Double-insulated appliances are common around us. The air purifiers in our classroom are one good example.
In these appliances, only two wires are needed, namely Live and Neutral wires. Earth wire is not required as the casing is made of insulator like plastic for this case.
A common mistake is to state that the reason for switch to be on the live wire is “so that the switch can turn on/off the appliance”. This explanation is wrong.
Refer to the view for the reason why switch and fuse must be on the live wire.
In general, in the event where the fuse is blown or the switch is open, it disconnect the appliance from the high potential of the live wire. So the appliance will no become ‘live’ and it is save to touch even though there is a fault.
Simply put, electric appliance can be very simple with just a live wire bring in the current and a neutral wire to bring the current out. The appliance can just work like this forever (without the hassle of having earth wire, fuse etc), provided there is no electric fault developed.
Both the earth wire and the fuse are safety features to protect the user and the appliance respectively. Take a look at the video below to understand the rationale.
Recall these notes given:
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Thankful to be able to share my work with this group of wonderful and nice science teachers from Ai Tong School. Thank you for the invitation! Hope your PLT will be a success!
How did he do that?!
Another type of gadget to hang your bags by the table is this. As long as the centre of gravity (CG) is directly below the support area (area of the hanger in contact with the table), the bag will not fall.
In theory, we have learned that
Dull (matt/rough) and black surface is a good emitter and good absorber of infrared radiation.
In this simple demonstration, the container sides are painted white matt, silver smooth and black matt.
Hot water is poured into the container and the thermal energy is conducted to the whole container. The temperature of the container is more or less uniform.
Using a infrared thermometer, we measure the temperature of the different surfaces.
From this simple demonstration, we can conclude that dull black surface is the best emitter of infrared radiation and smooth silver surface is the worst emitter of infrared radiation.
Radiometer is a device for measuring the amount of infrared radiation. It consists of freely pivoted rotor with four vanes perpendicular to one another. The vane surface is painted alternate black and silver. The rotor is enclosed in a partial vacuum glass bulb.
In general, when exposed to infrared radiation, the rotor will spin. The greater the amount of radiation, the faster the rotor spins.
The actual working principle is actually much more complicated. Refer to the videos below for detailed explanation.
For our context in O-level, we can briefly explained based on what we learned. As the vanes of the rotor are exposed to infrared radiation, the black side of the vane absorbs more radiation as it is a good absorber of radiation and hence its at higher temperature. The silver side reflects the radiation. The air molecules at the black side will get heated up and gain more kinetic energy. Hence rate of collision is higher and the air molecules collide on the black side with more force than the silver side. This results in a net force on the black surface and the rotor spins in a specific direction as shown in the video.
The more complicated theory how radiometer works
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This simple toy is made possible using convection current in the air.
The ‘fan’ of the carousel is similar to the windmill we are familiar with. Just that the kinetic energy of the wind is created by the stream of hot air rising up from the bottom. It is the opposite of an electric fan.
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The paper over metal or over plastic/wood will get burned faster?
Everyone knows that metal is a good conductor of thermal energy. But for experiments like this, many would have guessed it wrongly.
A paper is wrapped over metal and insulator (plastic, wood etc) and is exposed to the flame, the paper over the insulator becomes charred faster and burned faster.
The paper over the metal takes a longer time to be charred and burned. This is because metal is a good conductor of thermal energy. When the spot (paper over metal) is exposed to the flame, the metal conducts the thermal energy away from that spot to other parts of the metal. So the temperature increase at that spot is slower, hence the ignition temperature of the paper (approx. 230oC) will be reached much slower, compared to the spot where the paper is over an insulator.
In the three states of matter, in general, solid is the best conductor as the particles are closely packed in an orderly manner, hence thermal energy can be passed down by the collision of the particles in the solid faster.
On the other hand, liquid and gas are relatively considered poor conductor. Gas is the worst conductor as the particles are far apart.
The following experiment demonstrates that water is indeed a poor conductor of thermal energy.
An ice cube is kept at the bottom of the boiling tube by the net. The water at the top of the boiling tube is heated and started to boil. But the ice is not fully melted.
This shows that thermal energy transferred through the water from heated water at the top to the ice is weak, hence indicating that water is a poor conductor.
How about thermal transfer through convection current? In this experiment, the heated water at the top expands, the volume of the heated water increases, becomes less dense and remains at the top. The cooler water, which is denser, remains at the bottom. Hence there is no convection current formed throughout the entire water in the tube. So thermal energy transfer to the ice cube through convection is not present here.
A partially inflated balloon is placed inside a sealed container as air is pumped out of the by syringe. As the pressure of the container is reduced, the volume of the balloon increases. The pressure inside the balloon decreases.
Note that the pressure inside the balloon is not equal to the pressure of the container. The number of air molecules in the balloon is fixed. As the pressure of the container decreases, this creates a pressure difference inside and outside the balloon. This causes the balloon to expand and its volume to increase. The number of air molecules per unit volume inside the balloon decreases, hence pressure inside the balloon decreases.
But as the balloon is elastic, the wall of the balloon is stretched. Hence the pressure of the balloon will be greater than the pressure of the container. Refer to the video below.
Evan's Space 