Electromagnetism

Question 1

In charged particles, X charge goes to Y charge;

A) X = positive, Y = negative.
B) X = positive, Y = neutral.
C) X = negative, Y = positive.
D) X = negative, Y = neutral.

Answer 1

A) X = positive, Y = negative.

charges always go positive to negative or (+) → (-)

Question 2

In charged particles, the acceleration of the field is equal to the Electric field strength (Ef);

A) true.
B) false.

Answer 2

B) false.

the acceleration in an electric field is equal to force divided by the mass of the particle, force being equal to electric field times charge or (qEf), so a = (qEf)/m

Question 3

In charged particles, a stationary charge will experience a magnetic force;

A) true.
B) false.

Answer 3

B) false.

if a charge is stationary, it will experience a force by the electric field (F = Eq), but if it is in motion THEN it will experience a force by the magnetic field (F = qvBsinθ)

Question 4

In charged particles, for a charge experiencing a force in a magnetic field the angle of the force to the velocity and or magnetic field can vary;

A) true.
B) false.

Answer 4

B) false.

force in this case is ALWAYS perpendicular to the velocity and magnetic field

Question 5

In charged particles, for the angle θ between the velocity and the magnetic field, if θ = 0/180, θ = 1-89, θ = 90, the motion of the charge will be respectively;

A) no motion, straight (no force), spiral.
B) straight (no force), circular, spiral.
C) straight (no force), spiral, circular.
D) straight (no force), spiral, no motion.

Answer 5

C) straight (no force), spiral, circular.

if there is no angle of difference between the magnetic field and the velocity, it will not experience a force, so the velocity will be parallel to the field.
If there is an angle between 1 and 89, the charge will spiral because it is under some influence of the force and also the velocity.
If there is an angle of 90, it will move in circular motion and the F will also be mv^2/r (remembering centripetal force)

Question 6

In charged particles, the X and O represent respectively;

A) field line moving into page, field line moving out of page.
B) field line moving out of page, field line moving into page.

Answer 6

A) field line moving into page, field line moving out of page.

to remember this, pretend the X and O are like arrows. If it’s X it’s moving away from your point of view. If O it’s moving towards your point of view

Question 7

In charged particles, if the charge is negative, the force of the charge points;

A) the same as where you’d usually point F
B) opposite of where you’d usually point F

Answer 7

B) opposite of where you’d usually point F

when using your right hand rule, you’re thumb is representative of the force. It points this way when you’re charge is a positive. But if it’s a negative you point it the opposite way

Question 8

In the motor effect, the motor effect is defined as;

A) the magnetic field generated by a current carrying wire
B) the force experienced by a current carrying wire in an electric field
C) the force experienced by a current carrying wire in a magnetic field
D) the amount of magnetic field lines going through a current carrying loop

Answer 8

C) the force experienced by a current carrying wire in a magnetic field

the motor effect is the force that is caused when a current (the positive charge going through a wire) moves through a wire while in a magnetic field, the force itself denoted with the formula F = BILsinθ (θ being the angle between the wire and the magnetic field lines)

Question 9

In the motor effect, you can find the movement of the magnetic field lines going around a wire by;

A) curling your left hand and pointing up the ‘wire’ with your thumb, the thumb pointing where the current goes, and your curled fingers representing the movement of the field lines
B) curling your right hand and pointing up the ‘wire’ with your thumb, the thumb pointing where the current goes, and your curled fingers representing the movement of the field lines
C) curling your left hand and pointing up the ‘wire’ with your thumb, the thumb pointing opposite where the current goes, and your curled fingers representing the movement of the field lines
D) curling your right hand and pointing up the ‘wire’ with your thumb, the thumb opposite pointing where the current goes, and your curled fingers representing the movement of the field lines

Answer 9

B) curling your right hand and pointing up the ‘wire’ with your thumb, the thumb pointing where the current goes, and your curled fingers representing the movement of the field lines

this is self evident

Question 10

In the motor effect, for 2 wires, a current moving in the same way will;

A) do nothing
B) move parallel to each other
C) repel
D) attract

Answer 10

D) attract

use your left hand and point out your pinky, and with you hand point out your thumb (curl your fingers in both situations). If you can interlock your fingers, they attract. If not, they repel

Question 11

In the motor effect, the pole of the solenoid can be determined by;

A) how the solenoid is connected to a battery
B) only known in the question
C) left hand rule
D) right hand rule

Answer 11

D) right hand rule

use your right hand and curl your finger. Where the current goes, the fingers follow. Wherever your thumb points, is the north

Remember cause I forgot to add this, (N) → (S)

Question 12

In electromagnetic induction, EMF generated via Faraday’s law (ε = -(ΔΦ)/t) is negative because;

A) the generated EMF forces the change in flux to move backwards
B) the generated EMF opposes the original change in flux
C) non of the above, EMF is positive

Answer 12

B) the generated EMF opposes the original change in flux

when you change the flux over an area, another magnetic field is created which tries to balance out this new change in flux. This new change in flux is opposite the original change in flux, therefore it is negative i.e. ε is the negative derivative of Φ. This occurs because of the law of conservation of energy

Question 13

In electromagnetic induction, Lenz’s Law states and why does it occur?;

A) for a change in flux, when a current is induced in another wire, an opposing magnetic field (called a back EMF) will try to stop this change. This is because of the Law of Conservation of energy, because if this change was to continue without the back EMF, energy would be made out of nowhere
B) for a change in flux, when a current is induced in another wire, an equal magnetic field (called a back EMF) will double this change. This is because of the Law of Conservation of energy, where with this new change in flux, EMF must be made concurrently with this

Answer 13

A) for a change in flux, when a current is induced in another wire, an opposing magnetic field (called a back EMF) will try to stop this change. This is because of the Law of Conservation of energy, because if this change was to continue without the back EMF, energy would be made out of nowhere

well it’s in the pudding isn’t it?

Question 14

In electromagnetic induction, transformers are never perfect and lose energy through heat. Why is this and how can it be solved?;

A) higher current is responsible for loss of electric energy to heat, which can be solved by maximising voltage
B) higher current is responsible for loss of electric energy to heat, which can be solved by minimising resistance
C) higher voltage is responsible for loss of electric energy to heat, which can be solved by maximising current
D) higher voltage is responsible for loss of electric energy to heat, which can be solved by maximising resistance

Answer 14

A) higher current is responsible for loss of electric energy to heat, which can be solved by maximising voltage

looking at the formula P = VI, you can also get P = I^2R. The current makes up a large portion of the P formula, which itself is the reason why energy is lost through heat (when electrons bump into stuff causing heat). So by minimising current as much as possible, that is increasing voltage to as high as you can you can get the best amount of power and lose the least amount of energy to heat

Question 15

In electromagnetic induction, to stop losses of electrical energy into heat, how do cities and stuff not waste electricity when moving it around from place to place?;

A) lower voltage with a step down transformer before sending it away
B) raise distance of the power station to the city
C) raise voltage with a step up transformer before sending it away from station to city
D) raise resistance before sending it away

Answer 15

C) raise voltage with a step up transformer before sending it away from station to city

looking at the P = VI formula and P = I^2R formula, current changes electricity into heat. But by maximising voltage, this heat loss is reduced. So before sending it to cities, a step up transformer is used to convert high current into low current and to stop losing so much heat over typically long distances (remember formula VpIp = VsIs)

Question 16

In electromagnetic induction, high currents cause heat and sound energy to be generated. But what are these currents called and how can transformers more efficiently transfer energy with flux linkage?;

A) eddy currents, laminated sheets
B) eddy currents, thicker iron cores
C) eddy currents, increase in wire numbers
D) freddie currents, laminated sheets

Answer 16

A) eddy currents, laminated sheets

eddy currents are small spirals of currents that when they’re big, will generate lots of heat and sound, which is energy that is originally electrical. When the core is laminated, that is; stuck together in many layered sheets, instead of one massive collection of eddy currents, the eddy currents are limited by the size of the sheets they inhabit, allowing for more efficient energy transfer

Question 17

In electromagnetic induction, what is Lenz’s law?;

A) a change in flux over time will generate an EMF in the equation ε = -(ΔΦ)/t
B) the direction of the induced EMF is such that the current it produces creates a magnetic field which opposes the original change in flux
C) the ratio of the number of coils of a primary transformer compared to the secondary, is equal to the voltage ratio and inverse the current ratio
D) a metallic circle spinning in a magnetic field will slow down

Answer 17

B) the direction of the induced EMF is such that the current it produces creates a magnetic field which opposes the original change in flux

self evident. Not to be confused with Faraday’s Law which says that a negative change in flux over time produces an EMF, as opposed to Lenz’s law which is different in that the induced current is an important factor to mention

Question 18

In electromagnetic induction, what is energy equivalent to (kind of, sort of, it helps to remember is my point);

A) voltage
B) power
C) force
D) work

Answer 18

D) work

in physics, there is the very important energy-work principle. I say ‘kind of, sort of’ because they are not the same but related. They are both forces that effect over a distance, but energy is the RESULT of work. But for the sake of simplicity, know that Work = Energy (or kinetic energy)

Question 19

In electromagnetic induction, how should laminations of a transformer be set up?;

A) circles of one material, making a ‘polka dot pattern’ arranged vertically
B) sheets placed vertically
C) sheets placed horizontally
D) circles of one material, making a ‘polka dot pattern’ arranged horizontally

Answer 19

B) sheets placed vertically

this is because the eddy currents will have the least amount of distance to swirl around when arranged like this, allowing for better energy flow

Question 20

In electromagnetic induction, how do you define B?;

A) magnetic field strength
B) flux density
C) magnetic field times area
D) A and B

Answer 20

D) A and B

remember! B = magnetic field strength = flux density, in Teslas (T) or Webers (Wb)

Question 21

In electromagnetic induction, what happens to the face of a solenoid where the magnetic bar approaches and moves away respectively?;

A) Same pole, different pole
B) Different pole, same pole
C) Same pole, same pole
D) Different pole, different pole

Answer 21

A) Same pole different pole

because when a current induced by a changing flux, the solenoid will create a field which tries to stop this change in flux. If the change in flux approaches, the solenoid will try to keep it away (go away!). If the change in flux leaves, the solenoid will try to keep it in (don’t leave!)

Question 22

In applications of the motor effect, what is do a DC motors and generators have that AC doesn’t?;

A) carbon brushes
B) slip rings
C) split ring commutator
D) armature

Answer 22

C) split ring commutator

because a motor and a generator will always have an alternating current in its coils (armature). For an AC all that’s necessary are slip rings to configure this AC, but a DC can only go one way. A split ring commutator allows the armature to temporarily be cut off the DC current from the circuit, making momentum turn the armature and the alternating current in the armature is then configured in a way where the current is constant.

Question 23

In applications of the motor effect, what is a motor and a generator?;

A) motor = movement to electricity, generator = electricity to movement
B) motor = electricity to movement, generator = movement to electricity

Answer 23

B) motor = electricity to movement, generator = movement to electricity

self evident

Question 24

In applications of the motor effect, what is do a AC motors and generators have that DC doesn’t?;

A) carbon brushes
B) slip rings
C) split ring commutator
D) armature

Answer 24

B) slip rings

slip rings are configured to work with the AC already in the armature. But to not twist the coils around, there are two rings (one at the back, one at the front). The current will alternate from going into the back one and out of the front, to going out from the back and into the front ring

Question 25

In applications of the motor effect, what are the cons of AC motors?;

A) it is limited by an operation speed of 50 hz
B) cannot start itself
C) the load can stop the armature if it’s too heavy
D) all of the above

Answer 25

D) all of the above

they stink

Question 26

In applications of the motor effect, what are the cons of DC generators?;

A) they are subject to easy wear and tear
B) they can cause sparking when commutator is turning and are limited to low currents
C) A and B
D) they have steady voltage

Answer 26

C) A and B

they stink

Question 27

In applications of the motor effect, what is magnetic braking?;

A) the breaking of an electronic device’s internal magnetic structure
B) The use of magnetic induction to slow down a spinning thing
C) the increase of speed of creation of an EMF by slowing the rate at which a magnet approaches a solenoids
D) the stopping of creation of an EMF by slowing the rate at which a magnet approaches a solenoids

Answer 27

B) The use of magnetic induction to slow down a spinning thing

self evident

Question 28

In applications of the motor effect, for the formula of torque of a armature, T = nAIB, there is an angle, but where is this angle?;

A) same as angle for torque formula, T = Fr
B) same as angle for force on wire, F = BIL
C) same as angle for force on particle in B, F = qvb
D) same as angle for work W = Fs

Answer 28

A) same as angle for torque formula, T = Fr

you will see the formula T = nAIBsinθ or T = nAIBcosθ, and it gets confusing so just remember the torque formula. In the torque formula, the angle is in between the applied force (F) and the length of the thing turning (r)

Question 29

Formula for Force of Charge (in electric field)?

Answer 29

F = qE (N)

Question 30

Formula for Electric Field?

Answer 30

E = V/d (Vm^-1)

Question 31

Formula for Force of Charge (in magnetic field)?

Answer 31

F = qvBsinθ (N)

note remember to flip for negative charges i.e. electrons

Question 32

Formula for Force of Current Carrying Wire (in magnetic field)?

Answer 32

F = nBILsinθ (N)

[n = number of wires]

Question 33

Formula for Force between Two Current Carrying Wire?

Answer 33

F/l = I1I2μo/2πr (N/m)

[I1 = current of wire 1 (A)]
[I2 = current of wire 2 (A)]
[μo = magnetic permeability constant (4π x 10^-7)]

Question 34

Formula for Torque of Coil (in magnetic field)?

Answer 34

T = nAIBsinθ (Nm)

[n = number of wires]

note the paper we were given says cosθ, the take home from this is that it’s the angle of the torque formula (between force and device) just think about T = Frsinθ

Question 35

Formula for Magnetic Flux?

Answer 35

Φ = BAcosθ (Wb)

[θ = angle between normal and surface]

note B = flux density (Wb m^-2) = magnetic field strength (T)

Question 36

Formula for Faraday’s Law?

Answer 36

ε = -nΔΦ/t (V)

[n = number of wires]

note it’s negative because ε is the same as back emf

Question 37

Formula for Transformers (voltage-coil)?

Answer 37

Vp/Vs = Np/Ns

[n = number of wires]

Question 38

Formula for Transformers (voltage-current)?

Answer 38

VpIp = VsIs