EM 01 - Into Faraday's Law |

OPENING QUESTION: 1) Qualitatively state Gauss's Law for magnetic flux 2) By the by, what's a 3) While we're at it, what's so special about OBJECTIVES: I will be able to calculate induced emf using Faraday's Law during today's class. WORDS/FORMULAE FOR TODAY TERMS
CONSTANTS: UNITS: - Tesla = T defined as 1 N/C(m/s)
FORMULAE: - ε = -dΦ
_{B}/dt = - Φ
_{B}= ∫B ∙ dA - ∮B ∙ ds = μ
_{o}I: Ampere's Law - dB = (μ
_{o}I/4π)(ds x r̂ )/r^{2}(note r̂ here is the unit vector r̂ like i, j, or k. Biot-Savart Law - F
_{B}= ∫I ds x B NOTE: AP Version is: F_{B}= ∫I dℓ x B - v = E/B
- F
_{B}= qv x B (vector value) - F
_{B}= qvBsinθ (F_{B}magnitude of only) - F
_{B}= IL x B
WORK O' THE DAY: How is electric current created in a generator? Here's an interesting idea... how did the generation of electric current inspire Einstein as he was formulating his Work for today: Recall our equation for the magnetic flux through an enclosed area: Φ That equation states that for any enclosed area the magnetic flux in always equals the magnetic flux out.... which makes sense. Now let's combine that with the idea of inductance. Inductance is a property in physics whereby current can be started (induced) in a loop of wire by differing means: - a magnet passed through a loop of wire momentarily
*induces*current to flow through the wire - Current flowing through a wire
*induces*current to flow in a nearby coil of wire
For current to flow there must be an electric potential (specifically an emf) present which is directly related to the change in magnetic flux over time:
ε = -dΦ Recall back to our dim, dark days of TORQUE that the torque experienced by a rotating wire is dependant on the number of coils of wire: τ = NIAB Similarly, the emf due to changing magnetic flux also depends on the number of coils of wire (N) ε = -NdΦ Let's back up a bit--- recall that Φ We almost always evaluate the magnetic field as a If we also have a ε = -dΦ substituting for B ε = -d(BAcosθ)/dt Which means that we can generate an emf by..... changing what? Notice again there is some potential confusion here.... we say that B and A are constant in order to dodge the integration, but then we talk about changing them... what's up with that?
════════════════════ - Take a look at example
*31.1* - Example
*31.2*seems a little odd but take a run at that one also.
HOMEWORK: Problem #1 on page 958 (this one is a bit surprising. Take extra care dealing with the author's use of the term " 3, 5 & 9 on page 958-9 |

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