Forces on a coil
Definition
Electric motor
A device that uses the forces on a current-carrying coil in a magnetic field to produce continuous rotation.
Definition
Coil
A length of insulated wire wound into many loops, called turns, so that the effects of the current in each turn add together.
Definition
Moment
The turning effect of a force about a pivot, equal to the force multiplied by the perpendicular distance from the pivot to the line of action of the force.
- An electric motor uses the motor effect to turn a shaft, transferring energy from the electrical supply to the kinetic energy store of the rotating parts.
- A simple motor has a rectangular coil of wire placed between the poles of a magnet, mounted on an axle so that it is free to spin.
- When a current flows, it travels along one long side of the coil and back along the opposite long side, so the current in the two sides is in opposite directions.
- Both long sides lie across the magnetic field, so the motor effect produces a force on each of them.
- Because the currents are opposite, Fleming's left-hand rule gives forces in opposite directions, with one side pushed up and the other side pushed down.
- The two forces act on opposite sides of the axle, so each produces a moment in the same rotational direction, and together they make the coil turn.
- The short ends of the coil either lie along the field or feel forces that act along the line of the axle, so they add nothing to the turning effect.
- A coil with many turns has many wires along each side, so the forces on them add up and the turning effect is larger.
Keeping the coil turning
Definition
Split-ring commutator
A ring split into two halves that turns with a coil and reverses the connections between the coil and the external circuit every half turn.
- The turning effect is greatest when the plane of the coil is parallel to the field lines, because the two forces then act furthest from the axle.
- As the coil turns, the forces keep acting straight up and down, so the perpendicular distance from the axle to each force shrinks and the moment gets smaller.
- When the plane of the coil is perpendicular to the field lines, both forces act straight through the axle, so the moment is zero.
- The coil keeps moving past this position because of its momentum.
- If the current kept flowing the same way around the coil, the forces would now turn it backwards, so the coil would rock to and fro and settle in the perpendicular position.
- To keep the coil turning in one direction, the current in the coil must reverse every half turn, at the moment the coil passes the perpendicular position.
- In a simple d.c. motor a split-ring commutator does this by swapping the connections between the coil and the supply every half turn.
- After each swap, the side of the coil next to the north pole always carries current in the same direction, so it is always pushed the same way and the rotation continues.
Speed and direction of rotation
- The coil turns faster when the forces on its sides are larger.
- Increasing the current increases the force on each side of the coil.
- Using a stronger magnet increases the magnetic flux density and so increases the force on each side.
- Adding more turns to the coil increases the total force along each side.
- Reversing the current through the motor reverses the forces and so reverses the direction of rotation.
- Reversing the magnetic field, by swapping the poles, also reverses the direction of rotation.
- Reversing both the current and the field together leaves the motor turning the same way as before.
- Electric motors drive anything that needs electricity to produce rotation, including the wheels of electric cars, the drum of a washing machine, cordless drills and the fans in hairdryers.
Example
Direction of rotation
- Looking along the axle, the coil lies flat between a north pole on the left and a south pole on the right, so the field points from left to right.
- The side of the coil on the left carries conventional current out of the page, and the side on the right carries it into the page.
- For the left side, point your first finger to the right and your second finger out of the page, and your thumb points up.
- For the right side, your second finger points into the page, so your thumb points down.
- The left side is pushed up and the right side is pushed down, so the coil turns clockwise as seen from this end.
Practical
Building a simple motor
- Aim: to build a working motor from a coil, a magnet and a cell, and to see how the direction of the current and of the field affects the rotation.
- Apparatus: about 1 m1\ \text{m}1 m of enamelled copper wire, a 1.5 V1.5\ \text{V}1.5 V D-size cell, two large metal paper clips, a strong flat magnet, sticky tape, fine sandpaper and a marker pen or similar cylinder to wind the coil on.
- Method:
- Wind the wire tightly around the marker pen about 101010 to 151515 times to make a neat circular coil, leaving about 3 cm3\ \text{cm}3 cm of straight wire sticking out on opposite sides to act as the axle.
- Slide the coil off the pen and wrap each loose end twice around the turns to hold them together, checking that both straight ends lie on one line through the centre of the coil so it balances.
- On one end, sand off all of the enamel insulation all the way round.
- On the other end, hold the coil upright and sand the enamel off the top half of the wire only, leaving the bottom half insulated.
- Bend each paper clip into a support with a small loop at the top, and tape one to each terminal of the cell so the loops are level with each other.
- Stick the magnet to the side of the cell, directly below the space between the two loops.
- Rest the axle ends in the loops so the coil hangs just above the magnet, then give it a gentle flick to start it spinning.
- Turn the magnet over, and then turn the cell round, and note the direction of spin each time.
- Results:
- The coil spins steadily once it is balanced and the bare ends make good contact with the loops.
- The half-stripped end acts as a simple switch, so current flows only during the half of each turn when the bare wire touches the loop.
- The coil is pushed only during the part of each turn when the force turns it the right way, and it coasts round on its momentum for the rest of the turn.
- Turning the magnet over or reversing the cell makes the coil spin the other way.
- Watch out: a coil that only rocks back and forth usually has the wrong half of the enamel removed, and a lopsided coil wobbles and falls out of the loops.
- Safety: the coil has a very low resistance, so the current is large and the wire and cell become hot within a minute, so lift the coil out when you are not watching it.
Common Mistake
- Do not say that the coil turns because it is attracted to the magnet, since the turning comes from the motor-effect forces on its two sides.
- Do not say that the forces on the two sides act in the same direction, because they act in opposite directions and that is what produces the turning effect.
Exam technique
Explaining how a motor turns
- Build your explanation as a chain: current in the coil, a magnetic field around each side, interaction with the field of the magnet, forces on the two sides, and a turning effect on the coil.
- Use the phrase opposite directions for both the currents and the forces, because this is the step most often missed.
- When explaining why the motor keeps turning the same way, state that the current in the coil reverses every half turn.
- For ways to make the motor faster, give changes that increase the force, such as a larger current, a stronger magnet or more turns on the coil.
Self review
- Why do the forces on the two long sides of a motor coil act in opposite directions?
- In which position of the coil is the turning effect zero?
- Why must the current in the coil reverse every half turn?
- Give three ways of making a motor spin faster.
- State two ways of reversing the direction in which a motor turns.
