Month: July 2020

Describe the use of Fleming’s left-hand rule to determine the direction of the force on a current-carrying conductor cutting through the magnetic field of a magnet.

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Applying Fleming’s left-hand rule, the thumb, the index finger, and the middle finger on the left hand are positioned in such a way that they are pointing perpendicular to one another. The thumb will point in the direction of the force, the index finger will point in the direction of the magnetic field, while the middle finger will point in the direction of the conventional current flow.

This rule is also known as the Motor Rule. It is used in situations where electrical energy is converted to mechanical energy, similar to the energy conversion in a motor.

Define electrostatic charging by induction.

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It is a process in which electrical charges are redistributed in a neutral body by bringing an electrically charged object near to it.

Note: This type of induction is most effective in electrical conductors, but can also happen in some electrical insulators.

A metal plate being induced with a positive charge near the top surface, and a negative charge near the bottom surface.
Electrostatic induction in action! Bending water with a charged comb.

Better still, test it out yourself! 🙂

Burning biodiesel as a fuel is considered to be a “carbon neutral” activity. Explain what is meant by “carbon neutral” in this context.

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The source of biodiesel is from plant parts. During the growth and development of these plants, they absorb carbon dioxide from the environment for photosynthesis. During the combustion of biodiesel, the same amount of carbon dioxide is released to the environment.

The net amount of carbon added to the environment from the burning of biodiesel is considered to be zero, hence “carbon neutral”.

Explain, in terms of forces, how a speed-time graph change.

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Initially, the car’s speed is increasing. This causes the frictional forces to increase. With the forward driving force remaining constant, the resultant force acting on the car will be decreasing.

Since resultant force = mass x acceleration, with a decreasing resultant force, and constant mass, acceleration will be decreasing. This is represented by the decreasing gradient of the speed-time graph.

Eventually, the forward driving force will be equal to frictional forces. When this happens, resultant force acting on the car will be zero, so acceleration will be zero, and the car will be moving at constant speed. This is represented by the horizontal portion of the speed-time graph.

Explain why the fuse and switch are connected to the live wire.

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A live wire carries a high voltage and carries current from the electrical plug to the appliance.

The switch connected to the live wire will ensure that the appliance is at zero voltage every time the appliance is switched off, making the appliance safer to use.

The fuse connected to the live wire will ensure that high current flow to the appliance will immediately be cut off, preventing damage to the wirings and appliance due to the high current flow.

Suggest why an electrical appliance with a 2-pin plug is still safe to use.

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This appliance is missing an earth wire, but it most likely has double insulation.

Appliances with double insulation have their internal parts and wirings completely insulated from the outer casing, and the outer casing will be made of an electrical insulator, eg plastic.

This means that, even if a live wire touches the casing, it will not become “live”, hence it will be safe to use even without an earth wire.

Note: Appliances with double insulation will have this symbol:

Can a charged insulator transfer its charges to a neutral conductor when they touch?

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The short answer is no.

The insulator can be charged via electrostatic charging by friction, but not by electrostatic charging by induction. The charges do not travel easily from the insulator to the conductor or vice versa, simply by putting them in contact.

The electrons in the insulator are not delocalised, hence they are not able to move freely within its own body or cross over to another body to induce a charge.

Examples:

Answer is A

Answer is D

Explain, using structure and bonding, why alloys are stronger and harder than pure metals.

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Pure metals consist of a lattice of positive ions, arranged in regular pattern, surrounded by a sea of delocalised electrons. When a force is applied, the layers of positive ions slide over one another easily, without disrupting the metallic bonding. This makes the pure metal soft and malleable.

An alloy is a mixture of a pure metal with other elements. The atoms of the added element have a different size from those of the pure metal. This breaks up the regular arrangement of atoms in the pure metal. When a force is applied, the atoms of different sizes cannot slide over one another easily. This makes the alloy stronger and harder.

Why does an echo fill an empty room, but not one that is fully furnished?

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In an empty room, the hard and solid walls, floors and ceilings act as good reflectors of sound, so the sound waves reflect repeatedly off these surfaces, creating echo.

In a fully furnished room, there are usually soft, hollow surfaces, like the fabric sofa; the thick curtains; the wardrobe, etc, that act as good absorbers of sound. This reduces the effect of echo.