The Motor Effect Flashcards
All 26 cards in this deck
What is the magnetic field pattern around a straight wire carrying a current?
Concentric circles centred on the wire, at right angles to it. The circles are closer together (field is stronger) nearer the wire.
Which rule gives the direction of the magnetic field around a straight current-carrying wire, and how is it used?
The right-hand grip rule: point the right thumb along the (conventional) current, and the curled fingers show the direction of the field.
What are the steps to demonstrate the magnetic effect of a current?
e.g. a straight wire passed vertically through a card
- Place a plotting compass next to the wire with no current — it points north.
- Switch the current on — the compass needle deflects, showing a magnetic field is produced.
- Increase the current, or move the compass closer to the wire — the deflection increases, showing the field is stronger.
Describe the magnetic field of a solenoid carrying a current.
Inside: strong and uniform (evenly spaced parallel field lines). Outside: the same shape as the field of a bar magnet.
What is an electromagnet?
A solenoid with an iron core. The turns of the coil add their fields together and the iron core makes the field much stronger.
In a relay, what happens when a current flows in the input circuit?
The electromagnet is switched on and attracts an iron armature, which closes the contacts (switch) in the second circuit.
What is the motor effect?
When a current-carrying conductor is placed in a magnetic field, the magnet and the conductor exert a force on each other.
What three factors affect the size of the force on a current-carrying conductor in a magnetic field?
The magnetic flux density of the field, the current, and the length of conductor in the field.
What is the equation for the force on a conductor at right angles to a magnetic field carrying a current?
— force (N) = magnetic flux density (T) × current (A) × length (m).
What is magnetic flux density, and what is its unit?
A measure of the strength of a magnetic field, symbol , measured in tesla (T).
True or false? A current-carrying wire placed parallel to a magnetic field experiences the maximum force.
False. The force is maximum when the wire is at right angles to the field, and zero when it is parallel to it.
In Fleming's left-hand rule, what do the thumb, first finger and second finger represent?
Thumb = force (motion), First finger = magnetic Field (N to S), seCond finger = Current (+ to −).
How must the thumb and fingers be held when using Fleming's left-hand rule?
The thumb, first finger and second finger of the left hand are held at right angles to each other.
What are the steps to use Fleming's left-hand rule to find the direction of the force?
e.g. field pointing downwards, current out of the page
- Point the first finger (left hand) along the field: downwards.
- Point the second finger along the current: out of the page.
- The thumb then points along the force: horizontally, to the right.
True or false? In Fleming's left-hand rule the second finger points from the negative terminal to the positive terminal.
False. It points along the conventional current, from positive to negative.
What happens to the direction of the force on a conductor if the current through it is reversed?
The force reverses direction. (Reversing the magnetic field instead has the same effect.)
Why does a current-carrying coil in a magnetic field rotate?
The forces on the two opposite sides of the coil act in opposite directions (one up, one down), giving a turning effect.
What is the job of the split-ring commutator in a d.c. motor?
It reverses the direction of the current in the coil every half turn, so the coil keeps rotating in the same direction.
True or false? Without a split-ring commutator, the coil of a d.c. motor would carry on spinning in the same direction.
False. It would turn half a turn and then the forces would push it back, so it would just rock back and forth.
What are the brushes in an electric motor for?
They keep an electrical contact between the circuit and the rotating commutator, so current can flow in the spinning coil.
Name three ways to increase the turning effect (and speed) of an electric motor.
Increase the current, use a stronger magnet (greater flux density), and use more turns on the coil.
Which effect do moving-coil loudspeakers and headphones use to produce sound?
The motor effect.
What are the steps by which a moving-coil loudspeaker produces a sound wave?
e.g. an alternating current in the loudspeaker coil
- The current creates a magnetic field around the coil, which interacts with the field of the permanent magnet.
- The resultant force makes the coil and cone move.
- When the current reverses, the force reverses, so the cone vibrates and produces pressure variations in the air — a sound wave.
What do loudspeakers and headphones convert?
Variations in current in an electrical circuit into pressure variations in sound waves.
True or false? The frequency of the sound from a loudspeaker is the same as the frequency of the alternating current in its coil.
True. Each cycle of current gives one back-and-forth vibration of the cone.
What makes the sound from a loudspeaker louder?
A larger current in the coil — a bigger force moves the cone further, giving a larger amplitude sound wave.