Topic 13 - Electromagnetic Induction Flashcards
What happens when an electrical conductor moves relative to a magnetic field?
- A potential difference is induced across the conductor.
What happens to an electrical conductor when there is a change to the magnetic field that is placed in?
- The potential difference is also changed.
How do you produce an electric current using a magnet and a conductor on a small-scale?
- Moving a coil of wire between magnetic poles.
- Moving a magnet within a coil of wire.
- Once a voltage is induced, if there is a complete circuit, a current will also be induced.
How is electrical current produced on a large scale?
An electromagnet is rotated around a coil.
What factors affect the size of an induced voltage?
- The number of turns on the coil of wire
- How strong the magnetic field is
- The speed of the movement
What is the requirement for an induced potential difference to cause a current flow?
The conductor must form a closed loop or be part of a complete circuit.
What can be said about the direction of the magnetic field produced by an induced current?
- The direction of the field is such that it opposes the change that induced the current.
- This change is either the movement of the conductor or a change to the field
What are two ways that the generator affect is used to generate different types of current?
- In an alternator to produce alternating current
- In a dynamo to produce direct current
How does a dynamo generate current?
- A coil of wire rotates inside a magnetic field.
- A commutator is used to ensure it continues rotating in the same direction.
- This keeps the current flowing in the same direction (d.c current).
How is electromagnetic induction used in alternators to generate alternating current?
- A coil of wire rotates in a magnetic field.
- The end of this coil is connected to slip rings which will cause the current to change direction while rotating (a.c current).
What electromagnetic effect does a microphone take advantage of and how?
- The generator effect
- It converts the pressure variations in sound waves into alternating current.
How do loudspeakers make use of the motor effect?
- The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produced audible sound
Explain how a loud speaker works:
- A cone wrapped in wire is connected to an a.c power supply and is placed in a permanent magnetic field.
- When current flows through the wire, it creates a magnetic field which interacts with the permanent field.
- This produces a force which causes the cone to vibrate, producing sound.
How is the pitch from a loudspeaker changed?
- The frequency of the a.c current is altered.
- This creates a different frequency of vibration in the cone
How do microwaves convert sound into electrical signals?
- Sound produces pressure variations which causes the diaphragm to vibrate.
- A coil of wire is connected to the diaphragm, so as the dial moves, the coil also moves.
- Movement in a magnetic field = a p.d is induced
- As there is a complete circuit a current is also created
Describe the makeup of a basic transformer:
A primary coil and a secondary coil of wire wrapped around an iron core.
Why is iron used as the core for a transformer?
It is easily magnetised
How does a transformer work?
- An alternating current flows through the primary coil
- This induces a changing magnetic field in the core
- This changing magnetic field induces an emf, which produces a current int the secondary coil
Why must the current flowing through the primary coil of a transformer be alternating?
- For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing.
- For the magnetic field to be changing, the current in the primary coil must be alternating.
What can be said about the electrical power input and output of a 100% efficient transformer?
The electrical power input is equal to the electrical power output.
Equation linking number of turns with p.d in transformers
primary p.d / secondary p.d = primary turns / secondary turns
Power equation for transformers with 100% efficiency (uses voltage and current)
Vp x Ip = Vs x Is
On a step-down transformer, does the secondary coil have more or fewer turns?
The secondary coil has fewer turns
Advantage of a step down transformer in the national grid?
It means that voltage can be reduced to a value safe enough to be used in houses
Why is it important to have step up transformers in the national grid?
- It can increase efficiency
- For the same power, a higher voltage leads to a lower current (P = V x I)
- The lower the current, the less energy that is lost.
- Used for transmission across power lines.
How does a step-up transformer work?
- The primary coil has fewer turns
- An a.c current produced a magnetic field in the primary coil.
- The iron core passes the magnetic field to the secondary coil
- The magnetic field induces a higher voltage in the secondary coil
- Power is conserved, so this produces a lower current
Where are step-up and step-down transformers used in the national grid?
- Step-up transformers - used at power stations
- Step-down transformers - used domestically.