Magnetic fields

Question 1

Magnetic field

Answer 1

a field of force that is created by moving electric charge or permanent magnets

Question 2

Direction of magnetic field lines

Answer 2

Always north -> south
- the field lines are stronger when the lines are closer, and weaker when the lines are further apart

Question 3

Uniform magnetic field

Answer 3

the magnetic field strength is the same at all points
- represented by equally spaced parallel lines

Question 4

Dots and crosses

Answer 4

Dots - the magnetic field OUT of the page
Crosses - the magnetic field INTO the page

Question 5

Current-carrying conductor

Answer 5

• produces its own magnetic field
• experiences a force when PERPENDICULARLY interacting with an external magnetic field

Question 6

Magnetic flux density

Answer 6

F = BILsinΘ

B = F / IL

the force acting per unit current per unit length on a wire placed at right angles to the magnetic field

SI unit: Tesla (T)

Question 7

F = BILsinΘ shows that…

Answer 7

the greater the current or magnetic field strength, the greater the force on the conductor

Question 8

When does F = BILsinΘ reach maximum force?

Answer 8

maximum: sinΘ = 1 / 90°
- this is when the conductor is perpendicular to the B (magnetic) field
- hence… F = BIL

minimum: sinΘ = 0 / 0°
- this is when the conductor is parallel to the B field
- hence F = 0

Question 9

Flemings left hand rule

Answer 9

Thumb = motion / force
Pointer = magnetic field
Middle finger = current (direction of the current is the direction of conventional current flow)

Question 10

Tesla

Answer 10

a straight current carrying conductor carrying a current of 1A normal to a magnetic field of flux density 1T with force per unit length of the conductor of 1Nm^-1

Question 11

Force on a moving charge equation

Answer 11

F = BQvsinΘ

Q -> charge of the particle

Question 12

Equivalent to the force on a wire, if the magnetic field B is perpendicular to the direction of the charge’s velocity, the equation simplifies to:

Answer 12

F = BQv

Question 13

What does the equation F = BQv show?

Answer 13

If the direction of the electron changes, the magnitude of the force will change too

the force due to magnetic field is always perpendicular to the velocity of the electron

Question 14

Hall voltage

Answer 14

The potential difference produced across an electrical conductor when an external magnetic field is applied perpendicular to the current through the conductor

Question 15

Explain the hall effect

Answer 15

• When an external magnetic field is applied perpendicular to the direction of current through a conductor, the electrons experience a magnetic force
• This makes them drift to one side of the conductor, where they all gather and becomes more negatively charged
• This leaves the opposite side deficient of electrons, or positively charged
• There is now a potential difference across the conductor
• This is called the Hall Voltage, VH

Question 16

Direction of velocity, electric force and magnetic force of an electron

Answer 16

Velocity = forward
Electric force = upwards from electron
Magnetic force = downwards from electron

Question 17

Electric field strength of hall effect equation

Answer 17

E = Vh / d

Question 18

Derivation of the Hall voltage (Vh) equation

Answer 18

1. Fb = BQv and Fe = QE
2. Therefore, QE = BQV
3. Since E = VH/d , and cancelling the Q, hence Vh / d = Bv
4. Since I = nAvq, v = I/nAq
5. …therefore Vh/d = (B)(I/nAq)
6. Since A (cross sectional area) is the product of the width (d) and the thickness (t), A = dt
7. Vh/d = (B)(I/n (dt) q)

Vh = (B)(I/ntq)

Question 19

What is a hall probe used for?

Answer 19

To measure the magnetic flux density between two magnets based on the Hall effect

Question 20

How to use a hall probe to measure hall voltage?

Answer 20

1. The flat surface of the probe must be directed between the magnets, perpendicular to the magnetic field lines
2. The probe is connected to a voltmeter to measure hall voltage
3. Hall voltage depends on the angle between the magnetic field and the plane of the prob (maximum = perpendicular, minimum = parallel)

Question 21

Describe the path of a charged particle in a uniform magnetic field. Explain why.

Answer 21

A charged particle in uniform magnetic field which is perpendicular to its direction of motion travels in a CIRCULAR path

• This is because the magnetic force FB will always be perpendicular to its velocity v
• FB will always be directed towards the centre of the path, hence provides CENTRIPETAL FORCE

Question 22

Equation of centripetal force

Answer 22

F = mv^2 / r

Question 23

Derivation of the equation for the radius of the orbit of a charged particle in a perpendicular magnetic field

Answer 23

mv^2/r = BQv

r = mv / BQ

Question 24

Velocity selector

Answer 24

A device consisting of perpendicular electric and magnetic fields where charged particles with a specific velocity can be filtered

Question 25

Describe the construction of a velocity selector

Answer 25

1. Consists of two horizontal oppositely charged plates situated in a vacuum chamber
2. The plates provide a uniform electric field with strength E between them
3. There is also a uniform magnetic field with flux density B applied perpendicular to the electric field
4. If a beam of charged particles enter between the plates, they may all have the same charge but travel at different speeds v

Question 26

What happens when Fe = Fb on the charged particles in a velocity selector?

Answer 26

The particles travelling at the desired speed v will travel through undeflected due to the equal and opposite electric and magnetic forces on them

Question 27

Velocity v in a velocity selector equation

Answer 27

EQ = BQv

THEREFORE: v = E/B

Question 28

What happens when a particle has a speed greater or less than v?

Answer 28

The particle