Electricity and magnetism: current, voltage, field, induction

Use voltage, current, resistance, magnetic fields and induction to reason about real faults: mains interference on signals, hot connections, transformers, motors, coils and the energy that stays in a switched-off machine.

Why an engineer needs this first

Every machine in a hospital runs on electricity, and most of them also use magnetism somewhere: a transformer in the power supply, a motor in a table or pump, a coil in an MRI scanner, a stator spinning an X-ray anode. When you understand four ideas — voltage, current, resistance and induction — you can reason about faults on machines you have never seen. When you do not, you swap parts until the error disappears.

This lesson is not a circuit theory course. It gives you the working picture a senior engineer carries in his head, and ties each idea to a symptom you will meet on site.

Voltage, current, resistance and power

Voltage is the push: the difference in electrical potential between two points, measured in volts. Current is the flow of charge that the push produces, measured in amperes. Resistance is how hard the path makes it for current to flow, measured in ohms. Ohm's law ties them together: current equals voltage divided by resistance. Double the resistance at the same voltage and the current halves.

Power is voltage times current, in watts. Power lost in a resistance is current squared times resistance. That second form explains one of the most common real faults: a loose or corroded connection. Its resistance may be only a fraction of an ohm, but if it carries several amperes it turns that current into heat right at the contact. The plastic browns, the contact oxidises, the resistance rises, and it gets hotter still. A discoloured mains connector, a melted battery terminal and an intermittent power-off on a portable unit are often the same physics.

Direct current flows one way, as from a battery. Alternating current reverses direction many times a second — 50 times in Kazakhstan, Russia and Europe, 60 times in North America and in a number of other countries, including parts of South America and Asia and half of Japan. Check the rating plate of imported equipment. Mains is alternating because it can be transformed, which brings us to magnetism.

• Measure voltage across a component, with the circuit live and with the right category meter and probes.
• Measure current in series, or with a clamp meter around one conductor — never around a whole cable, where the currents cancel.
• Measure resistance only on a de-energised and discharged circuit.

Magnetic fields and the units you will see

Any current produces a magnetic field around it. Wind the wire into a coil and the fields add up; put iron inside the coil and the field becomes much stronger. Permanent magnets produce a field from the alignment of their own atoms. Field strength (more exactly, magnetic flux density) is measured in tesla. The Earth's field is roughly 25 to 65 microtesla depending on where you stand. A clinical MRI magnet is typically 1.5 or 3 tesla — tens of thousands of times stronger. You will also meet the older unit gauss: one tesla is ten thousand gauss, and the '5 gauss line' around an MRI magnet (0.5 millitesla) is the boundary used for pacemaker and access control.

A current in a magnetic field feels a force. That is how every motor works, how a loudspeaker moves, and why MRI gradient coils knock loudly: large pulsed currents in a strong static field push on the coil conductors. The same force throws a ferromagnetic object towards an MRI magnet — the static field pulls iron, and the effect grows very steeply as the object gets closer. Tools, gas cylinders and floor polishers have been turned into projectiles this way.

Induction: changing fields make voltages

Faraday's law: a changing magnetic flux through a loop induces a voltage in that loop. The faster the change and the more turns, the bigger the voltage. Lenz's law adds that the induced current opposes the change that caused it. These two sentences explain an astonishing amount of medical equipment.

A transformer is two coils on one iron core. Alternating current in the primary makes a changing flux; the flux induces a voltage in the secondary in proportion to the turns ratio. That is how an X-ray generator gets from mains to tens of kilovolts, and why its transformer must be fed alternating, not direct, current. Modern generators switch at high frequency precisely because a higher frequency allows a much smaller transformer.

A motor run backwards is a generator. Spin a stepper or brushless motor by hand and it produces a voltage — enough, on a heavy table or gantry, to back-feed electronics if someone pushes it with the drive disconnected. Eddy currents are induced in any conducting metal that sees a changing field; they heat the metal and produce their own opposing fields. In MRI they distort images after gradient pulses and are compensated in software; in a metal object moved quickly near the magnet they produce a braking force you can feel.

The receive coil of an MRI scanner is a pure induction device: the patient's precessing magnetisation induces a tiny voltage in it, which is the image signal. You will see this again in lesson 6.

Reading a circuit like a service engineer

When a machine is dead or misbehaving, the physics gives you an order of work. Start where the energy enters: is the mains voltage present at the outlet and at the device inlet, and does the fuse or breaker conduct? Then follow the energy inward: the output voltages of the power supply, measured under load, compared with the values on the label or in the documentation. A supply that reads correctly with nothing connected and collapses under load points to a weak supply or a short downstream; disconnect loads one at a time to find which.

Voltage drop tells you where resistance is. With the device running, measure across a connector, a relay contact or a cable: a good contact drops almost nothing; a bad one drops a noticeable fraction of a volt and gets warm. That one measurement, done live with care, finds faults that a resistance meter on a cold, unloaded circuit will never show. Live measurement is only for people who are trained and authorised for it under local electrical safety rules (in Kazakhstan and Russia this means the appropriate electrical safety group and the employer's permission); if you are not, or the procedure does not call for it, measure with the circuit de-energised.

Finally, remember that a measurement changes what it measures if you are careless. A meter in current mode across a voltage is a short circuit. A probe slipping between two pins can destroy a board. Choose the function before you touch, use probes with shrouded tips, and keep one hand away from the circuit on anything above safe low voltage.

Interference: induction you did not want

The same physics that makes transformers work also injects mains frequency into signals. A cable loop near a mains transformer picks up a 50 Hz voltage by induction. Two cables close together couple through their capacitance. Two separate earth connections at slightly different potentials make a ground loop and drive a current through a screen. The result is the classic 50 Hz hum on an ECG trace, ripple on an ultrasound image, or noise on a sensor reading.

Three tools fight it. Keep loop areas small: run signal and return together, twisted if possible. Screen sensitive cables and earth the screen at the point the design intends — usually one end, not both. Keep distance: field from a transformer or motor drops quickly as you move away. Before blaming a board, look at what was moved, added or re-routed recently.

Stored energy: off is not safe

Capacitors store energy in an electric field: the energy is half the capacitance times the voltage squared. Because voltage is squared, a high-voltage capacitor stores a great deal. Power supply bulk capacitors, defibrillator energy storage capacitors and the cable and smoothing capacitances in an X-ray high-voltage circuit can stay charged long after the plug is pulled. Inductors store energy in a magnetic field and release it as a voltage spike when their current is interrupted — that is why relay coils have flyback diodes and why breaking a motor or magnet circuit under current produces an arc.

A superconducting MRI magnet is the extreme case: it stores its energy in the field with no power applied at all, permanently. Switching off the room does not switch off the magnet.

The practical rule: isolate, lock out, wait the time the manufacturer specifies, then prove dead with a meter you have checked on a known live source. Discharge high-voltage parts only with the method and tool the manufacturer specifies. If the documentation for a high-voltage or defibrillator circuit is not in your hand, you are not ready to open it.

On-site checklist

• Isolate, lock out and tag before opening; wait the discharge time stated by the manufacturer.
• Prove dead with a meter checked before and after on a known live source.
• Look for heat marks at connectors and terminals: brown plastic is a resistance problem.
• Use a clamp meter around one conductor only.
• When hunting interference, ask what was added, moved or re-routed recently.
• Never upsize a fuse to make a fault go away.

Check yourself