# KPK Board 12th class Physics 15 AC Current short questions answers

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We provided 12^{th} class Physics short questions answers on this page of all KPK boards. Students of KPK boards can prepare the Physics subject for annual examinations.

In this List we have included all KPK boards and both Arts and Science students. These Boards students can prepare their exam easily with these short question answer section

Malakand Board 12th classes short questions Answer

Mardan Board 12th classes short questions Answer

Peshawar Board 12th classes short questions Answer

Swat Board 12th classes short questions Answer

Dera Ismail Khan Board 12th classes short questions Answer

Kohat Board 12th classes short questions Answer

Abbottabad Board 12th classes short questions Answer

Bannu Board 12th classes short questions Answer

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An induced emf is produced in an inductive coil when current flowing through it is changed. If Δl is the change in current during time Δt, then the induced emf say E is directly proportional to the rate of change of current in addition to the number of turns and other factors. So we have

E = LxΔI/Δt

L= E/ΔI/

Where L is called inductance of the coil. Since emf E and time rate of change of current are directly proportional to each other,

Significance: Taking the gradient or slope of the graph drawn between the induced

emf and rate of change of current we can calculate the value of the self inductance L as.

Gradient = ΔΕ/ΔI/

L= E/ΔI/

The unit of self inductance is volt. Se/Ampere which is known as Henery.

- When a resistor of resistance R is connected to an A.C generator, the expression for average power at any instant is equal to the product of I
_{rms }and V_{rms}. Therefore the power dissipated in the resistor can never be negative. This is because when resistor is connected to A.C source the current I and voltage V are in phase in the resistor. The mean power P at any instant t is given by

<P> = I_{rms}V_{rms}

V_{rms }= I_{rms }R

<P> = I^{2}_{rms}R = V^{2}_{rms }/R

Thus power absorbed by the resistor can never be negative when connected to A.C source.

- As we know that the mean power is given by

<P> = I_{rms}V_{rms}

<P> = I_{rms}V_{rms}

<P>=I_{m}/√2xV_{rms}/√2

<P>=1/2I_{m}V_{m}

2<P>= I_{m}V_{m}

Thus power varies at twice the frequency of either voltage or current.

The gradient or slope of a graph of inductive reactance against frequency determines the inductance of the inductor. Because from Ohm’s Law we know that the inductive reactance X_{L} is given by

V_{m}=I_{m}X_{L}

X_{L}=V_{m}/I_{m}

But V_{m}=wLI_{m}

X_{L}=wLI_{m}/I_{m}=wL=2πfL

X_{L} =2πfL

Thus the inductive reaction X_{L} is directly proportional to the frequency of the A.C source. A graph drawn between X_{L} and f is a straight line as shown. The gradient of the graph gives inductance L.

L=2πxΔX_{L}/Δf

(a) Inductor: Doubling the frequency of A.C source, the reactance of an inductor also becomes double. For the peak values of voltage V_{m} and current I_{m}, the reactance X_{L} of an inductor is

V_{m}=I_{m}X_{L}

X_{L}=2πfL

Now if the frequency f of the A.C source is doubled

F’=2f

Then X_{L }= 2πfL = 2π(2f)L

X_{L }= 2(2πfL) = 2X_{L}

X_{L }= 2 X_{L}

Thus doubling the frequency of A.C, the reactance of the inductor also become double.

- b) Capacitor: Doubling the frequency of A.C source applied, the capacitance the capacitor becomes half. Because for peak value of voltage V
_{m}and current I_{m}the reactance X, of the capacitor is

X_{c }= V_{m}/ I_{m}

I_{m }= wC V_{m}

X_{c }= V_{m}/ wC V_{m }= 1/Cw =1/2πfC

X_{c }= 1/2πfC

Now when the frequency of A.C source is doubled f’ =2f

X_{c }= 1/2πfC

X_{c }= 1/2(1/2πfC)

X_{c }= X_{c }/2

Thus doubling the frequency of AC source the reactance of capacitor becomes half.

_{eff }Value?

E_{m} = V_{m} =1000Volts

E_{eff }= E_{m}/√2 = 1000/√2 =1000/1.414 = 707.2V

(1) Inductors: The opposition offered by an inductor to the flow of alternating current is called inductive reactance denoted by X_{L}. According to Ohm’s Law the magnitude of inductive reactance X_{L} is equal to the ratio of AC voltage to the current.

V = I X_{L}

X_{L} = V/I =V_{m}sin wt/I_{m}sinwt

X_{L} = V_{m}/I_{m }(Voltage/Ampere) (Ohm)

- Capacitors: The opposition offered by a capacitor to the flow of alternating is called reactance of the capacitor, denoted by X
_{L}. The magnitude of capacitor reactance X_{L}is equal to ratio of A.C voltage to current.

V = IX_{L}

X_{c }= V/I (Voltage/Ampere) (Ohm)

Thus capacitor reactance is measured in Ohms.

The principle of ECG is based on the polarization and depolarization heart muscular layers. Due to heart electrical activity when the heart beats it produces time varying voltage. This varying voltage produces small potential differences between the points on the skin that can be measured by using electrodes.

The word electro cardio gram has been derived from Greek, meaning electrical heart activity. ECG is a medical test which is done to.

- Check the heart electrical activity.
- Find the cause of chest pain that causes heart attack.
- Find the cause of heart disease.