Unit 7 - Magnetism And Electromagnetism Flashcards
Where on a magnet are the magnetic forces the strongest
-Poles of the magnet
What happens when two magnets are brought close together
-they exert a force on each other
-two like poles repeal
-two unlike poles attract each other
-attraction and repulsion are non contact forces
Differences between a permanent and induced magnet
Permanent = produces its own magnetic field
Induced = material that becomes a magnet when it is placed in a magnetic field, always causes a force of attraction, when removed it loses most/all of its magnetism quickly
Example of magnetic materials
Iron, steel, cobalt and nickel
What is a magnetic field
-Region around a magnet where a force acts on another magnet or on a magnetic material
-the strengths of the magnetic field depends on the distance from the magnet
-the field is strongest at the poles
-the direction of magnetic field at any point is given by the direction of the force that would act on another North Pole placed at that point
-the lines of a magnetic field go from North Pole to South Pole
Finish sentence : induced magnetism always causes a force of ____________
Attraction
How does a magnetic compass show that the core of the earth is magnetic
-the earth has a magnetic field caused by molten iron in the earths outer core
-the earth is like a big bar magnet with a magnetic field
-the North Pole of the earth is actually magnetic south
-compasses point towards geographic north but this is magnetic south
How to plot the magnetic field pattern of a magnet using a compass
-place magnet on a sheet of plain paper
-place the compass near the end of the magnet
-mark the position that the compass needle points to
-move the compass so that the opposite end is at this position and mark the new position where the compass tip settles
-repeat above and below the magnet and then connect the lines to construct a field
What is the right hand rule
-when direction of current changes so does the direction of magnetic field
-so using your right hand point your thumb in the direction of current and curl your fingers, the direction of your fingers is the direction of the field
What happens to do with magnetic field in a current carrying wire
-when a current flows through a conducting wire a magnetic field is produced around the wire
-the strength of the magnetic field depends in the current through the wire and the distance from the wire
What is a solenoid
-coil of wire
-you can increase strength of magnetic field by wrapping the wire into a coil called a solenoid
-the magnetic field inside a solenoid is strong and uniform
-the magnetic field around a solenoid has a similar shape to a bar magnet
Ways to increase the strength of an electromagnet
-wind the wire do it has more turns
-increase the current in the wire
-add an iron core to the centre of the loops of wire
Why does a solenoid arrangement increase magnetic effect
-the coils of the wire causes the field lines around each loop of wire line up with each other
-this results in lots of field lines pointing in the same direction that are very close to each other
-the closer the field lines the stronger the field
Uses of electromagnetism
-used in cranes(to sort scarp metal) to attract and pick up things made from magnetic materials like iron and steel, using an electromagnet means the magnet can be switched on when you want to pick stuff up and off when you want to drop it
-can be used in circuits to act as a switches, when the switch is closed it turns on the electromagnet
Describe the magnetic field around a current carrying wire
Made up of concentric circles around the wire (with the wire at the centre)
What is the motor effect
-When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other
-the greatest force is experienced when the wire is perpendicular to the magnetic field
Factors that increase the size of the force on the conductor (from the motor effect)
-increase current
-increased magnetic flux density (magnetic field strength)
-increase length of wire in magnetic field
Equation for a conductor at right angles to magnetic field and carry a current :
Force (Newtons)= magnetic flux density (Tesla) x current (Amps) x length (metres)
What is Flemings left hand rule and what does each finger represent
thuMB = Motion
First finger= magnetic Field (north to south)
seCond finger = Current (positive to negative)
Direction of force acting on a current carrying wire can be predicted using Flemings left hand rule
-remember to use left hand
How do electric motors use the motor effect
-a current passes through the coil
-this current creates a magnetic field
-this magnetic field interacts with the magnetic field formed by the permanent magnet
-this causes a force on the coil
-the force causes the coil to rotate
-the split ring commutator reverses the current every half turn to keep the coil spinning in the same direction
Way to change the direction of the motor
-swapping the polarity of the dc supply (reversing the current)
-swapping magnetic poles over (reversing the field)
What is the generator effect
-induction of potential difference (and current if there’s a complete circuit) in a wire experiencing a change in magnetic field
-you can do this by moving a magnet in a coil of wire
-or moving a conductor (wire ) in a magnetic field (cutting magnetic field lines)
-of the magnet or coil is stationary then no current is induced
How to change the size of the induced potential difference
-increasing the speed of the movement = cutting more magnetic field lines in a given time
-increase the strength of the magnetic field (so there are more field lines that can be cut )
Uses of the generator effect
-used in an alternator to generate AC
-in a dynamo to generate DC
How does a generator work
-coil is moved in a magnetic field
-the coil experiences a change in magnetic field
-the coil cuts through the magnetic field lines
-this induced a potential difference
-if there is a complete circuit then a current is also induced
How do alternators generate alternating current
-alternators rotate a coil in a magnetic field (or a magnet in a coil)
-as the coil or magnet spins a current is induced in the coil
-the current changes direction every half turn
-alternators have slip rings and brushes so the contacts don’t swap every half turn
-this means they produce an alternating potential difference
How do dynamos generate direct current
-rotate a coil in a magnetic field or a magnet in a coil
-as the coil spins a current is induced in the coil
-they have split ring commutators
-this swaps the connection every half turn to keep the current flowing in the same direction
How to change the direction of the induced current or potential difference
-reverse the wire / coil direction
-reverse the magnet
How a speaker works (using the motor effect)
-current passes through the coil
-current creates a magnetic field
-this magnetic field interacts with the magnetic field formed by the permanent magnet
-this causes a force on the coil
-the current is alternating and changes direction which changes the direction of the magnetic field and therefore the force
-the changing force vibrates the cone
-this causes air particles to vibrate and create sound
Difference between AC generator and DC generator on a graph
-DC generator does not give negative values (so doesn’t go below x axis )
-AC generator varies from positive to negative values
How do microphones work (using generator effect)
-sound waves vibrate the diaphragm
-the diaphragm moves the coil
-the coil experiences a change in magnetic field
-the coil cuts through the magnetic field lines
-this induces a potential difference
-if there is a complete circuit a current is induced also
What does a basic transformer consist of
-primary coil
-secondary coil
-iron core (used because it’s easily magnetised )
How does a transformer work
-when an alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly
-this changing magnetic field induces an alternating pd in the secondary coil
-if coil is part of a circuit then a current is induced
-the ratio of the primary coil and the secondary coil is the same as the ratio between the number of turns as the primary and secondary coil
advantages of power transmission at high potential differences
-high pd is used so low current
-low current means that less energy is wasted
-so reduced power lost
