1. Which of the following can increase the strength of the electromagnet?
(1) Use a.c. current instead of d.c. current.
(2) Decrease the resistance of the rheostat.
(3) Increase the voltage of the battery.
2. Comment on the statements:
(1) The bell does not work if the spring is made of copper.
(2) The bell does not work if the polarities of the battery are reversed.
更新1:
For Q.1, the diagram is about the solenoid and a varying resistance in a electric circuit. For Q.2, the description is "A student designs a simple door bell. When switch is pressed and then released, two notes "ding-ding" are heard. The spring is made of iron.
更新2:
ferritic, what does it mean?
