# Electric Current Definition

In metals, there are a large number of free electrons which moves randomly from atom to atom.

When the electric potential difference is applied across a wire of metal, free electrons which are negatively charged, attracted by positive terminal of the cell. Due to this force of attraction, free electrons start moving towards the positive terminal of the cell.

**This moving stream of electrons is called the electric current.**

The flow of electrons in the wire is from Y to X i.e. from the negative terminal to the positive terminal of the cell through the external circuit. But, the direction of flow of electric current is considered from X to Y. It is a convention.

The charge flowing per second in an electric circuit is the measure of electric current in that circuit.

**Electric current, I = Q/t**

The charge is measured in coulombs and time in seconds, therefore, **the unit of electric current is coulombs/second or ampere.**

When, Q = 1 coulomb, t = 1 sec then, electric current, I = 1 A.

Therefore, one-ampere current is that amount of electric current when one-coulomb charge flows through any section of wire in one second.

One electron carries 1.6 x 10^{-19} C of charge. Hence, for 1 C of charge, 1/(1.6 x 10^{-19}) = 6.25 x 10^{18} electrons should flow in one second.

Thus, **1 ampere = 6.25 x 10 ^{18} electrons per second**.

This, however, does not mean that a wire carrying a current of 1 ampere, 6.25 x 10

^{18}electrons per second from one end of the wire to the other. Instead, it means that in one second, 6.25 x 10^{18}electrons enter at one end and the equal number of (other) electrons leaves at the other end.

The electric current is denoted in DC circuits by ‘I’ and in AC circuits by ‘i’. It is a scalar quantity although in diagrams we represent current in a wire by an arrow, it does not mean that the current is a vector quantity. The arrow simply indicates the direction of flow of positive charge in the wire.

# Electric Potential Definition

When a body is charged, either electrons are supplied to it or electrons are removed from it, thus work is done in charging it. This work done is stored in the body in the form of potential energy.

Now, the charged body has the capacity to do work by moving another charge either by a force of attraction or repulsion. This ability of a charged body is called electric potential.

The capacity of a charged body to do work is called electric potential.

The greater the capacity of a charged body to do work, the greater is its electric potential. Obviously, the measure of electric potential is the work done to charge a body to one coulomb i.e.

**Electric potential = work done/charge **

**V = W/Q **

The work done is measured in joules and charge in coulombs, therefore the **unit of electric potential is joules/coulomb or volts**.

If W = 1 joule; Q = 1 coulomb; then V = 1/1 = 1 volt.

Hence, a body is said to have an electric potential of 1 volt if one joule of work is done to charge the body to 1 coulomb.

## Electric Potential Difference Definition

When the electric potential of one charged body differs from the other charged body, there exists a potential difference between them. Both the bodies are under stress and strain and try to attain minimum potential.

Thus, the difference in the electric potential of two charged bodies is called potential difference.

The **unit of potential difference is volt**.

If one joule of work is done in transferring one coulomb of charge from one point to the other, the points are said to have a potential difference of one volt.

## Illustration

- Consider two charged bodies A and B. There is the excess of electrons on body A, therefore, it has an electric potential of 5 V (negative).
- Similarly, on a body B, there is the excess of electrons and it has an electric potential of 3 V (negative).
- There exist stress and strain between the two bodies which is called electric potential difference.
- If the two bodies are connected through a wire, the electrons will start flowing from body A to B till the two bodies attain the same potential. The electrons flow from one body to the other body only when potential difference exists.
- The flow of electrons stops when the two bodies attain the same potential and the potential difference is zero.

Potential difference = 5 – 3 = 2 V.

## AC Fundamentals — 1 | Objective Type Question Answers

### #1 A sine wave has a frequency of 50 Hz. Its angular frequency is …………. radian/ second.

100 π

### #2 The reactance offered by a capacitor to alternating current of frequency 50 Hz is 20 Ω. If frequency is increased to 100 Hz, reactance becomes ………….ohms.

10

### #3 The form factor is the ratio of

r.m.s. value to average value

### #4 The form factor is the ratio of

r.m.s. value to average value

### #5 An A.C. current is given by i = 200 sin 100 πt. It will achieve a value of 100 A after ………… second.

1/600

### #6 If two sinusoidals of the same frequency but of different amplitudes and phase angles are subtracted, the resultant is

a sinusoidal of the same frequency

### #7 If two sine waves of the same frequency have a phase difference of π radians, then

when one wave reaches its maximum value, the other will reach its minimum value

### #8 Two waves of the same frequency have opposite phase when the phase angle between them is

^{o}

^{o}

^{o}

^{o}

180

### #9 The power consumed in a circuit element will be least when the phase difference between the current and voltage is

^{o}

^{o}

^{o}

^{o}

90

### #10 The r.m.s. value and mean value is the same in the case of

square wave

### #11 For the same peak value which of the following wave will have the highest r.m.s. value ?

square wave

### #12 For the same peak value, which of the following wave has the least mean value?

half wave rectified sine wave

### #13 The current in a circuit is given by : i = 100 sin 314 t amperes. The maximum value and frequency of current are

100 A, 50 Hz

### #14 For a frequency of 200 Hz, the time period will be

0.005 s

### #15 The phase difference between voltage and current wave through a circuit element is given as 30 degrees. The essential condition is that

both waves must have same frequency

### #16 An A.C. voltage of 50 Hz has a maximum value of 50 V. Its value after 1/600 second after the instant the current is zero, will be

25 V

### #17 When two waves are in phase they have peak values at an interval of

^{o}

^{o}

^{o}

none of the above

### #18 For 200 V r.m.s. value triangular wave, the peak voltage will be

346 V

### #19 A sine wave of voltage varies from zero to maximum of 200 V. How much is the voltage at the instant of 30 degrees of the cycle ?

100 V

### #20 How much r.m.s. current does a 300 W, 200 V bulb take from the 200 V, 50 Hz power line ?

1.5 A

### #21 Two sinusoidal currents are given by i1 = 100 sin (ωt + π/3), and i2 = 150 sin (ωt — π/4). The phase difference between them is ………… degrees.

105

### #22 The r.m.s. value of a half-wave rectified current is 100A. Its value for full-wave rectification would be ………… amperes.

141.4

Do not forget to click the “finish” button to see the correct answers and result.

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