Section 11 - Magnetic Fields Flashcards
What is a magnetic field?
A region where a force is exerted on magnetic materials.
How can magnetic fields be represented?
Using field lines.
What is another name for magnetic field lines?
Flux lines
Which direction do magnetic field lines go?
North to south pole of a magnet (OUTSIDE of it).
How is the strength of a magnetic field represented using field lines?
The closer together the lines, the stronger the field.
What is a neutral point in magnetic fields?
Where magnetic fields cancel out, so there are no field lines.
What happens when a current passes through a wire?
A magnetic field is induced around the wire.
Describe the magnetic field induced around a current-carrying wire.
Concentric circles centred around the wire
What rule is used to work out the direction of the magnetic field around a current-carrying wire and how does it work?
• Right-hand grip rule
How it works:
• Thumb = Current (conventional)
• Fingers = Magnetic field
What happens to the shape of the magnetic field around a current-carrying wire when it is looped into a single coil?
Doughnut-shaped
What happens to the shape of the magnetic field around a current-carrying wire when it is looped into a solenoid?
Like a bar magnet.
What is the rule for working out the direction of the magnetic field around a current-carrying solenoid?
The right-hand rule, except the thumb and fingers are reversed.
What happens when you place a current-carrying wire in a magnetic field and why?
- The field around the wire and the magnetic field are added together
- This causes a resultant field so there is a force exerted on the wire
These bunched lines causes a pushing effect on the wire
What direction does the current have to be to the magnetic field lines?
Right angles - see in Flemings left hand
What rule is used to work out the direction of the force exerted on a current-carrying wire in a magnetic field and how does it work?
• Fleming’s Left-Hand Rule
How it works:
• Thumb = Force (motion) direction
• First finger = Magnetic field direction
• Second finger = Current (conventional) direction
What things is it important to remember when using Fleming’s left-hand rule?
- Magnetic field goes from north to south
* Current is conventional, so it goes from positive to negative terminals
When a current-carrying wire is in a magnetic field, what happens if the current is parallel to the field lines?
There is no force exerted.
What happens when you pass an AC current through a wire?
wire vibrates:
Direction of force in perpendicular to direction of current.
When current is reversed, direction of force is reversed.
Constant alternating current = constantly alternating direction of force
What is magnetic flux density?
The force on a current carrying wire at a right angle to a magnetic field is proportional to magnetic flux density, B.
So the definition is:
“The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.”
i.e. It is the magnetic field strength
In magnetic fields, what is B?
Magnetic flux density (a.k.a. Magnetic field strength)
What are the units for magnetic field density?
Tesla (T)
What is magnetic flux?
The number of field lines passing through an area.
It is how much the field is affecting the area.
In magnetic fields, what is φ?
Magnetic flux
What are the units for magnetic flux?
Wb
Is magnetic flux density scalar or vector?
Vector
Describe a experiment to investigate the effect of current on the force exerted on a current-carrying wire in a magnetic field.
1) Set up a top pan balance with a square loop of wire fixed to it, so that it is standing up and that the top of the loop passes through a magnetic field, perpendicular to it.
2) Connect the wire in a circuit with a variable resistor, ammeter and dc power supply. Zero the top pan balance when no current is flowing.
If the reading is negative swap the crocodile clips over
3) Vary the current using the variable resistor. At each current value, record the current and mass. Repeat 3 times at each current value and average.
4) Convert into force using F = mg.
5) Plot a graph of force F against current I. Draw a line of best fit.
6) Since F = BIl, the gradient of the rain is equal to B x l.
7) Alternatively, you could vary “l” by varying the length of wire that is perpendicular to the field or vary “B” by changing the strength of the magnets.
In the experiment to investigate F = BIl, how can you vary each of the variables?
What would need to keep constant when investigating length?
- B -> Use different magnets to vary field strength
- I -> Use variable resistor to vary the current
- l -> Use different loop sizes with different lengths perpendicular to the magnetic field (keep I constant with variable resistor, and also B)
What is 1 tesla equal to? Explain this.
1 Wb/m².
This is because magnetic flux density is the number of flux lines (Wb) per unit area.
Also 1 N/Am.
What is the equation for the force exerted on a current-carrying wire in a magnetic field?
F = B x I x l
Where: • F = Force (N) • B = Magnetic flux density (T) • I = Current (A) • l = Length of wire in the field (m)
(NOTE: This only applies when the current is at 90° to the magnetic field.)
Will a force still act on a wire even if it isn’t at 90 degrees?
Yes - but smaller (as long as it isn’t parallel)
What kind of force will F = BIl give you (For 90 degrees)?
Maximum force the wire could feel
Why does a current-carrying wire experience a force in a magnetic field?
A force act on the charged particles (electron) moving through it.
Do charged particles experience a force in a magnetic field?
Only if they are moving.
NOTE: Check this!
Derive the equation for the force on a charged particle in a magnetic field.
• F = B x I x l • I = Q / t • l = v x t Therefore: • F = B x (Q / t) x (v x t) • F = B x Q x v
When do F = BIl and F = BQv apply?
When the flow of charge is at 90° to the magnetic field.
And perpendicular to direction of travel (Flemings left hand)
And electron travels at a velocity of 2.00 x 10⁴ m/s perpendicular to a uniform magnetic field with a magnetic flux density of 2.00 T. What is the magnitude of the force acting on the electron?
F = BQv = 2.00 x (1.60 x 10⁻¹⁹) x (2.00 x 10⁴) = 6.40 x 10⁻¹⁵ N
What happens to moving charge in a magnetic field and why?
- They are deflected in a circular path
- Because Fleming’s left hand rule states that the force on a moving charge is always perpendicular to its direction of travel
How can Fleming’s left-hand rule be used for charged particles?
Like normal, except the second finger (normally used for current) is the direction of motion for a positive charge.
(i.e. This is intuitive)
When using Fleming’s left hand rule for moving charged particles, what happens if the charge is negative?
Point your second finger in the direction opposite to its motion.
How do you carry out an experiment investigating forces on a wire?
Is the force experienced by a moving charged particle in a magnetic field affected by mass?
What does depend on mass?
No, but the centripetal acceleration caused by it depends on the mass (since F = ma).
What is the equation for the force on a particle in circular orbit?
F = mv²/r
What is the equation for the radius of the circular path of a charged particle in a magnetic field?
r = mv/BQ
Where: • r = Radius (m) • m = Mass (kg) • v = Velocity (m/s) • B = Magnetic flux density (T) • Q = Charge (C)
Derive the equation for the radius of the circular path of a charged particle in a magnetic field.
Force on a charged particle in a magnetic field: • F = BQv Force on a particle in circular orbit: • F = mv²/r Therefore: • BQv = mv²/r • r = mv/BQ
How is a magnetic field going into the paper symbolised?
Circles with crosses in them
What happens to the radius of a charged particle moving in a magnetic field if the mass is increased?
Increases
What happens to the radius of a charged particle moving in a magnetic field if the velocity is increased?
Increases
What happens to the radius of a charged particle moving in a magnetic field if the magnetic flux density is increased?
Decreases
What happens to the radius of a charged particle moving in a magnetic field if the charge is increased?
Decreases
What does a cyclotron make use of?
A particle accelerator that makes use of the circular deflection of charged particles in a magnetic field.
What are some uses of cyclotrons?
- Producing radioactive tracers
* Producing high-energy beams of radiation for radiotherapy
What is the equation for the force experienced by a charged particle in a magnetic field?
F = BQv
Where: • F = Force (N) • B = Magnetic flux density (T) • Q = Charge on particle (C) • v = Velocity of particle (m/s)
What is the equation for frequency of rotation of charged particles?
What is is independent and dependent of?
Describe the structure of a cyclotron.
- Two hollow semi-circular electrodes with alternating potential difference.
- Slight gap between them.
- Uniform magnetic field applied perpendicular to the plane of the electrodes.
Describe how a cyclotron works.
- Particle (charged) is fired into one of the electrodes
- The magnetic field makes it flow a semi-circular path and return to the gap between electrodes
- The potential difference between them creates an electric field that accelerates the particle across the gap
- The velocity in now higher, so the particle takes a path with a larger radius before leaving the other electrode
- As it exits, the potential difference is reversed at this point so that the electric field is reversed and therefore the particle can accelerate across the gap
- This repeats as the particle spirals outwards, increasing in speed, before exiting the cyclotron
If p.d wasn’t alternating what would happen to the particle in a cyclotron?
It would slow down after leaving the second electrode
Since frequency of circular motion doesn’t depend on the radius, how long will the particle stay in each electrode?
What does this mean for the frequency of the alternating p.d?
Always spend the same time in the electrode (if B, m and Q are constant).
Alternating p.d has a fixed frequency
Remember to practise writing out the functioning and structure of a cyclotron.
Also research cyclotrons and alternating pd
What is the equation for magnetic flux?
φ = BA
Where:
• φ = Magnetic flux (Wb)
• B = Magnetic flux density (T)
• A = Area (m²)
What happens when a conductor is moved in a magnetic field?
If it cuts through field lines, an emf is induced in the conductor:
The electrons in the rod will experience a force, which causes them to accumulate at one end of the rod - this induces an emf across the ends of the rod = electromagnetic induction.
Why is an emf induced in conductor when it cuts through magnetic field lines?
- The electrons experience a force, so they accumulate at one end of the rod
- This induces an emf between the positive and negative ends of the rod
What is electromagnetic induction?
When an emf is induced in a conductor that cuts through magnetic field lines.
How can you induce an emf in a flat coil or solenoid?
- Moving the coil towards it away from the poles of the magnet
- Moving a magnet towards or away from the coil
An emf is induced in a conductor when what is changing?
The magnetic field
When is a current induced in a conductor that has had an emf induced in it?
When it is connected in a circuit.
What happens when the switch closes?
What will the ammeter do?
How do a change in magnetic field induce an emf and cause current flow?
A magnetic filed interacting with an electric field will generate a force.
When there is a change in magnetic field, force changes.
When force changes, energy changes.
This induces an EMF and current will flow.
What is the difference between flux and flux linkage?
Why is flux linkage used?
What is magnetic flux linkage?
The product of the magnetic flux passing through the coil and the number of turn in the coil they cut the flux.
What are the units for magnetic flux linkage?
Wb turns
What is the equation for magnetic flux linkage when the coil is perpendicular to the field?
Nφ = BAN
Where: • Nφ = Magnetic flux linkage (Wb turns) • B = Magnetic flux density (T) • A = Area of coil (m²) • N = Number of turns