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48 Cards in this Set
- Front
- Back
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Definition of 'Field' |
A field is a region of space in which an object or particle may experience a force due to some external influence |
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Electric field between two parallel plates |
Uniform - the force experienced is the same everywhere
Field lines parallel |
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Paths along which the p.d. is the same |
Equipotential lines - always perpendicular to field lines |
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Electric dipoles in technology |
- Microwave Oven: rapidly reversing electric field causes dipoles to 'jostle' and generate heat
- Liquid Crystal Displays: field aligns dipoles in LC to polarise light in display |
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Direction of field lines |
Arrows indicate the direction of the force a positive charge would experience |
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Use of capacitors |
- camera flash unit - super-capacitors used for hospital back up power - radio tuners - PC keyboard - timing circuit - defibrillator |
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Investigating a Uniform Electric Field (Equipment) |
- Two parallel metal plates connected to a variable p.d. - Microscope observes the gap between plates - Atomiser injects oil droplets through hole in plates into uniform field |
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Investigating a Uniform Electric Field (Method) |
- Atomiser creates fine mist of oil drops charged by friction as they leave the atomiser - When circuit is off the drops fall under weight - When circuit on the p.d. creates a uniform electric field & negative oil drops can be made to 'float' as electric force balances weight - Increased p.d. drops move towards top plate - Increased distance between plates drops fall |
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Capacitance definition |
Amount of charged stored per volt |
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Capacitance Dependent on: |
- Area of the plates - Separation of the plates - Dielectric (insulator) between the plates |
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Investigating Charged Stored on a Capacitor |
- Set up test circuit to measure p.d. - Constantly adjust variable resistor to keep charging current constant for as long as possible - Record p.d. at regular intervals until it equals the battery p.d. - I / t Graph: I is constant against time - area = Q - Q / V Graph: grad = capacitance, area = work |
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Work done on Capacitor |
Work done by removing charge from one plate and depositing it on the other
Comes from the electrical energy of the battery |
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Capacitor Time Constant |
- 𝛕 = RC (R is resistance in discharging circuit & C is capacitance) - The time it takes for charge to fall to 37% (100/e) or to rise by 63% - In practise time to fully charge/discharge usually taken as 5RC |
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Magnetic Field |
A region of space in which a force may be exerted on objects with magnetic properties |
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Magnetic field line direction |
North to South |
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Magnetic field around a wire |
Right Hand Rule - Thumb current & fingers field |
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Magnetic Field Strength |
- Magnetic Flux Density - Strength of magnetic field per unit area - Vector - Teslas, T |
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Magnetic Flux |
= BA
Perpendicular to magnetic field
Weber, Wb |
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Magnetic Flux (Coil) |
= N = BAN
Weber, Wb |
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Capacitors in circuits |
Opposite to resistors |
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Fleming's ________ hand rule (Motor Effect) |
- Left
Thumb - motion / force First Finger - magnetic field (N to S) Second Finger - current (+ to -) |
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Magnetic Field Strength Definition |
The force on one metre of wire carrying a current of one amp at right angles to the magnetic field |
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Electromagnetic Induction |
- EMF induced whenever there is relative motion between a conductor and a magnetic field - EMF produced whenever flux lines are cut - Flux cutting always induces EMF but will only induce a current if the circuit is complete |
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Faraday's Law |
The induced e.m.f across a conductor is directly proportional to the rate of change of flux linkage (or rate of flux line cutting) |
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Lenz's Law |
the induced e.m.f. will be directed such that the current which it causes to flow opposes the change that is producing it |
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Factors affecting e.m.f induced in a coil |
- Angle between the coil & field - Number of turns in the coil - Area of the coil - Magnetic Field Strength (Flux Density) - Angular speed of the coil |
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Alternator |
AC generator
Uses slip rings and brushes to connect the coil to an external circuit
V & I change direction with every half rotation of the coil causing an alternating current |
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Transformers |
- AC flowing in primary/input coil produces magnetic flux - Magnetic field is passed through laminated soft iron core to secondary/output coil where it induces an EMF of the same frequency (current if in closed circuit) - Step-Up: primary coil < secondary coil - Step-Down: primary coil > secondary coil |
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Motor Effect |
When a current carrying wire experiences a force when placed in a magnetic field |
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Motor Current Direction |
- Uses split ring commutator to cause current to change direction every half rotation |
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Factors affecting force experienced by a motor |
- Stronger magnetic field - stronger magnets, laminated soft iron core - More coils - single loops, multiple loops - Increased current (increased heating) |
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Current in Hand Rules |
Conventional Current - Positive to Negative |
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An Application of Magnetic Fields to Charged Particles |
Mass Spectrometer |
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Fleming's _______ Rule (Dynamo Rule) |
Right Hand |
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Reason for Soft Laminated Iron Core |
- Primary & Secondary coils wrapped around - Lamination reduces eddy currents - Soft means it responds quickly to changes in magnetic field - Has the effect of amplifying field strength |
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Energy Losses in Transformer |
- Heat due to - resistance of coil, eddy currents in iron core, magnetic hysterysis - Sound - Flux leakage - shape of primary & secondary coil |
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Coulomb’s law
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The electrostatic force between two point charges is proportional to the product of their charges and inversely inversely proportional to the square of the distance between them
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Electric field strength
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The electrostatic force per unit charge at a point in an electric field experienced by a small positive test charge placed at a point
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Electron volt
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The energy that an electron (or proton) gains (or loses) when it is accelerated (or decelerated) through a potential difference of 1 volt
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Electric Potential
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The work done per unit charge by an external force in an electric field in bringing a positive charge from infinity to a particular point without acceleration
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Electric Potential Energy
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The work done by an external force in an electric field in bringing a positive charge from infinity to a particular point
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Magnetic flux density
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The force acting per unit current per unit length on a wire placed at right angles to the magnetic field
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1 Tesla
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The magnetic flux density of a uniform magnetic field when a wire of length 1m , carrying a current of 1A, placed perpendicular to the field, experiences a force of 1N in a direction at right angles to both the field and the current
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Magnetic flux through a plane surface
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The product of the magnetic flux density normal to the surface BN and the area A of the surface
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1 Weber
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The magnetic flux through a surface if a magnetic field of flux density 1 T exists perpendicularly to an area of 1m2
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Magnetic Flux Linkage
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The product of the number of turns N of the coil and the magnetic flux linking each turn
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EMF
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Electromotive Force - The energy converted to electrical energy from other forms per Coulomb of charge across a component
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PD
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Potential Difference (Voltage) - The energy converted from electrical energy to other forms per Coulomb of charge across a component
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