Chapter 23 - Magnetic Fields

Question 1

Magnetic field lines

Answer 1

Much the same as electric field lines, point from north to south
Direct between, curve around the edges
Can be plotted using a plotting compass

Question 2

Magnetic metals

Answer 2

Iron, cobalt, nickel

Question 3

Magnetic field lines around conductors

Answer 3

Act around a wire carrying a conventional current
Circles that are closer nearer the wire as they are stronger
Use right hand grip rule for direction, thumb is current, curved fingers is field direction
Stronger with a greater current / coiled wire

Question 4

Current into plane and out of plane symbols

Answer 4

In: circle with an x in it
Out: circle with a point in it

Question 5

Coil field lines

Answer 5

Small circle around the sides, gradually flattening curves as you move inwards, straight line through the centre
Use RHG rule for direction

Question 6

Solenoid field lines

Answer 6

Uniform lines through the middle, curving wider each line from north to south outside.

Question 7

Fleming’s left hand rule

Answer 7

Thumb: Force
Index: Field direction
Middle: Current

Question 8

Wire in a field force equation

Answer 8

F = BILsin(θ)

where B is the magnetic flux density in T and θ is the angle between the magnetic field and right angles to the current

Question 9

Experiment for B

Answer 9

• Clamp a wire over a top-pan balance between two magnets
• Set the balance to 0 when the current in the wire is 0
• As the current flows, the wire experiences an upward force so the magnets experience a downward force (Newton’s third law)
• Vary the current and measure the force (mg)
• Plot graph of F against I and multiply the gradient by 1/l for the force (l is measured with a ruler)

Question 10

Electron deflection tube

Answer 10

Thermionic emission causes electrons to be emitted and move into the magnetic field
A force acting perpendicular to the motion acts on the electron (constant speed)
There is a screen where you can observe the motion

Question 11

Magnetic force on a charged particle

Answer 11

F = BQv

Question 12

Circular motion on a charged particle

Answer 12

Force is perpendicular to velocity so follows a circular path
Equate force equations for r = mv/BQ

Question 13

Velocity selector

Answer 13

Two plates with slits and a vacuum between with a potential difference between the plates and a magnetic field
V and B can be adjusted slowly so only one velocity passes through the slit in the second plate

Question 14

Velocity selector equation

Answer 14

v = E/B

Question 15

How to generate an electric current using a magnet

Answer 15

Move a magnet relative to a coil of wire so the field lines intersect the coil at different points

Question 16

Why does moving a magnet generate current

Answer 16

Work is done to move the magnet, some of this is transferred to electrical energy
Using F=Bev, the relative motion of the coil and the magnet cause the electrons to move

Question 17

Magnetic flux definition

Answer 17

The component of the magnetic flux density perpendicular to the area

Question 18

Magnetic flux and magnetic flux linkage units

Answer 18

Webers (Wb)

Question 19

Magnetic flux formula

Answer 19

Φ = BA cos(θ) where θ is the angle between the field lines and the normal to the surface

Question 20

Magnetic flux linkage formula

Answer 20

NΦ

The number of coils x magnetic flux

Question 21

Transformer process

Answer 21

1. Pass an AC to the primary coil
2. This produces a changing magnetic flux in the laminated iron core
3. The coils are linked by the changing flux of the magnetic field
4. Faraday’s Law states this produces a changing emf in the secondary coil

Question 22

Transformer equation

Answer 22

n(s)/n(p) = V(s)/V(p)

Question 23

Transformer if 100% efficient

Answer 23

V(s)/V(p) = I(p)/I(s)

Question 24

Faraday’s law definition

Answer 24

The magnitude of the induced emf is directly proportional to the rate of change of the magnetic flux linkage

Question 25

Faraday’s law formula

Answer 25

ε = -Δ(Nφ)/Δt

Question 26

Lenz’s Law

Answer 26

The direction of the induced emf or current is always such to oppose the change producing it

e.g. move a magnet towards, near side has the same polarity

Question 27

AC Generator

Answer 27

• Coil rotates at a steady speed
• Flux linkage changes with time
• Gradient of Nφ is emf
• ε ∝ Δ(cosθ)/t

Question 28

Where does a magnetic force act in relation to the field?

Answer 28

Perpendicular