In general, assuming temperature of a conductor is constant, the resistance of a conductor is affected by 1) length (L) – [L increases, R increases] 2) cross-sectional area (A) – [A increases, R decreases] 3) resistivity (p) = [p increases, R increases] As R is directly proportional to L and p, and is inversely proportional to A, R = pL/A MCQ questions from Olevel When radius or diameter is three times larger, cross-sectional area does not increase by three times!! Refer to this example: 2018PPp1q31 There are times when using the formula is more appropriate.

Answer: Option C View the videos below to learn how to solve it. There are more than one method. Alternatively, you may put values to work out using the 2 basic formulae of P = IV and V = IR. Refer the video below. Similar question 2018PPp1q35 Answer: Option D

Answer: Option D Key concept here is Newton’s 3rd Law. Action equals reaction. Both trolleys are connected by the spring, hence the tension force experienced by each trolley is equal and opposite. Fx    =    Fy ma       =    ma2 m x 2    =    m x a a        =   4 m/s2 Another important concept to know. In Fig A, a box of m is pulled with a force of 40 N. Given that the frictional force is 10 N. The resultant force is 30N. Box is accelerating. In Fig B, if another identical box m is added on top of the existing box (no change to the floor), the total mass is 2m , the frictional force now will double = 20 N. Hence resultant force now will be 20N. The acceleration of the box will be lower than previously.

Answer: Option A Of course if a heavy ball is suspended from the car we all know the ball will move in opposite direction of a decelerating car due to inertia. We are all familiar to this where inertia is applied to a body which is denser than the surrounding medium (air or liquid) which is less dense. That’s why this balloon’s behaviour surprises us! Let’s assume the water molecules are initially moving at constant speed with the tank before the deceleration.  Due to inertia, when the tank decelerates, the water molecules continue its state of motion forward. Hence the water molecules gush to the right side of the tank, displacing (pushing) the balloon to the left. Hence the balloon moves to the left! Refer to this Youtube video by Smarter Everyday and you can see the similar experiment of helium balloon (less denser) in the air (denser) of a car.

According to Fraraday's Law, the magnitude of the induced emg, hence induced current, is directly proportional to the rate of change of magnetic flux, i.e. rate in which the magnetic field lines cut the solenoid and vice versa. To get a higher induced current: 1) Increase the number of turns in the solenoid per unit length 2) Use a stronger magnet to increase the magnetic field strength 3) Insert the magnet in and out of solenoid at faster speed To reverse the direction of the induced current: 1) Reverse the winding of the turns on the solenoid 2) Reverse the pole of the magnet

When the current flows through the solenoid, the U-shaped iron rod will become magnetised easily and strongly. When the current is cut off, the iron rod will lose its magnetism easily. Iron is used as the core rod as it is a soft magnetic material, it can be easily and strongly magnetised. In addition, iron has the ability to concentrate the magnetic field lines generated by the current-carrying solenoid.

When a current flows through a wire, magnetic field is produced around the wire. This magnetic field can be detected using a compass. (compass needle is actually a magnet). Note that the compass needle follows the direction of the magnetic field. You can use your RHGR to determine the direction of the magentic field around the wire, hence knowing where the compass needle will turn.

It fine to use a multiplug as it brings convenience. The problem arises when too many high power appliances are connected and on at the same time. The current through the multiplug cable will be too high. Due to the cable resistance, this high current results in large thermal energy generated in the cable. The rubble insulation melts and leads to fire.