Flashcards in Electric Motors and Electromagnetic Induction Deck (37):
What happens when a wire carrying the current is put into a magnetic field?
A FORCE (motor effect)
Why is there the motor effect?
This is because the charged particles (electrons in a current) moving through a magnetic field will experience a force, as long as they are not moving parallel to the field lines
Describe the motor effect, and how does the force on a current-carrying conductor in a magnetic field increase with the strength of the field and with the current?
1. To experience the full force, the wire has to be at 90 degrees to the magnetic field. If the wire runs along the magnetic field it will not experience any force at all. At angle in between it will experience some force.
2. The force always acts in the same direction relative to the magnetic field of the magnets and the direction of the current in the wire
3. The strength of the force increases with the strength of the magnetic field
4. The force also increases with the amount of current passing through the conductor
5. Reversing the current or the magnetic field also reverses the direction of the force
How do you speed a motor up?
1. Increase current flowing through the wire
2. Increase number of turns on the coil
3. Increase strength of magnetic field
4. Use a soft iron core in the coil
Describe how a motor works
1. Current passes through a loop of wire, one side will experience a force pushing it upwards and the other side will experience a force pushing it downwards causing the loop to rotate
2. The split ring commutator means that when the loop is vertical, the connections to the supply through the brushes swap over, so that the current flowing through each side of the loop changes direction (it swaps the contacts every half-turn to keep the motor rotating in the same direction)
3. The wire at the bottom is now pushes upwards and the wire at the top is now pushed downwards , so the loop carries on turning
4. The arrangement of the brushes and split ring changes the direction of the current flowing through the loop every half turn, which means that the rotation can be continuous
How can a motor be adapted?
1. The direction of the motor can be reversed by swapping the polarity of the d.c. supply or swapping the magnetic poles over
2. Permanent magnets can be replaced with curved electromagnets capable of producing very string magnetic fields (practical motors)
3. The single loop is replaced with several coils of wire wrapped on the same axis, this makes the motor more powerful and allows it to run more smoothly (practical motors)
4. The coils are wrapped on a laminated soft iron core. This makes the motor more efficient and more powerful (practical motors)
5. The speed can be increased by adding more turns to the coil, increasing the current, increasing the strength of the magnetic field or by adding a soft iron core
How can you work out which way the coil will turn?
The left hand rule
Describe how loudspeakers work
1. They use the motor effect
2. A.c signals from an amplifier are fed to a coil of wire in the speaker, which is wrapped around the base of a cone
3. The coil is surrounded by a permanent magnet, so the a.c. signals cause a force on the coil and make it move back and forth
3. These movements make the cone vibrate and this creates sounds
What is electromagnetic induction?
When voltage is induced (and maybe current if the circuit is complete) in a wire/conductor/coil which is experiencing a change in magnetic field, or when it moves through a magnetic field
What is the dynamo effect?
Using electromagnetic induction to transform kinetic energy into electrical energy
What are the two situations of electromagnetic induction?
a) When an electric conductor (a coil of wire is often used) moves through a magnetic field
b) When the magnetic field through an electric conductor changes (gets bigger or smaller or reverses)
Describe how a generator works
1. If a wire is moved across a magnetic field at right angles a voltage is induced (or generated) in the wire
2. If the wire is part of a complete circuit then current flows
3. This is called electromagnetic induction
4. Generators use movement to make electricity
5. The voltage and current are generated when a conductor such as a wire cuts through the magnetic field lines
6. The faster the lines are cut, the larger the induced voltage and current
7. This can also be achieved by moving a magnetic through a coil of wire
-The generation of electricity is by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field
What is Faraday's Law of Electromagnetic Induction?
The size of the induced voltage across the ends of a wire (coil) is directly proportional to the rate at which the magnetic field lines of flux are being cut
How can the size of the induced voltage (and current) be increased by in a generator (move wire)?
1. Move wire more quickly
2. Using a stronger magnet
3. Wrapping the wire into a coil so that more pieces of wire move through the magnetic field
How can the size of the induced voltage (and current) be increased by in a generator (move magnet)?
1. Move magnet more quickly
2. Using a stronger magnet
3. Using a coil with more turns
4. Using a coil with a larger cross-sectional area (as cuts more field lines)
How can you test that the generator works both ways (a and b)?
-By connecting an ammeter to a conductor and moving the conductor through a magnetic field (or moving a magnet through the conductor)
-The ammeter will show the magnitude and direction of the induced current
-If the direction of the movement is reversed, the induced voltage/current will be reversed too
Give an example of how generators are used in everyday life
1. As the cyclist pedals, the wheel rotates and a small magnet within the dynamo spins around
2. As the magnet turns its magnetic field cuts through the surrounding coil, inducing a current in it.
3. This current can be used to work the cyclist's lights.
What is a generator that produces an alternating current called?
What are the properties of an alternator?
1. They rotate a coil in a magnetic field or a magnet in a coil, and as the coil spins, a current is induced in the coil. This current changes every half turn
2. They have slip rings and brushes so that the contacts don't swap every half turn, instead of a split-ring commutator
3. This means that they produce a.c. voltage. The faster revolutions produce not only more peaks, but higher overall voltage too
-E.G power stations use alternators to produce electricity, they just get the energy needed to turn the coil or magnetic field in different ways
-The frequency of an alternating current is the number of complete cycles it makes each second. If an alternator rotates twice in a second, the frequency of the alternating current it produces is 2Hz. The frequency of the UK mains supply is 50Hz
What are the similarities of a motor and generator?
-Both use magnets
-Both use a coil
-Both transfer energy
What are the differences of a motor and generator?
They have the reverse process of each other
Motor: electrical energy into kinetic energy
Generator: kinetic energy into electrical energy
Describe a transformer
They all have two coils, the primary and secondary joined with an iron core
What do transformers do?
They change the size of an alternating voltage by having different numbers of turns on the input and output sides
Describe how a transformer works
1. When an alternating current is passed through a coil, the magnetic field around it is continuously changing (reversing direction and as the magnetic field is continuously changing direction the voltage produced is an alternating voltage)
2. As the size of the current in the coil increases the field grows and as the size of the current decreases the field collapses
3. If a second coil is placed near the first, this changing magnetic field will pass through it and as it cuts through the wires of the second coil, a voltage is induced across that coil
4. The size and direction of the induced voltage changes as the voltage applied to the first coil changes (and the number of turns on each coil)
5. An alternating voltage is applied across the primary coil therefore produces an alternating voltage across the secondary coil
6. This combination of two magnetically linked coils is called a transformer
Where are transformers used?
-Mobile phone chargers
-National grid to transfer current from power stations to power lines to our homes and buildings
What is a step up transformer?
A transformer that is used to increase voltage (more turns on the secondary coil than the primary coil)
What is a step down transformer?
A transformer that is used to decrease voltages (more turns on the primary coil than the secondary coil)
How do the coils affect a transformer?
The ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils
What is the transformer equation?
input voltage/output voltage is equal to number of turns on primary coil/number of turns on secondary coil
Vp/Vs (Volts) = Np/Ns (no units)
What is the efficiency of a transformer?
Assume transformers are 100% efficient
Vp x Ip = Vs x Is
How are transformers used in the national grid?
-Step up and step down transformers are used when transmitting electricity across the country
-Transformers are used in the National Grid so that so that the electricity is transmitted at low currents and high voltages (so less energy lost in the form of heat)
1. The voltage produced by power stations is too low to be transmitted efficiently. P=VI so the lower the voltage the higher the current for a given amount of power and the current causes the wire to heat up
2. Therefore a step-up transformer is used to boost the voltage before it is transmitted (increased to approximately 400 kV but very dangerous so are supported high above the ground of pylons)
3. As cables enter cities and towns they are buried underground. Close to where the energy is needed the supply is passed through a step-down transformer. It is used at the end of the journey to reduce the voltage (to approximately 230 V, whilst at the same time increasing the current) so that it is more useful and safer to use.
How does an electric motor work when switch is closed? (marks scheme)
1. Current in coil
2. (Creates) a magnetic field (around the wires of the coil)
3. Interaction of this field with that of permanent magnets
4. There is a force on the wore of coil
5. Reference to LHR
6. Force up on one side and down on other side
How does a transformer change the size of an alternating voltage?
By having different numbers of turns on the input and output sides
What is the relationship between input (primary) voltage and output (secondary) voltage and the turns ratio for a transformer?
input (primary) voltage / output (secondary) voltage = primary turns / secondary turns
Vp/Vs = np / ns
What is the relationship between input power and output power?
input power = output power
VpIp = VsIs
for 100% efficiency
What is an electromagnet?
When a solenoid is wrapped around a magnetically soft core such as iron (combination of soft iron core and solenoid)