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4.1: Charge and Current

Conduc­tors: A material that allows the flow of electrical charge. Good conductors have a larger amount of free charge carriers to carry a current.
Conser­vation of Charge: The total charge in a system cannot change.
Conven­tional Current: The flow from positive to negative, used to describe the direction of current in a circuit.
Coulomb: The unit of charge.
Electric Current: The rate of flow of charge in a circuit.
Electr­olytes: Substances that contain ions that when dissolved in a solution, act as charge carriers and allow current to flow.
Electron Flow: The opposite direction to conven­tional current flow. Electrons flow from negative to positive.
Elementary Charge: The smallest possible charge, equal to the charge of an electron.
Insula­tors: A material that has no free charge carriers and so doesn’t allow the flow of electrical charge.
Kirchh­off's First Law: A conseq­uence of the conser­vation of charge. The total current entering a junction must equal the total current leaving it.
Mean Drift Velocity: The average velocity of an electron passing through an object. It is propor­tional to the current, and inversely propor­tional to the number of charge carriers and the cross-­sec­tional area of the object.
Quanti­sation of Charge: The idea that charge can only exist in discrete packets of multiples of the elementary charge.
Semico­ndu­ctors: A material that has the ability to change its number of charge carriers, and so its ability to conduct electr­icity. Light dependent resistors and thermi­stors are both examples.

4.4: Waves

Amplitude: A wave’s maximum displa­cement from its equili­brium position.
Antinodes: A position of maximum displa­cement in a stationary wave
Coherence: Waves with the same frequency and constant phase difference
Constr­uctive Interf­erence: The type of interf­erence that occurs when two waves meet in phase. The wave amplitudes are superp­osed.
Critical Angle: The angle of incidence that results in an angle of refraction of exactly 90o . It is when the refracted ray travels along the boundary line.
Destru­ctive Interf­erence: The type of interf­erence that occurs when the two waves are in antiphase. When one wave is at a peak and one is at a trough their addition results in a minimum point.
Diffra­ction: The spreading of waves as they pass through a gap of a similar magnitude to their wavele­ngth.
Displa­cement: The distance that a point on a wave is from its equili­brium position.
Electr­oma­gnetic Spectrum: The spectrum of electr­oma­gnetic waves, consisting of Gamma Rays, X-Rays, Ultrav­iolet, Visible Light, Infrared, Microwaves and Radiow­aves.
Electr­oma­gnetic Waves: Waves that consist of perpen­dicular electric and magnetic oscill­ations. All electr­oma­gnetic waves travel at the speed of light in a vacuum.
Frequency: The number of waves that pass a point in a unit time period. It is the inverse of the time period.
Fundam­ental Mode of Vibration: The oscill­ation of a wave at its natural frequency.
Intensity: The power transf­erred per unit area. It is propor­tional to the square of a wave’s amplitude.
Interf­erence: The superp­osition of the amplitudes of waves when they meet.
Longit­udinal Waves: A wave with oscill­ations that are parallel to the direction of energy propag­ation. Sound waves are an example of a longit­udinal wave. They cannot travel through a vacuum.
Nodes: A position of minimum displa­cement in a stationary wave.
Oscill­oscope: A device used to display and analyse waveforms.
Path Differ­ence: A measure of how far ahead a wave is compared to another wave, usually expressed in terms of the wavele­ngth.
Period: The time taken for a wave to complete one full cycle
Phase Differ­ence: The difference in phase between two points on a wave. It is usually expressed in radians
Polari­sation: The restri­ction of a wave so that it can only oscillate in a single plane. This can only occur for transverse waves.
Progre­ssive Waves: Waves that transfer energy from one point to another without a transfer of matter.
Reflec­tion: The bouncing of a wave at a boundary. The angle of incidence will equal to the angle of reflec­tion.
Refrac­tion: The changing of speed of a wave as it passes into a new medium. If it passes into an optically denser medium, it will slow down
Refractive Index: A material property that is equal to the ratio between the speed of light in a vacuum, and the speed of light in a given material.
Stationary Wave: A wave that stores, but does not transfer, energy.
Superp­osi­tion: When two waves meet at the same point in space their displa­cements combine and the total displa­cement at that point becomes the sum of the individual displa­cements at that point
Total Internal Reflec­tion: An effect that occurs in optical fibres, where full reflection occurs at the inside boundary of the fibre, meaning no radiation passes out. The angle of incidence must be greater than the critical angle for this to occur.
Transverse Waves: A wave with oscill­ations that are perpen­dicular to the direction of energy propag­ation. Electr­oma­gnetic waves are examples of transverse waves
Wave Speed: The product of a wave’s frequency and wavelength
Wavele­ngth: The distance between two identical positions on two adjacent waves. It is commonly measured from peak to peak or trough to trough.
Young Double­-Slit Experi­ment: An experiment that demons­trates the diffra­ction of light by passing monoch­romatic light across two narrow slits and observing the resulting pattern of bright and dark fringes.

4.2: Energy, Power and Resistance

Diode: A component that allows current through in one direction only. In the correct direction, diodes have a threshold voltage (typically 0.6 V) above which current can flow
Electr­omotive Force: The energy supplied by a source per unit charge passing through the source, measured in volts.
Filament Lamp: A bulb consisting of a metal filament, that heats up and glows to produce light. As the filament increases in temper­ature, its resistance increases since the metal ions vibrate more and make it harder for the charge carriers to pass through.
I-V Charac­ter­istics: Plots of current against voltage, that show how different components behave.
Kilowa­tt-­Hour: A unit of electrical energy. It is usually used to measure domestic power consum­ption.
Light-­Dep­endent Resistor: A light sensitive semico­nductor whose resistance increases when light intensity decreases.
Ohm: The unit of resist­ance.
Ohmic Conductor: A conductor for which the current flow is directly propor­tional to the potential difference across it, when under constant physical condit­ions.
Ohm’s Law: The current and potential difference through an ohmic conductor held under constant physical conditions are directly propor­tional, with the constant of propor­tio­nality being resist­ance.
Potential Differ­ence: The difference in electrical potential between two points in a circuit. It is also the work done per coulomb to move a charge from the lower potential point to the higher potential point. It is measured in Volts
Power: The rate of energy transfer in a circuit. It can be calculated as the product of the current and the potential difference between two points. It is measured in Watts.
Resist­ance: A measure of how difficult it is for current to flow through a material.
Resist­ivity: A measure of how difficult it is for charge to travel through a material. It is propor­tional to the object’s resistance and cross-­sec­tional area, and inversely propor­tional to the object’s length. It is measured in Ohm metres
Resistor: A device that has a fixed resistance and follows Ohm’s law
Volt: The unit of potential differ­ence.

4.3: Electrical Circuits

Conser­vation of Energy: Energy cannot be created or destroyed - it can only be transf­erred into different forms.
Conser­vation of Energy: Energy cannot be created or destroyed - it can only be transf­erred into different forms.
Kirchh­off's Second Law: A conseq­uence of the conser­vation of energy. The sum of the voltages in any closed loop must equal zero
Lost Volts: The difference between a source’s emf and the terminal voltage. It is equal to the potential difference across the source’s internal resist­ance.
Parallel Circuit: Components are said to be connected in parallel when they are connected across each other (separate loops).
Potential Divider: A method of splitting a potential differ­ence, by connecting two resistors in series. The total potential difference is split in the ratio of their resist­ances.
Resistors in Parallel: The potential difference across resistors connected in parallel is identical for each resistor. The current is split between the resistors. The total resistance is equal to the inverse of the sum of the inverses of the resist­ances of the resistors
Resistors in Series: The current through resistors connected in series is identical for each resistor. The potential difference is split in the ratio of their resist­ances. The total resistance is equal to the sum of the resist­ances of the resistors.
Sensor Circuits: A circuit that reacts to external condit­ions. They commonly involve a semico­nductor connected in a potential divider arrang­ement.
Series Circuit: Components are said to be connected in series when they are connected end to end (in one loop).
Terminal PD: The potential difference across the terminals of a power source. It is equal to the source’s emf minus any voltage drop over the source’s internal resist­ance.


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