# OCR - P3: Electric circuits

 P3.5 What are magnetic fields? Background to the topic What you should be able to do: Around any magnet there is a region, called the magnetic field, in which another magnet experiences a force. The magnetic effect is strongest at the poles. The field gets gradually weaker with distance from the magnet. The direction and strength of a magnetic field can be represented by field lines. These show the direction of the force that would be experienced by the N pole of a small magnet, placed in the field. The magnetic field around the Earth, with poles near the geographic north and south, provides evidence that the core of the Earth is magnetic. The N-pole of a magnetic compass will point towards the magnetic north pole. Magnetic materials (such as iron and nickel) can be induced to become magnets by placing them in a magnetic field. When the field is removed permanent magnets retain their magnetisation whilst other materials lose their magnetisation. When there is an electric current in a wire, there is a magnetic field around the wire; the field lines form concentric circles around the wire. Winding the wire into a coil (solenoid) makes the magnetic field stronger, as the fields of each turn add together. Winding the coil around an iron core makes a stronger magnetic field and an electromagnet that can be switched on and off. In loudspeakers and headphones the magnetic field produced due to a current through a coil interacts with the field of a permanent magnet. The 19th century discovery of this electromagnetic effect led quickly to the invention of a number of magnetic devices, including electromagnetic relays, which formed the basis of the telegraph system, leading to a communications revolution 1. Describe the attraction and repulsion between unlike and like poles for permanent magnets In practical sessions you will use plotting compasses to map the magnetic field near a permanent bar magnet, between facing like/opposite poles of two magnets, a single wire, a flat coil of wire and a solenoid. Investigate the relationship between the number of turns on a solenoid and the strength of the magnetic field. Build a loudspeaker. You will discuss how the developments of electromagnets have led to major changes in people's lives, including applications in communications systems (speaker and microphone), MRI scanners and on cranes in scrapyards. 2. Describe the characteristics of the magnetic field of a magnet, showing how strength and direction change from one point to another 3. Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic 4. Describe the difference between permanent and induced magets 5. Describe how to show that a current can create a magnetic effect 6. describe the pattern and directions of the magnetic field around a conducting wire 7. Recall that the strength of the field depends on the current and the distance from the conductor 8. Explain how the magnetic effect of a solenoid can be increased 9. Explain how a solenoid can be used to generate sound in loudspeakers and headphones (separate science only)