Topic 7 - Magnetism and Electromagnetism Flashcards
What causes the Earth’s magnetic field?
The Earth’s core
What materials can become magenetised?
1.Iron
2.Steel
3.Nickel
4.Cobalt
What is a magnetic field?
The region around a magnet where other magnets or magnetic materials experience a force.
Where is the magnetic field strongest?
The poles
What can we use to see a magnetic field?
1.Iron filings
2.Plotting compasses
Which direction do magnetic field lines travel?
North to south
How do we show the magnetic field getting weaker with a diagram?
The magnetic field lines get further apart
What is a uniform magnetic field?
A uniform magnetic field is one that has the same strength and direction at all points.
What is an electromagnet?
- When a conductor has a current flowing through it
- It creates a magnetic field around the wire
- Which can be turned on and off when the current is turned on and off.
Which direction does a conventional current flow?
Bottom of the page to the top
How can we find the direction of a magnetic field from an electromagnet?
The right hand rule
What is the right hand rule?
1.Thumb goes in the direction of current
2. The fingers curl in the direction of the field
What is a solenoid?
A solenoid is a long piece of wire that is curled to form a coil. When an electric current is passed through this coil, it creates a uniform magnetic field. A type of electromagnet
What is the relationship between the number of coils and magnetic field strength?
The more coils the stronger the field
What is created inside a solenoid?
A very strong uniform magnetic field
What does the direction of the magnetic field in an electromagnet depend on?
The direction of current
What is the motor effect?
Where current flowing in a magnetic field creates a force which causes motion
What is the left hand rule?
1.The thumb = motion
2.The first finger = magnetic field
3.The second finger = conventional current
Milwall Football Club
What does the size of the force from the motor effect depend on?
1.length of wire
2.Strength of magnets
3.Size of current
Bigger = more
What is the equation linking force, magnetic field strength, current and length?
force = magnetic field strength x current x length
F = B I l
What unit is magnetic field strength measured in ?
Tesla ( T )
How does the motor effect work?
1.The magnetic field from the current interacts with the magnetic field from the permanent magnet
2.On one side the magnetic fields cancel eachother out, and on the other they add together.
3.This creates a force in the direction away from the stronger magnetic field
How can we make an electric motor stronger/
- Add more turns coils
- Add a stronger magnet
- Add a higher current
What is a permanent magnet?
Objects which produce their own magnetic field
What is an induced magnet?
Magnetic materials which become magnets when in a magnetic field
What are 2 uses of electromagnets?
1.Used in cranes to pick things up
2.Used in the electric starters of motors
What must be true to experience the strongest force from the motor effect
The current must flow perpendicular to the field
How does dc motor work?
1.There is a coil of wire on a spindle, which loops around between 2 permanent magnets
2.The forces acting here are the same as the regular motor effect
3.As the current flows through the coil, one side of the loop experiences an upward force, whilst the other is forced downwards, causing it to spin
4.The split ring commutator swaps the contacts every half turn to keep the motor rotating in the right direction.
How do loudspeakers work?
1.An ac is sent through a coil of wire attached to the base of a paper cone
2.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, which causes the cone to move
3.When the current reverses, the force reverses and the cone comes back down
4.Different frequencies of ac can make the cone vibrate at different frequencies, creating different sounds
What is the generator effect?
The induction of a potential difference (and current if there is a permanent circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic fields.
What is the relationship between the induced current and the change that made it?
The induced current always opposes the change that made it
How does the generator effect work?
- A magnet in a coil of wire moves or vice versa
- THis cuts the magnetic field lines
- The shifting from side to side creates a pd and current if in a circuit
- If you keep moving the magnet or coil backwards and forwards, an alternating pd is made
How can you increase the size of an induced pd?
1.Increasing the speed of movement, which cuts more magnetic fields in a shorter period
2.Increasing the strength of the magnetic field, so there are more field lines to be cut
How do alternators work?
1.A coil is rotatesd in a field between to permanent magnets
2.As the coil spins, a current is induced in the coil, which changes direction every half turn.
3.The generator has slip rings and brushes so the contacts don’t swap every half turn.
4.So they produce an alternating pd, and an ac
How do dynamos work?
1.same as alternators
2.BUT - they have a split ring commutator instead of slip rings
3.This swaps the connection every half turn, so the current keeps flowing in the same direction, so u get dc
What does the oscilloscope look like for alternators and dynamos?
1.Alternators - a wavy line through the x - axis
2.Dynamo - a line which bounces along the x - axis
What does increasing frequency of revolutions do to a dynamo and alternator?
1.Increases the overall pd and creates more peaks
How do microphones work?
1.Loudspeakers in reverse
2.Sound waves hit a flexible diaphragm that is attached to a coil of wire around a magnet
3.This causes the coil to move in the field, which generates current
4.The movement depends on the sound wave
What do transformers do?
change the size of the pd of an ac
How do transformers work?
1.There are 2 coils of wire, a primary and secondary, joined with an iron core
2.When an alternating pd and ac is applied across the primary coil, it creates an alternating magnetic field around it.
3.This field creates a magnetic field in the core which changes directions.
4.This changing field induces an alternating pd in the second coil
What are the 2 equations to do with transformers?
- Vp/Vs = np/ns
- Vp x Ip = Vs x Is
1. pd1/pds = coils1/coils2
2. volt1 x current1 = volt2 x current 2
