Topic 11

peak voltage produced by the generator

wNBA

Motor effect

when a current is passed along a wire in a magnetic field, and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field - magnetic interaction between the wire and the field results in a force - direction of force is at right angles to the field and the current

half wave rectification

½ of power lost using a single diode

how can we use the experiment of two parallel wires to work out ampere

- 2 infinitely long wires carrying a current of one amp separated by a distance of one metre, force per unit length = 2 x 10⁻⁷N - coulomb = 1 ampere second ampere = flow of one coulomb of charge per second

difference between electric fields and magnetic fields

- a magnet does not feel a force when placed in an electric field - a positive charge does not feel a force when placed stationary in a magnetic field - isolated charges exist whereas isolated poles do not - The Earth itself has a magnetic field (magnetic south pole = geometric north pole)

transformer struture

- alternating pd across the primary creates an ac within the coil and hence an alternating magnetic field in the iron core - magnetic field links with the secondary coil and produce magnetic flux - magnetic field in both coils changing = flux changing, inducing emf (in sin function)

field pattern around a long straight wire

- as one moves away from the wire, strength of the field gets weaker constant mu = permeability (use permeability of vacuum) NA-2

calculating the value of emf generated

- choose the period of time, over which the motion of the coil is to be considered - at the beginning and end of the period, work out the flux passing through one turn of the coil. (shape of the coil is not relevant / only the cross sectional area is) - at the end: if the magnitude is the same but is passing through the coil in the opposite direction, flux initial = -flux final - determine the change in flux - if there's change in flux, emf induced

AC generator

- coil wire rotates in the magnetic field due to an external force = flux linkage changes with time and induces emf (Faraday's law), causing a current to flow - sides AB and CD of the coil experience a force opposing the motion (Lenz's law) - the work done rotating the coil generates electrical energy - coil rotating at constant speed will produce a sinusoidal induced emf

Lenz's law and conservation of energy

- electrical energy generated within any system must result from work being done on the system - when a conductor moves through a magnetic field and an induced current flows, an external force is needed to keep the conductor moving (external force balances the opposing force predicted by the Lenz's law) - the external force does work and this provides energy for the current to flow

The switch in the circuit is now moved to position B and the fully charged capacitor discharges. Describe what happens to the energy

- energy goes into the resistor/surroundings - energy transferred into thermal/internal energy form

role of transformers in transmission of electrical power

- if large amounts of power are being distributed, then the currents used will be high (power = VI) - the wires cannot have zero resistance = they must dissipate some power - power dissipated is (P=I^2R) if the current is large, then the power dissipated will be large - over large distances, the power wasted would be very significant - transmit the power at a very high potential difference - only a small current needs to flow, but very dangerous - step-up transformers increase voltage for the transmission stage, and step down to protect the end user

Explain how readings on the high resistance ac voltmeter can be used to compare the rms values of alternating currents in different cables.

- if the coil position/orientation relative to the cables is the same in all measurements - if the coil distance from the cables is the same in all measurements - the voltage measured across the coil is proportional to current in the cable

magnetic field in a solenoid

- parallel field lines = magnetic field inside the solenoid is constant

effect of increasing the speed of rotation in the AC generator

- reduce the time period of the oscillation - increase the amplitude of the induced emf (as the rate of change of flux linkage is increased) since w=2pi f, theta = wt

when test magnetic North pole is placed in a magnetic field, it will feel a force

- small magnet / iron placed in the field would rotate until lined up with the field lines

the emf induced depends on

- the speed of the wire - the strength of the magnetic field - the length of the wire in the magnetic field

The two cables in part (c) are suspended a constant distance apart. Explain how the magnetic forces acting between the cables vary during the course of one cycle of the alternating current (ac)

- wires/cable attract whenever current is in same direction - charge flow/current direction in both wires is always same «but reverses every half cycle» - force varies from 0 to maximum - force is a maximum twice in each cycle

Tesla

1NA-1m-1 Wbm-2

An alternating current (ac) generator produces a peak emf E0 and periodic time T. What are the peak emf and periodic time when the frequency of rotation is doubled?

2E0, T/2

Which of the following reduces the energy losses in a transformer? A. Using thinner wires for the windings. B. Using a solid core instead of a laminated core. C. Using a core made of steel instead of iron. D. Linking more flux from the primary to the secondary core.

D

Which of the following experiments provides evidence for the existence of matter waves? A. Scattering of alpha particles B. Electron diffraction C. Gamma decay D. Photoelectric effect

B.

magnetic field strength

L = length of the current that is in the magnetic field theta = between the field and current

A parallel-plate capacitor is connected to a cell of constant emf. The capacitor plates are then moved further apart without disconnecting the cell. What are the changes in the magnitude of the electric field between the plates and in the capacitance of the capacitor?

Magnitude: decrease capacitance: decrease

A parallel-plate capacitor is connected to a battery. What happens when a sheet of dielectric material is inserted between the plates without disconnecting the battery?

The energy stored increases

solenoid

a long current-carrying coil

magnetic force on a moving charge

a single charge moving through a magnetic field also feels a force in the same way that current fuels the force

flux linkage

amount of flux(total magnetic field which passes through a given area.) passing through one turn of coil x the number of turns

if the area is not perpendicular to the magnetic field line, the magnetic flux

angle = between B and normal to the surface Tm2 = Wb (webers)

magnetic field of two parallel wires with opposing charge

between the wires: magnetic field from the two wires point the same direction = total magnetic field will be large outside the wires: two magnetic field from the two currents are in opposite directions and cancel each other (weak)

losses associated with non-ideal transformers: flux losses

caused by magnetic 'leakage' transformer is only 100% efficient if all of the magnetic flux that is produced by the primary links with the secondary

rectification

conversion of ac into dc

transformer

device that takes a certain ac voltage as input and delivers a different ac voltage as output

full wave rectification

diode bridge (4 diodes) utilise all the electrical energy that is available positive half cycle: current flows from A C B D negative half cycle: D C B A - current always flows through the load resistor in the same direction (C to B) - diodes on parallel sides are in the same direction - positive output is taken from the junction of the negative side of two diodes - during each half cycle, one set of parallel side diodes conducts

what would happen if whole coil was inside the magnetic field

each side would generate an emf. Two emfs would oppose one another and no current would flow

in the middle of the wire in the magnetic field

electron is at an equilibrium induced electric force = magnetic force due to movement except for the initial short interval, there is no current

losses associated with non-ideal transformers: resistance of the windings (joule heating)

electrons carrying current in a wire lose energy to the metal atoms of the lattice.

When conductor moved through a magnetic field

emf is induced

alternative name for magnetic field strength

flux density

Define magnetic flux.

he product of (the magnitude of) the normal component of magnetic field strength;and area through which it passes/with which it is associated; or φ = BA cos θ ;all terms defined/shown on a diagram;

the value of induced emf in transformers depend on

increased number of turns on the secondary

losses associated with non-ideal transformers: magnetic hysteresis

magnetic energy stored in the field as magnitude increases is not all given back when it decreases - cause iron core to warm up due to continued cycle of changes to its magnetism

Faraday's law

magnitude of the induced emf is proportional to the rate of change of magnetic flux linkage equation when both Lenz's law and Faraday's law are combined

magnetic flux

number of magnetic field lines that cross or pierce the loop area

Outline the features of an ideal step-down transformer.

number of secondary coils < primary coils; coils wound around an iron core; ideally no flux leakage/zero resistive heating;

flux and emf are out of phase by

pi/2

graph of alternating voltage vs current

same

smoothing device using capacitor

since full wave rectification still pulsates - output is still fluctuating slightly: output ripple - capacitor is acting as a short term store of electrical energy - capacitor constantly charging and discharging - in order to ensure slow discharge, value of C chosen to ensure that the time constant is late

Lenz's law

the direction of the induced emf is such that if an induced current were able to flow, it would be in such a direction as to oppose the change in magnetic flux which caused it (so work must be done to achieve an induced current)

if the wire inside the magnetic field was part of complete circuit outside of the magnetic field,

the emf induced would cause a current to flow. If this situation was repeated with a rectangular coil with N turns, each section ab would generate an emf of Bvl. thus, E = BvlN

why is the resultant motion of the charge circular

the force on a moving charge is always at right angles to the velocity of the charge

why two parallel wires carrying currents in opposite directions repel each other

the magnetic field of the left hand current causes a force on the right hand current that is a repulsion from newton's third law, two currents repel each other

in an ideal transformer, the value of the input current is equal to

the output power

diode

two-terminal electrical device that has different electrical characteristics depending on which way around it is connected - allows current to pass through it in one direction when potential at A>B

losses associated with non-ideal transformers: eddy currents

unwanted currents induced in the iron core (due to free electrons moving in magnetic field and heating up the core) - laminating the core (into individually electrically insulated thin strips)

rms value

value of direct current that would produce the same average power dissipation as alternating current current through the resistor = rms current