Essential Physics for Manual Medicine

Electricity and magnetism (for convenience, try a six-inch nail) half a dozen times then connect the other end of the wire to the negative terminal. When you disconnect the circuit, you will find that the rod has become a magnet. The corollary also holds true and is a principle used to generate electricity from the humblest bicycle dynamo to the largest power station. If a loop of wire is rotated between two magnets, it will gener­ ate a current; the wire is moving with respect to the magnetic field. This is summarized in the first law of electromagnetic induction, which states: A change of magnetic flux through a circuit Electromotive force causes an electromotive force (i.e., a force that (conventional current) causes electrons to move) to be induced in the circuit, which will be proportional to the rate Figure 7_20 • Fleming's right hand rule for a moving of change of magnetic flux and to the area of the conductor. If you align your right index finger with the circuit. (Practically, the loop(s) of wire are direction of the magnetic field (i.e. from north pole to south often made stationary and the magnets rotated pole) and your thumb with the direction of movement of the around them - the motion is relative: the wire, then your middle finger (3rd digit) will indicate the electrons can 't spot the difference!) direction of the electromotive force and thus the (conventional) current . . . assuming that you haven't The direction of the current is dependent on two dislocated your wrist in the process! factors : the direction of the magnetic field and the direction of the motion; this is demonstrated by The transformer Fleming's Right-hand Rule (Fig.7.20). Voltage can be increased or decreased (,stepped-up' The implications of this rule are very interesting; or 'stepped-down ') easily and efficiently using a if we see what happens at different points during a device called a transformer, which again makes use single revolution of the loop (Fig. 7 . 2 1 ), we can see of the principles of electromagnetic induction that the current initially flows in one direction then, (Fig. 7.22). If we have a circuit, part of which is as the loop is moving increasingly parallel to the lines wrapped around one side of a ring of iron, when of flux, less and less current will be produced. When an alternating current flows though the circuit, it it passes this point, the current generated will again will generate a magnetic field in the iron. Because increase although it will flow in the opposite direc­ the current is not just changing (the requirement tion. This process is then repeated for the second for magnetic induction) but actually alternating half of each revolution. If we plot the current against from positive to negative, the magnetic field will time, we see a familiar pattern, a sinusoidal wave­ vary from north-south, to zero, to south-north, to form: the coil has generated an alternating current. zero and back to north-south each cycle. Whether the coil is spun by a bicycle wheel or a We therefore have changing lines of magnetic steam turbine, the result is the same, although the flux moving through the iron bar, which has become frequency of the alternating current will be deter­ an alternating electromagnet. If a second loop of mined by the number of rotations per second. wire is passed around the other side of the ring, this There are several ways in which the output can be changing magnetic field will induce a current inside increased, these are detailed in Table 7.6; indeed, the wires (remember, even though the wire is static, the typical output of a power station is measured it is still in motion with respect to the magnetic in gigawatts and the potential difference along a field . . . electrons can't tell the difference). high-tension power cable can be as high as 1 SO kV. Although this is quite convenient for moving large amounts of electricity across the countryside, it is not a practical voltage for use in domestic and clini­ cal situations; fortunately, induction comes to the rescue once again. 169