M6 Flashcards
What is the significance of Faraday’s and Lenz’s law?
A changing magnetic flux gives rise to an emf which induces a current. This current flows in a direction so as to oppose the original change (Lenz).
How is voltage transformed to ensure the safe transmission of power to homes?
Voltage is stepped up initially to prevent power loss across transmission lines. As the initial voltage is stepped up, this will reduce the current and as Ploss= I^2R, very low current results in minimal power loss across long distances. However, transformers at substations are needed to then step down voltage before reaching homes to prevent high voltages from entering homes (unsafe).
What are the components of a DC motor?
ROTOR:
Coil: consists of many loops of current-carrying conductors carrying a direct current. These coils experience a force as per the motor effect in an external magnetic field
Armature: a cylindrical frame of a laminated soft iron core mounted on an axle. The iron core intensifies the intensity of the external magnetic field and thus maximises the rotational torque experienced by the coil.
STATOR:
Electromagnets: provide external B-field
Split ring comm: an evenly segmented broad ring of metal mounted on the axle and attached to the coil. The SRC is responsible for reversing the direction of current flowing through the coil every half-turn to ensure that the coil experiences positive torque.
Carbon brushes: ensures electrical contact between the external circuit and the coil, allowing it to rotate freely without tangling.
What is the function of an AC induction motor?
Generates mechanical energy (rotation) from electrical energy (current) without electrical contact between the stator and the rotor. An AC induction motor produces a rotating magnetic field in the stator coils and thereby induces a current in the rotor, which gives rise to a net rotational torque.
Describe an AC induction motor
The stator is made up of 3 pairs of electromagnets or coils, connected to a 3-phase AC supply, these coils are mounted on a soft iron core to enhance the magnetic field they create, they create the rotating magnetic field without actually rotating themselves. The squirrel cage rotor consists of conducting bars made of aluminium and two conducting rings at either end. (As the external field rotates it experiences a change in flux -> emf -> current as a closed circuit)
What are the advantages of an AC induction motor?
Only having one moving part (rotor) prevents friction-related wear and tear. No brushes or commutators are required as the rotor is not connected to an external circuit.
What is the disadvantage of an AC induction motor?
The maximum speed is limited by the frequency of the supply current. Speed is fixed at the rate of an AC source (50Hz) which limits use only to very specific applications.
Describe a DC generator and outline the only difference to a DC motor
Similar to a DC motor, The rotor consists of a coil of many loops and armature. The stator consists of electromagnets, a split ring commutator (allows DC output) and spring-loaded carbon brushes. The main difference to the DC motor is a DC motor has an external power source that provides current so that the motor can do mechanical work. A DC generator takes an input of mechanical work (via a handle) so that it can provide a current to the external circuit and power a load.
What is the only difference between AC and DC generators?
The only difference between an AC and DC generator is the use of a slip-ring commutator instead of a split-ring commutator. These rings ensure the output current is AC.
Explain the phenomenon of magnetic braking
A metal conductive wheel passes through a magnetic field which is provided by a section of the rail/tracks. As there is relative motion between the metal conductive wheel and the magnetic field, an emf will be produced (Faraday’s law).
The surface of the conducting wheel will facilitate the formation of localised circular eddy currents.
Lenz’s law states that the current induced by an induced emf will always give rise to a magnetic field that opposes the initial change in flux. Therefore the direction of the eddy currents will be such that a magnetic braking force is exerted on the conducting wheel, opposite to its direction of motion.
However, the strength and size of eddy currents and hence the force that opposes the motion of the wheels is proportional to the relative speed between the conducting wheel and the external magnetic field. i.e. The faster the vehicle, the greater the braking force.
What are the advantages of magnetic braking?
The magnitude of induced eddy current is proportional to the rate of change of flux which is determined by the speed of the vehicle. Braking force decreases as the vehicle slows, allowing for a smooth stop.
Does not rely on physical contact between moving parts, so less wear and tear and hence maintenance costs
What are the disadvantages of magnetic braking?
Only works for vehicles with metal wheels that travel at high speeds. Not useful for slow vehicles as mag braking is not so significant
Cannot provide as strong a braking force as conventional brake-pad contact systems. So not good for sudden stops.
What is the ampere defined as
The ampere is defined as being equal to the amount of current needed through two identical parallel conductors of infinite length when they are 1 m apart, in order to produce a force per unit length of 2 × 10−7 N m−1.
What is a transformer?
A transformer is used to increase or decrease the voltage in an AC circuit. Transformers can be found in many electrical devices, such as laptop chargers. A transformer works on the principle of a changing magnetic flux inducing an emf. A transformer consists of two multi-turn coils wound onto a common core and is used to increase/decrease AC voltages
Why is AC preferred over DC in transformers?
AC is preferred as it can be transformed from one voltage to another. A transformer works on the basis of a changing current (AC) in the primary coil inducing a changing magnetic flux. This in turn induces a current in the secondary coil. A DC voltage has a constant, unchanging current and only produces a change in flux when the current is turned on or off.
