Magnetic Fields

Question 1

When a magnetic field is perpendicular to a current-carrying wire, does the wire feel a force?

Answer 1

Yes, magnitude F = BIl (B = flux density, I = current, l = wire length)

Question 2

Fleming’s left-hand rule: what properties are on what fingers?

Answer 2

Thumb – Force, First finger – Field, Second finger – Current

Question 3

What is magnetic flux density (B)?

Answer 3

Flux density in Tesla (T) or Wb/m², flux per m²

Question 4

A charged particle moving through a field feels a force along or perpendicular to the field lines?

Answer 4

Perpendicular to the field

Question 5

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

Answer 5

F = BQv

Question 6

Is the force applied perpendicular to particle motion or in one direction?

Answer 6

Perpendicular, causing circular motion

Question 7

Which fields do cyclotrons use?

Answer 7

Electric field and Magnetic field

Question 8

How does a cyclotron work and what are the field purposes?

Answer 8

Magnetic field makes particles move in circles; electric field accelerates them in the gap between “Dees”

Question 9

What is magnetic flux (Φ)?

Answer 9

Φ = BA (Flux Density × Area, B normal to A)

Question 10

What is flux linkage?

Answer 10

NΦ = number of turns cutting the flux at one time

Question 11

Flux linkage of a rectangular coil rotating in a magnetic field?

Answer 11

NΦ = BANcosΘ

Question 12

What is Faraday’s Law?

Answer 12

Induced emf ∝ rate of change of magnetic flux linkage

Question 13

What is Lenz’s Law?

Answer 13

Induced emf direction opposes the change that induces it

Question 14

What happens when you move a straight conductor through a magnetic field?

Answer 14

Electrons pushed to one end, creating emf (Faraday’s Law)

Question 15

EMF produced when rotating a coil at constant rate in a magnetic field?

Answer 15

𝜀 = BANωsin(ωt)

Question 16

Describe how to use an oscilloscope

Answer 16

X-axis = time base, Y-axis = potential difference; used to find peak voltage, time period, and frequency

Question 17

How does a transformer work?

Answer 17

AC in primary coil creates magnetic field; induces EMF in secondary coil → current flows

Question 18

What kind of current is produced by a transformer and why?

Answer 18

Alternating current, because EMF induced by changing magnetic field

Question 19

Why are transformers used?

Answer 19

Change voltage and current for power transmission, reduce energy losses, safe household voltage

Question 20

Equation linking number of coils and voltages in a transformer?

Answer 20

Ns/Np = Vs/Vp

Question 21

What is transformer efficiency?

Answer 21

Output power / input power = IsVs / IpVp

Question 22

In a step-up transformer, which coil has more turns?

Answer 22

Secondary coil (increases voltage)

Question 23

What is an eddy current?

Answer 23

Induced currents in the iron core due to changing magnetic field

Question 24

Why are eddy currents a problem?

Answer 24

Lenz’s Law → emf opposes primary coil, causes resistive heating → reduces efficiency

Question 25

How can eddy current losses be reduced?

Answer 25

Use a laminated iron core (thin insulated sheets to reduce current loops)