23.9 Inductance

AGFI uses both currents to induce an emf in the same coil.

Equal but opposite emfs will occur if the currents are equal.

Equal input and output voltages can be found in most isolation transformers.

They put a large resistance between the original source and the device being operated.
Even in the circumstance shown, this prevents a complete circuit between them.
The appliance has a complete circuit.
The person in the figure is touching only one of the transformer's output wires, and neither of the output wires is grounded.
The name isolation transformer refers to the appliance being isolated from the original voltage source by the high resistance of the material between the transformer coils.
A path with such a large resistance that the current is negligible is required for current to flow through the person.

An isolation transformer prevents a complete circuit between the device and the original source of power.

The basics of electrical safety are presented here.

It is possible to pursue electrical safety to greater depths.
There are problems with different earth/ground connections for appliances in close proximity.
Hospitals have many other examples.
Special protection is required for patients who are sensitive to shocks.
Currents as low as 0.1 mA can cause ventricular fibrillation for these people.
The material presented here can be used to further study.

Changing magnetic flux is the process in which an emf is created.

Some examples are more effective than others.
Transformer are designed to be particularly effective at inducing a desired voltage and current with very little loss of energy to other forms.

These coils can cause emfs in one another.

Their mutual inductance M shows the effectiveness of thecoupling between them.
A change in coil 1's current is seen to cause an emf in coil 2.

The geometry of the devices can be changed by varying current.

We focus on the rate of change of current as the cause.
A change in the current in one device causes a change in the other.

The minus sign is an expression of the law.

There is nature here.

The transformers run the same way.

A large mutual inductance is not desirable.

A transformer with a large mutual inductance is what we want.
An electric clothes dryer can cause a dangerous emf on its case if the inductance between its coil and the case is large.
Counterwind coils can be used to cancel the magnetic field.

The magnetic fields of the heating coil of the electric clothes dryer can be counter-wound so that they don't overlap.

The counter emf is caused by the magnetic field and flux increasing when the current through the coil is increased.

An emf is created if the current is decreased.
The change in current through the device is what causes the change in flux.
The rate of change of current is related to the physical geometry of the device.

The minus sign indicates that emf is against the change in current.

Both units of self-inductance and mutual inductance are henries.
A large coil with many turns and an iron core will not allow current to change quickly.

A large inductor is a 1 H. Consider a device with a 10 A current flowing through it.

The change will be opposed by an emf.

An emf will be given.
The positive sign shows that the large voltage is in the same direction as the current.
It may be necessary to change current more slowly because of the large emfs.

There are uses for a large voltage.

Camera flashes use a battery, two inductors that function as a transformer, and a switch to cause large voltages.
The battery voltage will be boosted to over one thousand volts, and the oscillator system will do this many times.
The high voltage in the Capacitor is used to power the flash.

1.5 V batteries can be used to induce emfs.

This voltage can be used to store charge for later use, such as in a camera flash attachment.

It is possible to calculate for an inductor based on its geometry and magnetic field.

It is difficult in most cases because of the complexity of the field.

The inductance is usually a number.
The solenoid has a very uniform field inside, a nearly zero field outside, and a simple shape.
It is important to derive an equation for its inductance.
The definition of self-inductance is given by the definition of self-inductance as.

The self-inductance of a device is always valid.

The last equation can be used to find an expression for the inductance of a solenoid.

The area of a solenoid has been fixed.
The magnetic field of a solenoid is given by.

The self-inductance of a solenoid is cross-sectional area and length.

The inductance is dependent on the physical characteristics of the solenoid.

The self-inductance of a 10.0 cm long, 4.00 cm diameter solenoid has 200 coils.

The only quantities that are known are the ones in the equation.

The cross-sectional area is 200 and the length is 0.100 m.

This is a moderate size.

Its inductance is moderate.

Inductance can be used in traffic lights to tell when vehicles are waiting at the intersection.

A waiting car stops at the place where an electrical circuit with an inductor is placed.
The circuit changes send a signal to the traffic lights to change colors as the body of the car increases inductance.
The same technique is used for airport security.
The metal detector frame has a coil or inductor in it.
The receiver receives a signal from the transmitter coil.
Any metal object in the path can affect the self-inductance of the circuit.
The approximate location of metal found on a person can be indicated by the sensitivity of the detectors.

The security gate at the airport can show the approximate height of metals above the floor.

Inductances oppose changes in current.

An alternative way to look at the opposition is based on energy.
There is a magnetic field.
There is an opposition to rapid change because it takes time to build up and deplete energy.
The magnetic field is proportional to the current and the inductance of the device.

The expression is similar to the one for the energy stored in a Capacitor.