Maybe your child knows the physics but keeps getting the electromagnetism questions wrong. They set up their fingers, choose a hand, and still get it the wrong way round. The left and right hand rules look almost the same, so the wrong hand can go down and lose the mark on a question your child understands. A useful way to tell them apart starts with working out which effect the question is about.
In short
- The left-hand rule and the right-hand rule set the fingers up in a similar way, which can make the two rules look alike.
- Use the left hand for the motor effect: a current in a magnetic field makes the wire feel a force.
- Use the right hand for the generator effect: moving the wire in a magnetic field sets up a voltage, and a current flows if the circuit is complete.
- The first thing to spot is which effect the question is about, then you know which hand to use.
- Your child can practise this on a marked A-level Physics paper they already have, in about ten minutes.
Where the left and right hand rules get mixed up
One place the mix-up can happen is in the middle of a question. Your child points their fingers for the force, the field and the current, then reaches for the hand they used last time. The fingers point the right way. The hand is wrong. So the answer comes out backwards, even though the method was fine.
A switch from one effect to the other can create the confusion. One part of a question is about the motor effect, where a current feels a force. The next is about the generator effect, where a moving wire sets up a voltage. The fingers look much the same, so the wrong hand can slip in. You can see where this sits among the mistakes that cost A-level Physics marks.
What each hand is actually for
Here are the two hands, side by side. In both, the first finger stands for the field and the second finger for the current. The thumb is the one that changes: on the left hand it is the force on the wire, and on the right hand it is the way the wire is moved. So the first thing to work out is which effect the question is about.
Left hand: the motor effect
This is the motor effect: a current flows through a wire in a magnetic field, and the wire feels a force. The thumb gives the direction of that force, the first finger the field, the second finger the current.
Right hand: the generator effect
This is the generator effect: the wire is moved in a magnetic field, which sets up a voltage across it, and a current flows if the circuit is complete. The thumb gives the way the wire moves, the first finger the field, the second finger the current.
So the choice is not really about the fingers. It is about which effect is happening, and that tells you which hand to pick up.
The one question to ask first
Before picking a hand, your child works out which effect the question is about. Is a current in a magnetic field making something move? That is the motor effect, so it is the left hand. Is a wire being moved to set up a voltage or a current? That is the generator effect, so it is the right hand.
Asking what the question gives and what it wants back is a useful way to spot the effect. Once your child names the effect, the hand follows, so they are not trying to recall which hand is which from memory.
This clip shows a maths lesson recorded in Pencil Spaces, the online classroom used by Greenhill Academics.
Not sure if it is the physics or the hand rule?
Tell us how your child’s electromagnetism answers go wrong. A tutor can look at the paper and see where the choice breaks down.
Ask about a tutorTry this at home: ten minutes on a marked paper
This takes about ten minutes. You need one of your child’s marked A-level Physics papers with some electromagnetism questions on it. So you are only checking the hand rule, not re-marking the paper.
- Find the hand-rule questions (2 minutes). Go through the paper and mark each question that uses a magnetic field with a force, a current or a movement.
- Name the effect (3 minutes). For each one, your child writes what the question gives and what it wants, then names the effect: motor if a current feels a force, generator if a moving wire sets up a voltage or current.
- Pick the hand (3 minutes). The motor effect is the left hand. The generator effect is the right hand. Write L or R next to each.
- Check against the mark (2 minutes). Compare each choice with the marked answer, and put a dot next to any where the hand was wrong.
What should you look for?
If the fingers were right but the hand was wrong, that may point to a hand-rule habit, and the step above is what to practise. If your child could not set up the fingers at all, that may be the idea to work on first. One ten-minute check cannot tell you why it is happening, and it cannot set the overall grade or cover the rest of the paper. It only shows whether the hand is being chosen the right way. A tutor can take the same paper and work on the choice with your child.
Want a tutor to practise this with your child?
Tell us the exam board and where the physics is going. A tutor can start with a paper your child has already sat.
Send an enquiryHow a tutor works on it
In a lesson, a tutor watches your child answer an electromagnetism question and sees the moment the hand goes down. They stop there, ask which effect the question is about, and have your child pick the hand out loud.
Then they practise it on the questions where it matters, from the motor effect to induced current, so choosing the hand becomes part of how your child reads the question. There is no promise of a set grade. The aim is simple: help your child pick the right hand for what each question is asking. It is one of the same habits behind how to get an A in A-level Physics.
The tutors who can help
These tutors studied physics and engineering at Oxford, Cambridge and Imperial, and teach A-level Physics. They work on the small choices, like which hand rule to use, that come up again in electromagnetism.
Lucas is completing a PhD in Aeronautical Engineering at the University of Oxford, after an MRes at Cambridge and top-of-cohort engineering degrees at Cagliari. He teaches Maths and Physics from GCSE through A level and IB, and prepares students for the ESAT and for Oxbridge and Imperial admissions.
Hugh holds a First-Class MSci in Theoretical Physics from Imperial College London and recently completed a DPhil at the University of Oxford. He teaches Maths, Further Maths and Physics from GCSE through to university entrance, including STEP, PAT and MAT.
Ashruf studied Information and Computer Engineering at the University of Cambridge (Clare College), with A* Maths at A level, ranked first in his cohort, alongside Further Maths and Physics. He teaches Maths, Physics and Further Maths at GCSE and A level, and coaches the MAT, TMUA and STEP.
Questions parents ask us about the hand rules
My child knows the physics, so why do they keep getting these questions wrong?
Knowing the physics and choosing the hand are two different steps. The two rules set the fingers up in a similar way, so the wrong hand goes down even when the method is right. A useful next step is to practise spotting the effect first, then choosing the hand.
What is the real difference between the left and right hand rules?
The left hand is for the motor effect, where a current in a magnetic field makes a wire feel a force. The right hand is for the generator effect, where moving a wire in a field sets up a voltage, and a current flows if the circuit is complete. The real difference is which effect is happening.
How do I tell which hand to use?
Work out which effect the question is about. A current in a magnetic field feeling a force is the motor effect, so the left hand. A moving wire that sets up a voltage or current is the generator effect, so the right hand.
How do I know if it is the hand rule or a bigger gap?
The ten-minute check above can give you a clue. If the fingers were right but the hand was wrong, the choice of hand is the thing to practise. If the fingers could not be set up at all, the idea may need work first.
What can a tutor do that we cannot do at home?
A tutor can watch your child answer under timed conditions, catch the moment the hand goes wrong, and practise the choice on the questions where it matters.
Want a tutor to work on the hand rules with your child?
Tell us the exam board and how the physics is going, and we can talk about a plan to practise choosing the right hand.
Send an enquiry