What is Electricity?
Electricity is a fundamental form of energy resulting from the movement of electric charges. It is characterized by the flow of electrons through conductors such as wires.
- Electricity powers various devices and systems, enabling them to perform functions like generating light, producing heat, and operating machinery.
- It is a crucial aspect of modern life and technology, with applications ranging from powering homes and appliances to driving electronic devices and industrial processes.
What is an Electric Charge?
Charge is a fundamental particle in an atom. It may be positive or negative.
Electric Charges: Positive and Negative
- Like charges repel each other.
- Unlike charges attract each other.
- Coulomb (C): S. I. unit of charge
- 1 Coulomb charge = Charge present on approx. 6 × 10^{18} electrons
- Charge on 1 electron = Negative charge of 1.6 × 10^{-19} C
i.e. Q = ne
Where, Q = Charge (total)
n = No. of electrons
e = Charge on 1 electron
Electric Current and Circuit
Electric current is vital for powering technology, enabling safety and convenience, and facilitating energy distribution. Circuits are essential pathways that control the flow of current, playing a key role in electronics, medical applications, transportation, research, and sustainability efforts. They are fundamental to modern life, impacting almost every aspect of our daily existence.
What is Electric Current?
Electric Current (I) is the flow of electric charge through a conductor, such as a wire.
Current = Charge/Time or,
I = Q/T
Electric Current is rate of flow of Charge
S. I. unit of current = Ampere (A). An ampere is a unit of measure of the rate of electron flow or current in an electrical conductor. One ampere of current represents one coulomb of electrical charge (6.24 x 10^{18} charge carriers) moving past a specific point in one second.
⇒ 1 A = 1 Cs^{-1}
⇒ 1 mA = 10^{-3} A
⇒ 1 µA = 10^{-6} A
- Current is measured by Ammeter. Its symbol is
- Ammeter has low resistance and always connected in series.
- Direction of current is taken opposite to flow of electrons as electrons were not known at the time when the phenomenon of electricity was discovered first and current was considered to be flow of positive charge.
Question for Chapter Notes: Electricity
Try yourself:A current of 0.5 A is drawn by a filament of an electric bulb for 10 minutes. What is the amount of electric charge that flows through the circuit?
Explanation
Current (I) = 0.5 A
Time (t) = 10 minutes = 600 seconds
We want to find the amount of electric charge (Q) that flows through the circuit.
The formula to calculate electric charge (Q) is given by: Q = It
Substitute the given values into the formula: Q = 0.5 A × 600 s Q = 300 C
Explanation:
- The formula Q = It represents the relationship between electric charge (Q), current (I), and time (t).
- In this case, the current is 0.5 amperes (A), and the time is 600 seconds (s).
- Multiplying the current (0.5 A) by the time (600 s) gives us the electric charge, which is 300 coulombs (C).
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Electric Potential and Potential Difference
Electric potential and potential difference are fundamental concepts in the field of electricity and electromagnetism. They are used in various ways, both in theoretical understanding and practical applications.
What is Electric Potential?
- Electric potential is the work done in carrying a unit positive charge from infinity to a point.
- If W is the work done and q is the charge, then electric potential:
V=W/q
- The SI unit of electric potential is Volts (V).
What is Potential Difference ?
- Potential difference is Work done to move a unit charge from one point to another.
V = W/Q
What is meaning of 1 volt potential difference?
- S. I. unit of Potential difference is Volt (V)
1 V = 1 JC^{-1} - When 1 joule work is done in carrying one Coulomb charge then potential difference is called 1 volt.
- Voltmeter: It is an instrument to measure the potential difference. It has high resistance and always connected in parallel. Symbol is:
- Current always flows from higher potential to lower potential.
What is an Electric Circuit?
An electric circuit is a closed loop or pathway.It allows electric current to flow through various components.
Electric Circuit
- Components can include wires, switches, resistors, and energy sources.
- Electrons typically move through conductors, creating the flow of electric current.
- To maintain current flow, the circuit must be closed without interruptions.
- Used in various applications, from lighting to powering electronic devices.
Circuit Diagram
A circuit diagram is a graphical representation of an electric circuit. In this diagram, various components of the circuit, such as cells (or batteries), plug keys, electrical components, and connecting wires, are symbolically represented using conventional symbols.
These symbols are standardized and commonly used to simplify the visual representation of complex circuits, making it easier for engineers, technicians, and students to understand and work with electrical circuits.
Circuit components are often represented by symbols in schematic diagrams.
Symbols of Some Commonly Used Components in Circuit
Ohm’s Law
Ohm's Law States that Potential difference across the two points of a metallic conductor is directly proportional to current passing through the circuit provided that temperature remains constant.
- Mathematical expression for Ohm’s law
V ∝ I
⇒ V = IR - R is a constant called resistance for a given metal.
V-I graph for Ohm’s law
- Resistance (R): It is the property of a conductor to resist the flow of charges through it.
- Ohm (Ω): S. I. unit of resistance.
- 1 ohm = 1 volt/1ampere.
When potential difference is 1 V and current through the circuit is 1 A, then resistance is 1 ohm. - Rheostat: Variable resistance is a component used to regulate current without changing the source of voltage.
Factors on which the Resistance of a Conductor depends
Resistance of a uniform metallic conductor is:
(i) directly proportional to the length of conductor,
(ii) inversely proportional to the area of cross-section,
(iii) directly proportional to the temperature and
(iv) depend on nature of material.
What is Resistivity (P)?
Resistivity is defined as the resistance offered by a cube of a material of side 1m when current flows perpendicular to its opposite faces.
- S.I. unit of Resistivity is ohm-metre (Ωm).
- Resistivity does not change with change in length or area of cross-section but it changes with change in temperature.
- Range of resistivity of metals and alloys is 10^{-8} to 10^{-6 }Ωm.
- Range of resistivity of insulators is 10^{12} to 10^{17} Ωm.
- Resistivity of alloy is generally higher than that of its constituent metals.
- Alloys do not oxidize (burn) readily at high temperature, so they are commonly used in electrical heating devices.
- Copper and aluminium are used for electrical transmission lines as they have low resistivity.
Question for Chapter Notes: Electricity
Try yourself:The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V?
Explanation
To determine the current the heater will draw when the potential difference is increased to 120 V, we can use Ohm's law, which states:
V=I⋅R
Where:
- V is the potential difference (voltage) across the heater.
- I is the current passing through the heater.
- R is the resistance of the heater.
We have the initial potential difference V_{1}=60 and the current I_{1}=4A.
we can use Ohm's law (V = I * R) to find the heater's resistance, which is 15 ohms. Now, if we increase the potential difference to 120 V, we can use the same Ohm's law to calculate the new current. Plugging in the values, we get 120 V = I * 15 ohms. Solving for I, we find that the heater will draw a current of 8 A when the potential difference is increased to 120 V. So, the answer is option B) 8 A.
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Resistors in Series
When two or more resistors are connected end to end, the arrangement is called series combination.
Resistors in Series
Total/resultant/overall/effective resistance in series
R_{s} = R_{1} + R_{2} + R_{3}
- Current through each resistor is same.
- Equivalent resistance is larger than the largest individual resistance.
- Total voltage = Sum of voltage drops
V = V_{1} + V_{2} + V_{3}
Voltage across each resistor
- V_{1} = IR_{1}
- V_{2} = IR_{2} [V_{1} + V_{2} + V_{3} = V]
- V_{3} = IR_{3}V = IR
⇒ V = IR_{1} + IR_{2} + IR_{3}
∝ IR = I(R_{1} + R_{2} + R_{3})
∝ R = R_{1} + R_{2} + R_{3}
Resistors in Parallel
When multiple resistors are connected in parallel in an electrical circuit, they are arranged so that both ends of each resistor are connected to the same two points, forming multiple paths for current flow.
Resisters in Parallel
- Voltage across each resistor is same and equal to the applied voltage.
- Total current is equal to sum of currents through the individual reistances.
I = I_{1} + I_{2} + I_{3}
⇒ V/R = V/R_{1} + V/R_{2} + V/R_{3} - Reciprocal of equivalent resistance is equal to sum of reciprocals of individual resistances.
1/Rp = 1/R_{1} + 1/R_{2} + 1/R_{3} - Equivalent resistance is less than the value of the smallest individual resistance in the combination.
Advantages of Parallel Combination over Series Combination
(i) In series circuit, when one component fails, the circuit is broken and none of the component works.
(ii) Different appliances have different requirement of current. This cannot be satisfied in series as current remains same.
(iii) The total resistance in a parallel circuit is decreased.
Heating Effect of Electric Current:
If an electric circuit is purely resistive, the source of energy continually get dissipated entirely in form of heat. This is known as heating effect of electric current.
As E = P×T ∝ VIt {E = H}
Heat produced, H = VIt {V = IR}
Or, Heat produced, H = I^{2}Rt
Question for Chapter Notes: Electricity
Try yourself:100 J of heat is produced each second in a 4 Ω resistance. What is the potential difference across the resistor?
Explanation
To determine the potential difference (voltage) across the 4 Ω resistor, we can use the formula V = √(P * R), where V represents the voltage, P is the power dissipated in the resistor, and R is the resistance of the resistor. We are given that 100 J of heat is produced each second (which is the power), and the resistance is 4 Ω. Plugging these values into the formula, we get V = √(100 W * 4 Ω) = √(400 V^2) = 20 V. So, the potential difference across the resistor is 20 volts, making option D) 20 V the correct answer.
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Practical Applications of Heating Effect of Electric Current
The heating effect of electric current, also known as Joule heating, is a fundamental concept in physics and electrical engineering. It's the phenomenon where electrical energy is converted into heat energy when current flows through a conductor with resistance. This heating effect has numerous practical applications in various fields.
Uses of Heating Effect of Electric Current
Joule’s Law of Heating Effect of Electric Current
It states that the heat produced in a resistor is (i) directly proportional to square of current, H ∝ I^{2}
- It is directly proportional to resistance for a given current, H ∝ R
- It is directly proportional to time for which current flows through the conductor, H ∝ t.
So, H = I^{2}Rt - Heating effect is desirable in devices like electric heater, electric iron, electric bulb, electric fuse, etc.
- Heating effect is undesirable in devices like computers, computer monitors (CRT), TV, refrigerators etc.
- In electric bulb, most of the power consumed by the filament appears a heat and a small part of it is radiated in form of light.
Filament of electric bulb is made up of tungsten because:
(i) it does not oxidise readily at high temperature.
(ii) it has high melting point (3380º C).
- The bulbs are filled with chemically inactive gases like nitrogen and argon to prolong the life of filament.
Electric Fuse: It is a safety device that protects our electrical appliances in case of short circuit or overloading.
- Fuse is made up of pure tin or alloy of copper and tin.
- Fuse is always connected in series with live wire.
- Fuse has low melting point.
- Current capacity of fuse is slightly higher than that of the appliance.
Electric Power: The rate at which electric energy is consumed or dissipated in an electric circuit.
P = VI
⇒ P = I^{2}R = V^{2}/R
S.I. unit of power = Watt (W)
⇒ 1 Watt = 1 volt × 1 ampere
Commercial unit of electric energy = Kilo Watt hour (KWh)
⇒ 1 KWh = 3.6 × 10^{6} J
⇒ 1 KWh = 1 unit of electric energy