A student wraps copper wire around an iron nail. Then she connects the ends of the wire to a battery, making an electromagnet. She notices that if she wraps more coils of wire around the bolt, then her electromagnet can pick up more paper clips. What conclusion can she draw?
More coils of wire increase the length of the nail
More coils of wire increase the density of the copper wire
More coils of wire increase the strength of the electromagnet
More coils of wire increase the mass of the batter