Chapter 15 - Electromagnetism Flashcards
What do all magnets have?
North and South poles
What do all magnets produce?
Magnetic field
How can you show magnetic filed?
Drawing magnetic field lines.
What is a magnetic field?
A region where other magnets or magnetic materials experience a force.
What direction do magnetic field lines go?
From north to south
When are magnetic fields the strongest?
When they are really close to each other.
Where is the magnetic field the strongest?
At the poles of the magnet, the lines of a magnetic field are always the most concentrated at the poles than elsewhere.
If 2 poles of a magnet are put near each other, what will happen?
They will exert a force on each other, Attract or repel.
Describe the force between a magnet and a magnetic material.
always attract.
What is a compass?
A bar magnet that is pivotal at the centre and is free to rotate in the presence of a magnetic field.
How does a plotting compass work?
Inside there is a tiny bar magnet.
The North pole of this magnet is attracted to the south pole of any other magnet it is near.
A plotting compass points in the direction away from the north pole of a magnet towards the south pole of the magnet.
How can you see a visible magnetic field around a bar magnet?
Place a sheet of paper over a bar magnet and sprinkle iron filings over the paper. The filings will form patterns of line - magnetic field lines.
How does Earth have a magnetic field?
The Earth’s core contains a lot of molten iron. This iron flows in convection currents and at the same time spins as the Earth rotates. The movements of iron are responsible for the Earth’s magnetic field.
Describe the Earth’s magnetic field.
The geographical north pole of the Earth is actually the south pole of its magnetic field.
when not near a magnet, where do compasses point? Why?
North (geographical south pole) - the Earth generates its own magnetic field so the Earth’s core must be magnetic.
What are some magnetic materials?
Iron, steel, nickel, cobalt.
These materials will be attracted to magnets and can become temporary magnets.
What are the 2 types of magnets?
Permanent and induced/temporary.
What do permanent magnets do?
Produce their own magnetic field.
What are induced/temporary magnets?
Magnetic materials that can be magnetised (turned into a magnet) when in a magnetic field.
Examples of inducing magnets
An unmagnetised iron rod placed in line with a bar magnet becomes a magnet with poles at each end. The nearest poles of the rod and bar magnet always have opposite polarity.
What happens when you take away? (induced magnets)
It quickly loses its magnetism and stops producing a magnetic field.
What happens when a current flows through a wire?
A magnetic field is created around the wire
How is the magnetic field around a wire arranged?
Concentric circles perpendicular to the wire, with the wire in the centre.
What are electromagnets?
Powerful temporary magnets can be made by wrapping a coil of wire around a soft iron core and passing an electric current through the wire.
What is the right-hand rule used for? How do you do it?
ou use the right-hand rule to find the direction of the magnetic field for each direction of the current.
thumb = current direction
fingers = direction of magnetic force
What does changing the direction of current in a wire do?
It changes the direction of the magnetic field.
What changes the strength of the magnetic field around a wire do?
The current and distance from the wire
- the greater the current the smaller the distance, the greater the strength of the magnetic field.
What is a solenoid?
A long coil of insulated wire. A magnetic field is produced in and around the solenoid when a current is passed through it.
How can you increase the strength of a magnetic field that a straight wire produces?
By wrapping the wire into a solenoid - the magnetic field is much stronger than if the wire was straight.
Describe the magnetic field inside a solenoid.
The field lines are parallel to the axis of the solenoid and they are all in the same direction. The magnetic form is stronger and uniform.
Why do solenoids have a stronger magnetic field than a straight wire?
The magnetic field lines around each loop of the wire line up with each other, resulting in lots of field lines pointing in the same direction that are very close together.
How can you increase the magnetic field strength of a solenoid?
The magnetic field strength will increase in strength if the current is increased, the number of coils increases and if a magnetic material is placed inside the coil, such as an iron bar.
What is an electromagnet?
A solenoid with an iron core. When current is passed along wire a magnetic field is created around it.
How are electromagnets used in cranes?
To pick up things that are made from magnetic materials and move them around. Electromagnets can be turned on and off which is very useful in cranes.
What can electromagnets be used within other circuits as
Switches
How can electromagnets work as a switch?
when the switch in a circuit is closed, it turns on the electromagnet which attracts the iron contact on the rocker
the rocker pivots and closes the contacts, completing circuit two and turning on the motor
What is the motor effect
The motor effect is when a conductor, carrying a current, is placed in a magnet, producing the field, and the conductor exerts a force on each other.
What happens when a current-carrying wire is put between 2 magnetic poles?
The magnetic field around the wire interacts with the magnetic field it has been placed in causing the magnet and wire to exert a force on each other.
this is the motor effect - the wire will move.
How can the size of the force be increased?
It can be increased by increasing the current and using a stronger magnet. The force is greatest when the wire is perpendicular to the magnetic field.
To experience the full force of the magnetic field a current-carrying wire is in, what position must the wire be in?
Perpendicular to the magnetic field - it has to cut across field lines.
What happens is a current-carrying wire runs parallel to the magnetic field it is in?
It will not experience any force
In what direction does the force on a current-carrying wire in a magnetic field?
Right angle to the magnetic field of magnets and to direction of current.
How can you find the direction of the force on a current-carrying wire in a magnetic field?
Fleming’s left-hand rule
What is meant by the “Magnetic flux density” of a magnetic field?
The strength of the magnetic field.
What is magnetic flux density measured in?
Tesla (T)
In a basic dc motor, where do forces act? What does this cause?
on the two side arms of a coil of wire that’s carrying a current
because the coil is on a spindle and the forces act one up and one down, the coil rotates
Describe an electric motor.
Rectangular coil that is forced to rotate. The coil is connected to the battery by two metal or graphite brushes. The brushes press onto a splitting commutator fixed to the coil.
What is the purpose of a split-ring communicator?
The split-ring commutator reverses the direction in which current flows through the coil every half-cycle (rotation).
This reverses the direction of the force exerted on each side of the coil every half-cycle (rotation). This allows for continuous rotation of the coil.
When current is passed through the wire, why does the coil rotate?
A force acts on each side of the coil due to the motor effect - force experienced by wire when current is passed through it in a magnetic field.
The force on one side is in opposite direction to the force on the other side.
How can the direction of a DC motor be reversed?
Reversing the polarity of the supply (reversing current)
Reversing the magnetic field (swap poles)
What type of current do loud speakers and head phones use?
Ac
How do loud speakers and headphones work?
Ac sent through a coil of wire on base of paper cone
Coil surrounds one pole of a permenant magnet and is surrounded by the other pole
The current causes a force on the coil, causing the cone to move
When the current reverses, force on coil acts in opposite direction so cones moves in opp direction
Variations in current = vibrations in cone, making air around cone vibrate and create the variations in pressure that cause a sound wave
How can you alter the sound wave produced by a loud speaker?
Alter the frequency of the ac as the frequency of the sound wave = the frequency of the ac
What is the generator effect?
The induction of a potential difference (and current if there’s a complete circuit) across a conductor which is experiencing a change in magnetic field.
What does the generator effect cause?
A potential difference in a conductor
A current if the conductor is part of a complete circuit.
How can you generate a potential difference across the ends of a conductor?
you produce an induced potential difference across the ends of a wire if the wire is in a changing magnetic field so that it cuts field lines or flux lines.
Moving the electrical conductor (wire) in a magnetic field will cause a potential difference to be induced across the ends of the wire.
Moving a bar magnet through a coil of wire will induce a potential difference across the ends of the wire.
What does the potential difference that is induced depend on?
The length of the wire in the field (more coils will increase the pd) the rate and which you cut the field line and using a stronger magnetic field.
Moving the wire faster and using stronger magnets means that you are cutting more field lines per second.
Aside from changing the direction of the movement of the magnet in a complete circuit, how can you reverse the current?
Swap the polarity of the magnet
If you keep the magnet/coil moving backwards or forward inside a coil/magnetic field, what is happening?
An alternating current is created. Why not dc???
How do generators work to produce an ac current?
They turn a magnet end to end in a coil or they turn a coil inside a magnetic field.
As the coil turns, the coil cuts through the magnetic field, inducing a pd and a current. After a half turn, the direction of field reverses so the potential difference reverses and the current flows in the opposite direction.
If the magnet keeps turning in the same direction, the potential difference will keep reversing and an alternating current will be produced.
How can you change the size of an induced potential difference?
The size of the magnetic field is the greatest when the plane of the coil is parallel to the direction of the magnetic field. This is because the sides of the coil cut through the magnetic field lines.
Using a magnet with a stronger magnetic field and a coil with a bigger area and more turns of wire on it will increase the potential difference.
Describe the movement of a generator?
They rotate a coil in a magnetic field or move a magnet in a coil.
What do ac generators have instead of a split-ring commutator and why?
Slip rings as brushes so the contacts don’t swap every half turn, producing an alternating potential difference.
What is an ac generator called?
An alternator
What is a dc generator called?
A dynamo
What is the main difference between a dynamo and an alternator?
Dynamos generate a direct current so they have a split ring commutator instead of slip rings.
What does the split ring commutator in a dynamo do?
As the coil spins, the split ring commutator reconnects the coil the opposite way around in a circuit every half turn. This happens each time the coil is perpendicular to the magnetic field lines.
The induced potential difference does not reverse its direction as it does in the alternator. The induced potential difference varies from zero to a maximum value twice each cycle and never changes polarity.
What do oscilloscopes show?
How the pd generated in the coil changes over time
For an alternator, what does an oscilloscopes show and how?
An ac
A line that goes up and down, crossing the horizontal axis
For a dynamo, what does an oscilloscope show and how?
Line stays above x axis but isn’t straight
What is the height of the line on an oscilloscope?
The generated pd at that time
What does increasing the frequency of revolutions on a generator do in terms regarding an oscilloscope?
Created more peaks
What are micro phones?
A device which generates electricity from sound waves.
How do microphones work?
Sound waves hit flexible diaphragm that is attached to a coil of wire wrapped around a magnet
Causes coil to move in the magnetic field, generating a current that is the same frequency as the original sound
Movement of coil depends on properties of sound wave (louder moves further)
Overall microphone has converted the signal back into the original sound
How does a speaker work?
The signal source causes current to flow through the coil and creates an electromagnet
The size of the current varies depending on the electric signal and so the strength of the electromagnet varies
The permanent magnet is attracted or repelled by the electromagnet
The movement of the permanent magnet causes the cup to vibrate
The vibrating cup pressurises the surrounding air producing sound waves.
What do microphones do?
Convert pressure variations of a sound wave into variations in a current in an electrical circuit.
What do transformers do?
Increase or decrease the size of an alternating potential difference.
Describe how a transformer works?
An alternating current is applied across the primary coil
This causes a changing magnetic field in the iron core
This changing magnetic field induces an alternating potential difference in the secondary coil
If the secondary coil is part of a complete circuit, this causes a current to be induced.
What current is needed for a transformer to work?
An alternating current is needed because with a direct current, there is no changing magnetic field, so the secondary potential difference is zero.
What is the ratio between the primary and secondary pds of a transformer the same as?
The ratio between the number of turns on the primary and secondary coils.
What does a step up transformer do? How?
Increases voltage on secondary coil.
They have more turns on the secondary coil than the primary coil.
What does a step down transformer do? How?
Decreases voltage on secondary coil
They have less turns on the secondary coil than the primary coil.
What is the transformer equation?
Potential difference across primary coil/potential difference across secondary coil = number of turns in a primary coil/number of turns on a secondary coil
Describe the efficiency of transformers?
Almost 100%
If you assume that a transformer is 100% efficient, what can you conclude?
The input power is equal to the output power
so
Secondary current x secondary pd
=
Primary current x primary pd
(Due to P=VI)
Why does the national grid raise the potential difference to supply power?
By increasing the potential difference, the current through the grid cables is reduced. Low current means less energy is wasted in heating the wires and surroundings. This makes the national grid more efficient.
Low current means high voltage due to P=VI
How can the number of turns needed to increase the pd to the right levels be calculated?
With the transformer equation.
How can you determine the movement/current/magnetic field of a generator?
Fleming’s right hand rule
How do you calculate the size of the force on a conductor?
Force (N) = magnetic flux density (T) x current (A) x length (m)
