# NSS Physics

Dear guys,

I'm a S.6 student who study Physics and I find some difficulties when I am having my revision of the coming mock exam. May you have me to solve the problems. Here are some of them.

1. I find it difficult to sketch the arrows to show the directions of the compass neddles. As the textbook have mentioned , the direction of the N pole can be determined by the right-hand grip rule. However, I don't know how to apply it to solve the problems, especially for the textbook E&M P.270- 271 checkpoint Q.2(SAM_0162.JPG & SAM_0163.JPG). I find it so differicult to direction of the compass needles when the compass is put outside the coil.

2. I find it difficult to find out the centre of compression and rarefaction in the s-t graph. I don't understand about the explanation which cover in "Last Minute"(SAM_0164). As it says, both B and C are setting closer to A. But how come both of them are getting closer to A? As B is in left-hand-side and C is at right-hand-side, why they will go closer to A at the same time?

3. Last but not least, I don't quite familar with the centre-zero galvanometer. Inside the textbook E&M P.365 checkpoint Q.3( SAM_0165.JPG), it says that the pointer will deflect to the left once wire AB is moved vertically upwards. But why it will deflect to the left instead of deflecting to the right? I just want to know how to determine whether the reading of the galvanometer will be positive or negative. Is there any method or trick for me to understand and memorize it?

### 1 個解答

• 天同
Lv 7
9 年前
最愛解答

1. A compass needle aligns itself along the direction of a magentic field line. On the first diagram (the current loop), field lines run from the fron to the back. Hence, the noth-pole of the needle is pointing towards the back for the compass placed at the centre of the coil.

For the other two, just treat the (small) section of the coil near the compass as a current carrying wire, you would find that needle is pointing towards the right for the both compasses.

For the solenoid, the sketched compasses shown are correct.

2. Generally, we take displacement of particles towards the right as +ve, and that towards the left as -ve.

In so doing, particel C is displaced towards the right from its equilibrium position, whereas particle B is displaced towards the left from its equilibrium position. Since particle A is at the equilibrium positon, just imagine that its left-hand side particle (particle C) is moving towards it (becuae it is moving towards the right), and its right-hand side particle (particle B) is also moving towards it (because it is moving towards the left). It is thus clear that particle A is at point of compression.

3. Generally speaking, the galvanometer needle deflects to the righ when current flows from it +ve terminal to its -ve terminal.

As you can see from the diagram, after applying Fleming's Right Hand Rule, the induced current flows from A to B. Thus the current will flow from the -ve to the +ve terminals of the galvanometer. The needle then deflects to the left.