CIE iGCSE Co-Ordinated Science P4.5.5 The d.c. motor Exam Style Questions Paper 3
Question
As the gasoline flows into the fuel tank, the gasoline becomes charged.
The gasoline evaporates and the driver’s hand cools down.
Explain this cooling effect.
Use ideas about molecules in your answer.
State the work done on the car by the car engine.
State the unit of your answer.
Describe what happens to the motion of the air particles in the tyres as the air warms up.
The turning effect on the current-carrying coil in the electric motor can be increased by increasing the magnetic field strength.
Complete the sentence to describe how to calculate the moment of a force:
The moment of a force is calculated by multiplying the ……………………….. by the perpendicular distance from the ………………………… .
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic P4.2.1 — Electrical charge: charging by friction (Part (a))
• Topic P2.2.2 — Melting, boiling and evaporation (Part (b))
• Topic P1.6.2 — Work (Part (c)(i))
• Topic P2.1.2 — Particle model: motion and temperature (Part (c)(ii))
• Topic P4.5.5 — The d.c. motor (Part (d)(i))
• Topic P1.5.2 — Turning effect of forces / moments (Part (d)(ii))
▶️ Answer/Explanation
(a)(i) Friction
As the gasoline flows and rubs against the surfaces of the pipe and tank, charge is transferred between the surfaces.
This process of charging by rubbing together is called friction.
(a)(ii) Name: electrons; Charge: negative
Charging by friction always involves the transfer of negatively charged particles.
These particles are electrons, which move from one surface to another, leaving one surface positively charged and the other negatively charged.
(b) The most energetic molecules escape from the liquid surface during evaporation, leaving behind molecules with lower average kinetic energy
Evaporation removes the fastest-moving, most energetic molecules from the surface of the gasoline.
This lowers the average kinetic energy (and so the temperature) of the molecules left behind, which is felt as a cooling effect on the hand.
(c)(i) Work done = \(5.8 \times 10^7\) joules (J)
Work done is defined as being equal to the energy transferred.
Since the engine transfers \(5.8 \times 10^7\,\text{J}\) of useful energy, the work done on the car has this same value, measured in joules (J).
(c)(ii) The air particles move faster
As the air in the tyres warms up, the particles gain kinetic energy.
This increase in kinetic energy means the particles move faster (with greater average speed).
(d)(i) Increase the current, or increase the number of turns on the coil
The turning effect on a current-carrying coil depends on several factors.
Besides increasing the magnetic field strength, increasing the current through the coil or increasing the number of turns on the coil will also increase the turning effect.
(d)(ii) The moment of a force is calculated by multiplying the force by the perpendicular distance from the pivot
Moment is a measure of the turning effect of a force about a pivot.
It is calculated using \(\text{moment} = \text{force} \times \text{perpendicular distance from the pivot}\).
Question
State the two quantities needed to calculate the pressure exerted by the car on the road.
Describe a simple test to show that a wheel is not made from steel.
He puts a spanner on a wheel nut as shown in Fig. 3.2.
Calculate the moment of the force about the centre of the wheel nut.
The turning effect of an electric motor can be increased by increasing the number of turns on the coil in the motor.
State one other way to increase the turning effect of a d.c. motor.
The force that causes the build-up of electrostatic charges on the car is …………………………. .
The charged particles that are added or removed during the build-up of charge are called …………………………. .
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic P1.7 — Pressure (Part (a))
• Topic C9.3 — Alloys and their properties (Part (b)(i))
• Topic P1.5.2 — Turning effect of forces (Part (b)(ii))
• Topic P4.5.5 — The d.c. motor (Part (b)(iii))
• Topic P4.2.1 — Electrical charge (Part (c))
▶️ Answer/Explanation
(a) Weight of the car; surface area of tyres in contact with the road
Pressure is calculated using \(\text{pressure} = \dfrac{\text{force}}{\text{area}}\).
The force here is the weight of the car pressing down on the road.
The area is the total contact area of all the tyres touching the road surface.
(b)(i) Use a magnet — the wheel is not attracted to it
Steel is magnetic, so a magnet will be attracted to a steel wheel.
An aluminium alloy wheel is not magnetic, so a magnet held near it will show no attraction.
This simple test distinguishes between the two materials.
(b)(ii) 100 N m
Moment is calculated using \(\text{moment} = \text{force} \times \text{perpendicular distance}\).
Moment \(= 250\,\text{N} \times 0.4\,\text{m}\).
Moment \(= 100\,\text{N m}\).
(b)(iii) Increase the current, increase the voltage, or increase the strength of the magnetic field
A stronger magnetic force is produced by increasing the current flowing through the coil.
Using a stronger magnet also increases the turning effect (torque) on the coil.
Either of these changes increases the force on the current-carrying coil.
(c) Friction; electrons
Friction between the tyres and the road (or the car and the air) causes electrons to be transferred.
This transfer of electrons results in a build-up of electrostatic charge on the car.
The charged particles involved in this process are electrons.