Electricity

Electric Current

Electric current is the flow of electric charge through a conductor, typically carried by moving electrons.
It is the amount of charge flowing through a particular area in unit time.

The SI unit of electric current is the ampere (A).

The SI unit of electric charge is the coulomb (C).
Current flows from the positive terminal to the negative terminal of a cell or battery in a circuit.
In an electric circuit the direction of electric current is taken as opposite to the direction of the flow of
electrons, which are negative charges.
Ammeter is used to measure the electric current in circuit which is always connected in series.

Electric Potential and Potential Difference

Electric potential is the amount of work done in bringing a unit positive charge from infinity to a point in an
electric field. The unit of electric potential is the volt (V).
Potential difference, also known as voltage, is the difference in electric potential
between two points in a circuit. It is measured in volts and is responsible for

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the flow of electric current in a circuit.
V = W/Q
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SI unit of electric potential difference is volt (V)
One volt is the potential difference between two points when 1 joule of

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work is done to move a charge of 1 coulomb from one point to the other
Voltmeter is used to measure the electric potential in circuit which is

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always connected in parallel.

ar Circuit Diagram

Conventional symbols used to represent some of the most commonly used electrical components are

Components

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An electric cell
A battery or a combination
of cells

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Plug key or switch
A wire joint

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Wires crossing without
Electric bulb
joining

Variable resistance or
A resistor of resistance R
rheostat

Ammeter Voltmeter

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 Ohm’s Law

The potential difference (V) across a wire in a circuit is directly proportional to the current
if the temperature is constant, known as Ohm's Law.
⇒
The resistance (R) of a metallic wire at a specific temperature inhibits charge flow. Measured in ohms (Ω),
resistance is a conductor property. SI unit of resistance is Ω

The current through a resistor is inversely proportional to its resistance, demonstrated by the halving of
current when resistance is doubled. To adjust current without altering the voltage source, a variable resistance
component like a rheostat is used in electric circuits.

Note -
The flow of electrons in an electric circuit forms an electric current.
Resistance from atoms in a conductor slows electron motion.

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Components with low resistance are good conductors, while those with moderate resistance are resistors.

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High resistance impedes current flow, found in poor conductors.
Insulators have the highest resistance, restricting electron flow significantly.

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Resistance of the conductor depends
Factors affecting Resistance

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on its length
on its area of cross-section
on the nature of its material.
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Electrical resistivity (ρ): measure of opposition to the flow of electric current. SI unit is Ωm.
resistivity of Metals/Alloys: 108 to 10

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resistivity of Insulators: 107 to 103
Both the resistance and resistivity of a material vary with temperature.

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The resistivity of alloys is higher than that of individual metals. Alloys are used in electrical heating devices as
they resist oxidation at high temperatures.

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Tungsten is preferred for electric bulb filaments
Copper and aluminum are commonly used in electrical transmission lines.

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 System of Resistors

Series Combination Parallel Combination

In a series combination of resistors, the current In a parallel combination of resistors, the voltage
remains constant throughout the circuit, ensuring an remains constant throughout the circuit and electric
equal current flow through each resistor and current across each resistor varies.
potential across each resistor varies.

The equivalent resistance is equal to,

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The equivalent resistance is equal to,

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It is impractical to connect an electric bulb and heater in series due to different required currents.
In a series circuit, if one component fails, the whole circuit stops working.

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Parallel circuits divide current, reducing total resistance. This setup is beneficial when gadgets have varying
resistance and current needs.

ar Heating Effect of Electric Current

The chemical reaction in a cell generates current by moving electrons. Source energy maintains the current,

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converting some energy into useful work and dissipating the rest as heat.
In purely resistive electric circuits, all source energy is converted to heat, utilized in devices like electric
heaters and irons.
Heat produced is governed by Joule’s law of heating. The law implies that heat produced in a resistor is

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directly proportional to the square of current for a given resistance

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directly proportional to resistance for a given current

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directly proportional to the time for which the current flows through the resistor.

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Application of Heating effect
Electric laundry iron, electric toaster, electric oven, electric kettle and electric heater are some of the familiar
devices based on Joule’s heating.
Electric heating in bulbs uses tungsten filaments with high melting points to emit light while retaining heat.
Filaments are thermally isolated for efficiency, and bulbs are filled with inert gases like nitrogen and argon to
extend filament life. The majority of power consumed by the filament is released as heat, with a small portion
emitted as light.

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 Electric Power

The rate at which electric energy is dissipated or consumed in an electric circuit is termed as electric power

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P = I R =V
2
R
The SI unit of electric power is watt (W).
In practice, the larger unit 'kilowatt' (kW) is used instead of the small 'watt' (W), with 1 kW equal to 1000 W.
Electric energy is measured in watt hour (Wh), where 1 Wh is the energy consumed when using 1 W for 1 hour.
The commercial unit is kilowatt hour (kWh), also known as 'unit'.

1 kW h = 1000 watt × 3600 second

= 3.6 × 106 watt second
= 3.6 × 106 joule (J)

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 Activity

Activity 11.1

Set up a circuit as shown in Fig., consisting of a nichrome wire XY of length, say 0.5 m, an ammeter, a voltmeter
and four cells of 1.5 V each. (Nichrome is an alloy of nickel, chromium, manganese, and iron metals.)
First use only one cell as the source in the circuit. Note the reading in the ammeter I, for the current and
reading of the voltmeter V for the potential difference across the nichrome wire XY in the circuit. Tabulate
them in the Table given.
Next connect two cells in the circuit and note the respective readings of the ammeter and voltmeter for the
values of current through the nichrome wire and potential difference across the nichrome wire.
Repeat the above steps using three cells and then four cells in the circuit separately.
Calculate the ratio of V to I for each pair of potential difference V and current I.

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Observation

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A straight-line graph shows that as the current through the wire XY increases, the potential difference across
the wire increases. This is called Ohm’s law.

ar Activity 11.2

Take a nichrome wire, a torch bulb, a 10 W bulb and an ammeter (0 – 5 A range), a plug key and some connecting

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wires. Set up the circuit by connecting four dry cells of 1.5 V each in series with the ammeter leaving a gap XY in
the circuit, as shown in Fig.

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Complete the circuit by connecting the nichrome wire in the gap XY. Plug the key. Note down the ammeter
reading. Take out the key from the plug.
Replace the nichrome wire with the torch bulb in the circuit and find the current through it by measuring the
reading of the ammeter.

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Now repeat the above step with the 10 W bulb in the gap XY.
Are the ammeter readings different for different components connected in the gap XY? What do the above
observations indicate?

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You may repeat this Activity by keeping any material component in the gap. Observe the ammeter readings in
each case. Analyse the observations.

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Observation
The ammeter readings are different in each case because current flows in each case are differentas electric
current flows through a component depending upon the resistance of the component
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 Activity 11.3

Complete an electric circuit consisting of a cell, an ammeter, a nichrome wire of length l [say, marked (1)] and a
plug key, as shown in Fig.
Now, plug the key. Note the current in the ammeter. Replace the nichrome wire by another nichrome wire of
same thickness but twice the length, that is 2l [marked (2) in the Fig].
Note the ammeter reading. n Now replace the wire by a thicker nichrome wire, of the same length l [marked
(3)]. A thicker wire has a larger cross-sectional area. Again note down the current through the circuit.
Instead of taking a nichrome wire, connect a copper wire [marked (4) in Fig.] in the circuit. Let the wire be of
the same length and same area of cross-section as that of the first nichrome wire [marked (1)]. Note the value
of the current.
Notice the difference in the current in all cases.
Does the current depend on the length of the conductor? Does the current depend on the area of cross-section
of the wire used?

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Observation

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When the length of the wire is doubled, the ammeter reading decreases to half of its previous value and when

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the nichrome wire is replaced with another nichrome wire of a larger cross-section area but of the same length ,
ammeter reading increases.

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A change in ammeter reading is observed when a wire of different material of the same length and the same
area of cross-section is used

ea Activity 11.4

Join three resistors of different values in series. Connect them with a battery, an ammeter and a plug key, as

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shown in Fig. You may use the resistors of values like 1 Ω, 2 Ω, 3 Ω etc., and a battery of 6 V for performing
this Activity.

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Plug the key. Note the ammeter reading.

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Change the position of ammeter to anywhere in between the resistors. Note the ammeter reading each time.

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Do you find any change in the value of current through the ammeter?

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Observation
For all three resistors in series combination, the value of current remains the same and value of the current in
the ammeter is the same, independent of its position in the electric circuit

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 Activity 11.5

In Activity 11.4, insert a voltmeter across the ends X and Y of the series combination of three resistors, as
shown in Fig.
Plug the key in the circuit and note the voltmeter reading. It gives the potential difference across the series
combination of resistors. Let it be V.
Now measure the potential difference across the two terminals of the battery. Compare the two values.
Take out the plug key and disconnect the voltmeter. Now insert the voltmeter across the ends X and P of the
first resistor, as shown in Fig.
Plug the key and measure the potential difference across the first resistor. Let it be V₁ . n Similarly, measure
the potential difference across the other two resistors, separately. Let these values be V₂ and V₃ , respectively.
n Deduce a relationship between V, V₁, V₂ and V₃.

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Observation

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The potential difference of the combination is equal to the potential difference across two terminals of the
battery

ni Activity 11.6

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Make a parallel combination, XY, of three resistors having resistances R₁, R₂ and R₃ , respectively. Connect it
with a battery, a plug key and an ammeter, as shown in Fig.

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Also connect a voltmeter in parallel with the combination of resistors.
Plug the key and note the ammeter reading. Let the current be I. Also take the voltmeter reading. It gives the
potential difference V, across the combination. The potential difference across each resistor is also V. This can
be checked by connecting the voltmeter across each individual resistor

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Take out the plug from the key. Remove the ammeter and voltmeter from the circuit. Insert the ammeter in

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series with the resistor R₁ , as shown in Fig. Note the ammeter reading, I₁ .

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Similarly, measure the currents through R₂ and R₃ . Let these be I₂ and I₃ , respectively. What is the

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relationship between I, I₁, I₂ and I₃ ?

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Observation
The total current I is equal to the sum of the current through each branch of the combination, I= I₁ +I₂ +I₃ The
potential difference across each resistor is same.
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