Friday, December 21, 2007

Electricity Problem

What do you think of this? This is part of a popular text book on Physics in Hong Kong for Form 5 students preparing for the School Certificate Examination.

At first I thought it was just indicative, meaning to show that the current (15A) in one side of the ring and the current (15A) in the other side of the ring add up to the total current (30A) delivered to the appliance. In reality, of course, that the resistance in the two sides may differ, hence the current may not be carried equally in the two sides.

Upon closer look into some of the exercises following this, I realized that the student is expected to answer the questions exactly this way, assuming an even split of current. This is misleading, as if there are some hidden, sophisticated equipment that ensures the current is evenly split in the two sides. Either that, or there electricity is so smart that it splits itself evenly, seeing that there are two paths to the same destination.




2 comments:

The Cat said...

Isn't this the "idealized" situation?

StephenC said...

But there is no such thing.

In reality the ring main is a mainly British technology, which was developed after the second world war because of a shortage of copper. This way smaller size cables can be used. It does require sophisticated equipment to control the flow of current in the two branches. Without it, current can easily be biased towards the side with lower resistance and burn out the cable, or blow the fuse.

By presenting a sophisticated system in a simplistic way, smart students get lost, and other students are encouraged to think in a simplistic manner.