Topic 7- Magnetism and electromagnetism Flashcards
What is produced when current flows through a conducting wire?
A magnetic field is produced around the wire
What determines the strength of the magnetic field around a current-carrying wire?
The magnitude of the current flowing through the wire
The distance from the wire
What is a solenoid?
A coil of wire which when current passes through creates a strong magnetic field
Describe the magnetic field found inside a solenoid
Strong and uniform
What is an electromagnet?
A solenoid with an added iron core
Adding the iron core increases the strength of the magnetic field
What is the motor effect?
When a force is exerted between a magnetic field and a current-carrying conductor placed in that field
What rule is used to determine the force experienced due to the motor effect?
Fleming’s Left-Hand Rule
When using Fleming’s left hand rule what does the fore finger represent?
The forefinger points in the direction of the magnetic field
When using Fleming’s left rule what does the middle finger represent?
The middle finger points in the direction of current flow in the conductor
What factors affect the size of the force on a current-carrying wire in a magnetic field?
The magnitude of the current flowing through the conductor
The strength of the magnetic field that the conductor is placed in
If the direction of current in a current-carrying wire placed in a uniform magnetic field is reversed, what happens to the force?
The direction of the force is reversed
If the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased, what happens to the force?
The strength of the force is increased
What criteria must be met for the equation linking force, magnetic flux density, current and length to hold?
The conductor must be at right-angles to the magnetic field it is placed in
What is the unit used for magnetic flux density?
Tesla , R
How does an electric motor work?
A coil of wire, carrying a current, is placed in a magnetic field
The forces on the two sides perpendicular to the field experience forces in opposite directions
This causes a rotational effect
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 produce audible sound
Explain how a loudspeaker works
A cone with a wire wrapped around it 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 second magnetic field, which interacts with the permanent field
This produces a force which causes the cone to vibrate
How is the pitch of the sound from a loudspeaker changed?
The frequency of the a.c current is altered
This creates a different frequency of vibration in the cone
What happens when an electrical conductor moves relative to a magnetic field?
A potential difference is induced across the ends of the conductor
What happens to an electrical conductor when there is a change to the magnetic field that it is placed in?
A potential difference is induced across the ends of the conductor
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 is an advantage of elctromagnets?
You can turn them on and off
How can you increase field strength in solenoids?
Increase current
Add more coils
Add iron core
What are two ways that the generator effect is used to generate different types of current?
In an alternator to produce alternating current
In a dynamo to produce direct-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 in a circuit
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
Explain how a transformer works
An alternating current flows through the primary coil
This induces a changing magnetic field in the core
This changing magnetic induces a current to flow in 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
What do step-up transformers do?
Increase p.d
Decrease current
What do step-down transformers do?
Decrease p.d
Increase current
What is the generator effect?
Using movement and a permanent field to induce a current
How can you increase the efficiency of a transformer?
Insulating layers in core (laminations)
Used in national grid to transfer electricity at low currents so less heat energy is wasted
