The motor effect
What you'll learn
- Why a wire in a circuit can feel a force near a magnet.
- How to predict the force direction using Fleming’s left-hand rule.
- How to calculate the force using F=BIlF = BIlF=BIl.
- How this force is used to make a simple electric motor rotate.
Edexcel marks this motor-effect content as Higher Tier. The electric motor explanation is also 12.14P, which is Separate Physics content, so it is part of GCSE Physics 1PH0.
The basic idea: a current can produce a force
A conductor is a material, such as a metal wire, that allows electric charge to flow. An electric current is the flow of electric charge around a circuit. In metals, electrons move, but in GCSE questions we normally use conventional current, which is the direction from the positive terminal to the negative terminal.
A magnetic field is the region around a magnet where another magnet or magnetic material can experience a force. Outside a magnet, magnetic field lines go from the north pole to the south pole.
The motor effect
The motor effect is the force experienced by a current-carrying conductor placed in a magnetic field.
The wire experiences a force because there are two magnetic fields interacting:
- the magnetic field from the permanent magnet
- the magnetic field produced around the current-carrying wire
Magnetic forces are due to interactions between magnetic fields. The combined field is stronger on one side of the wire and weaker on the other, so the wire is pushed.
Forces come in pairs
If the wire feels an upward force, the magnet feels an equal-sized downward force. These forces are equal and opposite, but they act on different objects, so they do not “cancel out” for either object.
Forgetting the force on the magnet
The question may ask about the force on the magnet, not the wire. If the wire is pushed up, the magnet is pushed down with the same size force.
When is the force biggest?
The force is greatest when the current direction is at 90 degrees to the magnetic field direction. At GCSE, this is usually described as the conductor being at right angles to the magnetic field.
If the conductor is parallel to the magnetic field, there is no motor-effect force on it.
Maximum force
A current-carrying wire gets the maximum motor-effect force when the current, magnetic field and force are all mutually perpendicular.
Mutually perpendicular means all three directions are at 90 degrees to each other. In the motor effect, the three directions are:
- magnetic field direction
- current direction
- force direction
Predicting direction: Fleming’s left-hand rule
Fleming’s left-hand rule is a hand rule for finding the relative directions of the magnetic field, current and force when they are mutually perpendicular.
Hold your left hand so your thumb, first finger and second finger are all at right angles to each other:
- First finger = magnetic Field, from north to south
- seCond finger = conventional Current, from positive to negative
- thuMb = Motion or force on the wire
Remembering the fingers
Use FBI in order across your hand: First finger = Field, second finger = Current, thumb = Force or Motion.
In diagrams, current going out of the page is often shown as a dot in a circle. Imagine the tip of an arrow coming towards you. Current going into the page is shown as a cross in a circle. Imagine the tail feathers of an arrow moving away from you.
Finding the force direction
A wire has current going into the page between a north pole on the left and a south pole on the right. Which way is the force on the wire?
- The magnetic field direction is from the north pole to the south pole, so it points left to right.
- Point your left first finger to the right for the field, then rotate your hand so your second finger points into the page for the conventional current.
- Your thumb points down, so the force on the wire is downward.
Using the wrong current direction
Fleming’s left-hand rule uses conventional current, not electron flow. Do not reverse the current direction just because electrons move the opposite way in a metal.
If you reverse the current, the force reverses. If you reverse the magnetic field, the force also reverses. If you reverse both the current and the magnetic field, the force direction stays the same.
Calculating the force: F=BIlF = BIlF=BIl
The size of the motor-effect force on a straight conductor at right angles to a magnetic field is:
F=B×I×lF = B \times I \times lF=B×I×lwhere:
- FFF is the force on the conductor in newtons, N
- BBB is the magnetic flux density in tesla, T
- III is the current in amperes, A
- lll is the length of conductor inside the magnetic field in metres, m
Edexcel lists this as a use equation rather than a recall-and-use equation, so it is provided on the equation sheet. You still need to recognise when it applies, substitute correctly, and rearrange it if needed.
Magnetic flux density
Magnetic flux density, symbol BBB, is a measure of the strength of a magnetic field. Its unit is the tesla, T, which can also be written as newtons per ampere metre, N/A m.
Only for right angles
The equation F=BIlF = BIlF=BIl applies when the conductor is at right angles to the magnetic field. The length lll means the length of wire actually inside the magnetic field, not necessarily the whole wire in the circuit.
Calculating the force on a wire
A 12 cm length of wire is at right angles to a magnetic field of flux density 0.25 T. The current in the wire is 4.0 A. Calculate the force on the wire.
- Convert the length to metres: 12÷100=0.1212 \div 100 = 0.1212÷100=0.12, so use 0.12 m.
- The wire is at right angles to the field, so the correct equation is F=BIlF = BIlF=BIl.
- Substitute the values: F=0.25 T×4.0 A×0.12 mF = 0.25\ \text{T} \times 4.0\ \text{A} \times 0.12\ \text{m}F=0.25 T×4.0 A×0.12 m.
- Multiply the values: F=0.12 NF = 0.12\ \text{N}F=0.12 N.
The equation also shows proportionality:
- increasing BBB increases the force
- increasing III increases the force
- increasing lll increases the force
So, for example, doubling the current doubles the force if the magnetic field strength and length stay the same.
Using the motor effect in an electric motor
An electric motor is a device that transfers electrical energy into kinetic energy using the motor effect. A simple dc motor uses a coil, which is one or more loops of wire, placed between magnetic poles.
A dc supply provides direct current, meaning the current in the external circuit has one direction. The coil is connected to a split-ring commutator, which is a ring split into two halves. Brushes are conducting contacts that press against the commutator and connect it to the external circuit.
The two vertical sides of the coil carry current in opposite directions. They are in the same magnetic field, so Fleming’s left-hand rule gives opposite forces on the two sides: one side is forced up, and the other side is forced down.
Because the forces act on opposite sides of the coil, they create a turning effect. This makes the coil rotate.
Explaining why the coil rotates
In the diagram, the magnetic field is from left to right. The left side of the coil has current out of the page, and the right side has current into the page.
- For the left side, apply Fleming’s left-hand rule with the field to the right and the current out of the page. The force is upward.
- For the right side, apply the same field direction but with the current into the page. The force is downward.
- The upward force on one side and downward force on the other side produce a turning effect, so the coil rotates.
- After each half-turn, the split-ring commutator reverses the current in each side of the coil, so the forces swap sides and keep the coil rotating in the same direction.
Without the split-ring commutator, the coil would turn for a short time and then the forces would oppose the motion after half a turn. The commutator reverses the current at the right moment so the motor continues rotating.
Making a motor stronger
A motor can be made stronger by using a stronger magnetic field, increasing the current, increasing the length of wire in the field, or using more turns on the coil.
In the exam
- Mark the magnetic field direction first: outside the magnet, it goes from north to south.
- Use Fleming’s left hand with conventional current. If the current is shown with a dot or cross, translate that into “out of the page” or “into the page”.
- For calculations, use F=BIlF = BIlF=BIl only when the conductor is at right angles to the field, convert lengths to metres, and use the length actually inside the magnetic field.
Check yourself
- A wire carries current out of the page in a magnetic field from left to right. Which way is the force?
- What happens to the motor-effect force if the current doubles but the length of wire in the field halves?
- Why does a simple dc motor need a split-ring commutator?
