vector equation of a reflected line
Month: November 2019
Heat Sinks
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.
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.
Describe a method of preparing sodium chloride from the reaction of sodium hydroxide and hydrochloric acid.
The sodium chloride salt can be prepared using the titration method.
First, use a pipette to draw an accurate fixed volume of hydrochloric acid to a conical flask.
Add 2 to 3 drops of phenolphthalein into the flask.
Titrate, drop by drop, the sodium hydroxide from the burette into the hydrochloric acid. Swirl the solution each time sodium hydroxide is added to it.
When the first appearance of pink color is observed in the solution, stop the titration and note the volume of sodium hydroxide used.
Mix the exact volume of hydrochloric acid and the required volume of sodium hydroxide, without adding the indicator, to obtain the sodium chloride salt solution.
The salt solution is then heated until saturated.
The saturated solution is allowed to cool so that crystals of sodium chloride can be formed.
Filter the mixture to remove excess water.
Wash the crystals with a little bit of cold distilled water and dry the crystals between two pieces of filter paper.
How does a fuse work?
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.
How is sound transmitted through a medium?
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.
Describe an experiment to study the speed of reaction between calcium carbonate and dilute hydrochloric acid, by measuring the loss in mass of reaction system over time.
The apparatus is set up as shown in the diagram below.
The initial mass of the reaction system is recorded.
The thread is pulled to mix the calcium carbonate and acid. The stopwatch is started at the same time.
The mass of the reaction system is recorded every one-minute.
From the data collected, a graph of mass of reaction system against time taken is plotted.
From the slope of the graph, the speed of reaction can be studied.
The steeper the gradient of the graph, the faster the speed of reaction.
Describe an experiment to study the speed of reaction between dilute hydrochloric acid and magnesium, by measuring the volume of gas produced over time.
The apparatus is set up as shown in the diagram below.
The thread is pulled to mix the magnesium ribbon and acid. The stopwatch is started at the same time.
The volume of hydrogen collected in the gas syringe is recorded every half-minute.
From the data collected, a graph of volume of gas produced against time taken is plotted.
From the slope of the graph, the speed of reaction can be studied.
The steeper the gradient of the graph, the faster the speed of reaction.
Explain, using structure and bonding, why copper(II) nitrate cannot conduct electricity when solid, but can conduct electricity when molten or aqueous.
Copper (II) nitrate is an ionic compound. In the solid state, the ions of copper (II) nitrate are held in fixed positions in a giant ionic lattice structure. There are no mobile ions to conduct electricity.
In the molten or aqueous state, the giant ionic lattice structure is broken down. The ions are now able to move about. These mobile ions can act as charge carriers.
force on a wire XY
a) The force on XY is acting downwards.
Applying Fleming’s Left Hand Rule, the middle finger points into the page, in the direction of the current flow from X to Y, the index finger points to the right, in the direction of the magnetic field, the thumb will be pointing downwards, which represents the direction of the force on XY.
b) (i) Pan moves upwards.
b) (ii) When the magnets are brought closer together, the magnetic field strength of the magnet increases, increasing downwards force on XY. The greater clockwise moment will then cause the pan to tilt upwards.
c) (i) When an alternating current flows through the iron bar, a rate of change of magnetic field is produced by the iron bar. The rate of change of magnetic field produced an induced electromotive force in the flat coil, and hence an induced current flow through the closed circuit of the coil, causing the lamp to light up.
c) (ii) The lamp will be dimmer.
The lower frequency produced a lower rate of change of magnetic field, which produce a lower electromotive force in the flat coil, causing the voltage supplied to lamp to decrease.