Magnetism and Electromagnetism Flashcards
(36 cards)
What are the forces produced when magnets are brought together?
When two like poles come together they repel each other.
Two unlike poles attract each other.
What is the difference between permanent and induced magnets?
Permanent magnet - produces its own magnetic field - they always have a magnetic field
Induced magnet - becomes a magnet when placed in a magnetic field - when the permanent magnet is taken away these magnets loose their magnetism.
Describe a magnet
A magnet has two poles, a north pole and south pole, this is where the force of the magnets are the strongest.
What is the force between magnets called?
The attraction and repulsion between poles are called non contact forces.
What is the force induced magnets always cause?
Attraction
What is a magnetic field?
This is region around a magnet that where a force acts on another magnet or magnetic material. The closer to the magnet the stronger the field - closer to the poles.
How do you plot magnetic field lines?
You can use a compass or iron fillings.
Compass - first place one near the north pole of the magnet than draw a cross at the north point of the compass move the compass so the south pole of the compass is on the cross. We continue until we have plotted a complete magnetic line join the dots. We show the direction by an arrow. We repeat this starting at different points around the magnet.
What are the four types of magnetic material?
Iron
Steel
Cobalt
Nickel
What direction does the magnetic field line go?
From North to South
How can we tell the Earth has a magnetic field?
When a compass is away from a magnet the compass will always point in the North South direction. This is due to the Earths core.
How does current flowing through a wire generate a magnetic field?
When a current flows through a conducting wire, a magnetic field is produced around the wire. We can prove this by a compass. The strength of the magnet depends on the amount of current flowing through the wire. The magnetic field is stronger closer to the wire. If the current direction is changed so is the magnetic field.
Explain how a solenoid an increase the magnetic effect of a current
A solenoid increases the strength of the magnetic field by -
Increase the current
Increase the coils
Place a piece of iron inside the iron - iron core, this is called an electromagnet - we can turn electromagnets on or off and vary its strength.
What is the magnetic field pattern for a straight wire carrying a current and for a solenoid?
When the current is on we get a strong uniformed magnetic field.
Where are electromagnets used?
A relay - used in high voltage circuits.
A doorbell
How do relays work?
It consists of two separate circuits a low voltage circuit with an electromagnet and a high voltage circuit with two metal contacts - one of the contacts is connected to a spring. There is an iron block attached to this contact. When the low voltage circuit is on the electromagnet produces a magnetic field this now attracts the iron block this cause the contacts to close. When the low voltage circuit is switched off the two contacts spring back and the circuit is broken.
How do doorbells work?
The switch of the circuit is closed when the buzzer is pressed, this causes a current to flow a magnetic field is then produced by the electromagnetic the iron contact is now attracted to the magnetic field. When the contact moves the clapper now hits the bell this then breaks the circuit so there is now magnetic field and the contact springs back and the circuit is closed again, this is repeating it giving a ringing sound.
What is the motor effect?
This is where a wire with a current is placed between two magnets this wire will move upwards due to a force called the motor effect. When a conductor such as a wire is carrying a current, it experiences a force in a magnetic field.
How do you calculate the size of the force generated by the motor effect?
F= magnetic flux densitycurrentlength
F(N)=B(T)I(A)l(m)
This equation applies to a wire that is at right angles to the magnetic field.
What is the left hand rule?
This is used to determine the direction of force
What is the useful application of the motor?
Electric motors
What happens when a loop of wire is placed in a magnetic field?
Each side of the loop which has an opposite direction of current experience force using flemmings left hand rule we can see the forces are different on either side one is moving upwards and the other downwards. This means the loop will rotate until 90 degrees.
How do electric motors work?
Using a loop of wire the current in an electric field experience different forces, the loop will rotate until 90 degrees then the current is switched every 90 degrees by a split ring commutator. Meaning the loop will continue to loop and not stay at 90 degrees.
How does a moving coil loudspeaker work?
We find these on stereos - There is a cone with a coil of wire wrapped around it supplying AC current and a permanent magnet inside the coil of wire. As the current passes through the coil, it generates a magnetic field. The magnetic field interacts with the permanent magnet this either repels or attracts this produces a resultant force this causes the cone to move when the current is switched the force is also switched this movement generates sound waves. We can also change the frequency of the AC supply this allows us to replicate high pitched or low pitched sound. If we increase the size of the current we increase the amplitude this is the volume of sound.
What is the generator effect?
If we have a complete circuit with induced potential difference we get induced current, the direction of the current switches when the direction of movement switches. When the movement stops so does the current.
