module 4

• Electric current, I, is defined as the rate of flow of charge, measured in Amperes (A), and can be measured using an ammeter placed in series
• Charge, Q, is a physical quantity measured in coulombs (C), where 1 coulomb is the flow of charge in 1 second when the current is 1 ampere
• In atoms, the net charge of a particle is due to the gain or loss of electrons, with a proton having a charge of +1 and an electron having a charge of -1, representing multiples of the elementary charge, e, 1.6x10^-19 C
• Electric current in metals is carried by electrons, while in conducting liquids like electrolytes, positive and negative ions allow for the flow of charge
• Conventional current is the rate of flow of charge from positive to negative terminals, regardless of the direction of charge carriers, with electrons flowing from negative to positive in metals
• Kirchhoff's first law states that the sum of currents into any point in an electrical circuit equals the sum of currents coming out of that point, a consequence of the conservation of charge
• Mean drift velocity, v, is the average velocity of electrons as they travel down a wire, influenced by the number density, n, representing the number of free electrons per unit volume
• Potential difference, V, measures the work done by charge carriers in a circuit, defined as the energy transferred from electrical energy to other forms, per unit charge
• Potential difference is measured in volts (V), where 1 volt is defined as 1 joule of energy transferred per coulomb
• Electromotive force is also measured in volts
• Resistance, R, of a component is defined as the potential difference across the component divided by the current in the component, measured in ohms (Ω)
• Ohm's law states that for a metallic conductor at a constant temperature, the current is directly proportional to the potential difference across it
• For metallic conductors, when the temperature increases, the resistance increases due to increased vibrations of metal ions, leading to more collisions with electrons
• Fixed resistors have constant resistance, following Ohm's law, while filament lamps show non-ohmic behavior with increasing current
• Diodes are non-ohmic components made from semiconductors, allowing current flow in one direction only
• Light-dependent resistors (LDRs) are non-ohmic components made from semiconductors, with resistance decreasing as light intensity increases
• The resistivity, ρ, of a material is a constant linking the resistance of the material with its area and length, measured in Ohm-meters (Ωm)
• Resistivity varies with temperature; for metals, increased temperature increases resistance, while for semiconductors, resistance decreases with increasing temperature
• Energy transferred by a component is calculated by multiplying power by time, with power defined as the rate of energy transfer, measured in watts (W) or Js-1
• Kilowatt-hours (kWh) are used as a unit for measuring energy for industrial or domestic purposes, where 1 kWh is the energy transferred by a device with a power of 1 kilowatt operated for 1 hour
• Kirchhoff’s second law states that in any circuit, the sum of the electromotive force is equal to the sum of the potential difference in a closed loop, a result of the conservation of energy
• In a series circuit, the current at every point is the same, and the total resistance is the sum of the resistance of each component, Rt = R1 + R2 + …
• In a parallel circuit, the current in each loop adds up to the total current, and the total resistance is given by the formula 1/Rt = 1/R1 + 1/R2 + …
• Internal resistance in a source of e.m.f. results in 'lost volts', where some energy is transferred to the internal resistance of the cell, causing a difference between the measured p.d. across the terminals and the actual e.m.f.
• To determine the internal resistance of a cell, connect it in series to an ammeter and a variable resistor, with a voltmeter in parallel around the cell, vary the resistance of the variable resistor, and record V and I readings to plot a graph of terminal p.d. against current
• Potential divider circuits distribute potential difference across 2 resistors, with one resistor connected to another circuit using the Vout potential difference, typically with a sensor or variable resistor to alter resistance
• In potential divider circuits, the potential difference across a component can be determined using the formula Vout = (R2 / (R1 + R2)) * Vin, where Vin is the e.m.f. of the circuit
• Diffraction gratings are optical devices with parallel slits or grooves that diffract light into bright and dark bands, used in spectrometers and lasers

  
• All progressive waves can be reflected, refracted, and diffracted
• Reflection occurs when a wave changes direction at a boundary between two media, remaining in the original medium
• Refraction happens when a wave changes direction as it changes speed upon entering a new medium
• Diffraction is the spreading out of a wave front as it passes through a gap, with no alteration to the wavelength and frequency of the wave
• Polarisation is unique to transverse waves, restricting oscillations to one plane, while longitudinal waves cannot experience polarization
• Intensity of a progressive wave is defined as the radiant power passing through a surface per unit area, with units of watts per meter squared
• Electromagnetic waves consist of magnetic and electric fields oscillating at right angles to each other, traveling at the speed of 3.0 × 10^8 m/s
• Destructive interference occurs when two waves of the same frequency and amplitude are combined in opposite phase, canceling each other out
• The principle of superposition states that when two or more waves overlap, they produce a single wave, with the resultant displacement depending on constructive or destructive interference
• Two waves are coherent when they have a constant phase difference; interference between coherent waves results in maximum displacement when the phase difference is an even multiple of π
• Stationary waves are formed when two progressive waves with the same frequency and amplitude, traveling in opposite directions, superpose to create nodes and antinodes
• To produce a stationary wave in a stretched string, a vibration generator oscillates the string until a stationary wave is formed, with nodes at the transmitter and pulley ends