Reshma bought three identical metal bars from the market. Out of these bars, two were magnets and one was just a piece of iron. How will she identify which two amongst the three could be magnets (without using any other material)?
Reshma can identify the magnets by testing for repulsion between the bars.
She should take any two bars and bring their ends close to each other in different orientations. If the two bars repel each other at some orientation, then both are magnets because repulsion occurs only between like poles of two magnets.
If they only attract each other in all orientations, then one of them is the iron piece and the other is a magnet.
She should then test the third bar with each of the first two. The two bars that show repulsion when brought together are the magnets, and the bar that only shows attraction with both others is the iron piece.
Explanation
The chapter explains in Activity 4.5 that 'like poles, that is, either the North poles or the South poles of both magnets, repel each other.' The key insight from Activity 4.5 is that 'a magnet can be identified by its property of repulsion' because an iron bar will be attracted by both poles of a magnet, whereas another magnet will show both attraction (unlike poles) and repulsion (like poles) depending on orientation.
Solution Steps
Test for repulsion between bars to identify magnets
Take any two bars and bring their ends close in different orientations
If bars repel each other at some orientation, both are magnets (repulsion only occurs between like poles)
If bars only attract in all orientations, one is iron and one is magnet
Test the third bar with each of the first two
The two bars showing repulsion are magnets; the bar showing only attraction is iron