Module 6 Magnetic Fields

Question 1

What is the symbol used for magnetic fields?

Answer 1

B fields

Question 2

What are the two ways of making a magnetic field?

Answer 2

With a permanent magnet

Moving chargers such as electric currents through wires

Question 3

What is the direction of magnetic field lines?

Answer 3

north to south

Question 4

How do you draw a magnetic field or current at 90º into a piece of paper?

Answer 4

a circle with a cross in it

Question 5

How do you draw a magnetic field or current at 90º out of a piece of paper?

Answer 5

circle with a dot

Question 6

How to remember which way is which for circles with dots and crosses when showing directions in and out of paper?

Answer 6

cross is into paper as that’s what you would see if you fired an arrow into a paper

Question 7

What is Maxwell’s right hand grip rule?

Answer 7

make a thumbs up. Direction of thumb is direction of current in a wire and fingers are direction of magnetic fields around the wire

Question 8

How can RH grip rule be used for solenoids?

Answer 8

thumb is direction of magnetic field, fingers are direction of current in the wire

Question 9

What is a solenoid?

Answer 9

a coil of wire with length much greater than the diameter

Question 10

How can the B field inside a solenoid be described?

Answer 10

strong and uniform

Question 11

What is a flat coil?

Answer 11

literally a single coil of wire

Question 12

What happens when a current carrying wire which is at 90º to a magnetic field?

Answer 12

wire will experience a force at 90º to both

Question 13

What is the formula for the force exerted on a wire by a magnetic field?

Answer 13

F=BILsinθ

Question 14

What are the units for magnetic fields?

Answer 14

Tesla (T)

Question 15

Define the tesla?

Answer 15

a flux density of one Tesla produce a force of 1 Newton on a wire of length 1 metre carrying a current of 1 Ampere at 90º to the field

Question 16

How to remember which finger represents what In Fleming’s left hand rule?

Answer 16

F B I thumb first second just like in the equation

Question 17

What happens if the magnetic field and current are in opposite directions?

Answer 17

no force is generated

(Current parallel to B field)

Question 18

What is another name for magnetic field strength?

Answer 18

magnetic flux density

symbol B (in Tesla)

Question 19

Describe an experiment to measure magnetic flux density

Answer 19

Place two magnetic poles at 90º to a current carrying wire over an electronic balance

Zero the balance once current is zero

Power supply is turned on to increase current which is measured on the ammeter

For each current measured the mass reading is taken on the electronic balance

Length of the wire inside the field is measured

Plot a graph of force against current, gradient = BL

Question 20

Describe how an electric motor works

Answer 20

a square wire with a current flowing through it is placed into a magnetic field

A force is exerted onto the wires which are at 90º to the magnetic field

This is considered a couple

Makes wire rotate (couple causes torque)

Question 21

Show that the torque of a wire in an electric motor is = BI(area)

Answer 21

Torque = Fd with F = BIL

Torque = BILd

Question 22

When using Flemings left hand rule for a moving negative particle, which direction is the current taken as?

Answer 22

opposite to velocity of negatively charged particle (ie conventional current)

Question 23

Explain why the motion of a moving charge in a magnetic field can be considered as circular motion?

Answer 23

F remains at 90º to direction of motion

(no displacement in direction of the force, no work done, constant KE and speed)

Question 24

Explain how the formula for centripetal force can be used with F = BQv

Answer 24

F = mv^2/r
This is the same F as in F = BQv

As F remains at 90º to direction of motion and thus can be considered a centripetal force

Question 25

What is another notation for F = BQv for an electron?

Answer 25

F = Bev

Question 26

What is a velocity selector?

Answer 26

When a moving charged particle

Passes through a B field and E field which are perpendicular to each other

And experiences no deflection if v=E/B

Question 27

Derive is the formula for a velocity selector?

Answer 27

BQv=EQ

Q’s cancel out

Bv=E

v = E/B

Question 28

Draw the arrangement of magnetic field and metal plates for electric fields to produce a velocity selector (for a negative particle)

Answer 28

Force from magnetic field B is vertically downwards
Force from electric field E is vertically upwards

Question 29

What are the three stages to mass spectrometer?

Answer 29

Acceleration of charged particle
Velocity selector
Deflection

Question 30

What happens during acceleration in mass spectrometer?

Answer 30

Acceleration of charged particles

QV = 1/2mv^2 so ions gain a speed towards a positive anode

Some ions pass through holes in an anode to form a narrow beam

Question 31

What happens during the velocity selector section of the mass spectrometer?

Answer 31

Crossed E and B fields

No deflection for ions whose speed is v = E/B

Question 32

What happens during deflection section of the mass spectrometer?

Answer 32

Only a magnetic field is present
Ions that pass through have a circular path
BQv = mv^2/r

Question 33

What does the value of r radius and d diameter depend on in the final stage of mass spectrometer?

Answer 33

mass of ion, magnetic flux density

and speed (although all ions have the same speed after the velocity selector)

Question 34

When drawing concentric rings around a wire what must you do?

Answer 34

Use the right hand grip rule to find direction of B field

Rings get further apart as B field gets weaker