Physics
25 flashcards
~13 min
Electricity and Magnetism
Master electricity and magnetism — two of the most practically important topics in all of physics — with this comprehensive flashcard deck. Electricity and magnetism underpin every piece of modern technology, from your smartphone to power grids to MRI...
About this deck
Master electricity and magnetism — two of the most practically important topics in all of physics — with this comprehensive flashcard deck. Electricity and magnetism underpin every piece of modern technology, from your smartphone to power grids to MRI machines.
This deck covers electric charge, current, voltage, resistance, Ohm's Law, series and parallel circuits, Kirchhoff's laws, power in circuits, electric fields, magnetic fields, electromagnetic induction (Faraday's and Lenz's laws), transformers, and AC vs DC current. Essential for GCSE, A-Level, AP Physics 2, and first-year university physics.
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Charge (Q): Coulombs (C). Current (I): Amperes (A) = C/s. Voltage/Potential difference (V): Volts (V). Resistance (R): Ohms (Ω). Power (P): Watts (W). Energy (E): Joules (J). Relationships: Q = It, V = IR, P = IV = I²R = V²/R.
V = IR — the potential difference across a conductor is proportional to the current through it, provided temperature is constant. A component that obeys Ohm's Law is ohmic (e.g., a resistor). Non-ohmic components: diodes, filament lamps, thermistors.
Components connected end-to-end (one path for current). Current: Same throughout (I = I₁ = I₂ = ...). Voltage: Splits between components (V = V₁ + V₂ + ...). Resistance: Adds up (R_total = R₁ + R₂ + ...). If one component fails, the circuit breaks.
Components connected across the same two points (multiple paths for current). Current: Splits between branches (I = I₁ + I₂ + ...). Voltage: Same across each branch (V = V₁ = V₂ = ...). Resistance: 1/R_total = 1/R₁ + 1/R₂ + ... (total resistance less than smallest branch).
The algebraic sum of currents at any junction (node) in a circuit equals zero: ΣI_in = ΣI_out. Conservation of charge — charge cannot accumulate at a junction.
The algebraic sum of all voltages around any closed loop in a circuit equals zero: ΣV = 0. Conservation of energy — the sum of EMFs equals the sum of voltage drops around a loop.
EMF: The total energy per unit charge supplied by a source (e.g., battery) — the "push" that drives current. Terminal voltage: The actual voltage across the battery terminals when current flows. V_terminal = EMF − Ir where r = internal resistance. They differ because internal resistance causes energy loss inside the source.
The rate of energy transfer: P = IV = I²R = V²/R. Energy transferred: E = Pt = IVt. A 100W bulb uses 100J of electrical energy per second. Units: Watts (W).
A region of space where a charged object experiences a force. Represented by field lines pointing from positive to negative charge. Field strength: E = F/Q = V/d (N/C or V/m). Uniform between parallel plates; radial around a point charge.
The force between two point charges: F = kQ₁Q₂/r² where k = 8.99 × 10⁹ N·m²/C², Q₁ and Q₂ = charges, r = separation. Like charges repel; opposite charges attract. Same inverse-square form as Newton's gravitational law.
A region where a magnetic force acts on moving charges or magnetic materials. Created by: moving electric charges (current-carrying wires), permanent magnets, changing electric fields. Represented by field lines from N-pole to S-pole outside a magnet.
A current-carrying conductor in a magnetic field experiences a force. F = BIL sin θ where B = magnetic flux density (Tesla), I = current, L = length of conductor in field, θ = angle between conductor and field. Direction given by Fleming's Left-Hand Rule (motor).
Used for the motor effect (force on a current in a magnetic field). Point the left hand with: First finger → Field direction. Second (middle) finger → Current direction. Thumb → direction of Motion (Force) on the conductor.
An EMF (and current if circuit is complete) is induced when a conductor cuts through magnetic field lines, or when the magnetic flux through a coil changes. EMF = −N × ΔΦ/Δt where N = number of turns, ΔΦ = change in magnetic flux.
The induced current flows in a direction such that it opposes the change that caused it. A consequence of conservation of energy — if it aided the change, energy would be created from nothing. Explains why moving a magnet into a coil requires effort.
A device that changes AC voltage using electromagnetic induction. Two coils (primary and secondary) wound on an iron core. V_p/V_s = N_p/N_s (turns ratio). Step-up transformer: N_s > N_p → voltage increases. Step-down: N_s < N_p → voltage decreases. Works only with AC (needs changing flux).
P = I²R — power lost in transmission lines depends on current squared. Stepping up voltage reduces current (P = IV; same power) → greatly reduces power losses as heat in cables. Then stepped back down for safe household use (~230V in UK; ~120V in USA).
DC (Direct Current): Flows in one constant direction. Source: batteries, fuel cells. AC (Alternating Current): Periodically reverses direction (frequency = 50Hz in UK/Europe, 60Hz in USA). Source: generators. AC is used for mains electricity because it can be easily transformed (stepped up/down).
An electrical component that stores charge (and energy) in an electric field between two conductive plates separated by an insulator (dielectric). C = Q/V (capacitance measured in Farads). Used in timing circuits, filters, energy storage, and camera flash units.
A semiconductor device that allows current to flow in only one direction (forward bias). Acts as a one-way valve. Symbol: arrow pointing in direction of conventional current flow. Used in rectifiers (converting AC to DC), LED lights, and circuit protection.
A resistor whose resistance decreases as temperature increases (most common type — NTC, Negative Temperature Coefficient). Used in thermostats, temperature sensors, and fire alarms. As temperature rises → resistance falls → current increases → can trigger a circuit.
A resistor whose resistance decreases as light intensity increases. In darkness: high resistance. In bright light: low resistance. Used in automatic lighting circuits (streetlights, cameras). When light falls → resistance drops → current increases → activates circuit.
Φ = BA cos θ where B = magnetic flux density (T), A = area of the coil (m²), θ = angle between B and the normal to the area. Units: Webers (Wb). Represents the total magnetic field passing through an area. Faraday's law: induced EMF = rate of change of flux linkage.
When a current-carrying conductor is placed in a magnetic field perpendicular to the current, charge carriers are deflected to one side → a potential difference (Hall voltage) builds up across the width. Used in Hall probes to measure magnetic flux density.
The opposition to current flow: R = V/I. For a wire: R = ρL/A where ρ = resistivity (Ω·m), L = length, A = cross-sectional area. Resistance increases with: longer length, smaller cross-section, higher temperature (for metals), higher resistivity material.