7. Electricity & Magnetism Flashcards
What is a magnetic field?
A region of space where magnetic material can experience a force
What type of poles attract?
Unlike poles
What type of poles repel?
Like poles
What is inside a compass?
A tiny bar magnet
How does a compass show that the earth generates its own magnetic field?
When the compasses are not near a magnet they point north
Which direction do field lines point in?
North to South
What does it mean when field lines are closer together?
They are stronger
Do field lines ever touch or cross?
No
Where is the magnetic field strongest?
At the poles of the magnet
What do the direction of the field lines show?
The direction of the force that would act on a tiny N-pole placed at that point
What are magnetic materials made of?
Molecular magnets
What are molecular magnets?
Tiny magnets with N and S poles
What are domains?
Groups of molecular magnets
How are the molecular magnets when a material is unmagnetised?
The unmagnetised domains point in random directions
How are the molecular magnets when a material is magnetised?
Most molecular magnets are aligned
What does saturated mean when talking about a magnet?
When all molecular magnets are aligned
What happens when a magnet breaks?
Two magnets are simply made
Is iron a soft or hard magnetic material?
Soft
What are the properties of a soft magnetic material?
- easy to magnetise
* will demagnetise easily (so useful in electromagnets)
What type of magnetic material is used for electromagnets?
Soft
Why is soft magnetic material used in electromagnets?
It is easy to magnetise and demagnetise
Is steel a hard or soft magnetic material?
Hard
What are the properties of a hard magnetic material?
- harder to magnetise
* also hard to demagnetise
What is hard magnetic material used for?
Permanent magnets e.g. needles in compasses
What is an electromagnet?
A solenoid with an iron core inside
What is a solenoid?
A coil of wire
What happens when a current is passed along the wire in an electromagnet?
A magnetic field is created and as a result the iron core is magnetised
What happens in an electromagnet when the current is switched off?
The iron bar loses it’s magnetism because it’s a temporary magnet
What can the strength of the magnetic field in an electromagnet be increased by?
- increasing the voltage or decreasing the resistance - to INCREASE THE CURRENT
- increasing the number of turns on the coil
- inserting an iron core
Where are electromagnets used?
- cranes
* in hospitals dealing with eye injuries - remove steel splinters
How does an electric bell work?
- bell-push completes circuit and current flows through electromagnet
- iron is attracted towards electromagnet and hammer hits gong
- movement breaks circuit so current stops and electromagnet is switched off
- spring pulls iron back so contact is made and sequence repeats
When will a wire NOT experience a force when placed in a magnetic field?
If it’s parallel to the direction of the field
What does Fleming’s left hand rule show?
The direction of movement of the wire
What does each finger represent in Fleming’s left hand rule?
- index finger - direction of field from N to S
- middle finger - direction of current from + to -
- thumb - direction of movement
How is the direction of the force in an electromagnet reversed?
- reversing the direction of the current
* reversing the direction of the magnetic field
How do loudspeakers work?
- ac current is passed through coil which is attached to paper cone
- current causes force on coil so paper cone moves
- current reverses and force acts on cone in opposite direction
- cone vibrates
What is 1 tesla equal to?
1 newton per amp per metre
What equation is used to calculate the force on a wire?
F=BIL
What is the motor effect?
When a wire is placed in a magnetic field and experiences a force
How does a d.c. motor work?
When a coil of wire is placed in a magnetic field and therefore the two sides of the coil experience a force due to the motor effect
Why does the coil rotate in a d.c. motor?
The current of the opposite side of the coil is in the opposite direction, so the forces are in opposite directions
Where are split ring commutators used?
In electric motors
How do split ring commutators work?
Every half turn it breaks the current so the coil of wire keeps turning in the same direction
What is an easy way to tell if a setup is a motor or a generator?
A motor will have a battery, a generator will not
What is the generator effect?
The induction of a potential difference in a wire which is moving relative to a magnetic field
How is a voltage induced in a wire using the generator effect?
• if a wire is moved downwards to cut through the magnetic field
When is a voltage not induced using the generator effect?
- if the wire is stationary
* if the wire is moved parallel to the field line
How is the voltage induced increased using the generator effect?
- move the wire faster
* use a stronger magnet
How is the voltage induced reversed in a wire?
- move the wire in the opposite direction
* reverse the magnetic field
During electromagnetic induction, why do you have to do work when you push the magnet into the coil?
The electromagnet is repelling the magnet so energy is transferred
During electromagnetic induction, why do you have to do work when you pull the magnet out of the coil?
The electromagnet is attracting the magnet so work is done against the force
What happens when a magnet is moved into a coil of wire which is a part of a complete circuit?
A current is induced in the wire - the coil is now an electromagnet
What is purpose of slip rings in an a.c. generator?
So the wire doesn’t twist as the coil rotates
What is the energy transfer in an electric generator?
Kinetic to electrical
What is the energy transfer in an electric motor?
Electrical to kinetic
What is an a.c. generator also known as?
An alternator
What is a d.c. generator also known as?
A dynamo
Why does an alternator produce an a.c. output?
- the two sides of the coil are always connected to the same side of the lamp
- when the LHS of the coil moves past the top it reverses the direction of movement
- so voltage and therefore current reverses
Why does a dynamo produce a d.c. output?
- the two sides the coil are connected to split rings so the two sides of the coil are not connected to the same side during a turn
- every half turn the connections are reversed so when induced voltage is reversed, the current flowing is not reversed
How does a microphone work?
- a magnet is moved near a coil and a current is generated so the diaphragm is attracted to coil
- diaphragm vibrates in response to incoming sound waves and coil vibrates
- this induces a current in the coil (a.c.)
What is a transformer made of?
A primary and secondary coil wrapped around an iron core
Why is the iron core of a transformer laminated?
So no induced currents flow in the iron core
What does it mean when an iron core is laminated?
It is made in layers of iron
How does a transformer work?
- a current flows in the primary coil and becomes an electromagnet
- this magnetises the iron core which is linked to the secondary coil
- a.c. voltage - magnetic field inside secondary coil will keep changing size and direction so a voltage is induced in the secondary coil
- voltage will change direction and br an a.c. current
What does the size of a secondary voltage in a transformer depend on?
- primary voltage
* number of turns on each coil
What is the equation to work out the voltage or number of turns in a transformer?
Vp/Vs = Np/Ns
