PHY EM

1)
since the internal resistance of the voltmeter is very large(10M Ohm),
so connect the voltmeter in series equal to connect a high resistance in series,high resistance to decrease the amperage,no current flows to ammeer.
since the relationship of current and resistance will not affect the voltage,i.e.
large resistance->small current or small resistance->large current..
those circumstances will not afeect the voltage supplied,
although the large resistance cause the zreo current,but the two point of the resistance still have potential difference(voltage).

2)
Current cause by the potential difference(voltage) and the resistance,since I=V/R,
but you can only have the potential difference(voltage),don't have the current and the resistance as the BATTERY,
to make the current,you must have a completed circuit,it should contain voltage,current and resistance,current cause by voltage and resistance.

3)
alternating current(AC),
which mean the sinusoidal wave with frequency 50Hz,the peak value is 311.08V(220x1.414),
alternating mean the principle of the live wire,
the live wire contain -220V-0V-200V continuosly in 50 revolution(50Hz)in one second,
from -ve 220V to +ve 220V,again +ve 220V to -ve 220V is called one period,
so 50Hz means the voltage from -ve 220V to +ve 220V,again +ve 220V to -ve 220V have 50 times,
or -220V to 220(220 to -220) 100 times,
you may observate the alternating current by cathode ray oscilloscope(CRO),and get mean information of AC Currnet.

4)what's voltage selector means?
if the voltage from the power station is high than singal phasee AC220V,
almost domestic electrical appliance will be broken out,
but if the voltage selected below 220V,almost domestic electrical appliance will not be operated,



2010-02-18 13:27:02 補充：
5)
a singal AC circuit is series wire,
in this case,the switch can be installed in difference position and still get a work.in the series wire,you have L and N,
if you connect in Neutal,

2010-02-18 13:27:14 補充：
the potential (+220V) from the live wire is directly going to the heating element in the iron since there no any switch,

2010-02-18 13:27:20 補充：
the heating element will not be work since the switch in neutral wire have not been switched on to be a closed circuit,the heating element have +220V,but don't have a way to be a completed circuit,in this case,if you touch the 220V heating element,you will be electrocuted.

2010-02-18 13:27:24 補充：
but inside the iron,after the live wire and before the heating element,it have the fuse,the fuse can protect the wrong wiring and the fuse protection from the plug to decrese the probability of the damage.

2010-02-18 13:28:17 補充：
因為吾夠位,所以我有分開咁多次.

1. A voltmeter has a very large resistance. By connecting a voltmeter in series in the circuit, this is equivalent to connect a large resistance in series in the circuit. As such, the current in the circuit is extremely small to be not measureable.

As said above, by considering the voltmeter as a large resistance, almost all of the battery voltage will be across this high (voltmeter) resistance. This is the reason why there is a p.d. across the voltmeter.

2. If the circuit is not complete (i.e. a open circuit), there will not be any current flowing, even a battery is connected to it. A simple example is a circuit with an open switch.

3. "Alternating" simple means "keep reversing (in direction)".

4. The electrical appliance will not give its "rated power (i.e. wattage)". By applying a voltage below 220v, the appliance will give less power. Conversely, the appliance will give more power if the voltage aplied is above 220v, but there is a risk of blowing up the appliance.

5. The "live" wire is always kept at a potential of 220v. By connecting the switch to the "live" wire, this will "cut off" the 220v live wire from the appliance. However, if the swtich is not connected to the live wire, the live wire will be connected directly to the appliance all the time. The appliance will always be kept at a potential of 220v.