Section 5 - Motors, Generators and Transformers Flashcards
What is a magnetic field?
A region where magnetic materials experience a force. Current-carrying wires also experience a force acting on them.
What are magnetic field lines?
Lines used to show the size and direction of magnetic fields.
Which direction do magnetic field lines always point?
From North to South (outside of the magnet).
What happens when the north and south poles of two permanent bar magnets are placed near each other?
A uniform magnetic field is created.
Diagram pg 81 of revision guide
What two things can be used to look at magnetic field lines?
- Compasses
* Iron fillings
How can compasses be used to look at the magnetic field lines of a magnet?
They always point towards the south pole, so they can moved around and used to trace the field lines.
How can iron fillings be used to look at the magnetic field lines of a magnet?
The magnet is placed under a piece of paper and iron fillings are scattered on top. They arrange themselves along the field lines.
Like magnetic poles…
Repel
Opposite magnetic poles…
Attract
What does a diagram showing repelling magnets look like?
Pg 81 of revision guide
What does a diagram showing attracting magnets look like?
Pg 81 of revision guide
Do both poles of a magnet attract magnetic materials?
Yes (as long as they are not magnets)
What happens when a magnet is brought near a magnetic material?
- Magnetism is induced in the material and it acts as a magnet.
- The closer the magnet gets, the stronger the induced magnetism
- A north pole will induce a south pole and vice versa
What happens in terms of magnetism when a current flows through a wire?
A magnetic field is generated.
What does the magnetic field around a straight wire look like?
Made up of concentric circles, with the wire in the centre.
What happens when the direction of the current in a wire is reversed?
The direction of a magnetic field is also reversed.
What is used to work out the direction of a magnetic field around a straight wire?
The Right-Hand Grip Rule
How does the Right-Hand Grip Rule work?
In a straight wire:
• Thumb -> Direction of current
• Fingers -> Direction of magnetic field
What happens when you bend a wire into a single flat circular coil?
- Concentric circles of magnetic field lines are still generated at each part of the coil
- In the centre, the magnetic field acts like a bar magnet
Give some examples of magnetic materials.
- Iron
- Nickel
- Cobalt
- Steel
What are hard magnetic materials?
Materials which retain their magnetisation once magnetised.
What are soft magnetic materials?
Materials which lose their magnetisation easily.
Give an example of a magnetically soft material.
Iron
Give an example of a magnetically hard material.
Steel
What is a solenoid?
A coil of wire carrying electric current.
What is the Right-Hand Grip Rule used for?
Working out the direction of the magnetic field around a straight wire when the current is given, or vice versa.
Describe the magnetic field in a solenoid.
- Inside the coil, the magnetic field is strong and uniform
* Outside the coil, the field is like that of a bar magnet
How can the direction of a magnetic field in a solenoid be worked out?
The Right-Hand Grip Rule is used, except:
• The thumb represents the direction of the magnetic field (i.e. Thumb point to north pole)
• The fingers represent the coil -> The fingers point in the direction the current is flowing
(See diagram pg 82 of revision guide)
How can the magnetic field in a solenoid be increased?
- Increasing current
- Increasing the number of coils
- Adding an iron core
What is an electromagnet?
A magnet whose electric field can be turned on and off with an electric current.
What are some uses of electromagnets?
- Scrapyard cranes
- Circuit breakers
- Electric bells
- Relays
When can the motor effect be observed?
When a current-carrying wire is put in a magnetic field.
What is the motor effect?
When a current-carrying wire is placed in a magnetic field, a force is exerted on the wire.
What happens when a current-carrying wire is placed in a magnetic field?
A force is exerted on it.
Why does the motor effect work?
The magnetic field produced by the wire and the magnetic field between the magnetic poles affect each other, exerting a force on the wire.
How does the direction of the wire affect the motor effect?
- Wire at 90* to the magnetic field -> Maximum force
* If the wire runs along the magnetic field, no force is exerted at all
What factors will increase the force exerted by the motor effect?
- Increasing the current
- Increasing the magnetic field
- Wire at 90* to magnetic field
Which rule is used for the motor effect?
Fleming’s Left-Hand Rule
How does the Fleming’s Left-Hand Rule?
- Thumb -> Force
- First finger -> Magnetic field
- Second finger -> Current
What effect do electric motors use to make them work?
The motor effect
What factors speed up an electric motor?
- Increasing current
- Increasing magnetic field
- Placing wire at 90* to magnetic field
How does an electric motor work?
- A coil of wire is placed in a magnetic field
- A current is passed through this
- Due to the motor effect, one side of the wire moves up, while the other side moves down, causing the motor to spin
- A SPLIT RING COMMUTATOR is used to swap the contacts every half turn to keep the motor turning in the same turn
(See diagram pg 84)
How can the direction that an electric motor spins be reversed?
- Swapping the polarity of the DC supply
2. Reversing the magnetic field
How are electric motors used inside appliances?
The coil is linked to an axle, making it spin round. A fan, blade, etc. can be attached to this to make it spin.
What is the generator effect?
The induction of a potential difference (and maybe current) in a wire which is intersecting a magnetic field.
What is another name for the generator effect?
The dynamo effect.
When does the generator effect create a current?
When the wire is connected in a circuit.
What happens when a wire is moved around inside a magnetic field?
If it is interesting the magnetic field lines, a potential difference is induced in the wire.
What are the two ways the generator effect can be used to generate an AC current in a wire?
- Moving a magnet back and forth in a coil of wire
2. Rotating a magnet end to end in a coil of wire
How can the direction of the potential difference generated by the generator effect be reversed?
- Moving the magnet in the opposite direction
- Moving the wire in the opposite direction
- Reversing magnetic field
What factors increase the potential difference generated by the generator effect in a coil?
- Strength of magnetic field
- Area of the coil
- Number of turns on the coil
- Speed of movement
What are the two types of generator?
- AC generator
* DC generator
How does an AC generator work?
• The construction is like a motor
• A coil rotates in a magnetic field
• As the coil spins, a current is induced in the coil. The direction of the current changes every half turn.
• The AC generator has SLIP RINGS AND BRUSHES instead of a split ring commutator.
• This produces an AC current.
(Diagram Pg 86)
How does a DC generator work?
• The construction is like a motor
• A coil rotates in a magnetic field
• As the coil spins, a current is induced in the coil. The direction of the current changes every half turn.
• The DC generator has a SPLIT-RING COMMUTATOR -> Contacts swap every half turn
• This produces a DC current.
(Diagram Pg 86)
Describe the display of a CRO connected to an AC generator.
The wave goes up and down, above and below the axis.
Describe the display of a CRO connected to a DC generator.
The wave ‘bounces’ up and down above the time axis.
What happens to the output of the generator if the speed of the rotating coil is increased?
- Increased frequency
* Higher peaks (current or voltage)
Which rule can be used for working out the direction of current in a generator?
Fleming’s RIGHT-hand rule
This is like Fleming’s left-hand rule, except on the other hand
What is the difference between AC and DC generators?
- AC generators -> Slip rings and brushes
* DC generators -> Split-ring commutator
Describe the contacts in motors and generators.
- AC generators -> Slip rings and brushes
- DC generators -> Split-ring commutator
- Motor -> Split-ring commutator
What do slip rings do?
Nothing - they allow the coil to spin without reversing the direction of the current.
What do split-ring commutators do in motors?
Reverse the direction of the current in the coil every half turn to keep the coil spinning in the same direction.
What do split-ring commutators do in DC generators?
Swap the contacts every half turn, keeping the output current flowing in the same direction.
What are the two types of transformer?
- Step-up transformer
* Step-down transformer
What does a transformer look like?
2 coils (primary and secondary) joined by a soft iron core.
What do step-up transformers do?
Step voltage up.
What do step-down transformers do?
Step voltage down.
Describe the number of turns on the primary and secondary coil of a step-up transformer.
More turns on the secondary than primary.
Describe the number of turns on the primary and secondary coil of a step-down transformer.
More turns on the primary than secondary.
Which effect do transformers use?
Generator effect.
What type of current is used in transformers and why?
AC - this produces a constantly changing magnetic field, so a potential difference can be induced in the secondary coil.
How does a transformer work?
- AC passes through the primary coil, producing a magnetic field that stays within the iron core
- Nearly all of it passes from the primary to secondary coil due to the iron core
- The changing magnetic field induced a potential difference in the secondary coil
- This in turn causes a current
What is the soft iron core used for in transformers?
Transferring the changing magnetic field from the primary coil to the secondary coil.
No electricity flows round the iron core!
What is the transformer equation (in terms of coils and potential difference)?
P.D. across Primary Coil / P.D. across Secondary Coil = No. of Turns on Primary Coil / No. of Turns on Secondary Coil
Vp / Vs = np / ns
(This equation can be written either way up)
How efficient are transformers and what does this mean?
Nearly 100%, so the power on each side of the transformer is equal.
What is the transformer equation (in terms of power)?
Power Input = Power Output
Vp x Ip = Vs x Is
What is a switch mode transformer?
- A transformer that operates at a higher frequency than traditional transformers.
- Used in chargers and power supplies (e.g. for laptops)
What is the frequency for normal transformers?
50Hz
What is the frequency for switch-mode transformers?
Between 50 kHz and 200 kHz.
What makes a switch-mode transformer different to a normal transformer?
- Lighter
- Smaller
- Operate at at higher frequency (50 kHz - 200kHz)
- More efficient
Why are switch-mode transformers efficient?
They use very little power when no load is applied but they are switched on.
e.g. If your charger is on, but not connected to a phone, it uses little power.
