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# phy 5 question on capacitors watch

1. (Original post by mik1w)
do you want a resistor in series with the capacitor when discharging?
like I said above it takes maybe a millisecond for a capacitor to discharge I think, like a spark, you won't be able to time that.

yes,

thats true, the charge drops fast. just like shorting it
2. Sorry! I was still thinking there was a power supply. Wow...

Okay just corrent myself here.

Capacitor with constant capacitance C in series with a resistor R. The capacitor carries a charge Q... to discharge, all the charge Q must travel through the resistor, so by kirchoff, the voltage across the resistor equals that across the capacitor.

Before you connect to discharge, voltage on the capacitor is Q/C. when you connect this is the voltage across the resistor and so the current is I=V/R = (Q/C)/R = Q/CR. as the capacitor discharges, Q falls and so I falls, and V falls across the capacotir and the resistor, as CR is a constant.

very sorry!! I hate it when that happens, I argue totally the wrong point. thanks for pointing that out, you are right as the capacitor acts as the battery.
3. (Original post by mik1w)
Sorry! I was still thinking there was a power supply. Wow...

Okay just corrent myself here.

Capacitor with constant capacitance C in series with a resistor R. The capacitor carries a charge Q... to discharge, all the charge Q must travel through the resistor, so by kirchoff, the voltage across the resistor equals that across the capacitor.

Before you connect to discharge, voltage on the capacitor is Q/C. when you connect this is the voltage across the resistor and so the current is I=V/R = (Q/C)/R = Q/CR. as the capacitor discharges, Q falls and so I falls, and V falls across the capacotir and the resistor, as CR is a constant.

very sorry!! I hate it when that happens, I argue totally the wrong point. thanks for pointing that out, you are right as the capacitor acts as the battery.

hehe, its good things were cleared up. i also made a mistake by saying V=QC. soz too

i like ur explanations too. i will not hve included the fact that CR is a constant so remains unchanged.
4. mik1w , ive got a simple question i think but dnt know whether im writing enough to get the 2 marks.

define Fields used in phy?

my ans - a field is a region where forces act. it is a vector quantity which describes the direction of the force which acts on a body/charge/current within it.

idnt know wot else to write

i
5. another on

State two differences between electric and magnetic fields. (2)
6. I'd leave it at "region where forces act"

btw CR is significant as it is the time (CR base units is the second) taken for I think the charge, current and voltage to fall to a 1/e th. Kinda like the half life. Also, RC is where you'd hit the time axis if you draw a tangent to the curve of voltage against discharging capacitor.

random knowledge!
7. (Original post by andyj72)
another on

State two differences between electric and magnetic fields. (2)

Id say the main one is that gravity is always attractive and magnetic can be both, also Id say gravity does not require the body to be moving but magnetic does (a charge which is stationary relative to the firld will not have a force on it)
8. (Original post by mik1w)
I'd leave it at "region where forces act"

btw CR is significant as it is the time (CR base units is the second) taken for I think the charge, current and voltage to fall to a 1/e th. Kinda like the half life. Also, RC is where you'd hit the time axis if you draw a tangent to the curve of voltage against discharging capacitor.

random knowledge!
Ah! , ill bear that in mind. thanx
9. magnetic is from fixed charge

electric is from moving charge

Thats all I can think right now...
10. (Original post by mik1w)
Id say the main one is that gravity is always attractive and magnetic can be both, also Id say gravity does not require the body to be moving but magnetic does (a charge which is stationary relative to the firld will not have a force on it)
11. cheers everyone.

ive got 2 more. i hope im not botherin everyone. its just that i want to understand some of these things
12. Whats the other two?

What are the simalarities??

Both decrease by inverse square law
+ & - similar to N&S poles, and opposite attract, and like repel

anymore?
13. here it is. thanx in advance

its in the word doc
Attached Files
14. PHY5January2004-2.doc (65.5 KB, 817 views)
15. oh and also, electric fields can be shielded with a conducter, but gravitational cant. Can this be done with magenetic?
16. (Original post by SinghFello)
oh and also, electric fields can be shielded with a conducter, but gravitational cant. Can this be done with magenetic?
not too sure
17. (Original post by mik1w)
Id say the main one is that gravity is always attractive and magnetic can be both, also Id say gravity does not require the body to be moving but magnetic does (a charge which is stationary relative to the firld will not have a force on it)
Gravity has nothing to do with it.

keep it simple:
Electric field strength indicates force per positive charge.
Magnetic field strength indicates force acting on a current. B=F/IL

Both can deflect and accelerate.
Both fields can exert attractive and repulsive forces.
Both forces are inversely proportional to separation distance squared.
18. (Original post by andyj72)
hehe, its good things were cleared up. i also made a mistake by saying V=QC. soz too

i like ur explanations too. i will not hve included the fact that CR is a constant so remains unchanged.
no no you made some errors..

you connect DC in series witha sweitch, in series with a capacitor, a micro ammeter and a variable resistor + voltmeter parallel to capacitor.
close the switch and you are CHARGING the capacitor (NOT DISCHARGE) then you time every 10 seconds and measure the current and potential difference.
the current is constant. so you use Q=It to work out Q for times.
and you have measured V. so you plot Q vs V and you should get straight line..
19. (Original post by ARCHK0VEN)
no no you made some errors..

you connect DC in series witha sweitch, in series with a capacitor, a micro ammeter and a variable resistor + voltmeter parallel to capacitor.
close the switch and you are CHARGING the capacitor (NOT DISCHARGE) then you time every 10 seconds and measure the current and potential difference.
the current is constant. so you use Q=It to work out Q for times.
and you have measured V. so you plot Q vs V and you should get straight line..
hmm , i think this is an alternative way of doing it. keeping the charging current constant by reducing the series resistance as the capacitor discharges. Q=It which u can relate to y=mx + c where c is the origin os y=mx. So u can draw a graph of V against Q which is a straight line thru the origin.

So, im guessing u do this for the range of P.d's for OV - 15V. the gradient will give u the capacitance. C=Q/V

20. talkin abt capacitors, i remember somewhere i was asked wats the reason for havin a resistor in a capacitor charging circuit
might b incredibly dumb, but i cant think of a plausible argument

suggestions?
21. so that a hight voltage isnt passed across the capacitor. if it was connected directly it will blow it. i think cos the capacitor has a very low internal resistance

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