Topic 12 - Magnetism & the Motor Effect Flashcards
What is the difference between permanent and induced magnets
Permanent magnets produce their own magnetic field all the time.
Induced magnets only produce a magnetic field when they’re in another, external magnetic field.
What do like and unlike magnetic poles do to each other
Like poles repel
Unlike poles attract
What is a uniform magnetic field
- Created between two opposite magnetic poles
- Have the same strength everywhere, so the field lines are evenly spaced, parallel and straight
What are the principles of drawing magnetic field lines
- Arrows go north to south
- 5 lines from each point
- Magnetic field is strongest at the highest density of lines
How do compasses show the direction of field lines around a bar magnet
The needle of a compass is a tiny bar magnet that will point towards the south pole of a nearby magnet - following the field lines. Multiple can be placed around a bar magnet so show their continuous direction of pointing to a south pole.
How do compasses show that the Earth’s core is magnetic
Their needle points towards the the Earth’s geographical north pole, which is the magnetic south pole.
How can you find the direction of a magnetic field, produced by a current through a wire
Right hand rule - point thumb upwards to represent the wire. The rest of your fingers show the field lines’ direction.
What does the strength of a magnetic field around a wire depend on
The size of the current
The distance from the wire
What is a solenoid
An elctromagnet produced by wires in a loop
What happens inside and outside a solenoid
- Inside - the fields from the individual coils add together to form a strong and almost uniform field along the centre
- Outside - the field lines overlap and cancel out, resulting in a weak field apart from the ends of the solenoid
What is the motor effect
How a current carrying conductor placed near a magnet experiences a force and exherts an equal and opposite force on the magnet.
What causes magnetic forces
Interactions between magnetic fields
What is fleming’s left hand rule
Point thumb upwards the represent force
Point index forwards the represent the magnetic field
Point middle finger sideways to represent the current
FBI
What is the equation F=BxIxL
Force from motor effect=magnetic flux density x current x length
What are the units for magnetic flux density
Tesla (T)
Newton per ampere metre (N/A m)
How do electric motors work
- A loop of wire that’s free to rotate about an axis is placed in a magnetic field
- When a direct current flows through the loop, the two side arms of the loop, each experience a force, due to the motor effect. The loop will start the rotate around its axis, as forces act in opposite directions.
- Without a communtator, the wire makes a half turn and gets stuck due to the forces acting outwards and the wire getting jumbled up.
- With a commutator, when a half turn is reached, no current passes through the circuit, as the commutator has a split in it.
- One half of the commutator is responsible for sending the current from the battery into the wire in the magnetic field. Upon every half turn of the wire, the commutator’s halves swap, so current is still sent from the same location.
- This swaps the direction of the current around the wire, so the direction of the forces swap compared to the wire, but the forces allow for rotation to continue in the same direction.
