Presentation on theme: "Electromagnetic Induction"— Presentation transcript:
1Electromagnetic Induction Induced EMF and Induced CurrentsNote: This is an AP topic only, so if they ask which way current will flow, they mean conventional current (+ with RIGHT hand rules)
2When there is no relative motion between the coils of wire and the magnet there is no current produces
3Current is created in the coil when the magnet is moved towards the coil Note: This is an AP topic only, so if they ask which way current will flow, they mean conventional current (+ with RIGHT hand rules)
4Current also exists when you pull it away from the coil, just in the opposite direction. The current in the coil is called an induced current. The coil itself acts as a source of emf known as an induced emf.
5Another way to look at it. Changing the area of a coil, in effect, reduces/increases the B field that the coil is subject to. Changing the B field strength experienced by the coil. This will also create a current.
6The EMF Induced in a Moving Conductor Motional EMFThe EMF Induced in a Moving Conductor
7A rod is being pushed to the right with constant speed v A rod is being pushed to the right with constant speed v. Suddenly the bulb lights. Why?Where is the current coming from ?Where is this opposing force coming from?
8ε=BLv We have been using the term emf, ε, or electro motive force. Potential Difference
21Basically, this is a transformer! Later, I will ask you why we need ALTERNATING CURRENTto make this work!Basically, this is a transformer!
22How many of those little Power Cube thingies do you have around your house? Here's how many I found around mine:The answering machineThe cordless phoneThe cell phoneThe electronic anti-flea ultrasonic noisemaker (we have 2 dogs)The digital cameraThe nicad battery rechargerThe video camera battery replacer/rechargerThe digital clock in the bedroomThe electric toothbrushThe electric razorThe electric screwdriverThe electric drillBoth sides of the baby monitorThe laptop computerThe office phoneThe ink jet printerThe speaker system on the computerThe tape recorder that records notes in the officeThe calculator rechargerThe 9-volt battery replacer for the radioThe radio shack univeral power cube
23So let's take one of these apart and see what's inside So let's take one of these apart and see what's inside. Here's the transformer we will be exploring today:
24Under the coverThis transformer came with a rechargeable electric screwdriver. This particular transformer is rated at 3 volts and 240 milliamps. Once you convince the cover to come off here is what you find inside:
25What you can see here are two windings What you can see here are two windings. The purpose of a transformer is to convert one AC voltage to another AC voltage. In this case the transformer converts the normal 120 volt AC current in your house down to three volts.
26Primary WindingThe 120 volts comes in on the primary winding on the left. Running down the middle of that winding (as well as around the outside) is an iron core. The AC current in the primary winding creates an alternating magnetic field in the iron just as it would in an electromagnet.Iron Core
27Secondary WindingThe other winding, known as the secondary winding wraps around the same iron core. In the secondary winding the magnetic field in the core creates current. The voltage in the secondary is controlled by the ratio of the number of turns in the two windings. So if the primary and secondary windings have the same number of turns, the primary and secondary voltage will be the same. If the secondary winding has half as many turns as the primary then the voltage in the secondary will be half that of the voltage in the primary.
28You can see in the following figure that the primary in this particular transformer uses very fine wire while the secondary uses much thicker wire. To drop down to 3 volts, there needs to be 40 times more turns in the primary than in the secondary.
29On the other side of the transformer you find two diodes wrapped in rubber insulation. The diodes act as a rectifier, turning the AC current into DC current. Most transformer cubes that you find around the house produce a low-voltage DC current (3 to 12 volts, and less than an amp of current).
30Turning AC into DCDC current is necessary because rechargeable batteries store DC current, because most electronics require low-voltage DC current and because small DC motors run directly from batteries and are the least expensive motors available.
31On the other hand, the picture tube in your TV requires 15,000 V to accelerate the electron beam, and a transformer is used to obtain this from a 120 V wall outlet.
32The current coming into the house is 30 amps, find a way to tell me the current in the High-voltage wire.