Upon successful completion of this module, the student will be able to:
Describe the characteristics of a magnetic field and differentiate between permanent magnets and electromagnets.
Explain the principle of electromagnetism using the right-hand rule for a current-carrying conductor and coil.
Explain the construction and operation of DC relays and solenoids.
Analyze the function of relays and solenoids within automotive and industrial circuits.
Describe the basic operating principle of a DC motor by explaining the interaction between magnetic fields.
Up to this point, we've treated electricity as something that happens inside wires and components. We've learned to control it, calculate it, and troubleshoot it. Now, we're going to unleash its true power: the ability to create physical motion.
This is the world of electromagnetism. It's the fundamental principle that allows a tiny current from your car key to crank a massive engine, and an electrical signal in a factory to move a powerful hydraulic arm. Understanding this connection between electricity and magnetism is the key to mastering almost every modern piece of technology.
Let's get started.
What is a Magnetic Field?
You've played with magnets. You know they have a North and a South pole and that opposite poles attract while like poles repel. The invisible energy field that causes this push and pull is the magnetic field. We can't see it, but we can see its effects.
Permanent vs. Electromagnets
Permanent Magnets: Like the magnet on your fridge, these are made from materials (like hard iron) that are "always on." Their magnetic domains are locked in alignment.
Electromagnets: This is where things get interesting. When you pass an electric current through a wire, it generates a magnetic field. If you wrap that wire into a coil around a soft iron core, you create a powerful magnet that you can turn on and off with a switch. This is the single most important concept in this module.
The Right-Hand Rule: Predicting the Field
How do you know which end of your electromagnet is North? Use the Right-Hand Rule. If you wrap your right hand around the coil so your fingers point in the direction of the conventional current flow, your thumb will point towards the North pole. It’s a simple trick every technician uses.
Now that we can create a magnet on command, we can use it to do useful things.
The Relay: An Electromagnetic Switch
A relay is a clever way to use a small current to control a very large current.
How it Works: A small current energizes a coil (the electromagnet). The magnetic field pulls on a tiny lever (the armature), which closes a separate, heavy-duty switch.
The Automotive Example: A Starter Relay. Turning your car key sends a small, safe current (maybe 1A) to the starter relay. The relay's electromagnet snaps shut, closing a massive switch that can handle the 150A+ needed to power the starter motor. The relay protects your delicate ignition switch from being vaporized.
The Solenoid: An Electromagnetic Plunger
A solenoid uses the exact same principle as a relay, but instead of closing an electrical contact, its goal is to create linear motion (a push or pull).
How it Works: A current energizes a coil. The magnetic field pulls a metal plunger into the center of the coil.
The Industrial Example: A Solenoid Valve. On a factory machine, a PLC sends a 24V signal to a solenoid valve. The electromagnet pulls a plunger back, opening a gate and allowing high-pressure hydraulic fluid to flow to a cylinder, moving a heavy component. Turn off the signal, a spring pushes the plunger back, and the flow stops. It's a simple, powerful way to convert an electrical signal into mechanical force.
Let's bridge the gap from a simple push/pull to continuous rotation.
The DC Motor Principle
A DC motor is a symphony of electromagnets.
It has stationary magnets (or electromagnets) on the outside called the stator.
It has a rotating electromagnet on the inside called the rotor (or armature).
When you send current to the rotor, it becomes a magnet. Its North pole is repelled by the stator's North pole and attracted to the stator's South pole. This creates a powerful rotational force (torque).
Here's the genius part: Just as the rotor is about to align, a mechanical switch called the commutator reverses the current flow in the rotor. This flips the rotor's magnetic poles! The pole that was just attracted is now repelled, keeping the motor spinning continuously in the same direction.
The Automotive Starter System: Putting It All Together
The car starter is the ultimate example. It combines all these concepts into one rugged unit.
You turn the key (low current).
This energizes the solenoid's coil.
The solenoid's electromagnet does two jobs at once:
It acts as a solenoid, pushing a plunger forward to engage the starter gear with the engine's flywheel.
It acts as a relay, closing the heavy-duty contacts to allow massive battery current to flow.
That massive current flows directly into the starter motor, which uses the principles of magnetic repulsion and attraction to spin the engine to life.
You are a technician. A customer says, "When I turn the key, my car just makes a single, loud 'CLICK' and nothing happens."
A rookie might say "you need a new starter" or "it's the battery." A pro evaluates the evidence to create a precise test plan.
Your Diagnostic Plan:
Evaluate the Symptom: The "CLICK" is the sound of the starter solenoid/relay energizing and closing its contacts. This is a HUGE clue. It tells you the entire low-current control circuit is working perfectly: the ignition switch, the neutral safety switch, and the relay's coil are all good. The problem is on the high-current load side.
Create a Hypothesis: The problem could be one of three things:
The high-current contacts inside the solenoid are worn out and not letting power through.
There is a bad connection (high resistance) in the heavy battery cables.
The starter motor itself has seized or failed.
Create a Test Plan (KVL Voltage Drop Test):
Test 1 (The Solenoid Contacts): Set your multimeter to DC Volts. Place one lead on the large battery cable terminal on the solenoid and the other lead on the large terminal going out to the starter motor. Have a helper turn the key to the "start" position (it will just "click").
The Expected Result: A good set of contacts is like a closed switch; the voltage drop across it should be very low (under 0.5V).
The Fault Finding: If you read a high voltage (e.g., 10V or more) across these two terminals while it's clicking, you have just proven the contacts are bad. The voltage is available on one side but isn't making it to the other. You have diagnosed a bad starter solenoid without ever removing a part.
(Remembering)
What is the name of the device that uses a low-current electromagnetic coil to operate a high-current switch?
According to the Right-Hand Rule for a coil, if your fingers curl in the direction of current, what does your thumb indicate?
(Understanding)
3. Explain why a soft iron core is used in an electromagnet instead of a hard iron core used for a permanent magnet.
4. In your own words, describe the two main functions that a starter solenoid performs simultaneously.
(Applying)
5. You are building an electromagnet by wrapping a wire around an iron bolt. What are two ways you could increase the strength of its magnetic field?
6. A relay has a coil resistance of 80Ω. It is connected to a 12V source. How much current is needed to energize the relay's electromagnet?
(Analyzing)
7. A technician is testing an industrial solenoid valve circuit. They measure 24V at the coil, but the valve doesn't open. They then measure the resistance of the coil and get a reading of "OL" (infinite). Analyze these two findings to determine the most likely fault.
8. A simple DC motor is spinning clockwise. What would happen to the motor's rotation if you reversed the polarity of the power source connected to it? Explain your reasoning.
(Evaluating)
9. A technician is designing a system and needs to choose between using a relay or a high-power transistor to switch a load. The load is a simple heater that is switched on and off infrequently. The relay is cheaper and simpler, but the transistor is solid-state with no moving parts. Which is likely the better choice for this specific application? Justify your answer.
(Creating)
10. A technician suspects the "hold-in" coil of a large industrial contactor (a heavy-duty relay) is failing, causing it to chatter. Create a simple, safe, off-vehicle test plan to determine if the electromagnet is strong enough to hold the contacts closed once they have been manually pushed in.
Various circuits using basic electrical kits, mobile modular, control circuit panels