7.5 Magnetic fields Flashcards
What is a magnetic field?
A region surrounding a magnet or current carrying wire that will exert a force on any other magnet or current carrying wire or moving charge placed within it.
What is the direction magnetic field lines around a magnet?
From north to south.
Wha is the right hand rule?
Magnetic field around a current (or vice-versa)
What is Fleming’s left hand rule?
(aka motor rule)
What is the equation for force on a wire perpendicular to a magnetic field?
What do the letters represent in this equation?
What is the force on a wire parallel to a magnetic field?
0
What is the force on a wire at an angle theta to a magnetic field?
What is magnetic flux density a measure of?
How tightly packed magnetic field lines are together or the strength of the field.
Define the Tesla.
1 Tesla is equal to 1 Newton per Amp per Metre.
What direction does force act on a charged particle as it moves through a magnetic field?
Perpendicular to the field and direction of motion.
What is the difference between a postive charge moving and a negative charge moving through a magnetic field?
The magnetic forces act in opposite directions.
What is the equation for force on a moving charged particle?
What do the letters represent in this equation?
If a charged particle is moving with constant speed through a magnetic field, what type of path does it take?
Circular.
(Hence can equate magnetic force with centripetal)
What is the equation for the radius of a charged particle moving through a magnetic field?
What do the letters represent in this equation?
Define magnetic flux.
The measure of magentic field that passes through a given perpendicular area.
What is the equation for magnetic flux?
What do the letters represent in this equation?
Define magnetic flux linkage.
The measure of magentic field that passes through a given perpendicular area enclosed by a coil of wire.
(aka number of turns of wire cutting the flux at one time)
What is the equation for magnetic flux linkage when the coil is perpendicular to the magnetic field?
What do the letters represent in this equation?
What is the equation for magnetic flux linkage when the coil is parallel to the magnetic field?
0
What is the equation for magnetic flux linkage if the normal of the coil is at an angle theta to the magnetic field?
What is Faraday’s law?
The induced emf is directly proportional to the rate of change of flux linkage.
What is the equation for Faraday’s law?
What do the letters represent in this equation?
What is the equation for the magnitude of induced emf?
What is the equation for induced emf in a conductor moving through a magnetic field?
What do the letters represent in this equation?
What is Lenz’s law?
Induced emf/current is always in such a direction as to oppose the change that induced it.
(hence the negative sign in Faraday’s law)
What is the explaination for Lenz’s law?
Energy is never created or destroyed. Induced emf/current could never be in a direction to help the change that causes it; that would mean producing electrical energy from nowhere.
What is the equation for induced emf for a rotating coil in a magnetic field?
What do the letters represent in this equation?
What is the gradient of a flux linkage vs time graph?
Induced emf.
What is the area of an induced emf vs time graph?
Flux linkage.
What is alternating current?
An electric current that reverses direction with constant frequency
(aka sinusoidal)
What is the frequency of an alternating current?
Number of cycles it passes through each second.
What is peak voltage?
Distance from the equilibrium to the highest (or lowest) point.
(Same goes for peak current)
What is peak-to-peak voltage?
Distance from lowest point to highest point.
(Same goes for peak-to-peak current)
What is rms voltage?
The average of all the squares of the possible voltages.
(Same goes for rms current)
What are the equations for rms voltage and rms current?
What do the letters represent in this equation?
What is a transformer’s function?
Steps up (or down) an alternating current.
What are the components of a transformer?
A primary coil and a secondary coil, both wound around a soft iron core.
How does a transformer work?
- AC in primary coil induces alternating magnetic field in core
- Alternating magentic field passes through secondary coil
- Resulting in changining flux linkage and inducing AC in the secondary coil
What is the transformer equation?
What do the letters represent in this equation?
What is a step-up transformer?
A transformer where the secondary coil has more turns than the primary coil.
What is a step-down transformer?
A transformer where the secondary coil has fewer turns than the primary coil.
What is the equation for transformer efficiency?
What do the letters represent in this equation?
What 4 things can reduce a transformer’s efficiency?
- Eddy currents
- Coil resistance
- Escaping flux
- Ease of magnetisation and demagnetisation
How would you reduce the inefficiency of coil resistance?
Use thicker wires with low resistance.
How would you reduce the inefficiency of escaping flux?
Designing the core such that the two coils are as close as possible.
How would you reduce the inefficiency of magnetisation and demagnetisation?
Using soft iron for the core, to reduce the energy needed to magnetise and demagnetise
(here ‘soft’ mean magnetically soft)
What are eddy currents?
Small currents induced in the core by the changing magnetic field.
Why do eddy currents cause inefficiency?
Due to Lenz’s law, eddy currents create magnetic fields that oppose the changing magnetic field that induced them in the first place. This causes heating in the core.
How would you reduce the inefficiency of eddy currents?
Laminating the core with insulating material, reducing the the size of the eddy currents and hence the magnetic field strength they create.
