- #1
RodionGork
- 6
- 2
Thread moved from the technical forums to the schoolwork forums
Hi Friends!
Regard the 2 kW electric kettle and 60 W incandescent bulb - what happens if we plug them in series? It is the problem I often ask my students somewhere at the end of the term to check if they remember basic "laws" about electricity. Recently I started collecting some exercises on a web-site, and added this one too - here is the full text - but it is pretty same, the main difference is that now there is precise question to find the power on the kettle in this situation.
This annoys me because calculations are made in assumption both devices behave in concordance with Ohm's law. It is quite close for the kettle (their heating elements made of alloys have almost constant resistance with temperature) - but it is definitely not so with the bulb (whoever tried measuring its resistance in the cold state knows it is about 10 times lower).
It still seems less or more OK for the school exercise, since the kettle will give negligible additional resistance and voltage on a bulb doesn't change much - hence its resistance too. But still I wonder if there exist some popular resistance-temperature approximation for pure metals used in school courses. When I tried to look up the topic in the wiki, I quickly faded out among formulas which seemed quite alien to me - but I understand they aim to be as precise as possible over the whole range of temperatures from zero to thousands.
Regard the 2 kW electric kettle and 60 W incandescent bulb - what happens if we plug them in series? It is the problem I often ask my students somewhere at the end of the term to check if they remember basic "laws" about electricity. Recently I started collecting some exercises on a web-site, and added this one too - here is the full text - but it is pretty same, the main difference is that now there is precise question to find the power on the kettle in this situation.
This annoys me because calculations are made in assumption both devices behave in concordance with Ohm's law. It is quite close for the kettle (their heating elements made of alloys have almost constant resistance with temperature) - but it is definitely not so with the bulb (whoever tried measuring its resistance in the cold state knows it is about 10 times lower).
It still seems less or more OK for the school exercise, since the kettle will give negligible additional resistance and voltage on a bulb doesn't change much - hence its resistance too. But still I wonder if there exist some popular resistance-temperature approximation for pure metals used in school courses. When I tried to look up the topic in the wiki, I quickly faded out among formulas which seemed quite alien to me - but I understand they aim to be as precise as possible over the whole range of temperatures from zero to thousands.
