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Physics Electromag
Physics Spring Y13
| Question | Answer |
|---|---|
| Arrow direction | The way a free north pole would move. From N to S. Earth direction opposite. |
| Uniform field | Equally spaced and parallel magnetic field lines |
| bar megnet | Strongest (lines closest together) at poles |
| Electromagnetism | When a wire carrying a current, a magnetic field is created around the wire. Any charged particle that moves creates a magnetic field in the space around it. ALWAYS use conventional current |
| Solenoid field shape | Bar magnet but line go through middle. |
| RH grip rule | Thumb in direction of CONVENTIONAL current, fingers show magnetic field lines. Can also do for solenoids, where thumb points north and fingers are current flow |
| Wire current symbols | Dot out of page. X into page |
| What is B? | Magnetic flux density |
| Current carrying wire in magnetic field | Where lines in same direction, add (more dense). Other side subtract (gap). Force in direction of the gap because of difference in magnetic field. Called a catapult field. Motor effect |
| Motor effect | Fleming's Left Hand Rule. FBI (force thumb, then magnetic field, then CONVENTIONAL current) Depends on angle between magnetic field and current (sin theta), length of wire *in magnetic field*, current, magnetic flux density (field strength) |
| SI unit for B | Tesla (T) Nm^-1A^-1 |
| When calculating gradient | ALWAYS state it to get working marks |
| Beam of electrons through mag field | Can be seen as a (reversed) current flow Electrons travel in a circular path in the region of the uniform perpendicular magnetic field, and the force on the electrons is always at right angles to motion |
| Wire N3L | Remember wire forms N3L pair on magnet too |
| Force on charges in wire | Electrons in of wire in L length of mag field. Force on every charge = BIl l = vt F = BIvt I = Q/t If N charges in length L, then I = NQ/t (current needs to use N) F = BNQv so on each F = BQv |
| Linking force of charges in wire | Centripetal force = F = mv^2/r F = BQv = mv^2/r r = mv/BQ = momentum/BQ |
| Velocity selector | Electric field pointing downwards. Mag field into page (so acting upwards). Electrons fired in to slit on far end. |
| Velocity selector purpose | Magnitude of force due to electric constant due to charge. Mag of magnetic field dependent on v Only particles of certain v will balance and go through slit. This is when qE = Bqv v = E/B |
| Velocity selector after slit | Mag field extends past. We know v (and we're looking for m) so can calc radius of turn for placing the electron detector after their circular path in mag field without electric. |
| Charges in mag field | Can only be affected if moving. This is because moving charges produce a mag field, which must interact with the other mag field for a magnetic force to act. |
| Velocity selector for mass spectrometer | Radius of curve dependent on mass as they all have the same velocity. |
| Shape of electric vs magnetic field electron flow | E force always same direction so parabola, in B force is always at right angles so circular. |
| Magnetic Flux unit | Webers (Wb) |
| Magnetic Flux | The magnetic flux density multiplied by the area perpendicular to the field direction. phi = BAcostheta where theta is the angle between the area used and the area perpendicular |
| Field lines aka | Lines of flux |
| Magnetic flux linkage | In electromagnetism, coils of wire are often used. Magnetic flux linkage = N x phi where N is the number of turns on the coil |
| Generator effect | Fleming's Right Hand Rule. Fingers the same. Kinetic to electric unlike opposite for motor |
| Faraday's Law | States that induced emf is equal to the rate of change of magnetic flux linkage. Emf = -N*phi/delta(t) where N*phi is the magnetic flux linkage. Minus sign is there to satisfy Lenz law |
| Check photos Taylor whatsapp 22:44 09/01/2026 | |
| Remember for scaling area in cm^2 | Squared scaling obvs |
| emf induction graph when frequency doubled but B halved | Half period, but same amplitude because deltaT is halved, so the delta(Nphi) being halved is cancelled for EMF |
| Check how flux loops inside circular iron core work | Seemingly pass continuously through whole circular core through every coil on it |
| For the grip rules and flow of charged particles | Check if +ve or -ve charged: matters |
| Magnets in a motor | Both have magnetic force on each other in opposite directions. Forms a couple - torque |
| Lenz's Law from wikipedia | From wikipedia: Lenz's law states that the direction of the electric current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current (FLH) opposes changes in the initial magnetic field. |
| Dynamo | Needs smoothing by capacitor. Split-ring commutator |
| Need AC for | transformers |
| Alternator | PMT says commutator to prevent tangling rather than split-ring commutator like for dynamo (Gcse said Slip rings - prevent tangling) |
| Induced emf vs flux linkage | Both sinacoidal out of phase 90 degrees. When turns on coil are parallel to field, flux linkage is zero but emf max because rate of change of flux linkage max. Opposite for perpendicular. Max flux linkage is BAN, minimum -BAN |
| Brushes | Connect generators to circuit |
| emf over time equation | e = e(subscript zero) * cos(wt) |
| Back emf | Occurs in motor. When motor turns, also produces dynamo effect due to Lenz's law. Arrow of V supplied to motor and opposite back emf on circuit. In the motor, V - emf = IR (high I if back emf small) |
| Back emf value depending on motor | Opposing emf high when spinning fast (unloaded i.e. low weight on motor), and low when spinning slowly (heavy load) |
| Diodes to protect motors from back emf | Specifically if cell turned off, still spinning so still back emf so would flip current - especially bad for DC motors. |
| What is a transformer | Metal core made of many thin sheets of soft iron insulated from all the others: done to reduce eddy currents. Primary and secondary side. Step-up/step-down depending on turns. |
| Eddy currents | Induced in the surface of the conductor by the changing magnetic field: allow for the 'drag effect' to happen. Lenz's law |
| How do transformers work | Alternating current in primary coil. Causes continuously changing magnetic flux in core. Secondary coil linked to the changing magnetic flux by core: changing magnetic flux linkage so emf induced in secondary coil by Faradays. |
| Coil vs turns | One coil has many turns |
| The turns ratio | emf across secondary/emf across primary = turns on secondary/turns on primary |
| Step-up | Has more turns on secondary, so higher induced emf for same primary, so current stepped down by same amount |
| For a 100% efficient transformer | Output power from secondary coil equal to input power into primary coil. This doesn't happen because the iron atoms constantly aligning with the changing field causes magnetic heating |
| Power loss | I = P0/V P loss = I^2*R = P0^2 * R/V^2 The higher the transmission voltage, the much, much smaller the power loss through heating: P loss proportional to 1/V^2 |
| Unit for magnetic flux linkage | Also Weber |
| Permanent magnets | Persistent magnetic fields because electrons in material move in an ordered way |
| Why iron filings show field lines | In a magnetic field its atoms tend to align with the field as electrons move in an ordered way. This creates an induced magnetic field, interacts for force, so north pole points in direction of mag field |
| Soft core of electromagnet | Induced field in core greatly enhances mag field produced by coil (also done by more turns). |
| Hard vs soft materials (italics on pmt) | Soft don't stay magnetised when away from mag field, hard do. Hard for permanent magnets, with crystal structure aligned by strong magnet in factory - removed by opposite strong field or hitting to make vibrate |
| For field around wire | Remember there are multiple field lines around it (getting increasingly far away faster than uniform) |
| F = BIlsintheta | Greatest force when current perpendicular to magnetic field, so theta is the angle between the current and magnetic field |
| 1T | the magnetic flux density required to generate a force of 1N on a wire carrying a current of 1A per metre perpendicular to the magnetic field |
| Measuring B | Horseshoe magnet (close to uniform) on scales. Clamp rigid straight wire, connect to DC. variable resistor and ammeter in series. Align so force up on wire. Measure length in field. Adjust R for values to graph. Remember scale gives force/9.81 |
| F = BQv | Velocity is using current velocity, not electron. Essentially only use this for magnitude, then do Fleming's Left Hand Rule for direction. |
| Derivation for F = BQv using the Mean Drift Velocity (in italics on PMT) | Sub I = -Anev (-ve for electron flow) into F = BIL n = N/V A/V is L Divide by N for single charge |
| Work done on charge in mag field when in circular motion | No displacement in direction of force bcos circular motion, so speed of particle stays the same |
| Equating circular motion force with BQv shows what? | r = mv/BQ Useful for proportionality |
| bubble chamber (italics on PMT) | make the tracks of ionizing particles visible as a row of bubbles in a liquid. Can be used for calculating particle mass if magnetic field strength known |
| Velocity selector equations | F = EQ F = BQv When in equilibrium, v = E/Q |
| Electromagnetic induction | Wire moved through non-uniform mag field, or changing the mag field strength over time Don't have to explain in detail - just say mag flux linkage changing so by Faraday's law there is emf |
| Example of electromagnetic induction | Bar magnet towards coil of wire would induce the wire to be an electromagnet pointing in the opposite direction, resisting the magnet going further |
| galvanometer | Sensitive ammeter |
| Lenz's Law from PMT | the induced e.m.f. is generated in a direction that so it opposes the change that produced it |
| Method for finding B | Hall Probe used (galvanometer connected to insulated coil). Moved through known B for calibration (so constant of proportionality found by probe), then through unknown B for value *Same motion each time needed* |
| Hall probe | Needs point of comparison - can use as a reason it's unsuitable for a situation if you're completely out of ideas for a 1 marker |
| Motor type of force | Forms a couple as net force is zero |
| Measuring AC voltage | oscilloscope |
| How to make transformers more efficient (italics on PMT) | Lower resistance wires so less heating Insulated laminated iron sheets in core so less eddy currents so less magnetic heating Soft iron cores easier to magnetise/unmagnetise |
| Shape of electric field around wire | Cylindrical and strongest closest to wire |
| Why coil much stronger electric field than wire | Adjacent fields overlap and strengthen |
| Adjacent wires where current in same direction | Attract Repel for opposite directions |
| Force from the top part of a rectangular coil rotating about vertical axis | Acts upwards (FLH) and downwards so cancel |
Created by:
Pyrogearos2