Magnetic Feilds

Question 1

Which fingers represent what in Flemings LHR and explain the symbol to show that current is either out or into the page

Answer 1

First finger= mag feild
2nd= current
Thumb = force
Each finger shld be at 90 degrees to eachother

A circle with a cross through means current is into the page
A circle with a dot = current out of the page

Question 2

Name 4 magnetic metals

Answer 2

Iron, steel, cobalt, nickel

Question 3

State 2 methods for producing a uniform magnetic feild

Answer 3

With 2 opposite bar magnets
And a solenoid ( multiple coils on a wire packed together )

Question 4

A solenoid produces a magnetic feild traveling from A to B what will happen to the magnetic feild pattern if current is both increased and reversed

Answer 4

The feild would be from B to A and the magnetic feild would get stronger therfore feild lines are closer together

Question 5

A student placed a permenant magnet ( has. A north and South Pole permanently fixed together) on a top pan balance. He clamped a straight peice of wire between the magnet so that it was suspended in the magnetic feild
When a current is passed through the wire the reading on the balance increased why?
(Current is out of page, feild is to the right)

Answer 5

The current creates a magnetic feild in the wire (if it was a rod , the feild would be around the rod) which interacts with the magnetic feild from the permanent magnet
Flemings LHR says the force on the wire is upwards so the force on the permanent magnet is downwards (N3) hence reading increased.

Question 6

Define magnetic flux density

Answer 6

The magnetic flux per unit area

Question 7

Define a tesla

Answer 7

The magnetic flux density that would give a 1N force on a 1m wire carrying a 1 A current, perpendicular to the feild

Question 8

Describe an experiment to show the validity of the equation F=BIL

Answer 8

Place a wire between a permenatn magnet and place the magnet on top of a balance. Connnect the wire to an ammeter in series and a variable resistor in series and a voltage supply.
Measure the length of the wire in the magnetic feild with a ruler
Use a set square to make sure that the current is entering the field at 90 degrees , camp the wire
Pass current through the wire and use an ammeter to measure the current
Measure mass using a balance F=mg
Change the current using a variable resistor and test 6 values, then record the reading on the balance, repeat 3 times and find a mean
Do this 6 times
Make sure balance is on 0 before starting
Then plot. A graph of F against I, straight line through origin

Question 9

a student uses a permanent mag and a top pan balance and current carrying wire, the current causes the reading on the balance to go from 85g to 85.4g, the poles on the mag are reversed ,what would be the new reading on the balance

Answer 9

change in mass due to current=+0 .4 g
therfore if poles are reversed new change in current= - 0.4g
therfore new reading=85-0.4 = 84.6g

Question 10

what is the meaning of the uniform in the phrase uniform magnetic feild

Answer 10

the mag feild lines are parallel and equally spaced

Question 11

what happens to the force prooduced on a wire in the magnetic feild if the poles wee placed further apart

Answer 11

the force will decrease as the magnetic feild strength decreases

Question 12

define a magnetic feild

Answer 12

a region of space in which moving charge particles are subject to a magnetic force

Question 13

what is a permanent magnet

Answer 13

an object made from magnetized material that creates its own persistent magnetic feild

Question 15

What direction do you use in Flemings left hand rule for the direction an electron is travelling

Answer 15

2nd finger point in opposite direction to path

Question 16

Describe the motion of an electron traveling perpendicularly into a uniform magnetic feild where the feild is into the page

Answer 16

Use flemmings LHR, current in opposit direction to electrons path, therfore the force acts perpendicular to to the electrons velocity at all times therfore has a circular path

Question 17

Derive an equation for the raduis of an electrons path in a magnetic feild (travels perp to feild)
Therefor explain what happens to radius as velocity increases

Answer 17

Circulator path therfore
Mv^2/r=Bev
Rearrange r=mv/Be
Radius will also increase if v increases

Question 18

Derive F=Bqv for an electron traveling perpendicular to a uniform magnetic feild

Answer 18

F=BIL
Sin theta=1 (sin 90 = 1)
L=Vt
I=Q/t
therefore F=Bqv

Question 19

Describe what a mass spectrometer does

Answer 19

It separates ions by mass
Ions are fed through a velocity selectors which uses electric and magnetic fields to separate particles by their velocities, and only the particles which satisfy Eq=bqv will make it through a straight line and reach the magnetic feild, (the slower and faster electrons will be deflected) which they are then separated by mass, as the velocity is now constant the haveier particles will have a larger radius and follow a circular motion until it hits a detector which will detect which particle hit it.

Question 20

A uniform extrication feild is produced with a p.d accross them, the plates are separated by 5cm and are 10 cm long , an electron with the horizontal velocity v passes through the feild , how can we find the horizontal distance at which it hits the bottom plate

Answer 20

First use E=V/d to find the electric feil
Then use E=F/Q to use the electrical force
Now use F=ma for acceleration
Then find time spent in the feild by using the hor distance and hor velocity as this is constant for the vertical and hor
Then use Suvat where u=0 to fins s