Topic 7 - Magnetism And Electromagentism Flashcards
What are the poles of a magnet?
The places where the magnetic forces are strongest
What happens when two magnets are brought close together?
They exert a force on each other
Give examples of a noncontact force?
Attraction and repulsion between two magnetic poles
What do permanent magnets produce?
Its own magnetic field
What is an induced magnet?
A material that becomes a magnet when it is placed in a magnetic field
Does induced magnetism always cause a force of attraction?
Yes
What happens to an induced magnet when it is removed from the magnetic field?
It loses most/all of its magnetism quickly
What is a magnetic field?
The region around a magnet where a force acts on another magnet or on a magnetic material
Give examples of magnetic materials
Iron, steel, cobalt and nickel
What kind of force is it between a magnet and magnetic material?
It is always one of attraction
What does the strength of the magnetic field depend on?
The distance from the magnet - it is strongest closest to the poles
How is the direction of the magnetic field at any point given?
It is given by the direction of the force that would act on another North Pole placed at that point
What is the direction of a magnetic field line?
From the north seeking pole of the magnet to the south seeking pole of the magnet
What does a magnetic compass contain?
A small bar magnet
How does a magnetic compass work?
The needle points in the direction of the earths magnetic field
What is the trick to find the direction of the magnetic field with the current?
Right-hand thumb rule
How does the right hand thumb rule work?
Point thumb in direction of current (in thumbs up position), the way that your forefinger curls is the direction of the magnetic field
What happens when a current flows through a conducting wire?
A magnetic field is produced around the wire
What does the strength of a magnetic field around a wire depend on?
The current through the wire and the distance from the wire
Why does shaping a wire to form a solenoid increase the strength of the magnetic field?
The field lines around each loop of wire line up with each other to result in lots of field lines pointing in the same direction that are very close to each other
Give two properties of the magnetic field inside a solenoid?
Strong and uniform
What is the shape of the magnetic field around a solenoid similar to?
That of a bar magnet
What increases the strength of the magnetic field of a solenoid?
- Adding an iron core
- increasing the current
- increasing the number of coils
What is electromagnet?
A solenoid with an iron core
What happens when a conductor carrying a current is placed in a magnetic field?
The magnet producing the field and the conductor exert a force on each other called The motor effect
How does Fleming‘s left-hand rule work?
Point your first finger in the direction of the field and your second finger in the direction of the current, your thumb will then point in the direction of the force
What factors affect the size of the force on the conductor?
- the magnetic flux density (how many field lines there are in a region)
- size of current
- length of conductor in magnetic field
What is the equation to calculate force for a conductor at 90° to a magnetic field and carrying a current?
Force = magnetic flux density (T - Tesla) x current x length
What is the basis of an electric motor?
A coil of wire carrying a current in a magnetic field tends to rotate
How do electric motors work?
Because the coil is on a spindle and the forces act one up one down, it rotates. The split ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction
How do you reverse the direction of the electric motor?
Swapping the polarity of the DC supply (reverse the current) or swap the magnetic poles over (reverse the field)
How do you loudspeakers and headphones use the motor effect?
Because that variations in current and electrical circuits to the pressure variations in sound waves
How do you loudspeakers work?
Alternating-current is sent through a coil of wire attached to the base of a paper cone. The coil surrounds one pole of a permanent magnet and is surrounded by the other pole, so the current causes a force on the coil, causing it to move. When the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction to. So variations in the current makes the cone vibrate which makes the air around the cones vibrate and creates the variations in pressure that cause the soundwave. By controlling the AC you can alter the frequency of the soundwave.
What causes a potential difference to be induced across the ends of the conductor?
If electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor
What is the generator effect?
The induction of a potential difference (and current if there is a complete circuit) in a wire which is moving relative to a magnetic field or experiencing a change in the magnetic field.
What does an induced current generate?
Magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field
What factors affect the size of the induced potential difference?
- increasing the speed of the movement (Cutting more magnetic field lines in a given time)
- increasing the strength of the magnetic field (so there are more field lines that can be cut)
What factors affect the direction of the induced potential difference?
- moving the magnet or conductor in the opposite direction
- reversing the polarity of the magnet
What is the generator affect used for?
In an alternator to generate AC and in a dynamo to generate DC
How alternators generate alternating-current?
Generators rotate a coil in a magnetic field, as the coil spins, a current is induced in the coil which changes direction every half turn. Instead of a split ring commutator, AC generators have slip rings and brushes so the contacts don’t swap every half term so that they produce an alternating potential difference
How do you dynamos generate direct current?
Dynamos work in the same way as alternators but they have a split ring commutator instead of slip rings to swap the connection every half turn to keep the current flowing in the same direction
What shows how the potential difference generated in a coil changes over time?
Oscilloscopes
How do microphones work?
They use the generator affect to convert the pressure variations in soundwaves into variations in current in electrical circuits
Describe how microphone works
Soundwaves hit a flexible diaphragm attached to a coil of wire, wrapped around a magnet. The soundwaves cause the coil to move in the magnetic field which generates a current
What does the movement of the coil in a microphone depend on?
The properties of the soundwave (louder sound make the diaphragm move further)
What does a basic transformer consist of?
A primary coil and a secondary coil wound on an iron core
Why is iron used in transformers?
It is easily magnetised
What is the ratio between the primary and secondary potential difference is the same as in transformers?
The ratio between the primary and secondary potential differences is the same as the ratio between the number of turns on the primary and secondary coils
What do you step down transformers do?
Decrease potential difference
- they have less turns on the primary coil than the secondary coil
What do step up transformers do?
Increase potential difference
-they have more turns on the primary coil than the secondary coil
What would happen if transformers were 100% efficient?
The electrical power output would be equal to the electrical power input
How do you calculate the power output of a 100% efficient transformer?
Potential difference (secondary) x current (secondary) = potential differenerce (primary) x current (primary)
How do you calculate the power input of a 100% efficient transformer?
Potential difference (primary) x current (primary) = potential difference (secondary) x current (secondary)
