Workbook 5 Flashcards
Define the direction of lines of force
The direction of the lines of force is defined as the path taken by a free magnetic North.
Define flux density
Flux density is defined as the amount of flux passing perpendicularly though a unit area.
Define the permeability of a material
The permeability of a material is defined as a measure of he ability of a material to support the formation of a magnetic field within it.
Define 1 Amp (Ampere’s Law)
The Ampere is defined as being the constant current which generates a force of 2 x10 to the -7 Newtons per unit length between two infinitely long, thin, straight parallel wires 1 metre apart in a vacuum.
Describe the Hall effect
When a wire carries current in a magnetic field, it experiences a force. Naturally, the individual electrons do so too. This force on the electrons in a wire causes them to shift to one side of the conductor along its width, leaving the other side positively charged.
Describe the DC motor
The DC motor consists of a coil carrying a current in a magnetic field. If the direction of the current remains the same, the coil will simply do a quarter of a turn and oscillates until it comes to a halt. By using a split ring (commutator), the polarity and hence direction of current in the coil is reversed enabling it to rotate continuously.
State Faraday’s Law
Faraday’s Law states that when a conductor is placed in a varying magnetic field an EMF is induced within it. The size of the EMF induced is dependent on the rate of change of flux linkage.
State Lenz’s Law
Lenz’s Law states that the polarity of induced EMF is such that it produced a current whose magnetic field opposes the change causing it.
How can one vary the EMF induced?
Using a coil with a different number of turns, using a different magnetic field strength and changing the speed of flux linkage.
What is mutual inductance?
Mutual inductance is the phenomenon in which a change in current of one coil causes an induced EMF in another coil next to it. The Mutual inductance of two circuits describes the size of the EMF in the secondary induced by changes in current of the primary.
Define 1 Henry of mutual inductance
1 Henry of mutual inductance is when a current of 1 Amp in the primary coil causes a flux linkage of 1 Wb in the secondary.
Define Self-Inductance
Self-Inductance is defined as being the induction of an EMF in a current-carrying conductor when the current in the wire itself is changing.
Define 1 Henry of self inductance
1 Henry of self-inductance is when a current of 1 Amp causes a flux linkage of 1 Wb in itself.
What happens in L-R DC circuits when trying to switch on an off?
In L-R DC circuits, due to Lenz’s Law, the self-inducting component in the circuit will try to oppose any change occurring. Therefore, if the circuit is switching on, the self-inductor will try to oppose current flow by inducing an opposing voltage. When switching off a circuit, the self-inductor will try to oppose this change by inducing a voltage that is in the direction of the current.
Describe the back EMF in an electric motor.
Upon starting the motor, the motor would start from rest. It will receive a large amount of current from the battery/supply and due to starting at 0 rpm, it will not have a back EMF. Running under normal load, the motor will be doing a particular amount of rpm and will generate a back EMF that will counteract the supply voltage and hence, reduce the current. The current will be less than the starting current. When running under heavy load, the rotating coil will slow down. Less rpm will result in a slower rate of change of flux, meaning less back EMF generated. This will result in an increase in current and an increase in the torque on the coil.
