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

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Malakand Board 12th classes short questions Answer

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Kohat Board 12th classes short questions Answer

Bannu Board 12th classes short questions Answer

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Sketch a graph of emf induced in inductive coil against rate of change of current. What is the significance of the graph?

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.

L= E/ΔI/

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

Current and voltage provided by an A.C generator are sometime negative and sometime positive. Explain why for an A.C generate connected to a resistor power can never be negative b. Explain, using sketch graphs, why the frequency of variation of power in A.C generator is twice as that of the current and voltage?

1. 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 Irms and Vrms. 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> = IrmsVrms

Vrms = Irms R

<P> = I2rmsR = V2rms /R

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

1. As we know that the mean power is given by

<P> = IrmsVrms

<P> = IrmsVrms

<P>=Im/√2xVrms/√2

<P>=1/2ImVm

2<P>= ImVm

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

What determines the gradient of a graph of inductive reactance against frequency.

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 XL is given by

Vm=ImXL

XL=Vm/Im

But Vm=wLIm

XL=wLIm/Im=wL=2πfL

XL =2πfL

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

L=2πxΔXL/Δf

How does doubling the frequency affect the reactance of (a) an inductor (b) a capacitor?

(a) Inductor: Doubling the frequency of A.C source, the reactance of an inductor also becomes double. For the peak values of voltage Vm and current Im, the reactance XL of an inductor is

Vm=ImXL

XL=2πfL

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

F’=2f

Then XL = 2πfL = 2π(2f)L

XL = 2(2πfL) = 2XL

XL = 2 XL

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

1. b) Capacitor: Doubling the frequency of A.C source applied, the capacitance the capacitor becomes half. Because for peak value of voltage Vm and current Im the reactance X, of the capacitor is

Xc = Vm/ Im

Im = wC Vm

Xc = Vm/ wC Vm = 1/Cw =1/2πfC

Xc = 1/2πfC

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

Xc = 1/2πfC

Xc = 1/2(1/2πfC)

Xc = Xc /2

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

If peak value of a sine wave is 1000 volts, what is effective Eeff Value?

Em = Vm =1000Volts

Eeff = Em/√2 = 1000/√2 =1000/1.414 = 707.2V

Show that reactance is measured in Ohms for both inductors and capacitors.

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

V = I XL

XL = V/I =Vmsin wt/Imsinwt

XL = Vm/I(Voltage/Ampere) (Ohm)

1. Capacitors: The opposition offered by a capacitor to the flow of alternating is called reactance of the capacitor, denoted by XL. The magnitude of capacitor reactance XL is equal to ratio of A.C voltage to current.

V = IXL

Xc = V/I (Voltage/Ampere) (Ohm)

Thus capacitor reactance is measured in Ohms.

Describe the Principle of ECG.

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.

1. Check the heart electrical activity.
2. Find the cause of chest pain that causes heart attack.
3. Find the cause of heart disease.
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