Category: Pure Physics

Describe in molecular terms, how thermal conduction occurs in a solid rod when one end of the rod is heated.

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When one end of the solid rod gains heat, the heat energy absorbed is converted to kinetic energy in the rod particles. The particles at this end of the rod vibrated more vigorously in their fixed positions. The increased vibration is passed on to the neighbouring particles, causing the neighbouring particles to also vibrate more vigorously. Hence the increased vibration of the particles is passed on from particle to particle, till the particles at the other end of the rod also vibrate more vigorously.

When the average kinetic energy of the particles in the rod increases, the temperature of the rod increases. Heat energy is hence transferred from the hotter end of the rod to the cooler end of the rod through vibration of particles.

In metals, besides particle vibration, there is also electron diffusion to transfer the heat energy. The delocalised electrons are able to move more freely, hence heat conduction in metals is faster compared to non-metals.

Why is the sky blue?

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Sunlight reaches Earth’s atmosphere and is scattered in all directions by all the gases and particles in the air. The visible light rays of sunlight consists of the spectrum of red, orange, yellow, green, blue and indigo light rays. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. The scattered blue light high up in the atmosphere causes the sky to appear blue.

Watch the video to get a better idea!

Total Internal Reflection and the Brilliance of Diamonds

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A diamond is cut in such a way that the flat shape of the crown allows for the maximum number of light rays to either reflect off the crown or refract into the diamond.

The critical angle of diamonds will be small as the refractive index of diamond is high.

The shape and angle of the pavillion is such that most light rays falling on it will have an angle of incidence greater than the critical angle. This allows for total internal reflection to occur, causing light rays to reflect multiple times within the diamond, and leave the diamond through the crown.

The multiple light rays leaving the crown of the diamond makes it look more brilliant to an observer looking at the crown of the diamond.

The ideal cut of a diamond will allow for total internal reflection of light rays to occur at the pavilion facet.

Describe how a force is exerted on a current-carrying conductor that is cutting through the magnetic field of a magnet.

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When a current flows through the current-carrying conductor, a magnetic field forms around the current-carrying conductor.

The interactions between the magnetic fields of the current-carrying conductor and the magnet produce regions of unequal magnetic field densities around the conductor.

A force will be exerted on the conductor, moving it towards the region of lower magnetic field density.

Why must the fuse/switch/circuit breaker be connected to the live wire?

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The live wire is at high voltage, and carries the current from the electrical mains to the electrical appliance. If any electric fault occurs, resulting in high current flow, the fuse/switch/circuit breaker on the live wire will immediately cut off the high current/high voltage before the high current/voltage can cause damage to the electrical wirings and the appliance.

What is the potential hazard when the switch of an electrical appliance is connected to the neutral wire instead of the live wire?

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The live wire is at high potential difference, while the neutral wire is at zero potential difference. When the switch is turned off, the electrical appliance will still be at high voltage, since the electrical appliance is still connected to the electrical mains via the live wire.

The electrical appliance being ‘live’ even though the switch is turned off is potentially hazardous as it increases the risk of electrocution and overheating and electric fire if a path to a lower potential difference is established.

Heat Sinks

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a(i) Copper (or any metal).
Copper, being a metal, is a good conductor of heat. The heat generated by the electronic component will be conducted away at a faster rate by the heat sink, preventing the component from overheating.
ii) Black color.
Black colored surfaces are better emitter of radiant heat than bright colored surfaces. This will help the heat sink to emit its heat to the surroundings at a faster rate, so that the heat sink do not get too hot, and will still be able to gain heat from the electronic component effectively.

b)(i) 1W –> increase by 2 deg C
so 10W –> increase by 20 deg C
Answer: 20 deg C
b)(ii) As the temperature of the heat sink increase, the rate of heat conduction from electronic component will slow down, and the rate of heat radiated to surroundings will increase. Eventually, the rate of heat conducted into the heat sink will be equal to the rate of heat emitted from it. At this point, its temperature will remain constant.

Some ice cubes are added to a glass of water at room temperature. Describe in terms of density changes, how convection currents occur in the water to cause the water to cool down faster.

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The water surrounding the ice cubes near the surface of the water lose heat to the ice cubes, contract and become denser.
The denser, cooler water then sinks downwards, causing the warmer, less dense water to rise upwards to take its place. This process repeats, setting up convection currents in the water, cooling down the water faster.

How does a fuse work?

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When current that is higher than the fuse rating flows through the circuit, the wire in the fuse melts and breaks the circuit.

This will prevent the high current from flowing through the wirings and the electrical appliance, preventing overheating of the wires, and damage to the appliance.

A glass fuse. The fuse wire can be seen.
The standard fuse ratings are: 1A, 2A, 3A, 5A, 7A, 10A and 13A. Most electrical appliances in Singapore use the 13A fuse.

How is sound transmitted through a medium?

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When the source of a sound wave vibrates, the surrounding particles are disturbed and vibrates in a direction parallel to the direction of the sound waves.

The vibrations are passed on from one particle to the next in a series of alternate regions of compressions and rarefactions, transmitting the sound waves through the medium.