Magnetic fields Flashcards
What is a magnetic field?
A region where a magnet exerts a force on objects made from magnetic materials or on other magnets
What happens when a current-carrying wire is placed in a magnetic field?
A force is exerted on the wire however if it is parallel the force is 0N
How is a magnetic field induced?
By passing a current through a wire
What are flux lines?
Lines that represent the direction of a force experienced by the north pole of a magnet at any point in the magnetic field
How does a diagram show areas of stronger flux density?
When the flux lines are closer together
What will happen to a freely suspended magnet in a field?
It will align itself with the field
What is magnetic flux density?
The strength of a magnetic field or
the number of magnetic flux lines that pass through an area of 1m²
What is the unit of magnetic flux density?
Tesla
What is a Tesla?
The flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux
What is a solenoid?
A current carrying coil of wire that produces magnetic flux.
(This is how an electromagnet works)
What do the poles in a solenoid depend on?
The direction of the current
How do you measure magnetic field strength?
using a hall probe
How does a hall probe work?
It contains a slice of semiconducting material. If a current flows in the semiconductor when it is perpendicular to the magnetic flux, a potential difference is generated across the side of the semiconductor and this potential difference s directly proportional to the flux density.
What does each finger represent in Flemings left hand rule?
Thumb-Force
pointer- magnetic field
middle- Current
What happens to a particle moving in a magnetic field?
It experiences a force
What direction does conventional current flow?
Positive to negative
What direction does a negatively charged particle flow?
from negative to positive / opposite to conventional current
How do charged particles move in a magnetic when at right angles to the uniform flux?
In circular motion
What is a cyclotron?
A particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic field and an alternating potential difference
What happens to the radius of a particle and time spent in the D as it accelerates?
It always spends the same amount of time in each D but the radius of the path increases as it accelerates
What is a synchrotron?
A particle accelerator with an increasing magnetic field as the particle accelerates. The radius remains the same even though they speed up.
What is magnetic flux?
The magnetic field lines passing through an area
magnetic flux density x cross-sectional area perpendicular to field direction
What is the unit of magnetic flux?
Weber ( Tesla metre squared)
What factors increase the number of flux line cut per second?
- The length of wire increases
- The speed of the wire increases
- The magnetic flux density increases
Can flux lines be “cut” when a conductor is moving parallel to the field lines?
No. Only when perpendicular so for parallel magnetic flux is 0Wb
What is magnetic flux linkage?
Magnetic flux x number of turns in a coil
What is the unit of magnetic flux linkage?
Weber-turns
What factors does the flux linkage depend on?
- Flux density
- Orientation of coil and flux lines
- coils cross sectional area
- number of turns on the coil
State Faradays law
The induced emf equals the rate of change of magnetic flux linkage
Describe electromagnetic induction
When a magnet is moved into a coil. When it is stationary inside, the current is 0. An emf is induced if there is relative movement between the coil and the magnetic field or if the flux linkage changes. This causes one end of the coil to become positively charged and the other negative
How can you change the flux linkage and generate an emf?
relative movement between the magnet and coil
rotating the coil at a constant frequency in the perpendicular plane to the field as it changes the cross sectional area through which the flux passes
State Lenz’s law and explain the effect
The direction of the induced emf causes effects that oppose the the change producing it.
when a magnet is moved north face in to the coil, the end of the coil becomes the north pole to oppose this motion, when it is removed from the coil, this end becomes a south pole to attract it/ oppose the motion.
What are eddy currents?
They are circulating Electric currents flowing in the plane of the metal. They are caused by the change of flux linkage when the metal moves perpendicular to the field
How are eddy currents induced?
By the alternating magnetic field in the primary coil
How can the effect of eddy currents be reduced?
By using a laminated iron core because the eddy currents cannot pass through the insulator and so their amplitude is reduced.
By using a core made of a material with a high resistivity
Why is a soft iron core used?
to allow easy magnetisation and demagnetisation
Name some things that use electromagnetic induction
Electric guitars and microphones
What frequency is the power supply from the national grid?
50Hz
What is the household rms power supply from the national grid?
230V
What is the peak value of the voltage?
The greatest magnitude in either the positive or negative direction with respect to 0.
Also half the peak-to-peak
What is the peak-to-peak value?
The voltage measured from one peak to the next
What is an oscilloscope?
A visual calibrated voltmeter where the operator can alter how the waveform is displayed
What is the timebase on an oscilloscope?
The time per division
What is the use of a transformer?
To increase or decrease the voltage of an alternating voltage supply.
How are transformers used in the national grid?
They are used to increase the the voltage generated in power stations up to 40000V so that energy can be saved as electricity is transmitted around the country. Transformers are then used to reduce this high voltage for safe use in the home.
What is the structure of a transformer?
It consists of two coils of wire linked by a soft iron core. The AC in the primary coil creates a changing magnetic field in the core. As the magnetic flux in the core changes, the magnetic flux linkage to the secondary coil changes and an emf is induced in the secondary coil.
Which supply do transformers work with?
AC only
Which coil has more turns in a step-up transformer?
the secondary coil
Which coil has more turns in a step-down transformer?
the primary coil
How can energy be lost in transformers and what can be done to prevent it?
Heat is produced in the coils when a current flows ( can be reduced by low resistance wire )
some magnetic flux produced by the primary coil doesn’t pass through (use coils that are close to each other or wound on top of each other)
Is a particles speed in a cyclotron affected by the magnetic field?
No
What happens when a conducting rod moves relative to a magnetic field?
The electrons in the rod will experience a force and build up on one side of the rod causing an emf to be induced
What does the variation of flux linkage depend on?
The angular speed of the coil
What does the waveform on an oscilloscope look like for an alternating current?
Sinusoidal
What is the root mean square voltage?
the average of all the squares of the possible voltages
What value does the root mean squared voltage give?
The average voltage of the voltage output by the supply
What is the peak voltage of UK mains?
325V (330 to 2 s.f.)
Why do transformers use alternating currents?
To change the size of their voltage
