IMO Practice Test — Magnetic Effects of Electric Current
14 Questions • 15 min • Olympiad level
15:00
Question 1 of 14
A current flows north in a horizontal wire. By the right-hand thumb rule, directly below the wire the field points:
north
east
west
down
Explanation: Fingers curl so that below the wire the field points east.
Question 2 of 14
If both the current and the magnetic field directions are reversed, the force on a conductor:
reverses
stays the same
becomes zero
doubles
Explanation: Two reversals cancel, so the force keeps its original direction.
Question 3 of 14
A motor uses the left-hand rule while a generator uses the right-hand rule because:
they use different magnets
one finds force, the other finds induced current
one is AC only
of friction
Explanation: Left hand: force from current; right hand: induced current from motion.
Question 4 of 14
When a magnet is held stationary inside a coil, the induced current is:
maximum
zero
alternating
doubled
Explanation: Current is induced only when the field is changing.
Question 5 of 14
Moving a magnet into a coil twice as fast makes the induced current:
smaller
larger
zero
unchanged
Explanation: A faster change in field gives a larger induced current.
Question 6 of 14
In India the mains current changes direction:
50 times per second
100 times per second
60 times per second
once per second
Explanation: At 50 Hz the AC completes 50 cycles, reversing twice each cycle = 100 reversals/s.
Question 7 of 14
Replacing the slip rings of an AC generator with a split ring would produce:
no current
direct current
stronger AC
heat only
Explanation: A split-ring commutator gives one-directional (DC) output.
Question 8 of 14
Two parallel wires carry current in the same direction. They will:
repel
attract
not interact
melt
Explanation: Currents in the same direction attract each other.
Question 9 of 14
A fuse of rating 5 A is best for an appliance drawing about:
8 A
3 A
10 A
12 A
Explanation: The normal current must be below the fuse rating, so 3 A is safe with a 5 A fuse.
Question 10 of 14
An electromagnet is preferred over a permanent magnet in a crane because it:
is cheaper only
can be switched on and off
needs no current
is always weak
Explanation: Switching the current lets it pick up and drop loads.
Question 11 of 14
The metal body of a refrigerator gives a shock; the most likely cause is:
good earthing
a broken or missing earth connection
a blown fuse
too low voltage
Explanation: Without earthing, a live body cannot discharge safely to ground.
Question 12 of 14
Running an iron, heater and geyser on one socket trips the fuse because of:
a short circuit
overloading
induction
earthing
Explanation: Too many appliances draw excess current — overloading.
Question 13 of 14
The force on a conductor placed exactly parallel to the magnetic field is:
maximum
zero
half of maximum
reversed
Explanation: No force acts when current is parallel to the field.
Question 14 of 14
Increasing the number of turns of a solenoid (same current) makes its field:
weaker
stronger
zero
unchanged
Explanation: More turns add more field in the same direction.