Physics: Resistance and Conductivity Concepts

Electricity Notes

• Electric Charge: A physical property defined by excess or deficiency of electrons. A scalar quantity. SI unit: Coulomb (C). Proton charge: +1.6 x 10^-19 C. Electron charge: -1.6 x 10^-19 C.

• Electric Current: The rate of flow of electric charge through a particular area. A scalar quantity. SI unit: Ampere (A). Formula: I = Q/t, where I is current, Q is charge, and t is time. Current direction is opposite to electron flow. Measured by an ammeter.

• Electric Potential: The amount of work done to move a unit positive charge from infinity to a specific point. A scalar quantity. SI unit: Volt (V). Formula: V = W/q, where V is potential, W is work, and q is charge.

• Electric Potential Difference: The difference in electrical potential between two points. A scalar quantity. SI unit: Volt (V). (Often called voltage). Formula: ΔV = V_B - V_A = W_AB/q, where ΔV is potential difference, V_B is potential at point B, V_A is potential at point A, W_AB is work done, and q is charge. Measured by a voltmeter.

• Electric Circuit: A closed path along which electric current flows. Includes a source (like a battery or cell), a load (like a bulb or appliance), a conducting path, a switch, and fuses. Diagram uses symbols for different components.

• Circuit Diagram: A pictorial representation of an electric circuit, using standardized symbols to show the connection of components.

• Ohm's Law: The current through a conductor between two points is directly proportional to the voltage across the two points. Formula: V = IR, where V is voltage, I is current, and R is resistance. Applies at constant temperature.

• Resistance: The opposition to the flow of electric current. A scalar quantity. SI unit: Ohm (Ω). Factors affecting resistance: material, length, cross-sectional area, and temperature.

• Resistivity: A material property representing its resistance to current flow. A scalar quantity. SI unit: Ohm-meter (Ω⋅m).

• Alloys: Mixtures of two or more metals. Often have higher resistivity than pure metals, making them suitable for heating elements.

• Series Combination: Resistors connected end-to-end. Total resistance is the sum of individual resistances. Current is the same through all resistors, but voltage drops across each resistor.

• Parallel Combination: Resistors connected across the same two points. Reciprocal of total resistance is the sum of reciprocals of individual resistances. Voltage is the same across all resistors, but current is divided among resistors.

• Heating Effect of Electric Current: When current passes through a conductor, some energy is converted into heat. Joule's Law of Heating: H = I²Rt, where H is heat produced, I is current, R is resistance, and t is time.

• Practical Applications: Heating effects are used in electric heating devices (e.g., toasters, heaters), electric bulbs, and fuses. Electrical resistance is used in circuits to limit current.

• Power: The rate at which electrical energy is consumed or supplied. SI unit is Watt (W). Formula: P = VI = I²R = V²/R where P is power, V is voltage, I is current, and R is resistance.

• Commercial Units: Electrical energy consumption can measured in kilowatt-hours (kWh).

• Fuses: Safety devices that protect circuits from excessive current by melting and breaking the circuit. This prevents overheating and damage of the appliance.

Previous Year Questions (1 Mark)

• Galvanometer function: detects current flow.
• Higher resistance lamp: the lower wattage lamp.
• SI unit of potential difference: volt (V).
• Potential difference definition: work done to move unit charge between two points.
• Device maintaining potential difference: battery/cell/power source.
• Physical quantity same in series: current.
• Electric current definition: rate of flow of charge.
• Resistance change with decreased potential difference: resistance remains constant, current halves.
• Cross-sectional area increase resistance: resistance decreases.
• Tungsten used for lamp filaments: high melting point and high resistivity.

Previous Year Questions (2 Marks)

• Alloy use in toasters: Higher resistivity, and don't oxidize as easily, preventing high temperatures.
• Wire resistance increase by lengthening : Resistance quadruples(if length doubles, resistance quads and area halves)
• Straight line graph in V-I graph means: proportionality between voltage and current. Method: pick two points, use the slope = ΔV/ΔI calculation.
• Meter zero readings: use substitution method to determine zero error; calculate and subtract or add based on the position of the pointer to get the true reading.
• Voltmeter connection: Connect in parallel. Ammeter connection: Connect in series.
• Factors for cylindrical conductor resistance: length, area of cross-section, temperature, material.
• Metal good conductors of electricity: higher number of free electrons to carry current. Glass has fewer free electrons, thus it's a bad conductor.
• Alloy use for heating devices: Higher resistivity, more durable at high temperatures.

Previous Year Questions (3 Marks)

• Parallel resistor connection: Method to connect to get minimum/maximum current. Calculation of current.
• Joule's law: H=I²Rt. Numerical examples (using the formula) to calculate heat.
• Parallel resistance: Formula to calculate total resistance in a parallel configuration with three or more resistors.
• Finding resistance: Method for connecting resistors in series or parallel to find equivalent resistance, showing connections and using the proper formulas.

Previous Year Questions (5 Marks)

• Deriving the heat produced by current through resistor (using Ohm's law and definition of current): Using the formula H=I²Rt.
• Different types of connections: Explain parallel and series connections, the differences of current and voltage in both cases.
• Specific types of connections with numerical examples: Numerical examples involving a combination of resistors in series and/or parallel.