Phase Angles

[m4tty]

Any chance someone could help me out with the 2 questions below as im stuck on these. If possible could someone explain how to get to the answer aswell please.

1. An inductor has an inductive reactance of 20Ω and a resistance of 20Ω. Which one of
the following gives the phase relationship between the voltage and current in this circuit ?
a) The current leads the voltage by 45º
b) The voltage leads the current by 45º
c) The voltage leads the current by 90º

As the inductance and reistance are equal the phase angle must be laggin meaning voltage is leading the current but as they are equal does it mean its (C) same as a purely inductive circuit??

2. A capacitor has a capacitive reactance of 500Ω. If the capacitor is connected in series
with a resistance of 500Ω which one of the following gives the phase relationship between the voltage and current in this circuit?
a) The current leads the voltage by 45º
b) The voltage leads the current by 45º
c) The current leads the voltage by 90º

Thanks in advance for any replies

 

[earlelectric]

seems to me, with the reactance and resistance being the same that they will cancel each other out so we are back to the basics of.... in an inductive circuit the voltage leads the current by 90,and in a capacitive circuit the current leads the voltage by 90.

hope this helps,hope im right.

 

[ezzzekiel]

sounds right to me

 

[ian.settle1]

If you go back to basics

Current in a resistance is in phase with the voltage

Current in a capacitor leads the voltage by 90 degrees

Current in a inductor lags the voltage by 90 degrees

that is why if you have a load which has a high inductive load power factor correction capacitors are used to try and bring the power factor as near to 1 as possible.

 

[Jurasic Spark]

Any chance someone could help me out with the 2 questions below as im stuck on these. If possible could someone explain how to get to the answer aswell please.

1. An inductor has an inductive reactance of 20Ω and a resistance of 20Ω. Which one of
the following gives the phase relationship between the voltage and current in this circuit ?
a) The current leads the voltage by 45º
b) The voltage leads the current by 45º
c) The voltage leads the current by 90º

As the inductance and reistance are equal the phase angle must be laggin meaning voltage is leading the current but as they are equal does it mean its (C) same as a purely inductive circuit??

2. A capacitor has a capacitive reactance of 500Ω. If the capacitor is connected in series
with a resistance of 500Ω which one of the following gives the phase relationship between the voltage and current in this circuit?
a) The current leads the voltage by 45º
b) The voltage leads the current by 45º
c) The current leads the voltage by 90º

Thanks in advance for any replies

See my reply to your post about impedance

 

[pushrod]

B and A are the answers ( as Jurasic Spark said). Resistance doesn't affect phase angle. Capacitance causes current to lead the voltage (or put another way it causes the voltage to lag) and inductance causes current to to lag (or, as in Q1, the voltage to lead).
The reason why the angle is 45 deg is the values are equal and there is 90 deg between resistance and capacitance and 90 degree between resistance and inductance. The resultant is halfway between ie 45 deg.

 

[Jurasic Spark]

To find the phase angle you need to find the Pf first.
This is cos (ADJ/HYp) should be a number less than 1.
Then INV cos (or 2nd func cos) on your calc to put it into degrees.

 

[ian.settle1]

To find the phase angle you need to find the Pf first.
This is cos (ADJ/HYp) should be a number less than 1.
Then INV cos (or 2nd func cos) on your calc to put it into degrees.

Don't confuse them

 

[Jurasic Spark]

Don't confuse them

I wasn't trying to, if you know an easier way then share the wealth (EDITED for carp typing)

"1. An inductor has an inductive reactance of 20Ω and a resistance of 20Ω. Which one of
the following gives the phase relationship between the voltage and current in this circuit ?"

Ok you need to find the impedance first and then the Pf:

Use Pythagoras to find Z (this is always the hypotenuse of a right angled triangle).
Therefore Z = SQRT of (20SQ + 20SQ) = 28.28.

Pf is found with cos = adj/hyp
= Res/Imp = 20 / 28.28 = 0.7072

Phase angle = 0.7072 2nd FUNC COS = 45 degrees

Because we have a predominantly inductive load rather than capacitve the current lags voltage by 45 degrees (or voltage leads current by 45 degrees.

SIMPLES