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.
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.
This simple circuit involves a resistance wire on the meter rule and the use of jockey tapping at different length L on the resistance wire.
Note that this is not a potential divider. Rather this set up works like a variable resistor (rheostat) in the circuit. When the switch is closed (jockey NOT tapping on resistance wire), there is no current flowing as voltmeter (infinity resistance) is connected in series with the ammeter and the battery. Hence the voltmeter is showing the battery’s electromotive force (emf) of 3.0 V.
When the switch is closed and the jockey is tapped on the resistance wire on the ruler, the longer the L (length of resistance wire), the higher the resistance of the circuit, the higher the potential difference (voltmeter) across the resistance wire and the smaller the current through the ammeter. Hence the tapping of the jockey on the resistance wire is similar to adjusting the resistance on the variable resistor.
(If you are wondering why the voltmeter is not showing the emf of the battery when the jockey touches the resistance wire L, it is because in reality there is internal resistance in the battery or even in the connecting copper wire. For olevel theory we assume no internal battery resistance or resistance in copper wire or ammeter).
To learn how to set up the experiment, refer to the video below.
Both ammeter and voltmeter have two terminals, (positive and negative). The conventional current must flow into the positive terminal (+) of the meters and out of the negative terminal (-). If the connection is the opposite, the needle will deflect below the zero marking. Refer to the video below.
Sometimes, the connection terminals on the voltmeter and ammeter are different. Likewise, different types of wire with different connection heads have to be used. Refer to the video to see how are they connected in general.
Evan's Space 