P2- Magnetism and Electromagnetism 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
What is a magnetic field?
The region around a magnet where a force acts on another magnet or on a magnetic material
4 examples of magnetic materials
Iron
Steel
Nickel
Cobalt
What kind of force is the forces in a magnetic field?
Non contact forces
Which direction do magnetic field lines go?
NORTH to SOUTH
The closer together magnetic field lines are…
The stronger the magnetic field
The further apart magnetic field lines are…
The weaker the magnetic field
The force between a magnet and a magnetic material is always…
Attractive
2 types of forces that could exert when 2 poles of a magnet are put near each other?
Attractive
Repulsive
What do compasses show?
The direction of the magnetic fields
What is inside a compass?
A tiny bar magnet
How does the bar magnet control where the compass points?
-The north pole of this magnet is attracted to the south pole of any other magnet
-So, the compass points in the direction of the magnetic field it is in
Why do compasses always point north even when they’re not near a magnet?
The Earth generates its own magnetic field showing that the inside (core) of the Earth must be magnetic
2 ways to show the shape of a magnetic field
-Plotting compasses (mark where the compass arrow points when around a magnet)
-Iron fillings (shake them around a magnet and they will align with the magnetic field)
What is the main difference between magnets and magnetic materials?
-Magnets ATTRACT and REPEL
-Magnetic materials only ATTRACT
What are the 2 types of magnets?
-Permanent magnets
-Induced magnets
What are permanent magnets?
Magnets that produce their own magnetic field
What are induced magnets?
Magnetic materials that turn into a magnet when put into a magnetic field
The force between a permanent and induced magnetic field is always…
Attractive
What happens to an induced magnet when you take away the magnetic field?
It quickly loses its magnetism and stops producing a magnetic field
What happens when a current passes through a wire?
A magnetic field is created/induced around the wire
What is the magnetic field around a wire made up of?
Concentric circles perpendicular to the wire
What does the strength of the magnetic field around a wire depend on?
-The current through the wire
-The distance from the wire
The larger the current through the wire or the closer to the wire you are…
The stronger the magnetic field around the wire
What is a solenoid?
A coil of wire which forms an electromagnet when a current flows through
How can you increase the strength of a magnetic field?
-Wrapping the wire into a coil called a solenoid
-Putting a block of iron in the centre of the coil
-Creating a large current
The magnetic field inside a solenoid is…
-Strong
-Uniform
Why is the magnetic field inside a solenoid strong and uniform?
It has the same strength and direction at every point in the region
What does the magnetic field around a solenoid have a similar shape to?
A bar magnet
What is an electromagnet?
A solenoid (a coil of wire) with an iron core
What are the 3C’s which effect the strength of a magnetic field?
1) Coils: More coils= Stronger field
2) Core: Easily magnetised core= Stronger field
3) Current: Larger current= Stronger field
What is the Motor Effect?
When a current carrying wire is placed in a permanent magnetic field, the two field interact and exert a force on each other
What is Fleming’s left hand rule?
Holding the thumb and first 2 fingers of your left hand at right angles to each other
What does the thuMb point at in Fleming’s left hand rule?
Direction of Force (Motion)
What does the First Finger point at in Fleming’s left hand rule?
Direction of the magnetic Field
What does the seCond finger point at in Fleming’s left hand rule?
Direction of Current
What 3 factors affect the size of the force on the conductor?
1)Current: Larger current= larger force
2) Length: Longer length of conductor= larger force
3) Magnetic Flux Density: More magnetic field lines= larger force on conductor
What equation is used for a conductor at right angles to a magnetic field?
Force (N)= Magnetic Flux Density (T) x Current (A) x Length (m)
2 examples that use the Motor Effect
-DC motor
-Loudspeaker (or headphones)
How does a DC motor work (7 steps)?
1) Coil connects to a battery inside a permanent magnetic field
2) When current flows in a coil, a magnetic field is induced
3) Induced and permanent fields interact and exert a force on each other
4) Direction of force can be found using Fleming’s rule (adjust to Q)
5) As current flows in opposite directions either side of coil, opposing forces occur
6) Coil rotates
7) Split ring commutators maintain a complete circuit
How do Loudspeakers work (7 steps)?
1) Alternating current in the coil creates an electromagnetic field
2) Electromagnetic field and permanent magnet interact, generating a force, pushing the cone outwards
3) Current is made to flow in the opposite direction
4) Direction of the electromagnetic field reverses
5) Force on the cone now pulls it back in
6) Repeatedly alternating current direction makes cone vibrate in and out
7) Cone vibrations cause pressure variations in the air (sound waves)
What is the Generator Effect/ Electromagnetic Induction?
When there is relative motion between an electrical conductor and a magnetic field, a potential difference will be induced across the ends of the conductor. A current will flow if this conductor is part of a complete circuit
2 factors that affect the size of the induced potential difference/induced current
1) Increase the speed of movement (cutting more magnetic fields in a given time)
2) Increase the strength of magnetic field (more field lines that can be cut)
2 types of generators
-Alternators
-Dynamos
What are alternators?
A device that makes use of the generator effect to generate
alternating current
How do alternators generate alternating current (5 steps)?
1) Coil rotates in a magnetic field
2) Current is induced in the rotating coil. Current changes direction every half turn
3) Split rings are connected to the coil
4) Brushes make continuous contact between the external circuit and the slip rings
5) Current flows in the external circuit and alternating potential difference is produced
What are dynamos?
A device that makes use of the generator effect to generate direct
current
How do dynamos generate direct current (5 steps)?
1) Coil rotates in a magnetic field
2) Current is induced in the rotating coil. Current changes direction every half turn
3) Split ring commutators are connected to the coil
4) The split ring commutators swap the connections every half turn to keep current flowing in the same direction
5) Current flows in the external circuit and direct potential difference is produced
What will the line on an oscilloscope look like for AC (Alternating current)?
The line goes up and down, crossing the horizontal axis
What will the line on an oscilloscope look like for DC (Direct current)?
The line always stays above the axis (not straight)
How do microphones work (6 steps)?
1) Pressure variations in sound waves cause flexible diaphragm to vibrate
2) Vibrations of the diaphragm cause vibrations in the coil
3) Coil moves relative to permanent magnet, potential difference is induced in coil
4) Coil is part of a complete circuit, induced potential difference= current flows
5) Changing size and direction of induced current matches vibrations of the coil
6) Electrical signals generated match the pressure variations in the sound waves
What are transformers?
Devices used to increase or decrease voltage
2 things to remember about Step Up Transformers
-Increase voltage, decrease current
-More turns on secondary coil than primary coil
2 things to remember about Step Down Transformers
-Increase current, decrease voltage
-More turns on primary coil than secondary coil
What kind of current are transformers used with?
Alternating current
What kind of core is used in transformers and why?
-Soft iron core
-It is easily magnetised
How is alternating current induced from transformers (4 steps)?
1) Alternating current in primary coil produces an alternating magnetic field
2) Magnetic field passes through the core and induces alternating potential difference in the secondary coil
3) Alternating potential difference induces alternating current (if the second coil is part of a complete circuit)
4) Size of output depends on ratio of coils and/or input current
Equation linking potential difference with number of turns on coils
Input potential difference (Vp)/ Output potential difference (Vs) = Number of turns on primary coil (np)/ number of turns on secondary coil (ns) (or flipped)
How efficient are transformers and what can you assume for this?
-Almost 100% efficient
-Assuming this input power is equal to output power
Equation linking input and output power
PD across secondary coil (Vs) x Current through secondary coil (Is)= PD across primary coil (Vp) x Current through primary coil (Ip)
