Flashcards in Magnetism and Electricity Generation Deck (27)
Which materials are magnetic?
Nickel, cobalt and iron (includes steel)
What happens when you put two magnets close to one another?
North - north and south - south (like) fields will repel. North - south (opposite) fields will attract.
What is the shape of a bar magnet's field? How would you find this out?
It is strongest at the poles, with fields originating at the North pole and going into the South pole. You would find this out by placing iron filings onto the magnet and observing their distribution.
What is the direction of a bar magnets field? How would you find this out?
The field comes from the North pole and goes to the South pole. You would find this out by moving a plotting compass around it; it would point away from the north end of the magnet and to the south end.
How can you induce magnetism on a nail?
Take a nail and stroke it with a bar magnet in one direction. If it is made of iron, cobalt or nickel it will become magnetic. This is called induced magnetism and is not permanent.
How do we know the Earth has a magnetic field? What is it like?
A compass will align with the Earth's magnetic field if there are no magnets near it. This shows there is a magnetic field, where the 'north pole' is the magnetic south pole. This comes from liquid iron and nickel convection currents in the outer core.
What is the magnetic field around a current carrying wire like?
There are concentric circles perpendicular to the wire around it. It is stronger the larger the current and the closer to the wire you are. Changing the current direction changes the field direction.
How can the direction of the magnetic field around a wire be measured?
Use the right hand thumb rule: point your thumb in the direction of the current and curl your fingers around. The direction of your fingers is the direction of the field.
What happens to a current carrying wire in a magnetic field?
The magnetic fields interact and the wire experiences a force (motion). The direction can be worked out using Fleming's left hand rule. To experience the full force, the wire has to be at 90 degrees to the magnetic field.
What is Fleming's left hand rule?
It is used to work out the direction of movement of a wire in a magnetic field. Put your first finger in the direction of the field and your second finger in the direction of the current. Your thumb will point in the direction of the force.
thuMb - Motion
First finger - Field
seCond finger - Current
How can the size of force on a conductor in a magnetic field be increased?
Increase the strength of magnet (magnetic flux density)
Increase the current through the current
Increase the length of current in the magnetic field
Increase the direction of the conductor to 90 degrees to the field
How can the force on a conductor at 90 degrees to a magnetic field be calculated?
F = B x I x l
Force (N) = Magnetic flux density (T) x Current (A) x Length (m)
How does a d.c. motor work?
A loop of wire is put in a magnetic field. This causes it to rotate, as forces act in opposite directions. A split ring commutator swaps the contacts every half turn so it rotates in one direction. Fleming's left hand rule dictates which direction it will turn.
What is a solenoid?
A solenoid it a loop of current carrying wire. The field lines around each loop line up, producing a strong field inside the solenoid and at the ends. Outside it is weak. It is an electromagnet and can be increased in strength by adding an iron core.
How can you increase the size of an induced p.d. from a field?
Increase magnetic field strength
Increase speed of movement
Increase coil density in the wire
How do transformers work?
They use induction to change the p.d. size in alternating current. There is a primary and secondary coil, joined with an iron core. The primary coil's alternating p.d. produces an alternating magnetic field. This means the magnetism in the core also alternates. This induces a p.d. in the secondary coil.
In a step-up transformer there are more turns on the secondary coil, so more voltage. In a step-down transformer there are less turns on the secondary coil, so less voltage.
What are the two types of generator?
Dynamos and alternators
What are dynamos?
Generators that rotate a coil in a magnetic field. This induces a current in the coil, which changes direction every half turn. There is a split ring commutator which swaps the connection every half turn to keep the current flowing in one direction (d.c.).
What are alternators?
Generators that rotate a coil in a magnetic field. This induces a current in the coil, which changes direction every half turn. There are slip rings and brushes which do not swap the current every half turn, so an alternating p.d. and current is produced.
How do microphones work?
Sound waves hit a flexible diaphragm attached to a coil of wire. This surrounds one pole of a magnet and is surrounded by the other pole. As the diaphragm moves, the coil moves, so a current is generated. The movement depends on the properties of sound, so the current is based on the sound.
How do loudspeakers work?
A coil is wrapped around one pole of a magnet, with the other pole surrounding it. The a.c. current therefore causes a force on the coil. This moves the paper cone attached. As the current is quickly reversing, the paper cone moves in different directions - it vibrates. This makes the air around vibrate, causing variations in pressure and making sound.
How does a power station generate power?
Fuels are burned, which generates heat.
This heats water in a boiler, turning it to steam.
The steam turns a turbine.
The turbine is connected to a magnet inside a generator, so it spins too.
This induces a large p.d. and an alternating current,
Why are transformers used in the National Grid?
A large current causes resistance, which causes ohmic heating in wires, so energy is lost. The current should therefore be as low as possible.
In order for it to be low, the p.d. must be high, as power = current x p.d.
This is achieved through step up transformers (to 400,000V). It is then lowered near homes through step down transformers so it is safer (to 230V)
What is the power equation in terms of energy?
Power = energy transferred / time
P (W) = E (J) / t (s)
What is the electrical power equation?
Power = Current x p.d.
P (W) = I (A) x V (V)
What is the power equation in terms of resistance?
Power = Current^2 x resistance
P (W) = I^2 (A) x R (ohms)