Module 6.3 - Electromagnetism (Basically done)

Question 1

In which direction do field lines go?

Answer 1

Field lines go from north to south

Question 2

What creates magnetic fields?

Answer 2

Magnetic fields can be created by:

• Permanent magnets
• Moving charges

Question 3

What is a solenoid?

Answer 3

Just a lot of loops of wire designed to be used as an electromagnet (best description possible right now)

Question 4

What does each finger represent in Fleming’s left hand rule?

Answer 4

Thumb - Force
First finger - Magnetic field
Third finger - Current

Question 5

Why does a current flowing through a conductor in a magnetic field case a force to act on the conductor?

Answer 5

The current generates a field around the coil of wire. This field interacts with the external magnetic field causing a force on the wire and (equal but opposite force) magnet

Question 6

Define magnetic flux density

Answer 6

Magnetic flux density is the force on one metre of wire carrying a current of 1 amp at right angles to the magnetic field. (measures strength of magnetic field)

Question 7

What is one tesla (T) equivalent to?

Answer 7

one tesla = 1 webber per square metre

Question 8

Why does the force on a current carrying wire decrease as the wire is rotated (from perpendicular to parallel)?

Answer 8

The force on a current carrying wire in a magnetic field is due to the magnetic field of the wire and fixed magnetic field interacting. This only occurs when the two fields are perpendicular to each other, rotating the wire causes a smaller vector component of the fixed magnetic field to be parallel to that of the wire, so a lower force.

Question 9

What is the equation for force on a moving particle in a magnetic field?

Answer 9

F = BQv

Question 10

How do you derive the equation F=BQv?

Answer 10

Equation for force on a current carrying wire is
F = BIL
I = Q/t > F = BQL/t
t = L/v > F = BQv

Question 11

Derive an equation for the radius of curvature for a charged particle travelling through a magnetic field? (perpendicular to its direction of motion)

Answer 11

1) F = mv^2/r and F = BQv
2) Set F equal
3) Re-arrange

Question 12

What is a velocity selector?

Answer 12

Velocity selectors are used to separate out particles of a certain velocity from a stream of accelerated charged particles moving at a range of speeds

Question 13

How does a velocity selector work?

Answer 13

1) In a velocity selector both a magnetic and electric field are applied perpendicular to each other at the same time
2) Particles in the selector experience opposing forces from electric and magnetic fields
3) Particles with a greater velocity will experience a greater force due to the magnetic field (but the same force from the electric)
4) If the forces cancel out the particles will pass on through the gap in the collimator. If the velocity is slightly different the path of the particle will curve and hit the collimator

The electric and magnetic fields interact with the charge of the particles equally, so velocity is the only property of the particle affecting its path. (velocity only magnetic)

Question 14

State Faraday’s law

Answer 14

The magnitude of the induced e.m.f is directly proportional to the rate of change of flux linkage

Question 15

How is e.m.f induced in a conducting rod moving through a magnetic field?

Answer 15

If a conducting rod is moving through a magnetic field, the electrons in the rod will experience a force. This causes them to accumulate at one end of the rod inducing an e.m.f across its ends.

Question 16

What does the induced e.m.f in a conducting rod moving through a magnetic field mean?

Answer 16

If the rod is connected to a circuit (in series) a current will flow through it.

Question 17

What is magnetic flux density?

Answer 17

Magnetic flux density (B) is a measure of the strength of a magnetic field

Question 18

What is the equation for total magnetic flux passing through an area?

Answer 18

Φ = BAcos(θ)
(measured in weber, Wb)

θ is the angle between magnetic flux and normal to the plane

flux linkage = NΦ

Question 19

State the equation for Faraday’s law

Answer 19

Induced e.m.f = - (NΔΦ)/Δt

Question 20

Define the weber

Answer 20

A change in flux linkage of 1 weber per second will induce an electromotive force of 1 volt in a loop of wire

Question 21

State Lenz’s law

Answer 21

The direction of any induced e.m.f is in a direction as to oppose the flux change that caused in (hence the - sign in Faraday’s law)

Question 22

How does a mass spectrometer work?

Answer 22

Ions with the same velocity are made to enter a magnetic field. This deflects them on a curved path towards a detector, the radius of curvature depending on the charge and mass of the particles.

Question 23

What does the gradient of a NΦ against time graph tell you?

Answer 23

e.m.f = - gradient

(from e.m.f = - (NΔΦ)/Δt)

DON’T FORGET - !!!

Question 24

What does the gradient of an e.m.f against time graph tell you?

Answer 24

Area under graph = - change in flux linkage (ΔNΦ)

Question 25

What is the equation for e.m.f induced in a rod moving through a uniform magnetic field

Answer 25

e = -Blv
(l = length)

Question 26

Where does the equation e = - Blv come from?

Answer 26

Faraday’s law states that the magnitude of the induced e.m.f is directly proportional to the rate of change of flux linkage. e = - ΔΦ/Δt
Φ = BA and area cut by the wire is vΔt * l
Φ = BvΔtl
e = - (BvΔtl)/Δt
e = - Bvl = -Blv

Question 27

The area of a flat coil is perpendicular to a magnetic field which collapses by 50%. What will be the direction of the current induced in the loop?
l Coil

Answer 27

A collapsing field is getting weaker, so by lenz’s law the induced current will induce a magnetic field in the same direction as the weakening magnetic field to try and preserve it. Using the right hand rule you can figure out which way the current flows

Question 28

In which direction will the magnetic field induced by the induced current in a coil act according to lenz’s law if:

1) The original magnetic field (which induced the current in the coil) gets stronger
2) The original magnetic field gets weaker

Tis pretty confusing to explain

Answer 28

• If the original magnetic field is getting stronger the induced magnetic field will be in the opposite direction to try and weaken it
• If the original magnetic field is getting weaker the induced magnetic field will be in the same direction to try maintain it

Question 29

How do you find the direction of the induced e.m.f / current in a conducting rod moving through a magnetic field (at right angles to the field)?

Answer 29

Lenz’s law says that the induced current will produce a force to oppose the motion of the conductor, a restive force. You can use Fleming’s left hand rule with the force being the direction of the restive force (opposite to the direction of motion!!!). You know the direction of the fixed magnetic field, so direction of current = direction of e.m.f.

Question 30

What is the name of an AC generator?

Answer 30

An alternator is an ac generator

Question 31

State the transformer equation

Answer 31

ns/np = Vs/Vp = Ip/Is

I is OPPOSITE to V !!!

Question 32

How does an AC generator work?

Answer 32

An AC generator consists of a rotating coil of wire in a permanent magnetic field. When the wire is turned it moves through the magnetic field which induces an e.m.f across it. As coil is connected to an external circuit it causes a current to flow through it. This current will be at a maximum when the normal to the coil is perpendicular to the magnetic field lines and 0 when it is parallel. Every half turn the current swaps direction so an ac current is generated.

MUST FIXO SOONO

Question 33

How does a transformer work?

Answer 33

1) An alternating current passes through the primary coil inducing a changing magnetic field alternating in direction in the iron core
2) This changing magnetic field is passed through the iron core to the secondary coil
3) The changing magnetic field induces an alternating e.m.f of the same frequency in the secondary coil
4) If the coil is connected an ac current 90* out of phase will be induced in it
5) The ratio of the number of turns in each coil determines the magnitude of the e.m.f induced in the secondary coil

Question 34

How do the number of turns in the primary and secondary coils compare for a:

1) Step-up transformer?
2) Step-down transformer?

Answer 34

Step-Up:
Ns > Np
Step-Down:
Ns < Np

Question 35

Why do transformers use a laminated iron core rather than a solid one?

Answer 35

It reduces energy losses in the form of heat due to eddy currents

Question 36

Why are transformers used in the national grid?

Answer 36

By stepping up the voltage a lower current is needed in the wires carrying electricity around the grid. Electricity can be sent around the national grid with much lower losses to heat.

P = I^2 R

Question 37

What are some uses for transformers?

Answer 37

• In the national grid

- To step down 230V ac to much lower voltage for charging and powering electronic devices