6.3 Electromagnetism

Question 1

What is a magnetic field

Answer 1

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

Question 2

How are magnetic fields created

Answer 2

A moving charge

Question 3

What does the arrow on a magnetic field line tell you

Answer 3

The direction in which a free North Pole would move - the arrow points from north to south

Question 4

What does it mean if magnetic field lines are parallel and equally spaced

Answer 4

Represents a uniform field

Question 5

How can you tell where the field is the strongest

Answer 5

When the magnetic field lines are closer

Question 6

What are the rules of repulsion and attraction of magnetic poles

Answer 6

Like poles repel and unlike poles attract

Question 7

What is the right hand grip rule

Answer 7

A way to determine the direction of the magnetic field

Question 8

What does the thumb represent in the right hand grip rule

Answer 8

The thumb point in the direction of the conventional current

Question 9

What do the fingers represent in the right hand grip rule

Answer 9

The direction of the field is given by the direction in which the fingers curl around the wire

Question 10

How does the magnetic field pattern of the solenoid change if the direction of the current is reversed

Answer 10

The direction of the field is reversed

Question 11

What happens when you place a current-carrying conductor in an external magnetic field

Answer 11

The two fields interact just like in the fields of two permanent magnets- they experience equal and opposite forces

Question 12

What is Flemings left hand rule

Answer 12

If fingers are positioned perpendicular to eachother, the thuMb will represent Motion, the First finger will represent Field and the seCond finger represents Current

Question 13

What does the magnitude of the force experienced by a wire in an external magnetic field depend on

Answer 13

Current, Length of wire in the field, sin θ (θ= angle between field and current direction), strength of field

Question 14

When’s the force on a current carrying wire in an external maximum and when is it zero

Answer 14

The force is maximum when the wire is perpendicular to the field and zero when parallel to the field

Question 15

State the equation for the force on a current carrying wire in an external field

Answer 15

F=BILsinθ (force= magnetic flux density x current x length of wire x sin(angle between field and current direction))

Question 16

State the equation for the force on a current carrying wire perpendicular to the field

Answer 16

F=BIL

Question 17

What are the units of magnetic flux density

Answer 17

Tesla (T)

Question 18

How is magnetic flux density defined

Answer 18

The strength of the field

Question 19

What are the SI units for magnetic flux density

Answer 19

Nm^-1A^-1

Question 20

Is magnetic flux density a vector or scalar quantity

Answer 20

Vector

Question 21

What happens when a charged particle moves in a magnetic field

Answer 21

The particle will experience a force

Question 22

How can you determine the direction of the force on a charged particle moving in a magnetic field

Answer 22

Fleming’s left hand rule

Question 23

why does the force on the beam of electrons in an electron gun with a visible screen is directed downwards

Answer 23

As the electrons enter the field, they experience a downwards force which always remains perpendicular to its velocity

Question 24

Describe the path of a beam of electrons travelling perpendicular to a magnetic field

Answer 24

Electrons travel in a circular path in the region of the field then keep moving in a straight line once out of the field

Question 25

why does the speed of the electrons remains unchanged when travelling perpendicular to a field

Answer 25

Because the force has no component in the direction of the motion

Question 26

State the equation for the force on a charged particle travelling perpendicular to a magnetic field

Answer 26

F=BQV (force= magnetic flux density x charge x speed of charged particle)

Question 27

What’s the expression for the radius of the circular path taken by charged particles moving perpendicular to a field

Answer 27

r=mv/BQ (radius= mass x velocity/ magnetic flux density x charge)

Question 28

do faster moving particles move in bigger or smaller circles

Answer 28

faster moving particles travel in bigger circles (r α v)

Question 29

do more massive particles move in bigger circles

Answer 29

more massive particles move in bigger circles (r α m)

Question 30

do particles with a greater charge move in smaller or bigger circles

Answer 30

particles with a greater charge move in smaller circles ( r α 1/Q)

Question 31

do stronger magnetic fields make the particles move in smaller or bigger circles

Answer 31

stronger magnetic fields make the particles move in smaller circles (r α 1/B)

Question 32

what equations are useful when solving problems including charged particles in circular paths

Answer 32

F=BQv F=ma F=mv^2/r F=2πr/T

Question 33

why is a velocity selector required in a mass spectrometer

Answer 33

so that the atoms emerge with the same speed v before entering a uniform magnetic field

Question 34

describe how a velocity selector works

Answer 34

-two parallel plates produce uniform electric fields - uniform magnetic field is applied perpendicular to the electric field - charged particles enter through a slit -if they travel at the right speeds the deflections cancel and they emerge from the second narrow slit

Question 35

what is electromagnetic induction

Answer 35

electromagnetic induction occurs whenever there is a change in the magnetic flux linking the circuit

Question 36

describe what happens when a magnet is pushed towards a coil of wire

Answer 36

an emf is induced across the ends of the coil

Question 37

describe what happens when a magnet is pulled away from a coil of wire

Answer 37

a reverse emf is induced

Question 38

what happens when a magnet is repeatedly pushed and pulled in a coil of wire

Answer 38

an alternating current will be induced in the coil- the faster the magnet is moved the larger the emf is

Question 39

how is energy conserved when a magnet is pushed and pulled in a coil of wire

Answer 39

some of the work done to move the magnet is transferred into electric energy

Question 40

define magnetic flux

Answer 40

the product of the component of the magnetic flux density perpendicular to the cross sectional area

Question 41

explain why a changing magnetic field induces a current in a wire

Answer 41

the motion of the coil (and the electrons in it) relative to the magnetic field makes the electrons move because they experience a magnetic force given by BEv. the moving electrons constitute an electrical current within the coil

Question 42

how is magnetic flux calculated

Answer 42

Φ=B x A Φ=B x A cosθ

Question 43

whats the SI unit of magnetic flux

Answer 43

Wber (Wb) -> Tm^2

Question 44

what is the value of the magnetic flux when the magnetic field is normal to the area being considered

Answer 44

B x A

Question 45

what is the value of the magnetic flux when the magnetic field is parallel to the area being considered

Answer 45

zero

Question 46

define magnetic flux linkage

Answer 46

the product of the number of turns in the coil N and the magnetic flux Φ

Question 47

what can you change to induce an emf

Answer 47

B A θ

Question 48

whats the difference between magnetic flux density and magnetic flux

Answer 48

magnetic flux density is the strength of the field whereas magnetic flux density is the product of the magnetic flux density perpendicular to the cross sectional area

Question 49

state Faraday's law

Answer 49

the magnitude of the induced emf is proportional to the rate of change of magnetic field linkage

Question 50

state Lenz's law

Answer 50

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

Question 51

Whats Faradays law in the form of an equation

Answer 51

ε α ΔNΦ/Δt

Question 52

whats the equation combining faradays law and lenzs law

Answer 52

ε = -ΔNΦ/Δt

Question 53

whats the purpose of a transformer

Answer 53

change alternating voltages to higher or lower values

Question 54

what does a transformer consist of

Answer 54

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

Question 55

how does a transformer work in terms of changing magnetic flux

Answer 55

-an alternating current is supplied to the primary coil which produces a varying magnetic flux in the soft iron core -the secondary coil is linked by the changing flux. -the iron core ensures that all the magnetic flux in the primary coil links the secondary coil and none is lost.

Question 56

what does faradays law tell us about transformers

Answer 56

a varying emf is is produced across the ends of the secondary coil

Question 57

state the turn-ratio equation

Answer 57

ns/np=Vs/Vp for an ideal transformer

Question 58

what is a step up transformer

Answer 58

a step up transformer has more turns on the secondary than the primary coil and Vs>Vp - increases the voltage

Question 59

what is a step-down transformer

Answer 59

a step-down transformer has fewer turns on the secondary than on the primary coil and Vs

Question 60

what is the relationship between current and voltage in a 100% efficient transformer

Answer 60

VsIs=VpIp or Ip/Is=Vs/Vp

Question 61

what does it mean if a transformer is 100% efficient

Answer 61

the output power from the secondary coil is equal to the input power into its primary coil

Question 62

why are transformers not 100% efficient

Answer 62

power loss due to the heating effect of the current

Question 63

which design features of a transformer increase efficiency and why

Answer 63

-low resistance windings -making a laminated core with layers of iron separated by an insulator helps to minimise currents induced in the core itself -core is made out of soft iron whidh iw very easy to magnetise which helps overall efficiency of the transformer