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Magnetic field lines show the direction a north-seeking pole would point, conventionally from north to south outside a magnet. Closer lines indicate a stronger field. Like poles repel and unlike poles attract. A current produces a magnetic field; a solenoid gives a field pattern like a bar magnet. Increasing current or coil turns, or adding a suitable iron core, can strengthen an electromagnet. A field line is a representation, not a visible wire in space.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain magnetic fields and electromagnets
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What does a current create around a wire?
A magnetic field. Current and magnetism are linked.
After watching 2: What is needed for sustained ordinary transformer action?
Changing magnetic flux. An alternating supply produces changing flux.
What you'll be able to do
Magnetic field lines show the direction a north-seeking pole would point, conventionally from north to south outside a magnet. Closer lines indicate a stronger field. Like poles repel and unlike poles attract. A current produces a magnetic field; a solenoid gives a field pattern like a bar magnet. Increasing current or coil turns, or adding a suitable iron core, can strengthen an electromagnet. A field line is a representation, not a visible wire in space.
A current-carrying conductor in a magnetic field can experience a force. Direction depends on current and field directions; reversing one reverses the force. A coil arranged with opposite forces can turn as a motor. The force is greatest when current is perpendicular to the field in the relevant straight-wire model. Distinguish a motor, converting electrical input to mechanical output, from a generator, transferring mechanical energy electrically.
Relative motion between a magnet and coil, or a changing current nearby, can induce a potential difference. A closed circuit then allows an induced current. Faster change, more coil turns or a stronger field can increase the effect. Transformers use changing magnetic flux to link two coils. For an ideal transformer, voltage ratio equals turns ratio and input power equals output power; real devices have losses. A steady direct-current input does not sustain ordinary transformer action.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: What happens to motor force if current reverses but field stays the same?
The force reverses.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.