7.9 - Electromagnetic Induction

Question 1

What is magnetic flux defined as?

Answer 1

The product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density

Question 2

What is the equation to calculate magnetic flux?

Answer 2

Φ = BA
B = magnetic flux density
A = Cross sectional area

Question 3

What are the units for magnetic flux?

Answer 3

Webers
Wb

Question 4

What is flux linkage defined as?

Answer 4

The product of the magnetic flux and the number of turns of the coil.

Question 5

What happens when an object experiences a change in a magnetic field?

Answer 5

It generates an electric field.

Question 6

What is the equation to calculate magnetic flux linkage?

Answer 6

ΦN = BAN
Φ = magnetic flux (Wb)
N = number of turns of the coil
B = magnetic flux density (T)
A = cross-sectional area (m²)

Question 7

What are the unit of magnetic flux linkage?

Answer 7

Weber turns.

Question 8

What is the equation for flux linkage when the field lines are not completely perpendicular to the area A?

Answer 8

NΦ= BANcosθ
θ = angle between magnetic field lines and the line perpendicular to the plane of the area

Question 9

When is magnetic flux at a maximum?

Answer 9

when cosθ = 1, hence θ=0 degrees (field lines perpendicular to the plane of the area)

Question 10

When is magnetic flux at a minimum?

Answer 10

When cosθ = 0, hence θ = 90 degrees (field lines are parallel to the plane of the area).

Question 11

When is an emf induced in a circuit?

Answer 11

When the magnetic flux linkage changes with respect to time. (e.g when a conductor moves through a magnetic field)
(so emf induced if there is a changing B,A or θ)

Question 12

What is electromagnetic induction defined as?

Answer 12

The process in which emf is induced in a closed circuit due to changes in magnetic flux.

Question 13

When a conductor cuts through the magnetic field lines what happens?

Answer 13

• This causes a change in magnetic flux
• which causes work to be done
• this work is transformed into electrical energy
• (therefore if attached to a circuit, a current is induced)

Question 14

What is electromagnetic induction used in?

Answer 14

• Electrical generators which convert mechanical energy to electrical energy
• Transformers which are used in electrical power transmission

Question 15

What factors increase the induced emf in the experiment which pushes a magnet in and out a coil?

Answer 15

• moving the magnet faster through the coil
• adding more turns to the coil
• increasing the strength of the bar magnet

Question 16

What factors will increase the induced emf in the experiment which moves a wire through 2 magnets?

Answer 16

• increasing the length of the wire
• moving the wire between the magnets faster
• increasing the strength of the magnets

Question 17

What is Faraday’s Law?

Answer 17

The magnitude of the induced emf is directly proportional to the rate of change in magnetic flux linkage.

Question 18

What is Lenz’s law?

Answer 18

The induced emf acts in such a direction to produce effects that oppose the change causing it.

Question 19

What is the equation to calculate the magnitude of the emf? (Faraday’s law)

Answer 19

ε = N Δɸ /Δt
ε = induced emf
N = number of turns of coil
Δɸ = change in magnetic flux
Δt = time intervals

Question 20

What is the significance of the minus sign in Lenz’s law?

Answer 20

It shows the direction fo the induced emf. (the induced emf will oppose the change that caused it) - conservation of energy

Question 21

How do you derive the equation ε = BLv?

Answer 21

movement of conductor through mag field =
s = vΔt
Area of mag flux that cuts through is:
A = LvΔt
total flux the conductor cuts through:
ΔΦ = BA = BLvΔt
ε = BLvΔt /Δt = BLv

Question 22

what does the equation ε = BLv show?

Answer 22

The emf induced increases if:
- A longer conductor is in the field
- the magnetic field strength is larger
- the conductor cuts through the field lines faster

Question 22

What is an alternator?

Answer 22

A generator of alternating current.

Question 23

How do alternators work?

Answer 23

• they induce electrical current by rotating a coil in a magnetic field
• slip rings and brushes are connect the coil to an external circuit
• The output voltage and current change direction with every half rotation of the coil, producing alternating current (a.c).

Question 24

When a coil rotates in a magnetic field, when is emf a maximum?

Answer 24

when θ = 90 degrees. The magnetic field lines are parallel to the plane of the area.

Question 25

When a coil rotates in a magnetic field, when is emf a minimum?

Answer 25

when θ = 0 degrees. The magnetic field lines are perpendicular to the plane of the area.
(THIS IS THE OPPOSITE OF THE MAX AND MIN FLUX THROUGH THE COIL)

Question 26

What is the equation for the induced emf in a rotating coil?

Answer 26

ε = BANω sin(ωt)
ε = emf induced in the coil (V)
B = magnetic flux density
A = cross-sectional area of the coil
ω = angular speed of the coil
t = time