Electromagnetism

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# Electromagnetism - PowerPoint PPT Presentation

Electromagnetism. Current-Carrying Wire. As you know from last year… Whenever a current flows, it creates a magnetic field. Field Around a Wire. The field is circular around the wire, the direction of the field can be found by using the Right Hand Rule. Magnets. Just a reminder….

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Presentation Transcript

### Electromagnetism

Current-Carrying Wire

As you know from last year…

Whenever a current flows, it creates a magnetic field.

Field Around a Wire

The field is circular around the wire, the direction of the field can be found by using the Right Hand Rule

Magnets

Just a reminder….

• Magnetic field lines always go from North to South.
• The closer the lines are together the stronger the field is.
• Like poles repel, unlike poles attract.
• Permanent magnets attract some things (like iron and steel) but not others (like wood or glass).
• A coil of wire (solenoid) with an electric current flowing through it becomes a magnet.
• Putting iron inside a current-carrying coil greatly increases the strength of the electromagnet.
Force on a Charge

Whenever a charge moves perpendicularly through a field it experiences a force. The direction of the force is determined by another hand rule:

The Thumb is: Velocity of the charge

Fingers are: The direction of the field lines

Out of the hand: Shows the direction of the force

Induction

So a current will create a magnetic field… A magnetic field can induce a current in a conductor.

When a conductor moves perpendicularly through a magnetic field a current is induced. This is called induction.

The opposite forces on positive and negative charges sets up an e.m.f and charges want to flow i.e. current

Moving a wire through a magnetic field creates a current!

Solenoid Torches

Induction is precisely how “shake torches” work.

How Can We Use This?

Well… This is how a generator works: http://www.school-for-champions.com/science/electrical_generation.htm

Summary
• Induction powers our houses and cities.
• The voltage (e.m.f) induced can be increased by:
• More coils of wire.
• Moving the wire/magnet faster.
• Stronger magnetic field.
• Make the area of the solenoid larger.
• A current is induced in a conductor if:
• The conductor moves through the magnetic field.
• The magnet moves in relation to the conductor.
• The magnetic field changes in strength or area.
Flux

Magnetic flux is essentially a measure of the number of magnetic field lines passing through an area.

You can think of it as the density of field lines.

A changing magnetic flux is the key to induction.

Magnetic flux is measured in Webers (Wb).

Remember we said we could get a bigger e.m.f () if we moved the wire/magnet faster or if we increased the magnetic field…

The induced voltage across the solenoid is equal to the change in magnetic flux per second.

Lenz’s Law

As you noticed with the generator the current changes direction… just cruises back and forth with the magnet moving.

What decides the direction?

Lenz discovered that the direction of the induced current is to oppose the change in magnetic flux.

As such Faraday’s Law is often written to show the opposing direction.

The reasons for the opposition are based on the conservation of energy.

In fact, if you drop a magnet down a copper or aluminium pipe it will take longer to fall.

Lenz’s Law

The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

Inductors

As if you needed another word for “coil of wire”…

An inductor is about as simple as an electronic component can get -- it is simply a coil of wire. It turns out, however, that a coil of wire can do some very interesting things because of the magnetic properties of a coil.

In a circuit diagram, an inductor is shown like this:

Inductors store magnetic energy. It will store the energy as long as a current flows. When the current stops, the inductor releases the stored energy.

This is often as a large voltage, given off as a spark.

Strength of an Inductor

Here by “strength” we mean the strength of the magnetic field produced by the inductor.

This depends on a number of factors:

• The number of turns of wire.
• What the core is made out of.
• Area of the coil.
• How close together the turns of wire are (turn density).

The “strength” of an inductor is termed, inductance.

Inductance is measured in Henries, H.

Named after

What’s the Point?

Surge Protectors Inductors are used as surge protectors because they block strong current changes.

Traffic Light Sensors Let's say you take a coil of wire perhaps 2 meters in diameter, containing five or six loops of wire. You cut some grooves in a road and place the coil in the grooves. You attach an inductance meter to the coil and see what the inductance of the coil is.Now you park a car over the coil and check the inductance again. The inductance will be much larger because of the large steel object positioned in the loop's magnetic field. The car parked over the coil is acting like the core of the inductor, and its presence changes the inductance of the coil. Most traffic light sensors use the loop in this way. The sensor constantly tests the inductance of the loop in the road, and when the inductance rises it knows there is a car waiting!

Spark Plug Older cars make use of the large voltage when current stops flowing.

Inductor in a Circuit

The coil of wire in this circuit is an inductor. The inductor is an electromagnet.

If you were to take the inductor out of this circuit, what you would have is a normal flashlight. You close the switch and the bulb lights up. With the inductor in the circuit as shown, the behaviour is completely different.

The light bulb is a resistor, so what you would expect when you turn on the switch is for the bulb to glow very dimly. Most of the current should follow the low-resistance path through the loop. What happens instead is that when you close the switch, the bulb burns brightly and then gets dimmer. When you open the switch, the bulb burns very brightly and then quickly goes out.

Inductor in a Circuit

The reason for this strange behaviour is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This current keeps the bulb lit for a period of time even though the switch is open. In other words, an inductor can store energy in its magnetic field, and…

…an inductor tends to resist any

change in the amount of current

flowing through it.

Current Growth and Decay

Just like with capacitors, inductors take time for their magnetic field to build up.

Time Constant for an Inductor

One time constant, , is again defined the time taken to get to 63% of the final value.

For inductors:

Inductance, in H

Time, in s

Resistance in circuit, in Ω

Energy in an Inductor

Inductors store energy in the form of a magnetic field.

It is stored as long as current flows, when disconnected this energy is released.

If done quickly, the rapid current drop will induce a very large voltage.

The energy stored in an inductor is given by:

Current, in A

, in H

Inductance, in H