P15 - Electromagnetism Flashcards
What is an alternator?
An alternating current generator
What is a dynamo?
A direct current generator
What is an electromagnet?
An insulated wire wrapped around an iron bar that becomes magnetic when a current flows through the wire
What is electromagnetic induction?
The process of inducing a potential difference using a magnetic field
What is Fleming’s left hand rule?
A rule that gives the direction of the force on a current carrying wire in a magnetic field to the directions of the current and the field
What is the generator effect?
If a conductor is moved through a magnetic field, a potential difference will be induced on the conductor. If this conductor is part of a complete circuit, the induced potential difference causes a current to flow around the circuit.
What is induced magnetism?
Magnetism of an unmagnetized magnetic material by placing it in a magnetic field
What is a magnetic field?
The space around a magnet or a current-carrying wire
What is a magnetic field line?
line in magnetic field along which a magnetic compass points
What is magnetic flux density?
A measure of the strength of the magnetic field defined in terms of the force on a current carrying conductor at right angles to the field lines
What is the motor effect?
When a current is passed along a wire in a magnetic field, and the wire is not parallel to the lines of the magnetic field, a force is exerted on the wire by the magnetic field
What is a solenoid?
A long coil of wire that produces a magnetic field in and around the coil when there is a current in the coil
What is a split-ring commutator?
Metal contacts on the coil of a direct current motor that connects to the rotating coil continually to its electric power supply
What is a step-down transformer?
Electrical device that is used to step down the size of an alternating potential difference
What is a step-up tranformer?
Electrical device that is used to step-up the size of an alternating potential difference
What is a transformer?
Electrical device used to change an alternating voltage
What 3 elements can be permanent magnets?
- Iron
- Cobalt
- Nickel
Describe the magnetic field around a straight current carrying wire?
Circular shape, with field lines closest together nearest the wire as the magnetic field s strongest nearest the wire
How do you find the direction of the magnetic field around a straight current carrying wire?
Use the “right hand grip rule”
What is a wire coiled into a spiral called?
A solenoid
What is the north pole of a magnet?
Where the field lines point outwards
Describe the magnetic field inside a solenoid:
The magnetic field is very strong inside a solenoid. The field lines are parallel to the axis of the solenoid, and they are uniform (in the same direction).
Describe the magnetic field outside a solenoid:
the magnetic field lines bend from the north end of the solenoid to the south end of the solenoid. The magnetic field lines from a complete loop as they pass through the inside of the solenoid
How do magnetic field lines represent the strength of a magnetic field?
If the magnetic field lines are closer together then the magnetic field is stronger. But if the magnetic field lines are further apart then the magnetic field is weaker
How do you make the magnetic field of a solenoid stronger?
- Increase the current
- Add more turns to the coil
- Add an iron core to create an electromagnet
What type of magnet is an electromagnet?
Temporary Magnet
How does an electromagnet work?
When a current is passed along the wire, a magnetic field is created around the wire. Because of this, the magnetic fields of the wire magnetise the iron core
How does an electromagnet pick up an unmagnetized magnetic material, such as iron?
The electromagnet’s pole induces the opposite pole on the material, therefore they are opposite poles and attract
What is the south pole of a magnet?
Where the field lines point in
Why are electromagnets useful?
- They can be turned on and off
- The direction of the magnetic field can be reversed
- They are much stronger than regular permanent magnets
What are 4 ways electromagnets are used in devices?
- Scrap yard cranes
- Circuit breaker
- Electric Bell
- Relay
Describe how an electromagnet is used in scrap yards?
A large electromagnet is attached to a crane. When a current is run through the wire, the electromagnet is turned on and is used to pick up and move heavy pieces of scrap metal. When the current is switched off, the electromagnet turns off and the scrap metal falls off the electromagnet
How is an electromagnet used in a circuit breaker?
A circuit breaker is a switch in series with an electromagnet. The switch is held close (allowing a current to flow) by a spring. When the current is too large, the strength of the electromagnet is increased and pulls open thee switch. The switch remains open until it is reset manually
Describe how an electric bell works:
When an electric bell is pushed, the circuit is completed causing the electromagnet to attract the iron armature, which hits a bell. Because the iron armature is pulled away, the metal strip is pulled away from the contact, causing the make-and-break switch to be broken, the circuit is no longer completed and the electromagnet loses its magnetism and the armature springs back, this causes the make and break switch to be complete, causing cycle to repeat rapidly
What is a relay?
A device used to switch an electrical machine with a large current on or off with a small current
Describe how a relay works:
A small current is ran through the coil of the electromagnet causing the electromagnet to turn on, this pulls the iron armature onto the electromagnet. As a result the contacts are pushed together and the switch gap is closed and the machine is switched on. in this way a small current is used to switch on a machine with a much bigger current.
What is induced magnetism?
An unmagnetized magnetic material becomes magnetised when in contact with a magnetic field
How can you increase the strength of the force produced in the motor effect?
- Increasing the current
- Increasing the magnetic field strength
- Increasing the length of the wire
How can you increase the strength of the force produced in the motor effect?
- Increasing the current
- Increasing the magnetic field strength
- Increasing the length of the wire
Describe how the force produced in the motor effect changes as the angle of the wire is changed
- The strength of the force produced is greatest when the wire is perpendicular to the magnetic field
- The strength of the force produced is 0 when the wire is parallel to the magnetic field
Which finger indicates magnetic field in Flemings left hand rule?
First finger = Field
Which finger indicates Current in Flemings left hand rule?
seCond finger = Current
Which finger indicated movement in Flemings left hand rule?
thuMb = Movement
In a circuit, where does the current flow from and to?
From the positive cell to the negative cell
In which direction will the conductor move when a current flows through it?
into the page
In which direction will the conductor move when a current flows through it?
out of the page
In which direction will the conductor move when a current flows through it?
out of the page
In which direction will the conductor move when a current flows through it?
into the page
What are the unites for magnetic flux density?
Tesla
What is the equation for a the force produced on a current carrying conductor at a right angle to the magnetic field in the motor effect?
Force = Magnetic Flux Density x Current x Length F = B x I x L
How do you reverse the direction of the force acting upon the wire?
- Reverse the direction of the current
* Reverse the direction of the magnetic field
Describe the movement of a current carrying coil placed within a magnetic field
The right side of a the coil will experience a downwards force due to the motor effect and the left side of the coil will experience an upward force due to the motor effect. This will cause the coil to begin rotating. However, once the coil has rotated 180, the force acting upon each side of the coil will swap as the direction of the current has swapped. This causes the coil to reverse its direction and rotate back to its original position. This process repeats with the coil never rotating a full 360, only 180*.
Describe the structure of an electric motor:
- A rectangular coil known as a armature is placed within the magnetic field of 2 magnets.
- The armature is attached to the split-ring commutator
- The split ring commutator and armature are connected to a circuit with a battery battery by two metal or graphite brushes which touch the split-ring commutator
Describe how an electric motor works:
When a current is passed through the coil, the coil begins to rotate as the motor effect causes a force to act in opposite directions on either side of the coil. As the coil rotates, the split-ring commutator rotates along with the coil. However, the positive and negative terminal’s remain where they are. Therefore, every 180* rotation, the current flows around the coil in the opposite direction, but continues to flow around the circuit in the same direction. This causes the coil to rotate in a continual direction
How do you change the speed of an electric motor?
- Increase current
- Increase magnetic flux density
- Increase number of turns in coil
How can a potential difference be induced in a wire?
- Keep the magnetic field still and move the wire through the magnetic field
- Keep the wire still and move the magnetic field
If a wire movement was parallel to a magnetic field, what would be the induced potential difference be and why?
0V as the wire is not experiencing a change in magnetic field
How can you increase the size of the induced potential difference in the generator effect?
- Increase the movement of the wire
- Increase the magnetic flux density
- Shape wire into a coil
What is the south pole of the solenoid?
The south pole is the end where the current is clockwise
What is the north pole of a solenoid?
The north pole is the end where the current is anti-clockwise
What happens when a bar magnet is moved through a solenoid?
The movement of the bar magnet induces a potential difference in the solenoid that causes a current to flow
If the north pole of a bar magnet is pushed into a solenoid, what will be the induced polarity and direction of current in that end of the solenoid?
Inducted polarity at end A = North
Direction of current = Anticlockwise
If the north pole of a bar magnet is pulled out of a solenoid, what will be the induced polarity and direction of current in that end of the solenoid?
Inducted polarity at end A = South
Direction of current = Clockwise
If the south pole of a bar magnet is pushed into a solenoid, what will be the induced polarity and direction of current in that end of the solenoid?
Inducted polarity at end A = South
Direction of current = Clockwise
If the South pole of a bar magnet is pulled out of a solenoid, what will be the induced polarity and direction of current in that end of the solenoid?
Inducted polarity at end A = North
Direction of current = Anticlockwise
Explain why when the north pole of a bar magnet is pushed into a solenoid, a north pole is induced on that end of the solenoid:
The induced current generates a magnetic field. This induced magnetic field always opposes the original change, causing the magnet and pole to repel each other. Therefore work is done by the person moving the magnet. Therefore the current generated is a result of work done.
Explain why when the north pole of a bar magnet is pulled out of a solenoid, a south pole is induced on that end of the solenoid:
The induced current generates a magnetic field. This induced magnetic field always opposes the original change, causing the magnet and pole to attract each other. Therefore work is done by the person moving the magnet. Therefore the current generated is a result of work done.
Describe the structure of an alternator
A rectangular coil spins within a uniform magnetic field. One end of the coil connects to 1 metal slip ring whereas the other coil connects to another metal slip ring
Describe and explain the graph showing the PD produced by an alternator
The potential difference produced is greatest when the plane of the coil is parallel to the direction of the magnetic field, as the coil is “cutting” through the magnetic field lines. However, the induced PD is 0 when the plane of the coil is perpendicular to the magnetic field lines as the coil is moving parallel to the magnetic field lines, therefore it is not “cutting” through the magnetic field lines and therefore no PD is produced. As the coil continues to rotate back into a horizontal position, a potential difference is produced, however the potential difference has reversed direction as the 2 sides of a coil are moving in opposite directions to their original movement
Describe how an alternator works:
As the coil rotates within the uniform magnetic field, the coil “cuts” across the magnetic field lines and produces a potential difference. This potential difference causes a current to flow. This current changes direction every half turn as the direction of movement of each end of the coil switches between upwards and downwards
What happens if you increase the rotation speed of the coil in an alternator?
- The size of the PD produced increases
* The frequency of the current produced increases
What is the role of slip rings?
Slip rings cause the contacts to remain the same in an alternator. This causes an alternating potential difference to be produced
Describe the structure of a dynamo:
A rectangular coil spins within a uniform magnetic field. The end of the coil is connected to a split ring commutator
Describe and explain the graph showing the PD produced by a dynamo
As the coil rotates and cuts through the magnetic field lines, a potential difference and coil and induced. When the coil is perpendicular to the magnetic field lines, it is moving parallel to the magnetic field lines and therefore the potential difference is 0. As the coil continues to rotate, the split ring commutator swaps the connection causing the potential difference to remain positive
Describe how a dynamo works:
As the coil rotates within the uniform magnetic field, the coil “cuts” across the magnetic field lines and produces a potential difference. This potential difference causes a current to flow. This current remains in the same direction as the split ring commutator swaps the connection every half turn. This means that one side of the split ring commutator is always connected to the side of the coil that is moving upwards and one side of the split ring commutator is always connected to the side of the coil that is moving downwards.
How many peaks are there in 1 rotation of the coil in a dynamo?
2
How can the peak value of the induced pd be increased?
- increasing the speed of the coil
- increasing the strength of the magnet
- increasing the number of coils of wire on the coil
What is the structure of a loudspeaker?
How does a loudspeaker work?
An alternating current is sent through a coil of wire attached to the base of a paper cone. The coil surrounds 1 pole of a permanent magnet and is surrounded by the other pole, therefore the coil is between the 2 poles of the magnets. When a current flows through the coil, a force is exerted on the coil due to the motor effect, causing the cone to move. When the current is reversed, the force acts in the opposite direction, causing the cone to move in the opposite direction. The variations in current make the cone vibrate, making the air around the cone vibrate creating a sound wave. The frequency of the sound wave is the same as the frequency of the ac.
How do you increase the volume of a loudspeaker?
Increase the size of the current, causing a greater force to be produced, increasing the amplitude of the vibration
What is the structure of a microphone?
How does a microphone work?
Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet. This causes the coil of wire to move in the magnetic field, inducing a potential difference in the wire. This potential difference causes a current to flow. The movement of the coil (and therefore generated current) depends on the properties of the soundwave.
What does a microphone do?
Microphones convert the pressure variations of a sound wave into variations in current in an electrical circuit
What does a loudspeaker do?
Convert variations in current in electrical circuits into pressure variations in sound waves
What is the difference between electromagnetic induction and the generator effect?
Electromagnetic induction is when a potential difference is induced in a wire, the generator effect is when this wire is part of a complete circuit meaning a current flows.
What is the structure of a step-up transformer?
A step-up transformer contains more coils on the secondary coil than the primary coil
How does a transformer work?
- An alternating potential difference is supplied to the primary coil which causes a current to flow
- This generates an alternating magnetic field in the iron core
- The alternating magnetic field lines of the alternating magnetic field in the iron core pass through the secondary coil
- The alternating magnetic field in the secondary core induces an alternating potential difference in the secondary coil
How does a transformer induce a p.d on the secondary coil if there is no movement of iron core or coil?
The magnetic field of the iron core is alternating, therefore there is a change in magnetic field
Why are the wires insulated in transformers?
so the current cannot flow through the iron core
What is the structure of a step down transformer?
A step-down transformer contains more coils on the primary coil than the secondary coil
Why do transformers not work with a direct current?
There is no changing magnetic field
What is the transformer equation?
potential difference across primary coil / potential difference across secondary coil = number of turns on primary coil / number of turns on secondary coil
Vₚ / Vₛ = nₚ / nₛ
What is the transformer efficiency equation?
Primary potential difference x Primary current = Secondary potential difference x secondary current
Vₚ x Iₚ = Vₛ x Iₛ
Explain why transformers are used:
The heating effect of a wire is proportional to the square of the current. By increasing the potential difference, the current through the cables is reduced. Therefore, the heating effect is smaller and less power is wasted
If a step-up transformer made the potential difference 4x greater, how many times less is the power wasted?
16x less
Why would 2 iron rods placed in a solenoid move apart?
The rods would become induced magnets with the same polarity at each end. Therefore they would move apart as like poles repel
How often does the potential difference reverse in an alternator?
every half turn
How often does the current reverse in a alternator?
every half turn
