Problem 3 – Magnets in conducting liquid
In total, 62 solutions were submitted for problem 3, out of which 18 were correct.
Congratulations to everyone who managed to solve this problem!
The Best Solution Awards are usually selected from those solutions which were submitted before any hints, sometimes with the exception of Hint 1 which is usually very subtle. This time, however, the first correct solution was submitted after the second hint. Because of that, half of the best-solution award goes to the single participant who solved the problem with two hints, Luka Passek-Kumerički. The remaining half is distributed evenly, between the only two contestants who solved the problem with three hints, Anindya Guria and Chung Wang Lam; both of them earn one-quarter of the award. All these solutions are clear and well-written.
When seeing that the problem turned out to be more difficult than expected, with no correct solutions for a long time, I asked a few former Physics Cup winners to try out this problem. As a result, we have also two excellent solutions, written by Dobrica Jovanovic, currently a second-year undergraduate student at MIT, and Thomas Foster, currently a graduate student at Princeton University. These are both really excellent solutions. The solution by Thomas Foster in particular provides additional insight into the problem and useful discussions.
Final results for Problem 3
Pre-university students:
| First name | Surname | Country | School/University | P3 time | P3 Score |
| Chung Wang | Lam | Hong Kong | St. Paul’s Co-educational College | 25/2/23 15:52 | 2.0982 |
| Alexandar | Roussev | Bulgaria | – | 27/2/23 10:02 | 1.4148 |
| Ilan, Ivan (Team) | Mihelja, Hegedić | Croatia | XV. gymnasium Zagreb, V. gymnasium Zagreb | 27/2/23 11:05 | 1.2862 |
| Eppu | Leinonen | Finland | Munkkiniemen yhteiskoulu | 6/3/23 18:27 | 0.8858 |
| Filip | Vučić | Croatia | I. gymnasium Zagreb | 1/3/23 17:30 | 0.8524 |
| Tomas | Babelis | Lithuania | Vilniaus jėzuitų gimnazija | 20/3/23 12:52 | 0.6588 |
| Ruicheng | Ma | China | Harbin Normal University High School | 19/3/23 22:38 | 0.6523 |
| Dženan | Midžić | Bosnia and Herzegovina | JU Gimnazija “Bihać” | 1/4/23 11:46 | 0.599 |
University students:
| First name | Surname | Country | School/University | P3 time | P3 Score |
| Luka | Passek-Kumerički | Croatia | University of Zagreb | 17/2/23 20:32 | 3.079 |
| Anindya | Guria | India | Indian Institute of Science | 26/2/23 13:38 | 1.9074 |
| Kerim | Cherkezov | Turkmenistan | Turkmen State University | 28/2/23 04:47 | 1.2862 |
| Lukas | Schicht | Germany | University of Rostock | 4/3/23 18:29 | 1.1108 |
| Alessio | Biccari | Italy | La Sapienza | 11/3/23 21:23 | 0.7972 |
| Gil | Ronen | Israel | Hebrew University of Jerusalem | 12/3/23 09:04 | 0.7247 |
| Tejas | Acharya | India | University of Oxford | 28/3/23 20:33 | 0.605 |
| Soumyadeep | Sarma | India | Indian Institute of Science | 26/3/23 06:19 | 0.599 |
| Konstantin | Tripunovski | North Macedonia | University of Ljubljana – Medical Faculty | 21/3/23 15:33 | 0.593 |
| Marinus | Lehmann | Germany | University of Potsdam | 30/3/23 10:56 | 0.495 |
The list of countries:
Algeria, Argentina, Armenia, Australia, Austria, Azerbaijan, Bahrain, Bangladesh, Belarus, Belgium, Bosnia and Herzegovina, Brazil, Bulgaria, Cambodia, Cameroon, Canada, China, Croatia, Cuba, Czech Republic, Ecuador, Egypt, El Salvador, Estonia, Finland, France, Georgia, Germany, Greece, Guatemala, Honduras, Hong Kong, Hungary, India, Indonesia, Iran, Israel, Italy, Japan, Kazakhstan, Kenya, Kosovo, Kyrgyzstan, Latvia, Lithuania, Macau, Malaysia, Mexico, Moldova, Montenegro, Nepal, Netherlands, Nigeria, North Korea, North Macedonia, Pakistan, Peru, Philippines, Poland, Romania, Russia, Rwanda, Saudi Arabia, Serbia, Singapore, Slovakia, Slovenia, South Africa, South Korea, Spain, Sri Lanka, Suriname, Sweden, Switzerland, Syria, Taiwan, Tajikistan, Thailand, Turkey, Turkmenistan, Ukraine, United Arab Emirates, United Kingdom, United States, Uzbekistan, Vietnam.