1. Do now: Eventually, the current will build up to it’s maximum value: A L R V In your own words, describe what happens in this circuit when the switch is closed Paraire, 29 Kohi-tātea 2016 Describe Faraday / Lenz’s law in detail!

2. Homework: Questions 15 – 20, due Monday 22 nd Sep End-of-Term test: Tuesday 23 rd Sep Kirchhoff, internal resistance, capacitors, inductors

3. Changing Current I t I max I t Without Inductor With Inductor

4. Time Constant The time constant  is defined as the time taken for the current to reach 63% of it’s maximum value. A circuit is said to be at maximum current after 3 time constants (same as caps) 63%  I t I max

5. What do you think the time constant depends on? The size of the inductor (bigger L means more back e.m.f / magentic field to fill, so longer for current to reach max) The resistance of the circuit (larger R means smaller circuit current, so less mag field, so less back e.m.f, so shorter time for current to reach max)

6. Sketch back E.M.F vs time? The back e.m.f will be large to begin with as the current is changing most rapidly It drops to zero as the rate of change of current reduces. Again the time constant is the time taken to fall 63% of max value V t V max

7. Read p254 -256: Take notes if you haven’t already Do Activity 15D