3.2 — Convection (Part (b))
(a)(i) Similarity: Both boiling and evaporation are changes of state from liquid to gas.
Difference 1: Boiling occurs at the boiling point, while evaporation occurs below the boiling point.
Difference 2: Boiling occurs throughout the liquid (forms bubbles), while evaporation occurs only at the surface of the liquid.
(a)(ii) Three factors which increase the rate of evaporation:
1. Increased temperature (particles have more kinetic energy, more can escape)
2. Increased surface area (more particles at the surface can escape)
3. Increased air movement over the liquid surface (removes vapour, maintaining a steep concentration gradient)
(b) Convection in the beaker of water:
The heated water at the bottom becomes less dense (as it expands).
This less dense water rises to the top.
The cooler, more dense water sinks to the bottom, where it is heated and the cycle repeats, creating a convection current.
This circulation transfers thermal energy throughout the water, causing all the water in the beaker to increase in temperature.
