幾題physics questions plz~

1. The structure of a simple dc dynamo and an ac dynamo is basically the same. The difference lies on the connection to the external circuit. A pair pf commutators is used in the dc dynamos, but a pair of slip rings is used in an ac dynamo. Thus, the induced current in one side of the rotating coil always changes its direction in half a cycle, it is the use of the commutators that rectify the induced current to dc.

2. Use ideal gas equation, PV = nRT
assume one mole of gas ( n = 1)
At STP, 10^5(Vo) = R(273)where Vo is the volume at STP
But volume = mass/density
hence, 10^5(m/po) = R(273)
where m is the mass of gas and po is the density at STP

At 800 kPa and 42'C (= 315 K),
8x10^5(V) = R(315)
where V is the new volume
i.e. 8x10^5(m/p) = R(315)
where p is the new density

Dividing the two equations: (p/po)(1/8) = 274/315
p = 8 x 273/315 x 1.25 kg/m^3 = 8.67 kg/m^2
3. Statement (1) is wrongAlthough there is cutting of field lines by both the sides AD and BC, you could, by using Fleming's right hand rule, find out that any induced current tends to flow from D to A and also from C to B. Thus these two currents cancel away and no net current is flowing.
Be aware that the existence of induced emf in the coil doesn't not necessarily indicate that there is induced current. In this case, the induced emf on AD and BC are opposite in direction and would counter-balance one another, causing no current to flow.
Statement (3):
The coil rotates with AD as the axis of rotation. This is similar to, but not exactly the same as, what happens in a dynamo with a coil rotating in a magnetic field. There is change of magnetic flux linkage with the coil (or the side BC cuts through field lines). Hence, induced emf and current are produced.