L1 - Electricity PDF

# L1 - Electricity ## Special Class Padhle - Oct 2, 2024 ## Electricity - A diagram of a power line and wires. The word "Electricity" is written underneath the diagram in bold font and L1 is written on a white sign held by a hand. - A diagram of an arrow pointing to a box with "Cover" written ins...

# L1 - Electricity ## Special Class Padhle - Oct 2, 2024 ## Electricity - A diagram of a power line and wires. The word "Electricity" is written underneath the diagram in bold font and L1 is written on a white sign held by a hand. - A diagram of an arrow pointing to a box with "Cover" written inside, and another arrow pointing to the same box with "3K likes" written underneath. The arrow is labelled "Nationalism in India" to the left. Next to the box is an arrow pointing to the text "Sunehre" to the right. - The word **Brain** is written underneath **Nationalism in India**. ## Introduction - An atom has a positively charged nucleus and negatively charged electrons revolving around it. - Valence electrons in metals are free to move within the conductor and constitute an **electric current**. ## Charge: - In physics, charge, also known as electric charge, is a fundamental property of matter that describes the presence of positive or negative electrical properties in particles. - Charge is a fundamental particle of matter. - It may be positive and negative. - S.I. unit of charge is Coulomb (C). - A diagram shows two blue circles with negative charges and two blue circles with positive charges. The negative charges repel and the positive charges attract. The caption underneath the diagram is "Interaction between charges." - $S = ne$ no. of electrons - $Q = ne$ charge on an electron - $Q = n \times e$ - $Q = 100,000 \times 1.6 \times 10^{-19}$ ## Current: - The electric current is defined as the rate of flow of electric charge through any cross section of a conductor. - **Electric current = Charge/Time or I = Q/t** - Electric current is a scalar quantity ## Ampere: - The unit of current is the ampere. - If one Coulomb of charge flows through any cross-section of a conductor in one second, then current through it is said to be one ampere. ## Conductors and Insulators: - A substance which offers comparatively less opposition to the flow of current is known as a conductor, and substances which offer larger opposition are called insulators. - A list of 5 electrical conductors: - Silver - Gold - Copper - Steel - Sea water - A list of 5 electrical insulators: - Rubber - Glass - Oil - Diamond - Dry wood ## Electric Potential and Potential Difference: - The **electric potential** at a point is defined as work done in bringing a unit positive charge from infinity to that point. - The **potential difference** between two points is defined as the difference in electric potentials at the two given points. - The electrons move only if there is a difference in electric pressure called the potential difference. - One Volt is defined as energy consumption of one joule per electric charge of one coulomb. - Electric potential is the amount of electric potential energy at a point while potential difference is the difference in the amount of electric potential energy between two points in an electric circuit. - It is known as voltage, which is equal to the work done to move a unit charge from one point to another against a static field. - **Voltage (V)= Work done(W)/Charge(Q)** - S.I unit of potential difference is Volt (V). ## Electric Circuit: - The closed path along which an electric current flows is called an "electric circuit." - A diagram of a circuit is shown with the following: - A cell - Connecting wire - Resistor - Ammeter - Voltmeter - The **flow of current** is from the negative terminal of the cell to the positive terminal. ## Conventional Direction of Current: - Conventionally, the direction of motion of positive charges through the conductor is taken as the direction of current. - The direction of conventional current is opposite to that of the negatively charged electrons. ## Electrochemical or Voltaic Cell: - It is a device which converts chemical energy into electrical energy. - A diagram showing an electrochemical cell with: - A voltmeter - Two beakers, one with Copper Sulphate solution and one with Zinc Sulphate solution - A Copper electrode - A Zinc electrode - A salt bridge ## Galvanometer: - It is a device to detect current in an electric circuit. - An image of a galvanometer. ## Ammeter: - It is a device to measure current in a circuit. - It is a low resistance galvanometer and is always connected in series in a circuit. - An image of an ammeter. ## Voltmeter: - It is a device to measure the potential difference. - It is a high resistance galvanometer and is always connected in parallel to the component across which the potential difference is to be measured. - An image of a voltmeter. ## Drift Velocity of Electron: - It is the average velocity that an electron attains inside a metallic conductor due to the application of an electric field due to the potential difference. - A diagram illustrates the concept of drift velocity of an electron. ## Symbols of Some Commonly Used Components in Circuit Diagrams - A table showing symbols of commonly used electrical components. - SI. No. | Components | Symbols ----|-------------|---------- 1 | An electric cell | -|- 2 | A battery or a combination of cells | -|||- 3 | Plug key or switch (open) | (-) 4 | Plug key or switch (closed) | (.) 5 | A wire joint | - 6 | Wires crossing without joining | ) 7 | Electric bulb | ( ) or | 8 | A resistor of resistance R | -w-w- 9 | Variable resistance or rheostat | -w-w- or -ww- 10 | Ammeter | + A - 11 | Voltmeter | + V - ## Ohm's Law - It states that "the current through a conductor between two points is directly proportional to the voltage across the two points provided external conditions remain constant". ## Mathematical Expression for Ohm's Law: - $I \propto V$ - $V = IR$ - (Where, R = Resistance) ## V-I Graph for Ohm's Law - The graph between V and I is always a straight line with slope equals to R. - A diagram shows the V-I graph. ## Resistance (R): - It is the property of a conductor to resist the flow of charges through it. - (i) S.I. unit of resistance is Ohm (Ω). - (ii) 1 Ohm = 1 Volt/1 Ampere ## 1 Ohm: - When potential difference is 1 V and current through the circuit is 1 A, then resistance will be 1 ohm. ## Rheostat: - Rheostat is a variable resistor used to regulate current without changing the source of voltage. - A diagram of a rheostat is shown. ## Factors on Which The Resistance of A Conductor Depends: - Resistance of a uniform metallic Conductor is: - (i) Directly proportional to the length of the conductor. - (ii) Inversely proportional to the area of cross-section. - (iii) Directly proportional to the temperature. - (iv) Depends on the nature of the material. ## Resistivity (p): - The resistance offered by a wire of unit length and unit cross-sectional area is called resistivity. - Its S.I. unit 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 Ω.m. - Range of resistivity of insulator is 1012 to 10 17 Ω.m. - 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 ## Q&A 1. A complete circuit is left on for several minutes, causing the connecting copper wire to become hot. As the temperature of the wire increases, the electrical resistance of the wire: - (A) decreases. - (B) remains the same. - **(C) increases.** - (D) increases for some time and then decreases. - **Explanation**: A complete circuit is left on for several minutes, causing the connecting copper wire to become hot. As the temperature of the wire increases, the electrical resistance of the wire also increases. 2. Plastic insulation surrounds a wire having diameter D and length L as shown. A decrease in the resistance of the wire would be produced by an increase in the: - (A) length L of the wire - **(B) diameter D of the wire** - (C) temperature of the wire - (D) thickness of the plastic insulation - **Explanation**: The thinner or smaller the diameter (cross sectional area), the greater is the resistance. This means that resistance is inversely proportional to the area of the wire. 3. A cylindrical conductor of length 'l' and uniform area of cross section 'A' has resistance 'R'. The area of cross section of another conductor of the same material and same resistance but of length '2l' is: - (A) A/2 - (B) 3A/2 - **(C) 2A** - (D) 3A - **Explanation**: Resistivity of the conductor in the first case, p= RA/l...(i) - Resistivity of the conductor in the second case, p= RA'/2l...(ii). Since both conductors are of the same material and are at the same temperature, the resistivity of both conductors will be the same. Therefore, from equations (i) and (ii), we have: RA/l = RA'/2l => A' = 2A. 4. When a 4V Battery is connected across an unknown resistor there is a current of 100 mA in the circuit. The value of the resistance of the resistor is: - (A) 4 Ω - **(B) 40 Ω** - (C) 400 Ω - (D) 0.4 Ω - **Explanation**: V= IR, V=4V, I= 100mA = 0.1A. Hence, R= V/I = 40Ω. 5. Electrical resistivity of a given metallic wire depends upon: - (A) its length. - (B) its thickness. - (C) its shape. - **(D) nature of the material.** - **Explanation**: The resistivity of any wire is its intrinsic property. It depends on the nature of the material of the wire.