AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Magnetism and electromagnetism › The motor effect
Practise Electromagnetism. 9 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
A current in a wire produces a magnetic field around it. You need to describe how to show this, draw the fields around a straight wire and a solenoid with their directions, and explain how a solenoid and an iron core make the field stronger. In GCSE Physics (not Combined Science) you also use diagrams to explain how electromagnetic devices such as relays and electric bells work.
Key facts
Current in a wire → magnetic field around the wire
Straight wire: circles centred on the wire; stronger for more current, weaker further away
Right-hand grip rule: thumb = current, curled fingers = field
Solenoid: field inside is strong and uniform; outside it is like a bar magnet's field
Electromagnet = solenoid with an iron core
Stronger electromagnet: bigger current, more turns, iron core
Iron core: easily magnetised, and loses its magnetism quickly when the current is switched off
Notes
Field around a straight wire
diagram
When a current flows through a wire, a magnetic field is produced around the wire.
The field lines are circles centred on the wire.
The field is stronger for a bigger current, and weaker further from the wire (draw the circles further apart as you go out).
Reversing the current reverses the direction of the field.
Right-hand grip rule: point your right thumb in the direction of the current (+ to −). Your curled fingers show the direction of the field lines.
Right-hand grip rule: thumb along the current, fingers curl the way the field goes.
To show it: put plotting compasses on a card around a vertical wire. With the current on, they line up in a circle; reversing the current reverses them.
Solenoids and electromagnets
diagram
A solenoid is a wire wound into a long coil. Shaping the wire into a solenoid makes the magnetic field much stronger.
Inside a solenoid the field is strong and uniform: straight, parallel, equally spaced lines.
Outside, the field has the same shape as a bar magnet's field. One end acts as a north pole and the other as a south pole.
Solenoid (cut through lengthways; dots: current out of page, crosses: into page). Strong, uniform field inside; bar-magnet shape outside.
An electromagnet is a solenoid with an iron core. The iron becomes magnetised, which makes the field much stronger.
To make an electromagnet stronger: increase the current, add more turns, or add an iron core.
Iron is used because it is easily magnetised and loses its magnetism quickly when the current is switched off.
How to answer each type of question
Describe how to show the magnetic effect of a current
2 to 3 marksGrade 4
Describe the set-up: a wire through a card, connected to a power supply.
Say where the plotting compasses go (around the wire, on the card).
Say what you see when the current is switched on (and reversed).
Example. Describe how you could show that there is a magnetic field around a wire carrying a current. [3 marks]
Show the model answerHide the model answer
Pass the wire vertically through a horizontal piece of card and connect it to a power supply (1). Place several plotting compasses on the card around the wire (1). When the current is switched on, the compasses turn to point in a circle around the wire (and they reverse when the current is reversed) (1).
Describe or draw the field around a wire or solenoid
2 to 3 marksGrade 5
Straight wire: circles centred on the wire, getting further apart further out.
Find the direction with the right-hand grip rule and show it with arrows or words (clockwise or anticlockwise, and from which viewpoint).
Example. A long straight wire passes vertically through a horizontal piece of card. The current in the wire is upwards. Describe the magnetic field pattern on the card, and give its direction as seen from above. [3 marks]
Show the model answerHide the model answer
Concentric circles, further apart further out, anticlockwise from above. Circles centred on the wire (1). The circles get further apart further from the wire (1). The field is anticlockwise as seen from above (1).
Don’t lose marks
Drawing the circles round a wire evenly spaced: they get further apart further from the wire.
Drawing the lines inside a solenoid curved or spreading out: inside they are straight, parallel and equally spaced.
Using the left hand to find the direction of the field round a wire.
Suggesting a steel core: steel would stay magnetised after the current is switched off.
Forgetting the arrows on field lines.
More tips
Memory tricks
Right hand for fields round a current (thumb = current, fingers = field). The left hand is only for the motor-effect force.
Solenoid ends: look at one end. If the current goes anticlockwise, that end is a north pole; if clockwise, it is a south pole.
Stronger electromagnet: more Current, more Turns, an Iron core.
Outside a solenoid, draw the field as if the coil were a bar magnet.
Exam technique
When asked how a solenoid 'increases the magnetic effect', say that the fields from all the turns add together inside the coil.
Give directions clearly: 'clockwise' or 'anticlockwise' plus the viewpoint, e.g. 'as seen from above'.
For 'explain how it works' questions, write a chain of steps, joining each to the next with 'so'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State that a current produces a magnetic fieldWhen a current flows through a wire, a magnetic field is produced around the wire.
Grade 4
Describe how to show a wire's magnetic fieldPut plotting compasses around a vertical wire: they line up in a circle when the current is switched on.
Grade 5
Draw the field around a straight wireCircles centred on the wire, further apart further out, with arrows showing the direction.
Grade 5
State what affects the strength of the fieldThe field is stronger for a bigger current, and weaker further from the wire.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
When there is a current in a wire, what is produced around the wire?
A magnetic field
A solenoid is a long coil of wire. When there is a current in the solenoid, it produces a magnetic field. Describe the magnetic field inside the solenoid.
Strong and uniform.
A large direct current is switched on in the wire. Describe what happens to the compasses.
The needles turn so that together they form a circle around the wire.
What is a solenoid?
A long coil of wire.
Give one way to reverse the polarity (which end is the north pole) of an electromagnet, without changing anything about the coil itself.
Reverse the direction of the current.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) When there is a current in a wire, what is produced around the wire?[1]
(b) Give two factors that affect the strength of the magnetic field at a point near a straight wire.[2]
(c) Describe the shape of the magnetic field lines around a long, straight wire that is carrying a current.[1]
(d) What is an electromagnet? Tick (✓) one box.[1]
A solenoid with an iron core
A solenoid with a copper core
A permanent magnet made of steel
A straight wire carrying a current
Show the answer and mark scheme
(a)Answer: A magnetic field
a magnetic field
(b)Answer: The size of the current and the distance from the wire.
the size of the current (in the wire)
the distance from the wire
(c)Answer: Concentric circles centred on the wire.
(concentric) circles around the wire
(d)Answer: A solenoid with an iron core
Question 2Medium8 marks
A solenoid is a long coil of wire. When there is a current in the solenoid, it produces a magnetic field.
(a) Describe the magnetic field inside the solenoid.[2]
(b) Describe the shape of the magnetic field outside the solenoid.[1]
(c) Give three ways of increasing the strength of the magnetic field of the solenoid.[3]
(d) Electromagnets are used on cranes in scrapyards to lift old cars. Explain why an electromagnet is more useful for this than a permanent magnet.[2]
Show the answer and mark scheme
(a)Answer: Strong and uniform.
strong
uniform / the field lines are parallel and equally spaced
(b)Answer: Like the field around a bar magnet.
similar to the field of a bar magnet
(c)Answer: Increase the current, add an iron core, use more turns of wire.
increase the current
put an iron core inside the solenoid
increase the number of turns (on the same length of solenoid)
(d)Answer: It can be switched off, so the crane can release the car.
an electromagnet can be switched on and off
so the car can be picked up and then released (dropped) where it is needed
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