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In 1820 the Danish scientist Hans Christian Ørsted noticed that a compass needle twitched whenever he switched on a nearby electric current — and with that accidental observation, the link between electricity and magnetism was discovered. We now know that an electric current always produces a magnetic field around it. This is called the magnetic effect of a current, or electromagnetism, and it is the principle behind electromagnets, electric bells, relays and the giant cranes that lift scrap cars. Crucially, unlike a permanent magnet, an electromagnet can be switched on and off and its strength controlled — which is what makes it so useful. This lesson, part of Topic P4 (Magnetism and magnetic fields) of OCR Gateway Science A, builds up from the field around a single wire to the field of a solenoid and on to the electromagnet and its everyday uses.
By the end of this lesson you should be able to describe the magnetic field around a straight current-carrying wire, use the right-hand grip rule to find its direction, describe the field of a solenoid, explain how an electromagnet works and what increases its strength, and describe uses of electromagnets such as the scrapyard crane, relay and electric bell.
Whenever a current flows along a straight wire, it creates a magnetic field in the space around the wire. The field lines form a pattern of concentric circles — circles centred on the wire, lying in planes at right angles to it. The lines are closest together near the wire, where the field is strongest, and spread further apart as you move away, where the field is weaker.
The direction of these circular field lines is given by the right-hand grip rule:
Imagine gripping the wire with your right hand so that your thumb points in the direction of the (conventional) current. Your curled fingers then point in the direction of the magnetic field around the wire.
Two consequences follow directly from this rule:
The diagram below shows the circular field lines around a wire carrying current out of the page.
Exam Tip: For the field around a straight wire, remember concentric circles and the right-hand grip rule: thumb = current, curled fingers = field. Always state that reversing the current reverses the field direction.
A single wire's field is weak. To make a much stronger, more useful field, the wire is wound into a long coil of many turns called a solenoid. Inside the solenoid the circular fields of all the individual turns add together to give a strong, uniform magnetic field that runs straight along the inside of the coil — the field lines are parallel and evenly spaced, just like a uniform field.
Strikingly, the field outside a solenoid looks exactly like the field of a bar magnet: it loops from one end (which behaves as a north pole) round to the other (which behaves as a south pole). So a current-carrying solenoid is, in effect, a bar magnet that can be switched on and off with the current.
Exam Tip: A solenoid produces a strong, uniform field inside (straight parallel lines) and a field outside that looks like a bar magnet's. The big advantage over a permanent magnet is that it can be turned on and off and its strength changed.
An electromagnet is a solenoid with a core of soft iron placed inside it. When the current flows, the iron core becomes strongly magnetised (it becomes an induced magnet), greatly increasing the strength of the magnetic field — far beyond what the coil alone could produce. When the current is switched off, the soft iron loses almost all of its magnetism at once, so the electromagnet switches off cleanly. (This is why soft iron is used rather than steel: soft iron magnetises and demagnetises easily, whereas steel would keep its magnetism and stay magnetic after the current stopped.)
There are three ways to make an electromagnet stronger:
| To increase the strength of an electromagnet… | …because |
|---|---|
| Increase the current through the coil | A bigger current makes a stronger magnetic field |
| Increase the number of turns on the coil | More turns add more field together in the same space |
| Add a soft-iron core (or use a better one) | The iron becomes an induced magnet, hugely boosting the field |
The great advantage of an electromagnet is control: unlike a permanent magnet, it can be switched on and off, and its strength can be varied by changing the current or the number of turns. Its direction can also be reversed by reversing the current.
Exam Tip: To make an electromagnet stronger: more current, more turns, add a soft-iron core. Use soft iron (not steel) for the core because it loses its magnetism when the current is switched off.
The on/off control of an electromagnet makes it useful in many devices.
The scrapyard crane. A large electromagnet on a crane is switched on to attract and lift steel objects (such as scrap cars), then moved over a skip and switched off to drop the load exactly where it is wanted. A permanent magnet could not do this — there would be no way to release the load.
The electric bell. When the push switch is pressed, current flows through an electromagnet, which attracts a springy iron armature. As the armature moves across, it strikes the gong and at the same time breaks the circuit. With the circuit broken, the electromagnet switches off, the spring pulls the armature back, which remakes the circuit — and the cycle repeats rapidly, so the hammer strikes the gong again and again to make a continuous ringing.
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