6.4 The Magnetic Effect of a Current, The Motor Effect, and Magnetic Field Strength
Not only can a magnetic field be produced by a magnetic body, but it can also be produced by a current. The result of moving charges (as current) in a wire, for instance, is a magnetic field around it. The magnetic field produced is perpendicular to the wire, and therefore the current (as the current necessarily moves along the wire), and can be found by simply using your right hand.
Make a "thumbs up" sign with your right hand. Point your thumb in the direction of the (conventional) current. The direction outwards that the rest of your fingers coil is the direction of the magnetic flux lines perpendicular to the wire. Because, when viewed as a cross-section, a wire is simply a point, the flux lines are in the form of concentric circles. As in this diagram:
a dot indicates current flowing towards you (out of the cross-section of the wire), while a cross indicated current flowing away from you (into the cross-section). Use your right hand method to match the magnetic flux lines shown in the diagram.
A because a current creates a two-dimensional magnetic field around a wire, we can also use a wire to make a magnet. A solenoid is a coil of wire, which turns into a magnet when a current flows through it. The field patterns of a solenoid are similar to those around a normal bar magnet, except that flux lines also flow through the center of the coil. As with a bar magnet, a solenoid has poles, which can be identified using a simple method. Looking in at either end of a solenoid, determine which way (clockwise or counter clockwise) the current flows. A clockwise current indicates a south pole, and a counter clockwise current is a north pole. The powers of a solenoid can be extended to create other magnets. If a metal core is placed inside the coils of a solenoid, it turns into an electromagnet when the current is switched on. If the core is of a magnetic material such as iron or steel, the dense magnetic flux lines that flow through the center of a solenoid will magnetize the piece of metal. The effects of this will often last even after the current is switched off!
What happens if two current-carrying wires come near each other? Lets see!
As the two wires of the previous diagram move together, their flux lines overlap: This effectively strengthens the magnetic field in between the two wires. Remember though, the diagrams do not have all the possible flux lines drawn in. In a scale diagram, the concentric circles would extend much farther from the center. In fact, on the far sides of the pari of wires, flux lines from both would meet once again. These lines, as you may have guessed, would be in opposite directions, and therefore cancel each other out. The result of having an increased magnetic field between them, and a decreased magnetic field on their outsides pushes the two current-carrying wires back out to a more stable position.
This is one of the few cases in electromagnetism where opposites do not attract! When two wires with current in the same direction are moved together, the opposite happens. Flux lines in the middle are in opposite directions, and so cancel out, whereas lines on the outside are increased. The result is that the two wires move even closer.
You may recall from the previous section that magnetic flux lines are also produced by magnets. When a current-carrying wire is placed in a magnetic field, as in the diagram, it too is forced to move.
As the diagram shows, magnetic flux line are increased on the right side of the wire, but on the left side, they are canceled out. The result is that the wire gets pushed out of the field, towards the left. We do not consider any other part of the circular flux lines of the wire except those in the same line of action as the magnets field lines. This is because the wires field lines become more and more perpendicular to the magnets field lines, until they have no effect on each other. If you were to reverse either the direction of the current in the wire, or the poles of the magnet, the wire would move out to the right instead.
The movement of a current-carrying wire from a magnetic field is called the motor effect. You can determine which way the wire will move by using your left hand:
Spread your first three fingers (including the thumb) apart so that they are at right angles to each other. Keeping them in position, point your first finger in the direction of the field, and your second finger in the direction of the current. Your thumb will then tell you the direction the wire will move. This rule is called Flemings Right Hand Rule. The actual force on a current-carrying wire in a field is dictated by a simple equation, the variables of which are the amount of current in the wire(I), the length of wire perpendicular to the field lines, and the magnetic field strength (B):
Force is measured in Newtons, current in Amps, magnetic field strength in Teslas (T), and length in meters. q is the angle the wire is from the magnetic fields line of action.
The motor effect is useful for just that --motors. A motor is simply a coil (usually, several coils) of wire placed in a magnetic field. Current flows in and out of the circuit by a special device called a split-ring commutator that keeps the coil of wire from ever getting twisted or tangled. The continuous movement created is used for many things in everyday life.
Here is a simplified diagram of a motor:
The two blue half-circles make up the split-ring commutator, and the two green parts are the carbon brushes, not attached, only touching the commutators, through which the current enters the circuit. In the circuit shown, conventional current enters from the right, goes through the left side of the commutator, and enters the ring of wire (shown in red). The current is now inside a field, so the wire carrying it must move, due to the motor effect. Using Flemings Left Hand Rule, we find that the wire goes down. Now lets take a look at the other side of the loop. The field is still going in the same direction, but this time the current is going in the opposite direction. This means that this part of the wire moves up to escape the field. The wire has now completed a one-quarter turn. But since the commutators are attached to the wire loop, they have also completed quarter turn. At this point, it is the two slits, separating the two sides of the commutators, that are lined up with the carbon brushes. This prevents any current from entering the loop, but momentum keeps the loop rotating clockwise. Now the side of the commutator that was on the right is on the left, and vice versa. But current still enters through the right carbon brush (and leaves through the left). The new right side goes down, and the new left side of the loop goes up, and the turning continues.
It is important that the current entering the motor is D.C. because this means that the positive side of the power source is always positive. Were A.C. being used in the motor, the wire loop would get a quickly changing current input, which wouldnt allow the loop to move very far in either direction. In reality, the motor would have many, many more coils in the wire, and the magnet would be much closer-fitted to the coil.. There would also be an axle attached throught the center of the coils of wire, so that the turning of the motor could be used, for instance, to turn a drill, or an electric fan.