1. Unit 9, Lesson 6
Generators and Motors

2. How Generators Work (Motion → Electricity)
A turbine turns a coil in a magnetic field.
A current is induced in the coil, which flows in such a direction as to create an induced magnetic field that opposes the movement, as per Lenz’s law.
The current changes direction every half turn, so alternating current (AC) is produced.
10 min

3. How Motors Work (Electricity → Motion)
Current flows through the coil, producing a magnetic field. This induced magnetic field interacts with the surrounding permanent magnetic field, causing the armature to move.
In order to keep the motor spinning in the same direction, the commutator switches the current direction every half turn.
10 min

4. Back EMF As a motor spins, the spinning coil also acts as a generator!
The emf generated is opposite the applied voltage (Lenz’s law).
This is called “counter emf” or “back emf”.
V = IR = ε(source) – ε(back)
5 min

5. Brain Break!

6. Back EMF Example
A motor in a fan draws 6.0 A when jammed and 1.5 A when running at full speed connected to a wall outlet (120 V).
a) Find the back emf.
b) What is the current if the fan runs at half speed?
R = V/I = 120/6 = 20 Ω
ε(back) = ε(source) – IR = 120 – (1.5)(20) = 120 – 30 = 90 V
b) if the frequency of turns is half the induced emf is also half
ε(back) = 45 V
IR = ε(source) – ε(back)
I = (120-45)/20 = 3.25 A
10 min

7. Design Challenge!
Choose a partner and collect a piece of poster paper.
Rules: No electronic devices allowed! You may only use your notes and your brainpower.
Challenge: Design a motor that outputs as much power as possible (hint: maximize the magnetic force). Sketch your design and note how each feature makes your motor more powerful.
20 min - more powerful permanent magnet, or increase the electric current flowing through the wire, or make the coil so it has many "turns"

8. Unit 9, Lesson 7
Transformers

9. Transformers
Alternating current* in the primary coil induces a fluctuating magnetic field. In turn, this changing magnetic field induces an alternating current in the secondary coil.
Since ΔΦ/Δt is the same in both coils, and Ԑ = N·ΔΦ/Δt, the voltage is directly proportional to the number of turns in each coil.
In a step-up transformer, the voltage is increased by having more turns in the secondary coil than in the primary. In a step-down transformer, the voltage is decreased by having fewer turns in the secondary coil.
*Why won’t direct current work?
20 min

10. Brain Break!

11. Try This!
An ideal (100% efficient) step-up transformer that converts 100 V generated at a power station to V for transmission has turns in the secondary coil and an input current of 1200 A. Find the number of turns in the primary coil and the current in the secondary coil.
Np = 400
Is = 2.4 A
15 min

12. Physics Scattergories!
Choose a partner.
Put away electronic devices and have a sheet of paper and a pencil ready.
Name as many examples of transformers in the home, workplace, and community as you can!
Examples: phone charger, laptop charger, large transformer (front yard) to move from power lines to homes, or for supplying high voltage to factories or machine shops – 15 min

13. Homework:
Pg. 259 #2-4