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Electricity is derived from the Greek word ‘electron’. You must have noticed that when we rub a comb against our hair and then bring it near the small pieces of paper, the paper gets attracted towards the comb and gets lifted above the surface. The force that is pulling the paper towards itself is known as electrostatic force. This is happening due to the charge present on the material. When we rub one material against another, they acquire a charge, one acquires a positive charge and the other acquires a negative charge. The concept lying behind is that like charges repel each other and opposite charges attract each other.
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Important Concepts
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COULOMB’S LAW
There is a force of attraction or repulsion between two charges known as the electrostatic force. The electrostatic force is proportional to the product of charges and is inversely proportional to the distance between them.
F = K(q1q2)/r2
Note: Here K is the proportionality constant and is equal to 9 x 109 Nm2/C2.
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INSULATOR AND CONDUCTORS
Conductors are the materials that allow free flow of charge through them.
Insulators however do not allow the flow of charge as they are bad conductors of charge but can be charged by friction easily.
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ELECTRIC CURRENT
The term electric current refers to the rate of flow of charge through a conductor.
The SI unit of current is ampere (A) and it is denoted by the letter i or I. Current is a scalar quantity.
I = q/t
The flow of current is dependent on the potential. If there is a potential difference, then there will be a flow of charge that leads to the generation of electric current. The current is carried by the movement of electrons. The direction of the flow of electric current is opposite to the direction of movement of electrons.
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ELECTRIC POTENTIAL
The term electric potential is referred to the amount of work done in carrying a unit positive charge from infinity to a point. The SI unit given is Volts(V).
V=W/q
Here V is the electric potential. W refers to the amount of work done and q refers to the charge.
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ELECTRIC POTENTIAL DIFFERENCE
The electric potential difference between any two points refers to the amount of work done in carrying a unit positive charge from one point to another in a circuit. The SI unit of potential difference is also Volts (V).
The potential difference between two points A and B can be written as :
VAB = work done is carrying a charge q from point A to B/charge q.
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ELECTRIC POTENTIAL ENERGY
This refers to the work done in bringing one unit to a respective location, against the electric field, with the help of an external source of energy.
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CIRCUIT DIAGRAM
A closed path loop that defines the flow of current and the arrangement of components in a circuit using conventional symbols for the components is known as a circuit diagram.
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AMMETER
An ammeter is a measuring instrument that can be used for measuring the current in a circuit.
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VOLTMETER
A voltmeter is a measuring instrument that is used to measure the potential difference between two points in a circuit.
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OHM’s LAW
Ohm’s law states that the amount of current flowing in a conductor is directly proportional to the potential difference between two points in a circuit.
VαI
V = I x R
Here, R is the resistance offered.
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RESISTANCE
The term resistance refers to the property of a conductor that resists the flow of current through it. The SI unit of resistance is Ohm(Ω). The component that resists the flow of current is known as a resistor. 1 ohm (Ω) of resistance(R) is equal to the flow of 1A of current through a conductor between two points having a potential difference equal to 1V.
FACTORS AFFECTING RESISTANCE
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The resistance of a conductor depends on:
- The length of the conductor.
R α l
It is directly proportional to the length of the conductor.
- Nature of the conductor (Resistivity).
R α ρ
It is directly proportional to a conductor’s resistivity.
- The temperature of the conductor.
R α T
It is directly proportional to the conductor’s temperature.
- Area of the cross-section of the conductor.
R α 1/A
It is inversely proportional to the cross-sectional area.
Therefore,
R = ρ x L x T/ A
- RESISTIVITY
It is referred to as electrical resistance or we can say it is the property of a material of how strongly it resists electric current.
- COMBINATION OF RESISTANCE
- If resistors are combined in series the current flow in the resistors is the same.
- Resistors are said to be in parallel if the potential difference between all the resistors is the same.
- EQUIVALENT RESISTANCE
The equivalent resistance across two resistors can be calculated as:
- If connected in series -
Req = R1 + R2
- If connected in parallel -
1/ Req = 1/R1 + 1/R2
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HEATING EFFECT OF CURRENT
In everyday life, we see that whenever we use fans for a longer time, they get heated. A part of the energy produced due to the chemical reactions within the cells produces heat in the device. This is prominently recognized as the heating effect of the current.
Read more : Resistors in series
Joule’s Law of Heating:
Joule’s law of heating states that the amount of heat produced in a resistor is:
(i) Always directly proportional to the square of the amount of current for a given resistance,
(ii) Always directly proportional to the resistance for a given current, and
(iii) Always directly proportional to the period of time during which the current flows through the resistor.
It can be expressed as-
H=I2Rt
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ELECTRIC POWER:
Electric power (P) is defined as the rate of electrical energy consumed.
The SI unit of power is Watt (W).
Power is expressed in terms of work (W) and time (t):
P=W/t
Frequently Asked Questions
Ques: Explain the function of a device known as a galvanometer in a circuit?(1 mark)
Ans: The galvanometer refers to a device that is used for measuring the amount of current flowing through a circuit.
Ques: While studying an experiment about the dependence of potential difference (V) across a resistor, when the current (I) is passing through it, 5 readings for different values of current was taken by a student and plotted on a graph between V and I. A straight line graph passing through the origin has been made. What does the straight-line signify? Also, give the method for determining the resistance using the plotted graph.(2 marks)
Ans: The straight line in the graph plotted signifies that potential difference and current are directly proportional to each other.
By using Ohm’s law, the resistance of a resistor can be calculated
V = IR
and calculating the slope of the graph using the points:
Ques: what is the reason for using alloys in heating devices?(1 mark)
Ans: Alloys are commonly used in electrical heating devices because the resistivity of an alloy is generally higher than that of pure metal and they do not oxidize at higher temperatures.
Ques: Define electric circuit? Differentiate between an open and a closed circuit.(1 mark)
Ans: An electric circuit is defined as a closed path where the current flows.
Open Circuit refers to a circuit where there is no flow of current whereas a closed circuit refers to the circuit where current flows. When the switch is off, the circuit is open and when the switch is on, the circuit becomes a closed circuit.
Ques: Solve the given numerical.
A wire having a radius of 0.01 cm has a resistance of 10 Ω. When the resistivity of the wire is of the value 50×10-8 Ωm, find the length of the wire.(2 marks)
Ans: Given values;
Radius of wire, r =0.01cm
Resistance of wire, R=10 Ω
Resistivity, ρ = 50×10-8 Ωm
The resistance is calculated using:
R= ρl/A
Cross-sectional area, A=πr2
Hence, the length of wire can be written as;
l=(Rx πr2)/ ρ
on substituting the values, we get
l=62.8 cm
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