InductorsInductors

An inductor is essentially a coil of wire with an iron core.An inductor is essentially a coil of wire with an iron core.

They store energy in a magnetic fieldThey store energy in a magnetic field

The symbol for an inductor is: The symbol for an inductor is:

A

B

What do inductors do?What do inductors do?

Consider this circuitConsider this circuit

What will happen What will happen when the switch is when the switch is closed?closed?

Lamp A will glow Lamp A will glow immediatelyimmediately

Lamp B will take Lamp B will take some time to reach some time to reach full brightness.full brightness.

Why?Why?

As the current starts to flow through the As the current starts to flow through the inductor, a magnetic field starts to form inductor, a magnetic field starts to form around it…..around it…..

As this field forms, the magnetic field lines As this field forms, the magnetic field lines cut through the coils in the inductor and cut through the coils in the inductor and induce a voltage across the inductor….induce a voltage across the inductor….

According to Lenz’s Law, this voltage will According to Lenz’s Law, this voltage will oppose the voltage that caused it…..oppose the voltage that caused it…..

Why?Why?

This opposing voltage restricts the current This opposing voltage restricts the current build-up in the circuit so it takes a while for build-up in the circuit so it takes a while for the bulb to reach full brightness.the bulb to reach full brightness.

Opposing voltage (EMF)

Current building up slowly

InductorsInductors

Once the current has reached it’s full Once the current has reached it’s full value, it is no longer changing, so the field value, it is no longer changing, so the field lines in the inductor are no longer building lines in the inductor are no longer building up, so there is no induced back EMF. Ie up, so there is no induced back EMF. Ie the inductor has no effect on the circuit the inductor has no effect on the circuit any longer.any longer.

InductorsInductorsThe reverse happens when the current is The reverse happens when the current is switched off.switched off.

A collapsing current will cause the field A collapsing current will cause the field lines around the inductor to collapse.lines around the inductor to collapse.

The collapsing field lines induce a voltage The collapsing field lines induce a voltage across the inductor to try and keep the across the inductor to try and keep the current flowing. (Lenz’s Law again)current flowing. (Lenz’s Law again)

This induced voltage can be very large, This induced voltage can be very large, creating sparks – dangerous!creating sparks – dangerous!

InductanceInductance

The effect of an inductor in a circuit is The effect of an inductor in a circuit is measured as measured as inductance Linductance L

The changing current produces a The changing current produces a changing field which produces a changing changing field which produces a changing flux ie flux ie ΦΦ αα I I

The constant of proportionality is the The constant of proportionality is the inductance Linductance L

So So ΦΦ = L x I = L x I

InductanceInductance

Faraday’s Law states:Faraday’s Law states:

t

ILV

t

ILV

tV

)(

InductanceInductance

So the size of the induced back EMF So the size of the induced back EMF depends on the depends on the inductance of the inductorinductance of the inductor and the and the rate of change of currentrate of change of current..

The unit for inductance is the The unit for inductance is the Henry HHenry H

An inductor only affects circuits when the An inductor only affects circuits when the current is changing and will always oppose current is changing and will always oppose what is happeningwhat is happening

Changing CurrentChanging Current

When the switch is When the switch is closed, current starts closed, current starts to flow in the circuitto flow in the circuit

It will build up to it’s It will build up to it’s maximum value:maximum value:

A

LR

V

R

VI max

Changing CurrentChanging Current

I

t

Imax

I

t

Imax

Without Inductor With Inductor

Time ConstantTime Constant

The time constant The time constant is is defined as the time defined as the time taken for the current taken for the current to reach 63% of it’s to reach 63% of it’s maximum value.maximum value.

A circuit is said to be A circuit is said to be at maximum current at maximum current after 3 time constantsafter 3 time constants

(Same as caps)(Same as caps)

63%

I

t

Imax

Time ConstantTime ConstantThe time constant depends on:The time constant depends on: The size of the inductor The size of the inductor

(bigger L means more back (bigger L means more back e.m.f so longer for current to e.m.f so longer for current to reach max)reach max)

The resistance of the circuit The resistance of the circuit (larger R means smaller (larger R means smaller circuit current, so less mag circuit current, so less mag field, so less back e.m.f, so field, so less back e.m.f, so shorter time for current to shorter time for current to reach max)reach max)

R

L

Back E.M.FBack E.M.FThe back e.m.f will be The back e.m.f will be large to begin with as large to begin with as the current is the current is changing most rapidlychanging most rapidly

It drops to zero as the It drops to zero as the rate of change of rate of change of current reduces.current reduces.

Again the time Again the time constant is the time constant is the time taken to fall 63% of taken to fall 63% of max valuemax value

V

t

Vmax

EnergyEnergy

Energy is stored in the magnetic field Energy is stored in the magnetic field around the inductor. around the inductor.

If the current is suddenly interupted,a If the current is suddenly interupted,a spark may occur as the energy is spark may occur as the energy is dissipated. Self inductance can be a major dissipated. Self inductance can be a major problem in big circuits because of this.problem in big circuits because of this.

2

2

1LIE

TransformersTransformersThese consist of 2 These consist of 2 coils wound close to coils wound close to each other.each other.Changing the current Changing the current in one coil makes the in one coil makes the field around it change. field around it change. This changing field This changing field induces current in a induces current in a nearby coil.nearby coil.

TransformersTransformers

The ratio of the windings The ratio of the windings determines how much determines how much voltage/current is inducedvoltage/current is inducedThe voltage can be The voltage can be calculated using:calculated using:

N=number of turnsN=number of turnsV=VoltageV=VoltageP=primary coilP=primary coilS=secondary coilS=secondary coil s

p

s

p

V

V

N

N

TransformersTransformers

No transformer is 100% efficient, but No transformer is 100% efficient, but assuming it was:assuming it was: Power in = Power outPower in = Power out

sspp

outin

IVIV

PP

Transformers Transformers

3 Types:3 Types: Step up : VStep up : Vss > V> Vpp

Step down : VStep down : Vss < V < Vpp

Isolating (a safety device) : VIsolating (a safety device) : Vss = V = Vpp

They only work with alternating current or They only work with alternating current or switched DCswitched DC