Magnetic Fields Flashcards
What is a magnetic field?
A region in which a force will act on magnetic materials or induced magnets
What are field lines?
- A way of representing a force field
- Called flux lines in a magnetic field
Which way do field lines travel for bar magnets?
From north to south
Describe the diagram for two north magnets facing each other
- Lines curving away from each other
- Arrows pointing outward
What happens when a current flows through a wire?
- A magnetic field is induced around the wire
- The field lines are concentric circles centred around the wire
How is the direction of the magnetic field induced by a wire identified?
By using the right hand rule
What happens when a current carrying wire is put into a external magnetic field?
- The field from the wire and magnets combine, causing a resultant field.
- Where the field is stronger, the lines are closer together
- These bunched up lines cause a resulting force on the wire, perpendicular to the direction of the magnetic field
How is the direction of the force identified?
- Fleming’s left hand rule
- Upward: Force
- Forward: External magnetic field
- Right: Current (Positive to negative)
Define magnetic flux density
The force on one metre of wire carrying a current of one amp, perpendicular to the magnetic field
Derive the equation for the circular path of particles
- F = mv²/r = BQv
- mv²/r = BQv
- mv = BQr
- r = mv/BQ
State the equation, which includes v, for the frequency of an object in circular motion
f = v/2πr
Derive State the equation for frequency of rotation in terms of B, Q and m.
- f = v/2πr and r = mv/BQ
- f = v/2π(mv/BQ)
- f = 1/2π(m/BQ)
- f = BQ/2πm
State one application of circular motion of particles
Particle accelerators, i.e cyclotrons
Describe how a cyclotron works
- Charged particles are produced and fired into one of the two hollow semicircle electrodes with a uniform magnetic field
- The uniform magnetic field is applied perpendicular to the plane of the electrodes which have an alternating p.d between them
- Charged particles are accelerated in a circular path by an applied p.d in the electrodes
- As particles speeds up, path radius is bigger until it exits the cyclotron at high speed
Define magnetic flux
The number of field lines passing through an area, in Webers
How is an electromotive force induced?
When there’s relative motion between a conducting rod and a magnetic field
Define electromotive force
- Energy transferred from chemical energy to electrical energy in a power supply
- Electrical energy per unit charge
State Faraday’s Law
Induced e.m.f is directly proportional to rate of change of flux linkage
State Lenz’s Law
The induced e.m.f is always in such a direction as to oppose the change that caused it
What do generators do?
- Convert kinetic energy into electric energy
- They induce an electric current by rotating a coil in a magnetic field
State the equation for average power
Average power = I(rms) x V(rms)
What is the value for UK mains electric supply?
230V(rms)
What is a transformer?
Devices which make use of electromagnetic induction to change the size of the voltage for an alternating current
Describe how a transformer works
- An alternating current flowing in the primary coil causes the core to magnetise, demagnetise and re-magnetise continuously in opposite directions
- This produces a rapidly changing magnetic flux across the core which is why a magnetically soft material is needed(iron or special alloy)
- Rapidly changing flux in iron core passes through secondary coil
- This induces an alternating voltage of the same frequency
State the 2 types of transformers and their roles
- Step up transformers: Increase the voltage by having more turns on the secondary coil than primary coil
- Step down transformers: Decrease the voltage by having more turns on the primary coil than secondary coil
State the factors which cause a transformer to have a decrease in efficiency
- Eddy currents
- Heat by resistance in the coils
- Heat by magnetising and demagnetising the core
- Magnetic flux loss over distance
What are eddy currents?
Looping currents induced by the changing magnetic flux in the core
How do eddy currents decrease efficiency in a transformer?
- The rapidly changing flux cuts the metallic core, inducing an e.m.f
- This e.m.f induces looping currents in the core
- The currents create a magnetic field which opposes the field which induced them, reducing field strength
- They also dissipate energy by generating heat
How can a transformer be adapted to reduce the effect of eddy currents?
By having layers of the core separated out by a thin insulator, so a current can’t flow (laminating the core)
How can a transformer be adapted to reduce energy lost by heat as resistance in the coils?
Use wires with low resistance, such as thick copper wire which has a low resistivity and a larger diameter meaning smaller resistance
How can a transformer be adapted to reduce the energy lost as heat by magnetising and demagnetise the core?
By using a magnetically soft material which magnetises and demagnetises easily, such as iron
How can a transformer be adapted to reduce the magnetic loss from the primary coil to the secondary coil?
By designing the core so that the coils are as close as possible
For the national grid, state the equation for the power losses due to resistance of the cables
P = I²R
Define Terminal Potential Difference
- Energy transferred from electrical energy to another type
- Work done per unit charge
