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unit 11 notes Physics Oxford IB Diploma Programme
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these are detailed notes of unit 11: electromagnetic induction from ibdp physics hl course
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Describing magnetic fields- A constant current creates a constant magnetic field.
- Faraday’s experiment proved that changing magnetic fields create current.
Field lines:
- North → south
- The distance between lines tells us the magnetic field strength (B) at a point.
Magnetic field strength (B) can be quantified as:
a. 𝐹 = 𝐵𝐼𝐿𝑠𝑖𝑛θ,
- F is the force on the wire,
- L is the length of the wire,
- I is the current through the wire
- θ is the angle between the magnetic field lines and current.
Φ
b. 𝐵 = 𝐴𝑐𝑜𝑠θ
Φ = 𝐵𝐴𝑐𝑜𝑠θ
−2
- B is the 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑓𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ [𝑇] ≡ 𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝑓𝑙𝑢𝑥 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 [𝑊𝑏𝑚 ]
- Flux density: # field lines (i.e magnetic flux) per unit area. [Wbm^-2]
- Φ, is the Magnetic flux (i.e total, fixed, number of field lines [Wb - Webers]).
- θ is the angle between the field lines and the normal to area A.
- If given the angle between the field and the plane (α), you must use
90 − α = θ, or 𝑠𝑖𝑛θ as the angle
- A, when generating electricity, is the area inside one loop of a wire.
,Magnetic flux linkage:
λ = Φ𝑁 = 𝐵𝐴𝑁𝑐𝑜𝑠θ
- λ is the flux linkage (i.e the total number of times any field line passes through every
turn of the coil).
- N is the number of turns/loops in a coil/wire,
- A is the area inside each loop,
, Changing magnetic fields
Electromagnetic induction: the process in which an e.m.f is induced in a closed circuit due to
changes in magnetic flux.
∆ϕ
- Faraday’s law: ε =− 𝑁 ∆𝑡
If the interval of time becomes very small (i.e., in the limit of Δt → 0):
𝑑(𝑁ϕ)
- Faraday’s law: ε =− 𝑑𝑡
This can occur either when:
a. A conductor cuts through a magnetic field
b. The direction of a magnetic field through a coil changes
How to induce e.m.f:
∆𝐵𝐴
a. Moving magnet or coil changes the magnetic field strength: ε =− 𝑁 ∆𝑡
1. When the bar magnet is held still inside, or outside, the coil, the rate of change of flux
is zero, so, there is no e.m.f induced
2. As the bar magnet enters the coil, a change in magnetic flux (ΔΦ) induces an e.m.f
within the coil.
3. When the bar magnet exits the coil, an e.m.f is induced in the opposite direction. As
the magnet changes direction, the direction of the current changes.
4. Increasing the speed of the magnet, increases the rate of change of flux increases,
inducing an e.m.f with a higher magnitude
𝐵∆𝐴
b. Rolling a bar along two contacts, changes the loop’s area: ε =− 𝑁 ∆𝑡
- ε=− 𝑁𝐵𝑣𝑙, where v is the velocity of the rolling bar, and l is its length.
∆𝐵𝐴
c. Rotating a coil: ε =− 𝑁 ∆𝑡
- When rotating by 180° the field is going from B to -B so ∆B = 2B
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