Demo Equipment

Motional
- wire, strong horseshoe
- Galvanometer, or big ammeter? Test both, and update demo number accordingly

Notes: Motional emf

If a conductor passes through a magnetic field, the charges within it feel a force.
... regardless of whether the conductor has a loop shape for computing flux
... or whether the magnetic field changes
F = q v B sin(θ)

Potential difference

B points up, 0.005 T
Flat, horizontal, square shaped metal conductor moves to the right.
Electrons feel force in, metal ions feel force out
Potential difference is created, which eventually stops further charge separation.
ΔV = B v l
B is the component of the magnetic field perpendicular to the velocity
l is the length of the conductor perpendicular to BOTH the magnetic field and the velocity.

Demo: Motional emf

Wire is connected to galvanometer (sensitive current meter)
Wire is placed within horseshoe magnet.
Wire will be pulled outwards, out of the magnet.

Questions

What direction is the magnetic field the wire will travel through when I pull it out? (up)
What direction will the force on electrons be in? (right)
What direction will the current be in? (left)
Leftward current → negative reading
Demonstrate

Numbers

Magnet is 15 cm wide, with a field of 0.10 T
I pull the wire ~ 1.0 m/s
ΔV = B l v = 0.015 V
If the resistance of the wire is 0.1 Ω, the current will be 0.15 A

Example: Motional emf

A car drives east on the highway, 30 m/s
The Earth's magnetic field points northward, 1.2×10^-5 T
The car is 3.0 m long, and 1.6 m wide, and 1.5 m high.
Which two ends (front-back, driver-passenger, or top-bottom) of the car develop a potential difference?
Which end is positive, and which is negative?
What potential difference develops?

Solution

top is positive, bottom is negative
ΔV = B l v = 5.4×10^-4 V

Problem: Airplane

An airplane flies through a region where the Earth's magnetic field is nearly vertical, 2.4×10^-5 T down.
The plane's length is 56 m, its wingspan is 60 m, and its height is 20 m.
The plane flies at 250 m/s.
Which two ends (front-back, left-right, or top-bottom) of the plane develop a potential difference?
Which end is positive, and which is negative?
What potential difference develops?

Solution

left is positive, right is negative
ΔV = B l v = 2.4×10^-5 T × 60 m × 250 m/s = 0.36 V