Magnetic Forces & Electromagnets
Learn how magnetic poles interact, what makes magnetic forces stronger or weaker, and how electromagnets work by changing coil turns and electric current.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Magnetic Forces & Electromagnets, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every magnet has two poles — north and south — and they follow a simple rule: opposite poles attract, but like poles push away from each other. This repulsion and attraction happen without the magnets touching, which makes magnetism a force that works at a distance. But not all magnetic forces are equally strong. In this lesson, you will discover what makes magnetic forces stronger or weaker — the distance between magnets, how powerful the magnet is, and, if you are working with an electromagnet, how tightly you wind the coil and how much electric current flows through it. Understanding these factors helps you predict which design will create the strongest electromagnet for a real application.
Magnetic Poles and How They Interact
Every permanent magnet has two poles: a north pole and a south pole. If you try to bring the north pole of one magnet close to the north pole of another magnet, they push away from each other. The same happens when you bring south pole to south pole. We say that like poles repel. But if you bring a north pole near a south pole, they pull toward each other — opposite poles attract. This happens because magnetic force is a force field that surrounds every magnet. You cannot see the field, but you can observe its effects when you move one magnet near another or when you bring a magnet close to a magnetic material like iron. The magnetic force acts at a distance, meaning the magnets do not have to touch for the force to work. This is similar to gravity, which also works at a distance between objects.
How Distance Affects Magnetic Force
Just as with gravity, the strength of a magnetic force depends on how far apart the magnets are. When two magnets are very close together, the magnetic force between them is strong. As you move them farther apart, the force gets weaker. If you conducted an investigation where you measured how much force it takes to push two magnets together (or pull them apart), you would find that the force drops rapidly as distance increases. This is why you can feel a strong pull when a magnet is almost touching a paperclip, but barely any pull from across the room. When you are interpreting data from a magnetic force investigation, look for this pattern: as distance increases, force decreases. The exact relationship is not as simple as with gravity, but the general trend — closer magnets interact more strongly — is a reliable guide for predicting what will happen when you change how far apart the magnets are.
Permanent Magnet Strength and Other Factors
Not all magnets are equally strong. A large, well-made permanent magnet pulls harder on iron objects than a weak, small magnet does. When you investigate how magnet strength affects the magnetic force, you test magnets of different sizes or quality while keeping everything else the same — the distance, the type of object being pulled, the environment. If you double the strength of a magnet, the magnetic force roughly doubles as well. You can also change the direction of a magnetic force by rotating a magnet. For example, if the north pole of one magnet is attracting an iron object, turning the magnet around so the south pole faces the object still produces attraction — but now it is the south pole doing the attracting. The key insight is that you must vary one factor at a time while keeping others constant, so you can clearly see which factor is responsible for the change in force.
Electromagnets: Current and Coil Turns
An electromagnet is a temporary magnet created by running electric current through a coil of wire. Unlike a permanent magnet, an electromagnet can be turned on and off, and its strength can be controlled. Two factors change the strength of an electromagnet: the number of coils (or turns) in the wire, and the amount of current flowing through the wire. When you increase the number of turns in the coil, you increase the magnetic field strength. Wrapping the wire around a soft iron core (like an iron nail) also makes the electromagnet stronger. When you increase the electric current, the electromagnet becomes stronger. Running twice as much current through the same coil roughly doubles the magnetic force. This is why electromagnets are so useful in real applications — you can adjust their strength by changing the current or rewinding the coil. In an investigation, if you wind more coils while keeping the current the same, you should observe a stronger magnetic force. If you increase the current while keeping the number of coils constant, you should also observe a stronger force.
Designing and Improving a Magnetic Investigation
When you conduct or plan an investigation about magnetic forces, you must think carefully about what question you are trying to answer and which factor you want to test. A valid investigation question is specific and testable — for example, 'How does the number of coil turns affect the lifting force of an electromagnet?' rather than 'Do electromagnets work?' To test this question, you would build electromagnets with different numbers of turns, connect each to the same power source (so current is constant), and measure how many paperclips each one can pick up. You would keep the coil diameter, the wire type, the iron core, and the current all the same so that only the number of turns changes. A good next trial in a sequence of investigations might be to test whether current also affects lifting force, or to test whether the type of iron core matters. When you see an investigation result, ask yourself: was only one factor changed? Were all other conditions kept the same? If yes, then the data can clearly tell you what caused the change in force.
Key terms
- Magnetic pole.
- One of two ends of a magnet where the magnetic field is strongest; labeled north or south based on direction.
- Like poles.
- Two poles of the same type (both north or both south); they repel each other.
- Opposite poles.
- A north pole and a south pole; they attract each other.
- Magnetic force.
- The push or pull that one magnet exerts on another magnet or magnetic material, acting at a distance.
- Electromagnet.
- A temporary magnet made by running electric current through a coil of wire, typically wrapped around an iron core.
- Coil turns.
- The number of times wire is wrapped around the core of an electromagnet; more turns increase magnetic strength.
- Magnetic field.
- The invisible region around a magnet where magnetic force can be detected and felt.
Worked example
You are designing an electromagnet to pick up metal scraps in a recycling factory. You test three different electromagnets, all connected to the same 12-volt power supply. Electromagnet A has 10 coil turns, Electromagnet B has 20 coil turns, and Electromagnet C has 40 coil turns. You hold each electromagnet the same distance above a pile of steel scraps and count how many scraps each one picks up. Your results: A picks up 8 scraps, B picks up 16 scraps, and C picks up 32 scraps. (a) What factor did you change in this investigation? (b) What pattern do you observe? (c) If you built a fourth electromagnet with 80 coil turns, what would you predict it would pick up?
(a) The factor you changed is the number of coil turns. Electromagnet A had 10, B had 20, C had 40. Everything else stayed the same: the voltage from the power supply (12 volts), the distance from the pile, the wire type, and the core material. So you are isolating the effect of coil turns on electromagnet strength.
(b) Look at the data: as the number of turns doubled (10 → 20 → 40), the number of scraps picked up also roughly doubled (8 → 16 → 32). This shows a pattern: doubling the coil turns doubles the magnetic force. The relationship is proportional in this range.
(c) If you double the coil turns from 40 to 80, you would expect to double the scraps picked up from 32 to 64. Based on the pattern in your data, an electromagnet with 80 turns should pick up approximately 64 scraps (or close to it, accounting for small variations in the experiment). This prediction assumes the same distance, power supply, and other conditions hold.
(b) Look at the data: as the number of turns doubled (10 → 20 → 40), the number of scraps picked up also roughly doubled (8 → 16 → 32). This shows a pattern: doubling the coil turns doubles the magnetic force. The relationship is proportional in this range.
(c) If you double the coil turns from 40 to 80, you would expect to double the scraps picked up from 32 to 64. Based on the pattern in your data, an electromagnet with 80 turns should pick up approximately 64 scraps (or close to it, accounting for small variations in the experiment). This prediction assumes the same distance, power supply, and other conditions hold.
Practice questions
You have two bar magnets. You test how the force between them changes as you move them closer and farther apart, keeping them in the same orientation (north pole of magnet 1 always facing south pole of magnet 2). Which statement correctly describes what you would observe?
- The magnetic force stays exactly the same no matter how far apart the magnets are.
- The magnetic force increases as the magnets get closer together.
- The magnetic force decreases as the magnets get closer together.
- The magnetic force is strongest when the magnets are on opposite sides of a room.
Answer: The magnetic force increases as the magnets get closer together.
Magnetic force depends on distance. When magnets are very close, the force is strong. As you move them farther apart, the force becomes weaker. This is the key pattern: distance and force are related inversely — greater distance means weaker force, and smaller distance means stronger force. The other options ignore this distance dependence or reverse the relationship.
An electromagnet is made by wrapping 50 turns of wire around an iron nail and connecting it to a 6-volt battery. In a test, it picks up 12 paperclips. You want to increase the pulling force without changing the battery. What should you do, and why?
Answer: You should increase the number of coil turns (for example, wrap the wire 100 times instead of 50 times). You could also use thicker wire or a stronger core, but increasing turns is the most direct method.
Since you cannot change the voltage (the battery is fixed at 6 volts), and the current depends on the voltage and resistance of the circuit, the most reliable way to increase electromagnet strength is to increase the number of coil turns. Each additional turn of wire adds to the magnetic field. Doubling the turns should roughly double the lifting force, so your electromagnet should pick up close to 24 paperclips. This is a better approach than trying to change the battery, which you said you cannot do.
You conduct an experiment in which you test how the current through an electromagnet affects its strength. You use four different power supplies (giving 3 volts, 6 volts, 12 volts, and 24 volts) and measure the magnetic force at each setting. You wrap the wire the same way each time (same number of turns, same core, same wire diameter). The data show that doubling the voltage roughly doubles the magnetic force. What can you conclude?
Answer: You can conclude that increasing the electric current through the electromagnet increases its magnetic force. Since higher voltage pushes more current through the wire, and doubling the voltage doubled the force, the data support a proportional relationship between current and magnetic strength in this range.
Your experiment held all factors constant except voltage. By varying the power supply, you indirectly varied the current flowing through the coil. The pattern — doubling voltage led to doubling force — tells you that current and magnetic force are proportional in this experiment. This is a valid conclusion based on your controlled investigation, and it matches what we expect from electromagnet physics.
FAQ
- If I break a bar magnet in half, will I get one north pole and one south pole?
- No. When you break a magnet in half, each piece becomes its own complete magnet, with its own north pole and south pole. The south pole does not stay attached to one end and the north pole to the other. This happens because magnetic properties come from the alignment of electrons throughout the material, not from two separate poles sitting at the ends.
- Why does an electromagnet stop working when you turn off the power?
- An electromagnet creates its magnetic field only when electric current flows through the coil. When you disconnect the power, the current stops, and the moving electrons no longer create a magnetic field. A permanent magnet, by contrast, has magnetic properties built into its material that persist without any power source.
- Can I make an electromagnet stronger by using a thicker wire or a longer wire?
- Thicker wire can help because it has lower resistance, allowing more current to flow at the same voltage, which strengthens the magnet. However, using longer wire (for the same number of turns) actually makes resistance higher, which reduces current and weakens the magnet. The most straightforward ways to strengthen an electromagnet are to increase the number of coil turns or increase the voltage (current).
- What does the iron core in an electromagnet actually do?
- The iron core amplifies the magnetic field created by the current in the coil. Iron is a magnetic material that easily aligns with the field, and it concentrates the field lines through the center of the coil. Without the core, the same number of coil turns carrying the same current would produce a much weaker electromagnet. This is why electromagnets usually wrap around an iron nail or rod.
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The Crimsora tutor teaches Magnetic Forces & Electromagnets live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.