Why does pin I fall before pin II? Why did all the pins not fall together?
Answer
Pin I falls first because it is the nearest to the candle flame, and the pins do not fall together because heat takes time to travel along the strip.
Why it happens: conduction is a step-by-step process. The particle at the heated end gets hot, passes heat to the next particle, that one to the next, and so on. So the strip becomes hot gradually, first near the flame and only later at the far end. The wax under pin I therefore reaches its melting point before the wax under pin II, and so on down the line. If heat could jump instantly from one end to the other, all four pins would indeed fall at the same moment — they do not, and that is the evidence for conduction.
Q2.
From your observations, what can you infer? Do you think that heat is being transferred along the metal strip from the end that is being heated?
Answer
Yes. The observations show clearly that heat is being transferred along the metal strip, from the heated end towards the colder end.
The inference is:
Heat travels from the hotter part of an object to its colder part. This process is called conduction.
The transfer is not instant — it moves along the strip step by step, which is why the pins fall in order.
The metal itself does not move, and neither do its particles. A heated particle simply passes the heat on to its neighbour.
Metals allow this to happen easily, so they are good conductors of heat.
Tip: the falling pins are only an indicator. The real event you are watching is the melting of wax, and wax melts only when the strip beneath it becomes hot.
Q3.
If we use a strip made of a material like wood or glass in place of a metal strip to perform Activity 7.1, the pins will not fall. Can you think of the reason for this based on our learning from the chapter ‘The World of Metals and Non-Metals’?
Answer
Because wood and glass are non-metals, and non-metals such as these do not allow heat to pass through them easily. They are poor conductors (insulators) of heat.
Why it happens: in a metal, heat is passed on rapidly from particle to particle, so the whole strip warms up quickly. In wood or glass the transfer is extremely slow. The heat stays bunched up near the flame, and the part of the strip under the pins never becomes hot enough to melt the wax. So the pins stay stuck.
Did you know? Clay and porcelain are poor conductors too. That is exactly why tea or coffee served in a kulhad or a porcelain cup stays hot much longer than in a steel glass — and why the steel glass burns your fingers while the clay cup does not.
Q4.
List some materials around you and classify them as good or poor conductors of heat in Table 7.2.
Answer
Here is Table 7.2 filled with materials found in any home or classroom.
Table 7.2: List of good or poor conductors of heat
S.No.
Material
Good or Poor conductor of heat
1.
Steel
Good conductor
2.
Wood
Poor conductor
3.
Aluminium (pressure cooker, tawa)
Good conductor
4.
Copper (wire, bottom of a vessel)
Good conductor
5.
Iron (nail, ladle)
Good conductor
6.
Glass (tumbler, window pane)
Poor conductor
7.
Clay / porcelain (kulhad, cup)
Poor conductor
8.
Plastic (mug, bucket)
Poor conductor
9.
Wool (sweater, blanket)
Poor conductor
10.
Paper and cardboard
Poor conductor
11.
Rubber (chappal sole, handle grip)
Poor conductor
12.
Air
Poor conductor
Tip: a quick test — touch a metal object and a wooden object lying in the same room. The metal feels colder although both are at the same temperature. It feels colder only because it is a good conductor and carries heat away from your hand quickly.
Q5.
Does your list include air? If it is there on the list, where have you placed it?
Answer
Yes — and air must be placed in the column of poor conductors (insulators) of heat.
Why it happens: the particles of a gas are far apart, so heat cannot be passed easily from particle to particle. Trapped air is therefore one of the best insulators we have, and we use it everywhere:
Woollen clothes trap air in their pores; the trapped air cuts down the flow of heat from our body to the surroundings, so we feel warm.
Two thin blankets keep us warmer than one thick blanket, because a layer of air is trapped between them (Fig. 7.2).
Hollow bricks in outer walls hold air inside them, keeping houses warm in winter and cool in summer.
Did you know? Air is a poor conductor, but it is an excellent convector — when it is free to move it carries heat very well. It insulates only when it is trapped and cannot flow.