6.3 Electromagnetism Flashcards
Magnetic fields are due to what?
moving charges or permanent magnets
What direction do field lines go in?
north to south
How do you know a field is uniform?
If field lines are equally spaced and in the same direction the field is uniform
What is the Magnetic field patterns for a long straight current-carrying conductor
Field lines are concentric circles centred on the wire
What is the Magnetic field patterns for a flat
coil?
Single coil-looks like: wire into page and out of page (circle with cross and dot) use right hand rule to know the field line directions
What is the Magnetic field patterns for a long solenoid?
Row of wires into page and out, with uniform looking field lines between. Use right hand rule to draw field lines.
What is Fleming’s left-hand rule?
Shows the direction of the force on a conductor carrying a current in a magnetic field.
Thumb: Force
First finger: Field
Second finger: Current
Techniques and procedures used to determine the uniform magnetic flux density between the poles of a magnet using a current-carrying wire and digital balance
Place the horseshoe magnet on a set of digital balance scales and zero the scales.
Then, connect a straight wire to a DC power supply, a variable resistor and an ammeter in series.
Align the wire so that the force produced on the wire
is upwards using Fleming’s left-hand rule. This will ensure that the force on the magnet is downwards by Newton’s third law of motion i.e. the force on the wire will produce a force equal in magnitude but opposite in direction of the magnet.
Also, measure the length of the wire that is in the field using a ruler. The scales will then register the magnetic force as weight and give a value for the extra mass it assumes has been place on the scales.
Record a series of values for the current and the mass registered by the scales. The current can be varied by altering the resistance of the circuit using the variable resistor.
Finally plot a graph of the mass registered versus the current in the wire. By balancing the forces
the magnetic flux density can be determined from the gradient. BL/g (g and L constant)
Define Magnetic flux density (B)
A measure of the strength of the magnetic field, defined by the equation for the force on a current carrying conductor in a magnetic field, F=BILsin(x)
How do you find the Force on a charged particle travelling at right angles to a uniform magnetic
field
F=BQv
What does Fleming left hand rule tell us about the motion of a charge particle in a uniform magnetic field?
magnetic field is always perpendicular to its direction of travel (mathematically a condition for circular motion), creates circular path
how do you derive r=mv/BQ
f=mv^2/r and F=BQv
What is a velocity selector?
Separate out particles of a certain velocity from a stream of accelerated charged particles at a range of speeds.
How do velocity selectors work?
By applying both a magnetic and electric field at the same time perpendicular to each other, while a stream of particles is fired perpendicular to both fields at a device with a collimator.
What are velocity selectors used for?
Mass spectrometer
Only particles with what velocity can pass through the collimator in a velocity selector?
v=E/B
What must you remember when looking at a velocity selector diagram?
Fleming left had rule
Like charges repel/opposites attract
Magnetic field from positive to negative
What is magnetic flux symbol and unit?
φ
unit: Wb
Define Weber
A change in flux linkage of one weber per second will induce an electromotive force of 1 volt in a loop of wire
Define Magnetic flux linkage
the product of the magnetic flux and the number of turns on the coil. Unit = Weber turns
Define Faraday’s law of electromagnetic induction
The induced e.m.f. is directly proportional to the rate of change of flux linkage
Define Lenz’s law
The induced e.m.f. is always in such a direction as to oppose the change that caused it.
What does lenz’s law say?
- If the og magnetic field is getting stronger, the induced magentic field will be in the opposite direction to try weaken it
- if the og magentic field is getting weaker, the induced magnetic field will be in the same direction to maintain it
Techniques and procedures used to investigate magnetic flux using search coils
- 2 bar magnets a small distance apart, opposite poles facing
- Get search coil, connect to data recorder to record induced emf with small time interval between
- place search coil in middle of magnetic field, so area is parallel to surface of magnet, immediately move coil out of field
- e.m.f. induced changing from max to zero.
- Plot graph of induced e.m.f against time
- Area under curve=total flux linkage change, so can find B by dividing by A & N
- repeat and find mean B.
What is a search coil?
Small coil of wire with a known number of turns (N) and known area (A)
How do generator, or dyanmos induce current?
Convert KE into electrical energy by rotating a coil in a magnetic field
when does the output of pd and current change for a generator?
with every half rotation of the coil, producing ac current
What do slip ring and brushes do?
Slip ring and brushes connect the coil to an external circuit
What is a transformer?
Devices that use electromagnetic induction to change the size of the voltage for an alternating current
Why should you use a laminated core?
reduces energy lost
does the national gird transmit current at high or low value?
Low, P=I^2R, so doubling current, quadruples power loss.
What contains transformers?
plug, to lower pd for appliance
How do you investigate the number of turns, voltage across the coils?
- 2 C-cores together, wrap wire around each to make coils. 5 tuns in primary, 10 turns in secondary (1:2) connected to low voltage ac supply (with voltmeter attached to each coil)
- turn on ac supply to primary coil, record voltage across each coil
- keep Vp the same, try 1:1 ratio, 2:1 ratio.
Divide ns by np and Vs
How do you investigate the number of turns, voltage and current of transformers in a circuit?
- 2 C-cores together, wrap wire around each to make coils. 5 tuns in primary, 10 turns in secondary (1:2) connected to low voltage ac supply (with voltmeter attached to each coil)
but also add variable resistor around primary coil and ammeter to both coils, - turn on power supply and record currents and voltages
- leave number of turns constant, adjust variable resistor to change input current. record voltages and currents, repeate for range of input current
- should find ns/np=vs/vp=is/ip
Define electric potential
Work done per unit charge in bringing a positive charge from infinity to the point
Define magnetic flux
Magnetic flux density * area normal to the field
