Current and electricity

Class 10th
Current and Electricity

Charge: The electric charge is equal to the number of electrons in it and the magnitude of charge on the electron i.e.
q = ne
where n = number of electrons and
e = charge of electron
= 1.6 x 10-19 C          
Positive and negative charges: The charge acquired by a glass rod when rubbed with silk is called positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.


Electric Current:The flow of electric charge is known as electric current.

Electric current is denoted by letter ‘i’. Electric current is expressed by the rate of flow of electric charges. Rate of flow means the amount of charge flowing through a particular area in unit time.
       I = Q/t, Where I = Current
                                    Q = Net charge flowing.
                                    t = time
                •  Unit:
                     The unit of current is Ampere.
                     Q = Coulomb(C)
                     I = Ampere(A)
                     t = Second(s) 1 A = 1C/1s
Potential Difference: The potential difference between two separate points is defined as the work done to move a unit positive charge from one point to another.
       V = W/Q
                •  Unit:
                    The unit of potential difference is Volt
                    1 Volt = 1 Joule/ 1 Coulomb
Cell: The Difference of potential may be produced by a battery, consisting of one or more electric cells.

Potential difference across the terminals of the cell generated due to chemical reaction within the cell. When cell is connected to a conducting wire, current flows from high potential to low potential.
Ammeter: An apparatus to measure the value of current. It is always connected in series in a circuit through which the current is to be measured.
Voltmeter: An apparatus to measure the potential difference between two points in an electric circuit. It is always connected in parallel across the points between which the potential difference is to be measured.
Ohm’ law: Under constant physical conditions (i.e., constant temperature, pressure etc.), the current flowing through a conductor is directly proportional to the potential difference across the conductor.
  Potential difference (which is measured in Voltage) is the cause of current (which is measured in Ampere).
  In conductors, flow of electrons constitute current. In a circuit current flow from positive terminal of the battery to the negative terminal, but electrons travels from negative terminal to the positive terminal. The negative terminal of a battery is said to be at lower potential and the positive terminal is said to be at higher potential.

  When a battery is not connected to any circuit, the potential difference across the terminals of the battery is equal to the EMF of the battery. (EMF = Electro Motive Force). Electromotive force, also called EMF, (denoted and measured in volts), refers to voltage generated by a battery or by the magnetic force according to Faraday Law, which states that a time varying magnetic field will induce an electric current.

1 ohm (Ω) of Resistance (R) is equal to the flow of 1 A of current through a conductor between two points having potential difference equal to 1 V.
V / 1A
Rheostat
As we know that
V=IR

So to increase or decrease the current accordingly in the circuit a component used is called “Rheostat”, that regulates the current without changing potential difference. Represented by “Rh”.
Its symbol is

If a conductor has less Resistance, then more current will flow through it.

Resistance- It is the obstruction to flow of charge.
Formula: R=V/I
Unit: ohm
We can define 1ohm as - resistance is said to be 1 ohm when 1volt of potential difference is applied which allows 1 ampere current to flow through it.

Factors on which resistance depends :

Dependence of resistance on length of wire: Resistance is directly proportional to the length of the wire. More is the length of wire, more is the number of collisions and more is the resistance.

Resistance on area of wire: Resistance is inversely proportional to the area of the wire. More is the area, less is the resistance as number of collisions is less as they can pass easily.

Resistance on nature of wire: Depending upon the material used for making wires we have:

Conductors: have almost nil resistance
Semiconductors: have resistance
Insulators: have high resistance
Resistance on temperature: Resistance is directly proportional to temperature for pure metals but for alloys it decreases. With increase in the temperature, number of collisions are more, therefore, resistance is more. Combining all factors we get :
R= rho L/A
Specific resistance or resistivity: In the above formula of resistance, if we consider it for wire of length 1m and area 1m2 then resistivity is defined as the measure of resistance of wire 1m length and 1 meter square area .

formula :                       R= rho L/A
unit is ohm –metre
Resistance can be introduced  in a circuit in 2 ways:

Resistance in series: In series, the resistances are attached as shown


In series, the current remains constant and potential difference changes.

Resistance in parallel: They are connected as shown in figure :


In parallel, Potential Difference remains constant and  the current changes.

Joules heating effect of current

When current flows, the electrons move and when they move, they collide with each other. When they collide, heat is produced. Heat produced depends upon the square of current, resistance and temperature.

i.e.,:  H=I^2RT

Also called as joules heating effect.
👉You know that nichrome wire is used in heating devices and the reason being
because

It has high resistance
It does not burn even at high temperature
It has high coefficient of linear expansion

Power

We come across ‘power’ that is mentioned on all electrical appliances. So, if we define, it is the rate of doing work.

Formula : P=w/t
Unit : joule/sec
Bigger units : kilowatt=1000watt
megawatt=1000000 watt
gigawatt=1000000000watt
horsepower =746 watts
Further derived formulas of power :
P=v.i
P=v2/r
P=i2/r

Ncert questions current and electricity

EXERCISES

1. A piece of wire of resistance R is cut into five             equal parts. These parts are then connected in          parallel. If the equivalent resistance of this                combination is R′, then the ratio R/R′ is –

      (a) 1/25 (b) 1/5 (c) 5 (d) 25

2. Which of the following terms does not represent      electrical power in a circuit?

     (a) I2R (b) IR2 (c) VI (d) V2/R

3. An electric bulb is rated 220 V and 100 W. When      it is operated on 110 V, the power consumed will      be–

    (a) 100 W (b) 75 W (c) 50 W (d) 25 W

4. Two conducting wires of the same material and      of equal lengths and equal diameters are first          connected in series and then parallel in a circuit      across the same potential difference. The ratio          of heat produced in series and parallel                        combinations would be –

    (a) 1:2 (b) 2:1 (c) 1:4 (d) 4:1

5. How is a voltmeter connected in the circuit to          measure the potential difference between

    two points?

6. A copper wire has diameter 0.5 mm and                    resistivity of 1.6 × 10–8 Ω m. What will be

    the length of this wire to make its resistance              10Ω? How much does the resistance change if          the diameter is doubled?

7. The values of current I flowing in a given resistor for the corresponding values of

potential difference V across the resistor are given below –

I (amperes) 0.5 1.0 2.0 3.0 4.0 

V (volts) 1.6 3.4 6.7 10.2 13.2

Plot a graph between V and I and calculate the resistance of that resistor.

8. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

9. A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω , 0.5 Ω

and 12 Ω, respectively. How much current would flow through the 12 Ω resistor?

10. How many 176 Ω resistors (in parallel) are required to carry 5 A on a 220 V line?

11. Show how you would connect three resistors, each of resistance 6 Ω, so that the

combination has a resistance of (i) 9 Ω, (ii) 4 Ω.

12. Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?

13. A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each of 24 Ω resistance, which may be used separately, in series, or in

parallel. What are the currents in the three cases?

14. Compare the power used in the 2 Ω resistor in each of the following circuits:

(i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel

with 12 Ω and 2 Ω resistors.

2018-19