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Transcript
1
Mutual / Self-Induction – Learning Outcomes
 HL: Define and discuss mutual induction for two adjacent
coils.
 HL: Demonstrate mutual induction.
 HL: Define and describe self-induction.
 HL: Demonstrate self-induction.
 Give the structure and principle of operation of a
transformer.
 Demonstrate a transformer.
 Solve problems about transformers.
 Give some uses of transformers.
2
Mutual / Self-Induction – Learning Outcomes
 Describe the effect of inductors on a.c.
 Describe the effect of capacitors on a.c.
 Give the uses of inductors.
3
HL: Mutual Induction
 Mutual induction occurs when two coils are adjacent to
each other. If the magnetic field in one coil changes, an
emf is induced in the other. This happens with a.c.
 The size of the induced emf depends on:
 the distance between the coils (closer yields more emf),
 whether the coils have the same core (yields more emf),
 the number of turns in each coil (more yields more emf).
aka Faraday’s experiment
4
HL: Demonstrate Mutual Induction
1. Set up two circuits – one consists of a galvanometer and
a coil of wire (circuit 1), while the other consists of a d.c.
power supply, open switch, and coil of wire (circuit 2).
2. Move the coils next to each other.
3. Close the switch in circuit 2 and note an instantaneous
deflection in the galvanometer needle in circuit 1.
4. Leave the switch closed, noting no deflection in the
galvanometer needle in circuit 1.
5. Open the switch in circuit 2 and note an instantaneous
deflection in the galvanometer needle in circuit 1.
6. Deflection only occurs when the magnetic field due to
circuit 1 changes. There is no deflection with constant
current.
5
HL: Demonstrate Mutual Induction
 To get “Faraday’s apparatus”, include an iron ring with a
coil wrapping around either side.
6
HL: Self Induction
 Whenever the current passing through a coil changes, its
magnetic field changes. This change induces an emf in
the same coil which opposes the change. This is called a
back emf and the phenomenon is called self induction.
 Coils which exhibit noticeable levels of self induction are
called inductors.
7
HL: Demonstrate Self Induction
1. Connect a coil with a soft iron core in series with a bulb,
power supply, and switch.
2. Close the switch.
3. Note that the bulb does not reach full brightness
immediately: it gradually gets brighter.
4. Thus, the coil exhibited self-induction: as the current
increased, it created a changing magnetic field,
inducting a back emf in the coil.
8
Transformers
 We discussed transformers
previously in domestic
circuits.
 Recall that electricity is sent
around the country at high
voltage to avoid heat losses
by Joule’s Law (𝑃 ∝ 𝐼 2 ),
then transformed down to
~230 V for local use.
 Transformers consist of two
coils wrapped around the
same iron core with
different numbers of turns.
by Fizped – CC-BY-SA-3.0
9
Transformers
 The primary coil with 𝑁𝑃
turns is connected to a.c. of
voltage 𝑉𝑖 , which results in a
changing magnetic field.
 This flux induces an emf 𝑉𝑜 in
the secondary coil which
has 𝑁𝑆 turns.
 The ratio of the voltages
between the coils is the
same as the ratio of the
number of turns.

𝑉𝑖
𝑉𝑜
=
𝑁𝑃
𝑁𝑆
by Fizped, edited under CC-BY-SA-3.0
10
To Demonstrate a Transformer
1. Set up circuit 1 with a coil in series with an a.c. supply,
measuring the voltage of the coil with a voltmeter.
2. Set up circuit 2 with a voltmeter measuring the voltage
of a second coil.
3. Wrap both coils around the same iron core.
4. Note that the ratio of the voltages is the same as the
ratio of the number of turns in the coils.
symbol for a transformer
11
Uses of Transformers
 Mains electricity is generated at low voltage, then a
“step-up transformer” increases the voltage for national
transmission. A “step-down transformer” is then used to
reduce the voltage to ~230 V for local use.
 Many appliances (e.g. tvs, computers, microwaves) will
not work well at 230 V, so will have an internal
transformer to get the appropriate voltage.
12
Inductors and a.c.
 Inductors act as regular resistors when in a d.c. circuit.
 In an a.c. circuit, the changing current causes a
changing magnetic field, causing self-induction (and
thus, a back emf) in the inductor.
 Inductors are used to:
 smooth out variations in d.c.
 tune radios,
 dim lights as part of dimmer switches.
13
Capacitors and a.c.
 Recall that capacitors will not conduct d.c. once they
are charged.
 In a.c., the changing direction of the current means that
capacitors are in a constant cycle of charging and
discharging, which effectively means they conduct a.c.
 Higher capacitance capacitors will offer less resistance
to a.c.