It seems there's a mix-up in your description of the electromagnet experiment and a discussion of charging by induction using rods and spheres. These are two separate topics in physics.
For the first part regarding the electromagnet:
The student's experiment with changing the number of turns around the nail while keeping the battery voltage constant is a way to see how the strength of the electromagnet changes with the number of coils. The key variable that the student is manipulating is the number of turns of the wire. The result would typically show that as the number of turns increases, the magnetic field strength also increases, thus allowing the electromagnet to attract more pins.
For the second part regarding charging by induction using rods and spheres:
It's unclear which experiment scenario you are referring to since there's no direct connection to the initial electromagnet setup. If you're discussing how to induce charge on spheres by using charged rods, here are some considerations:
1. When you bring a negatively charged rod near a neutral metallic sphere, it will cause the electrons in the sphere to repel away from the point closest to the rod. This process is called induction, and it results in the side of the sphere nearest to the rod becoming positively charged while the far side becomes negatively charged.
2. Bringing a positively charged rod near a neutral metallic sphere would cause electrons in the sphere to be attracted towards the side closest to the rod, making that side negatively charged and the far side positively charged.
3. Placing two negatively charged rods near each sphere would repel electrons away from the rods, making the sides of the spheres closest to the rods positively charged.
4. Placing two positively charged rods near each sphere would attract electrons towards the rods, making the sides of the spheres closest to the rods negatively charged.
If charging by induction and resultant force on pins were related to the electromagnet experiment (which does not seem to be the case), you would need to provide more context to accurately address what the student should do with charged rods and spheres. However, based on the scenarios given, no clear connection is established between charging by induction and the student's electromagnet experiment.
A student conducted an experiment where they made an electromagnet. They used a battery that was 10V, but changed the varying number of turns around the nail. Then the student tested how many pins it attracted.
She should place a negatively charged rod near the left sphere.
She should place a positively charged rod near the left sphere.
She should place two negatively charged rods near each sphere.
She should place two positively charged rods near each sphere.
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