5.5 Magnetic Fields Flashcards
Define a magnetic field
the space surrounding a magnet where a magnetic force is experienced
What is the direction of a magnetic field?
North to South
In the right hand grip rule, what direction does the thumb point in
the direction of conventional current
How do you increase the strength of a magnetic field in a solenoid? (3 things)
- increasing current in coil
2.increasing no. turns in coil - placing a rod of soft iron into the coil
Define magnetic flux density
the force per unit current carrying length
What are the units of magnetic flux density?
T, teslas
Define a tesla
the strength of the magnetic field when a current of 1A is flowing in a conductor perpendicular to a magnetic field causes a force of 1N to be produced on each metre of conductor in the field
In Fleming’s left hand rule, where does the thumb point?
the direction of the force
In Fleming’s left hand rule, where does the pointer finger point?
The magnetic field direction
In Fleming’s left hand rule, where does the middle finger point?
the direction of conventional current (because fuck conventional current)
The force acting on a current carrying conductor depends on what 2 things?
- The length of the conductor, l, in the magnetic field
- The current, I, flowing in the conductor
What is the purpose of the split-ring commutator in an electic motor?
To reverse the direction of current every time the coil passes through the vertical, so that the coils keep rotating in the same direction
What is magnetic flux?
It represents the total number of magnetic flux lines that pass at 90degrees thru’ a given area
What is magnetic flux measured in?
Wb, Webers
Define magnetic flux linkage
the total magnetic flux linking a coil of N turns of current carrying conductor and an area of cross-section A
What is ‘Faraday’s Law of Electromagnetic Induction’?
The magnitude of the increased emf is equal to the rate of change of magnetic flux linkage
What 3 things is the magnitude of increased emf proportional to?
- strength of magnet
- number turns on coil
- speed of moving magnet
For an emf to be induced, what MUST there be between the magnet and conductor?
relative motion
Define Lenz’s Law
The direction of the induced current is such that it opposes the changes in the magnetic flux that is producing it
In step up transformers, are there more turns in the primary or secondary coils?
There are more turns in the secondary coil
In step up transformers, does voltage increase or decrease?
voltage increases
In step up transformers, does current increase or decrease?
current decreases
In step down transformers, are there more turns in the primary or secondary coils?
There are more turns in the primary coils
In step down transformers, does voltage increase or decrease?
voltage decreases
In step down transformers, does current increase or decrease?
current increases
How do resistive heat losses in transformers cause power losses?
As coils heat up, resistance increases and so the amount of energy lost increases
How is the iron core in a transformer heated, causing power loss?
Repeatedly magnetising the iron core in one direction then the other results in its heating
How do eddy currents in a transformer cause power losses?
The iron core is a conductor in a changing magnetic field, so large currents, known as eddy currents, are induced to flow in it. They result in heating the iron core
How can eddy currents in a transformer be reduced?
laminating the iron core
In a transformer, the fact that not all of the magnetic flux of the primary coil passes thru’ the secondary coil causes what?
power losses
To reduce power loss in electricity transmission, Steve suggests using cables with a very large cross-sectional area because, he says, this will reduce resistance. Why do electricity companies not use Steve’s suggestion?
This would be very expensive.
What is a good way to reduce power loss in electricity transmission?
Utilise step-up transformers to increase voltage and reduce current in transmission cables. Then utilise step-down transformers to reduce voltage for uses in industry and homes
