P7 - Magnetism and Electromagnetism Flashcards
What is an alternator?
A device that makes use of the generator effect to generate alternating current.
What is attraction?
Opposite poles experiencing a force towards each other.
What is a dynamo?
A device that makes use of the generator effect to generate direct current.
What is an electromagnet?
A solenoid with an iron core. (iron core increases the strength of the magnetic field)
What is the generator effect?
When there is a relative motion between an electrical conductor and a magnetic field, so a p.d. is induced across the ends of the conductor, causing a current to flow.
What is a magnetic compass?
A device containing a small bar magnet that points in the direction of the Earth’s magnetic field.
What are magnetic field lines?
Lines representing the strength and direction of a magnetic field.
What do you know about the field line direction?
It is in the direction that a force would act on another north pole if placed at that point.
What is a magnetic field?
The region around a magnet in which another magnet or magnetic material will experience a force.
What are magnetic poles?
The regions of a magnet where the magnetic forces are at their strongest.
What is the motor effect?
When a force is experienced between a current carrying wire placed in a magnetic field and the magnet responsible for the field, creating motion.
What is a permanent magnet?
A magnet that produces its own magnetic field.
What is repulsion?
When like-poles experience forces in opposite directions.
What is a solenoid?
A wire wrapped into the shape of a coil which when current passes through creates a strong magnetic field.
How can you increase a solenoid’s magnetic field strength?
Adding an iron core.
More turns.
What is a step-down transformer?
A transformer which has a smaller p.d. in the secondary coil than in the primary coil.
What is a step-up transformer?
A transformer that has a larger p.d. in the secondary coil than in the primary coil.
What is tesla?
The unit of magnetic flux density.
What is a transformer?
An iron core with a primary and secondary coil of wire wound in opposite ends.
At what part of the magnet are the magnetic forces the strongest?
The poles.
What happens when 2 magnets are brought close together?
They exert a force on each other.
What type of force is exerted is 2 like poles of a magnet are brought close together?
A repulsive - non-contact force.
What type of force is exerted is 2 unlike poles of a magnet are brought close together?
An attractive - non-contact force.
What is a permanent magnet?
Produces it’s own magnetic field.
What is an induced magnet?
A material that becomes magnetised when placed in a magnetic field.
What type of force does induced magnetism always cause?
A force of attraction.
What happens when an induced magnet is removed from a magnetic field?
It removes all/most of it’s magnetism
Give 4 examples of magnetic materials.
Iron.
Cobalt.
Nickel.
Steel.
How does the strength of a magnetic field alter as you move further away from the magent?
Magnetic field strength decreases.
In what direction does a magnetic field point?
From the north seeking pole to the south seeking pole.
What is produced when current flows through a conducting wire?
A magnetic field is produced around the wire.
What determines the strength of a magnetic field around a current-carrying wire?
The magnitude of the current.
The distance from the wire.
Number of coil turns.
Describe the magnetic field found inside a solenoid.
Strong and uniform.
What rule is used to determine the force experienced by the motor effect?
Fleming’s left-hand rule
What does the forefinger represent (left-hand rule)?
Direction of magnetic field.
What does the second finger represent (left-hand rule)?
The direction of current flow.
What does the thumb represent (left-hand rule)?
Direction of the force/movement.
What factors affect the size of the force on a current-carrying wire placed in a magnetic field?
The magnitude of the current.
The strength of the magnetic field.
If the direction of current in a current-carrying wire is reversed what happens to the direction of force?
Is reversed aswell.
If the strength of the current in a wire placed in a magnetic field is increased, what happens to the force?
Is increased.
What is the criteria for the equation linking force, magnetic flux density, length and current to hold?
Conductor must be perpendicular to the magnetic field it is placed in.
How does an electric motor work?
A coil of wire, carrying a current, is placed in a magnetic field.
The wire experiences opposite forces on each side of it causing a rotational effect.
What equation links force, magnetic flux density, current and length of wire in field?
force = magnetic flux density x current x length of wire in field
What is the unit for current?
Amps (A)
What is the unit for length?
metres
What is the force involved in the motor effect called?
Lorentz force.
What happens when an electric conductor moves relative to a magnetic field?
A potential difference is induced across the ends of the conductor.
What is the requirement for an induced p.d. to cause current flow?
The conductor must 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.
The change is either the movement of the conductor or a change to the field.
2 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?
Generator effect.
How does a microphone use the generator effect?
It converts the pressure variations in sound waves into alternating current in a circuit.
Describe the make-up of a basic transformer.
A primary coil and a secondary coil wrapped around an iron core.
Why is an iron core used for a transformer?
It is easily magnetised.
How does a transformer work?
- AC flows through the primary coil.
- Primary coil current induces a changing magnetic filed.
- Iron core increases strength of the magnetic field.
- Changing magnetic field induces a changing potential difference in the secondary coil
- the induced potential difference produces an alternating current in the secondary coil.
Why must the current flowing through a primary coil be alternating?
For current to be induced in the secondary coil, the magnetic field must be constantly changing.
What can be said about the electrical power output and input of a 100% efficient transformer?
Electrical power output = electrical power input
How do loudspeakers use the motor effect?
Converts variations in the current of an electrical circuit into the pressure variations which produce audible sound.
How does a loudspeaker work?
- A cone with a wire wrapped around it is connected to an a.c. power supply and placed in a magnetic field.
- When current flows through a wire it produces a second magnetic field which interacts with the permanent magnetic field.
- Which produces a force causing the cone to vibrate.
How is the pitch of the sound from a loudspeaker changed?
The frequency of the a.c. is altered.
Creating a different frequency of vibration in the cone.
What does the direction of the magnetic field depend on in a current-carrying wire?
The direction of current.
How can you determine the direction of the magnetic field by knowing the direction of current?
Right-thumb rule.
What 2 factors increase the strength of the magnetic field strength of a solenoid?
Higher current.
Greater number of coils.
What is a relay made up of?
An electromagnet cell and a magnetic switch.
How does a solenoid produce a strong magnetic field?
The magnetic field lines at each loop of the wire result in lots of magnetic field lines pointing in the same direction.
What is the opposite of the generator effect and explain why?
Motor effect. Generator effect you put electricity in and get motion out. Motor effect you put motion in and electricity out.
How is an alternating current produced in the generator effect?
When a magnet is moved into a coil a current is generated. When it is moved out of the coil the current reverses. So therefore, moving it in and out generates an alternating current.
What 3 factors increase the size of the induced p.d in the generator effect?
- Speed of movement of the magnet or coil is increased.
- Number of turns on the coil is increased.
- Magnetic field strength is increased.
What creates the p.d. in the generator effect?
The change in the magnetic field that the wire experiences, as it moves through the field.
What happens to the current produced when the wire stops moving in the generator effect?
No current is produced as there is no change to the magnetic field.
2 ways we can change the direction of the current in the generator effect.
- swapping poles
- moving magnet in and out of coil
Why is iron a good material for a core?
It is easily magnetised.
Describe the steps in which an alternating p.d. in the primary coil creates an alternating p.d. in the secondary.
- Alternating p.d. applied to primary
- Produces a magnetic field, which changes as the current changes.
- Changing magnetic field induces an alternating p.d. in the secondary coil.
What is true about the induced voltage compared to the applied voltage in a step-down transformer?
Induced voltage is lower than applied voltage.
What is true about the induced voltage compared to the applied voltage in a step-up transformer?
Induced voltage is larger than applied voltage.
In a step-down transformer, what is true about the number of turns in the primary coil and the secondary coil?
Primary coil has more turns than secondary coil.
In a step-up transformer, what is true about the number of turns in the primary coil and the secondary coil?
Primary coil has less turns than secondary coil.
What does a high amount of turns mean in a transformer?
A high p.d.
What is the equation for the ratio of p.d. and no. turns?
V(p) / V(s) = N(p) / N(s)
Voltage of primary / Voltage of secondary = Number of turns of primary / Number of turns of secondary.
What is the efficiency of a tranformer?
100% - meaning power in = power out.
What is the equation using the concept that power in = power out?
V(s) x I(s) = V(p) x I(p)
Because P=IV
What does V(p) x I(p) determine?
The power input.
What does V(s) x I(s) determine?
The power output.
Why is the iron core important in transformers?
It carries magnetic fields from the primary coil to the secondary coil.
How is a p.d. produced in the generator effect?
Magnetic field around the coil changes.
Why is the p.d. produced in the generator effect alternating?
Because the magnet changes direction.
When drawing the magnetic filed lines around a current, how many do you have to draw?
3
How does a commutator work in the motor effect?
Every half-revolution the commutator swaps form one carbon brush to another - which reverses the current - but ensures the forces work in the same direction.
