P14,P15 Flashcards
What are the 2 poles of a magnet?
North and South
Where is the magnetic force the strongest?
Near the magnet’s poles
What sort of force is magnetic force?
A non-contact force
Name 3 magnetic materials
Iron, Cobalt, Nickel
What is a permanent magnet?
An object that gets magnetized and creates its own persistic magnetic field, always causing a force on other magnets
What are induced magnets?
Induced magnets are magnetic materials that turn into a magnet when they are put into a magnetic field.
What direction do magnetic field lines go?
North to South
What produces Earth’s magnetic field?
The molten iron in the outer core of the Earth which due to the Earth’s rotation and axis, creates a magnetic field
What is the force between a permanent and induced magnet?
The force between a permanent and induced magnet is always attractive, irrespective of the poles.
What happens when an induced magnet moves far away from a permanent magnetic field?
Its magnetism weakens, until it moves to a point where its magnetism is completely lost
When not near a magnet, why does a compass always point north?
Because that’s where Earth’s south magnetic pole is located, meaning they attract
What happens when a current flows in a wire?
It creates a circular magnetic field around the wire
What is an electromagnet?
A magnet made by wrapping a coil of wire around an iron bar and passing an electric current through the coil
What are electromagnets most commonly used for?
- Electric motors
- Headphones
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 the motor effect?
A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles.
How can the size of the force be increased in the motor effect?
- increasing the current
- using a stronger magnet
When is the force in the motor effect greatest?
When the wire is perpendicular to the magnetic field
When is the force zero in the motor effect?
When the wire is parallel to the magnetic field lines
What is the magnetic flux denisty?
The measure of the strength of the magnetic field
What is the unit for magnetic flux density?
Tesla (T)
How can the direction of a motor be reversed?
- Reversing the current
- Reversing the field by swapping the magnetic poles
Why does a motor spin when a current is passed through the coil?
- A force acts on each side due to the motor effect
- The force on one side is opposite to the force on the other side
What does the split-ring commutator do in a motor?
Reverses the current around the coil every half-turn (90 degrees). The momentum of the motor, carries it past 90 degrees. The sides then swap over meaning the coil is pushed in the same direction every half-turn.
What is electromagnetic induction?
When motion between a conductor and a magnetic field creates electricity, ie a magnet is moved into a coil of wire.
What is electromagnetic induction often referred to as?
The generator effect
When does an induced voltage produce an induced current (generator effect)?
If the conductor is connected in a complete circuit.
How can you increase the induced current/p.d? (generator effect)
- speed of movement increased
- magnetic field increased
- number of turns coil is increased
What factors affect the direction of the induced potential/current (generator effect)?
- magnet moved out of coil
- other pole of magnet is moved into the coil
Why does the induced current always oppose the original change that made it?
The magnetic field created by an induced current always acts against the change that made it.
For example, if the north pole of a magnet enters a coil, the coil will repel it to return things to the way they were. If the magnet moves away, the coil will attract it.
Why can transformers only work with alternating current?
A changing magnetic field is required to induce an alternating current in the secondary coil
What are transformers used for?
Increasing/decreasing the size of an alternating potential difference
How does a transformer work (5 marks)?
- primary voltage carries ac through primary coil
- primary coil produces magnetic field which changes current
- iron core increases
- changing magnetic field induces potential difference in secondary coil
- induced pd produces ac in external current
Difference between step-up and step-down transformers
- Step-up transformers have more turns on the secondary coil than the primary coil (increases p.d)
- Step-down transformers have less turns on the secondary coil than the primary coil (reduces p.d)
How does a transformer produce an induced current?
An alternating voltage is applied to one coil (the primary coil). This causes a changing (alternating) magnetic field to be set up in the core. The other coil (the secondary coil) is in this changing magnetic field and so it has an alternating voltage induced in it.
Why is iron used as the core of a transformer?
It is easily magnetised
What would happen if transformers were 100% efficient?
The electrical power output would equal the electrical power input
What type of current do dynamos produce?
Direct
2 types of generators
- Dynamos
- Alternators
How can the size of the induced voltage in a generator be increased>
- rotating coil or magnet faster
- using a magnet with stronger magnetic field
- having more turns of wire in coil
- having iron core inside coil
What does the thumb indicate in flemings left hand rule?
Force/Motion
