Explain why an electromagnet usally has a core of iron or steel

If the speed of a particle triples ,by what factor does its kinetic energy increase?

Two point charges are separated by a certain distance. how does the strength of the electric field produced by the first charge, at the position of the second charge, change if the second charge is doubled? a. the field does not change b. the field strength decreases by half. c. the field strength doubles d. the field strength quadruples

Which of the following best describes the circuit shown below?