P13.2 Magnetic Fields of Electric Currents

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the field around a straight wire and a solenoid, the right-hand grip rule, uniform fields and electromagnets. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe and draw the magnetic field pattern around a long straight wire carrying a current
• Use the right-hand grip rule to find the direction of the field from the direction of the current
• Describe how the strength of the field near a wire depends on the current and the distance from the wire
• Describe and draw the field in and around a solenoid, and explain what a uniform field is
• Describe an electromagnet, why its core is iron, and how to make it stronger

Key words

• Electro- magnetism: The magnetic effect of an electric current: any current produces a magnetic field around it.
• Concentric circles: Circles that share the same centre. The field lines around a straight wire form concentric circles centred on the wire.
• Solenoid: A long coil of insulated wire. A current in it produces a strong magnetic field.
• Uniform field: A field that has the same strength and direction everywhere: the field lines are parallel and equally spaced.
• Electro- magnet: A solenoid wound around an iron core. It is a magnet only while there is a current.
• Core: The bar of material (usually iron) inside a coil.

The field around a straight wire

When there is a current in a wire, a magnetic field is produced around the wire. You can show this with plotting compasses placed on a horizontal card with the wire passing vertically through it. Use a wooden or plastic stand so that no iron is nearby.

• The field lines are concentric circles centred on the wire, in a plane at right angles to the wire.
• The field is stronger if the current is bigger.
• The field is weaker further from the wire (the circles are drawn further apart).
• Reversing the current reverses the direction of the field (the compasses all turn round).

Exam tip: If you move a compass away from the wire, its needle turns back towards magnetic north. Far from the wire the Earth’s field is stronger than the wire’s field.

The right-hand grip rule

Imagine gripping the wire with your right hand so that your thumb points the way the current flows. Your fingers then curl round in the direction of the field lines.

Worked example: which way does the compass point?

A vertical wire carries a current downwards. A compass is placed due east of the wire. Looking down from above, which way does the field go?
Thumb pointing down: the fingers curl clockwise when seen from above.
Going clockwise, the field at a point due east of the wire points due south.

Answer: the N end of the compass points south (if the wire’s field is much stronger than the Earth’s)

The field of a solenoid

Winding the wire into a long coil makes the fields of all the turns add together. This makes a much stronger field.

• Inside the solenoid the field is strong and uniform: the lines are parallel to the axis, equally spaced and all in the same direction.
• Outside, the field has the same shape as the field of a bar magnet. One end acts as a N-pole and the other as a S-pole.
• A bigger current or more turns per metre makes the field stronger. Reversing the current swaps the poles.

To find which end is which, look at the end of the coil and follow the current:

Exam tip: You can also use the right-hand grip rule on a coil: curl your fingers the way the current flows round the turns, and your thumb points to the N-pole.

Electromagnets

An electromagnet is a solenoid wound on an iron core. The field of the coil magnetises the iron (induced magnetism), which makes the field many times stronger. When the current is switched off, the iron loses almost all its magnetism, so the electromagnet can be turned on and off.

• increase the current; Why it works each turn of wire produces a stronger field
• add more turns of wire, or wind the turns closer together; Why it works more turns per metre add their fields together
• use an iron core; Why it works the iron becomes magnetised and adds its own field

The core is made of iron, not steel. Iron magnetises easily and loses its magnetism as soon as the current stops. Steel would stay magnetised, so the electromagnet could not be switched off.

Key fact: The wire must be insulated so that the current goes round every turn instead of short-circuiting between touching turns or through the iron core.

Common mistakes

• Drawing the field round a wire as straight lines or as a bar-magnet shape. It is concentric circles.
• Using the left hand for the grip rule. The grip rule always uses the right hand.
• Saying the field inside a solenoid is weak. It is strong and uniform.
• Choosing steel for an electromagnet core. Steel stays magnetised, so the magnet would not switch off.
• Forgetting that more turns only helps if the current stays the same.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the shape of the magnetic field lines around a long straight wire carrying a current? (Choose from: circles centred on the wire, straight lines parallel to the wire, the same shape as a bar magnet, straight lines pointing away from the wire.)

Answer: circles centred on the wire

Working: The field lines are concentric circles centred on the wire, further apart further from the wire.

Question 2. The diagram shows a wire seen end-on, carrying a current into the page. Four plotting compasses are placed at 1, 2, 3 and 4. Use the right-hand grip rule. Which way do the field lines go round the wire, as seen in the diagram? (Choose from: clockwise, anticlockwise, straight out from the wire, straight into the wire.)

Answer: clockwise

Working: With the thumb pointing into the page, the fingers curl clockwise, so the compasses point clockwise round the wire.

Question 3. Which change would not make an electromagnet stronger? (Choose from: reversing the direction of the current, increasing the current, adding more turns of wire, putting an iron core inside the coil.)

Answer: reversing the direction of the current

Working: Reversing the current swaps the N and S poles but does not change the strength.

Related lessons

• Previous lesson: P13.1 Magnetic Fields
• Next lesson: P13.3 The Motor Effect

Continue in the full course

This lesson is section P13.2 (AGCSC4202) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.