Magnetic Fields

Question 1

Define a Magnetic Field

Answer 1

A field surrounding a permanent magnet or a current-carrying conductor in which magnetic objects experience a force

Question 2

Describe Magnetic Field lines

Answer 2

Arrows point from north to south
Equally spaced and parallel magnetic field lines represent a uniform field - strength of the magnetic field does not vary
Field is stronger when the magnetic field lines are closer
Like poles repel and unlike poles attract

Question 3

What happens when a wire carries a current

Answer 3

A magnetic field is created around the wire, the field is created by the electrons moving within the wire.

Any charged particle that moves creates a magnetic field in the space around it

Question 4

What are magnetic field lines for a current-carrying wire

Answer 4

Concentric circles centred on the wire and perpendicular to it - right hand grip rule can be used to determine the direction of magnetic field

Thumb points in the direction of the conventional current

Question 5

Magnetic field lines in a coil

Answer 5

North and south poles at the opposite faces

Refer to text book to check descriptions

Question 6

What is the strength of magnets and magnetic fields measured in

Answer 6

Tesla (T)

Question 7

What happens when a current-carrying conductor is placed in an external magnetic field

Answer 7

Two fields interact just like the fields of two permanent magnets, they experience equal and opposite forces

Question 8

What happens when a current-carrying wire is placed between the poles of a magnet

Answer 8

The direction of the force experienced by a current-carrying conductor placed perpendicular to the external magnetic field can be determined using Fleming’s Left Hand rule

Question 9

Flemings Left Hand Rule

Answer 9

Fuck - Force
Man - Magnetic Field
City - Current

Question 10

What is the magnitude of the force experienced by a wire in an external magnetic field dependent on

Answer 10

Force is directly proportional to current, length, sin theta and strength of the magnetic field

F = BILsin(theta)

Question 11

What is Magnetic Flux Density

Answer 11

Strength of a magnetic field
Measured in Tesla

Question 12

Is magnetic flux density a vector or scalar

Answer 12

Vector

Question 13

Motion of a charged particle in a magnetic field

Answer 13

Circular

Straight line once out of the field

Question 14

Why does a current-carrying wire experience a force in a magnetic field

Answer 14

Each electron in the wire experiences a tiny force

Question 15

What is the force on a charged particle in a magnetic field

Answer 15

Perpendicular to its velocity, therefore, has no effect on velocity

F = BQV

Question 16

What can = force on an electron in a magnetic field

Answer 16

mv^2 / r

Question 17

What is radius of a charged particles circular path proportional to

Answer 17

velocity and mass

Question 18

What is radius of a charged particles circular path inversely proportional to

Answer 18

Magnetic fields, charge

Question 19

How to induce an emf

Answer 19

A coil, magnet and sensitive voltmeter are needed.

Attach the voltmeter to a coil.

When the magnet is pushed towards the coil, an emf is induced across the ends of the coil

When the magnet is pulled away, a reverse emf is induced.

Repeatedly pushing and pulling will induce an alternating current.

Faster the magnet is moved, the larger the induced emf.

Question 20

Explain EM Induction

Answer 20

Energy is always conserved.

Some of the work done to move the magnet is transferred into electrical energy. The motion of the coil relative to the magnetic field makes the electrons move because they experience a magnetic force Bqv.

The moving electrons constitute an electrical current within the coil, so the process has produced electrical energy

Question 21

Define Magnetic Flux

Answer 21

Product of the component of the magnetic flux density perpendicular to the area and the cross-sectional area

Magnetic Flux = BAcos(theta)

Units = Wb

Question 22

What is Magnetic Flux linkage

Answer 22

Product of the number of turns in the coil and the magnetic flux

Question 23

When is an emf induced in a circuit

Answer 23

When there is a change in the magnetic flux linking the circuit

Question 24

What is Faradays Law

Answer 24

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

The constant of proportionality is -1

Question 25

What happens when a magnet is moved towards a coil with no voltmeter connected across it

Answer 25

If the magnet is moved towards the coil, the induced current is such that the end facing the magnet is north pole - work needs to be done in order to push the magnet towards the coil. If the end was a south pole, then principal of conservation of energy would be violated, as the attraction between the coil and the magnet creating electrical energy out of nowhere Work done is equal to the electrical energy produced in the coil

Question 26

What happens when a magnet is moved away from a coil with no voltmeter connected across it

Answer 26

The motion of the magnet must once again be opposed so that you do work. The end the magnet is moving away from becomes the south pole

Question 27

What is Lenz's Law

Answer 27

Direction of the induced emf or current is always such as to oppose the change producing it

Question 28

What is Lenz's Law

Answer 28

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

Question 29

Explain how an AC generator works using Faraday's Laws

Answer 29

Has a flux linkage As the coil rotates at a steady frequency, the flux linkage changes with time t. This variation is sinusoidal and is caused by the changing cos(theta) For a given generator, induced emf is directly proportional to rate of change in cos(theta)

Question 30

What does a simple transformer consist of

Answer 30

A laminated iron core, a primary (input) coil and a secondary (output) coil

Question 31

How do transformers work

Answer 31

An alternating current is supplied to the primary could This produces a varying magnetic flux in the soft iron core The secondary coil is linked by this changing flux The iron core ensures that all the magnetic flux created by the primary coil links the secondary could and none is lost A varying emf is produced across the ends of the secondary coil

Question 32

Describe a step-up transformer

Answer 32

Has more turns on the secondary than on the primary coil

Question 33

Describe a step-down transformer

Answer 33

Fewer turns on the secondary coil than on the primary coil

Question 34

How can transformers be made efficient

Answer 34

Low-resistance windings help reduce power losses due to the heating effect of the current Making a laminated core with layers of iron separated by an insulator helps to minimise currents induced in the core itself (eddy currents) - minimises loss due to heating

Question 35

What is a velocity selector

Answer 35

Device that uses both electric and magnetic fields to select charged particles of specific velocity Two parallel horizontal plates connected to a power supply produce a uniform electric field of field strength E between the plates. A uniform magnetic field of flux density B is also applied perpendicular to the electric field. Charged particles travelling at different speeds that are to be sorted enter through a narrow slit Y. Electric and Magnetic Field Lines deflect them in opposite directions - only for particles with a specific speed v will these deflections cancel so that they travel in a straight line and emerge from a narrow slit Z For an undeflected particle - Electric force = Magnetic force EQ = BQv v = E / B