Magnetism Flashcards
Is magnetism a contact force?
No, non-contact.
What direction do magnetic field lines always go in?
North to south.
There’s a higher magnetic flux density of there are more or less lines in a region of space?
More.
Define magnetic flux, Φ. What is it measured in?
Net number of magnetic lines passing through a region of space, Webers.
Define magnetic flux density, B. What is it measured in?
It is the amount of magnetic flux per unit area, Teslas.
What does everything carrying an electric current cause?
Magnetic field.
What rule do we use to find the direction of the magnetic field in a wire?
Right hand thumb rule. Thumb points in direction of the current. Magnetic field circles the wire in the direction of your fingers.
What do circles and crosses mean in regards to the way the current is flowing?
Circle = current flowing towards you
Cross = current flowing away from you
What is a solenoid?
A current carrying wire wrapped up into a coil.
How do the magnetic field lines look for a solenoid?
Like a bar magnet around the solenoid, inside there’s a uniform field everywhere.
What is the motor effect?
Put a current carrying wire in an external magnetic field and it experiences a force.
How do we increase the motor effect?
External magnetic field gets bigger, current gets bigger.
Where does a wire in a magnetic field feel the most force?
90 degrees.
Where does a wire in a magnetic field feel no force?
Parallel.
How does Fleming’s left hand rule work?
First finger along magnetic field line
Second finger at a right angle to it – point that along current
Thumb points in the direction of the force
What is the equation for radius of curvature, how do we derive it?
r = mv / BQ
By combining F = mv^2 / r and F = BQv
What is the equation for the frequency of rotation, how do we derive it?
f = BQ / 2πm
By combining f = v / 2πr and r = mv / BQ
What are the six steps to how a cyclotron works?
- Charged particles fired into electrode
- Magnetic field makes them follow a semi-circular path
- Applied p.d. between electrodes accelerates particles across the gap into the
next semi-circle - Speed of particle now higher – so follows a path with a slightly bigger radius
- When it gets to next gap, p.d. has reversed so it is accelerated again –
speed goes up – radius gets even bigger - Particle spirals outwards getting faster
What is the generator effect (Faraday’s Law)?
The induction of a potential difference in a conductor placed in a changing magnetic field.
What do you have to do in order to have a changing magnetic field?
- Actually change the B field
- Move a conductor in a constant B field
How does Fleming’s right hand rule work?
Thumb is the direction of motion
First finger is the direction of the B field
Second finger is the direction the current will move it
What is the equation for magnetic flux linkage?
NΦ = BAN
What is the equation for magnetic flux linkage at an angle?
NΦ = BANcosθ
What is the equation for Faraday’s law?
ε = NΔΦ / Δt
What is Lenz’s law?
The B field produced by an wire due to an induced current that acts opposite to the external B field that made it.
What is flux linkage in a rotating coil?
NΦ = BAN cos ωt
What is the emf in a rotating coil?
ε = BANω sin ωt
What are the two different types of current?
Direct current – current flows in same direction all the time
Alternating current – current constantly switching direction back and forth
What does the graph of V over t look like for an AC current?
Sine curve. Varies between Vp and -Vp (peak voltage)
What is Vpp?
2Vp
What is Vrms and what is the equation for it?
The root mean square voltage
Vrms = Vp / root2
For AC what is the average power equation?
Average power = Irms x Vrms
What is the UK mains Vrms value?
230V
Explain how transformers change the p.d using the genrator effect in five steps.
- a.c. Current passes into primary coil – makes a magnetic field
- Magnetic field of coil magnetises iron core (induced magnetism)
- Magnetic field in iron core is alternating – because current in coil is a.c.
- Alternating B field in the iron core makes an alternating p.d. in second coil (generator
effect) - Get a.c. out of the second coil
What is the transformer equation?
Ns / Np = Vs / Vp
What is the difference between a step up and step down transformer?
Step-up = more turns in the 2nd coil than 1st coil - so get bigger p.d. in second coil than in first coil
Step-down = less turns in the 2nd coil than first, so get a p.d. out that is smaller than the p.d. you put in
How do transformers lose energy?
- The iron core being continuously cut by flux causes currents in the core, called eddy currents
- Eddy currents dissipate energy as heat
- Heat generated from resistance in the coils
- Energy is also needed to magnetise and demagnetise the core which is wasted
- Not all the flux from the first coil cuts through the second coil in practice
What is the transformer efficiency equation?
η (efficiency) = IsVs / IpVp
How do we reduce power loss in transmission lines?
Use a small current (high p.d.) to reduced energy lost to heating.
