P15 Flashcards
Electromagnetism
Where on a magnet are magnetic forces the strongest ?
at the poles
What do magnetic poles do if they are alike/opposite ?
like poles repel whilst opposite poles attract
Why are permanent magnets made of steel rather than iron or other magnetic materials ?
doesn’t lose its magnetism as quickly as other magnetic materials
What is a permanent magnet ?
a magnet that is always magnetic
- it produces its OWN magnetic field
[] bar magnets are good examples of these
What is an induced magnet ?
an unmagnetised magnetic object that becomes a magnet when placed in a magnetic field
- these magnets lose their magnetism when taken out of the magnetic field
- induced magnetism always causes a force of attraction
Which are the key four magnetic materials ?
- iron
- steel
- cobalt
- nickel
What is a magnetic field ?
a region around a magnet where the non-contact force of magnetism acts on other magnets or magnetic materials
- the strength of the magnetic field depends on the other magnet/magnetic material’s distance from the magnet
Explain how to plot the magnetic field of a bar magnet using a compass
- place bar magnet in middle of piece of paper
- place compass near north pole of magnet
- draw x at north pole of compass
- place compass so its south pole is on the cross
- again mark where the compass’ north pole is
- repeat until one complete magnetic field line has been plotted, and join with a smooth curve
- draw an arrow on the line to represent the direction of the magnetic field
[] ALWAYS runs from north to south pole - repeat whole process starting at different points around the north pole of the bar magnet
How can you tell that the Earth has its own magnetic field due to its core ?
- compasses have a small bar magnet inside
- if you hold the compass away from any magnets, the needle always points in the north-south direction
When is a magnetic field produced around a CONDUCTING wire ?
when a current flows through it
How can you prove that there is a magnetic field around a conducting wire with a current through it ?
- place a plotting compass next to the wire with the current off
[] the pin should point in the north-south direction due to the Earth’s magnetic field - turn on the current and observe that the needle has deflected, proving the magnetic field produced by current through a conducting wire
What happens to the direction of the magnetic field around a conducting wire if the direction of current is reversed ?
the direction of the magnetic field also reverses
What shape are the lines of force around a straight conducting wire with a current ?
(from above) a series of concentric (centred on the wire) circles
What does the strength of the magnetic field of a straight conducting wire depend on ?
the size of the current and distance from the wire (stronger nearer)
How can you tell the direction of the magnetic field of a wire ?
- the right hand grip rule
[] make a thumbs-up with your right hand
[] place your right hand in this shape on the paper with your thumb pointing in the direction of the current
[] MAKE SURE FINERS ALWAYS FACE UP - YOU SHOULD NOT SEE THE BACK OF YOUR HAND AT ANY TIME
[] the magnetic field is in the direction that your fingers point
What is a solenoid ?
long coil of insulated wire
Describe the magnetic fields in and outside of a solenoid
inside:
- strong and uniform (in the same direction)
- parallel to the axis of the solenoid
outside:
- similar to a bar magnet BUT with the exception that each field line is a complete loop as it passes through the inside of the solenoid also
What is an electromagnet ?
a solenoid with current through it and an iron core
- the iron core is magnetised by the magnetic field of the solenoid/wire
- can be turned on/off
How can you increase the strength of an electromagnet/solenoid’s magnetic field ?
- increase the size of the current
- increase number of turns/coils of solenoid
Give 4 examples of devices where electromagnets are used
- scrapyard crane
- electric bell
- circuit breaker
- relay
What is a relay and how does it work ?
- device used to switch dangerously high voltage circuits on and off
how it works: - two circuits; high voltage and low voltage
- in the low voltage circuit, there is a switch and an electromagnet
- in the high voltage circuit, there are metal contacts in place of a switch, connected to an iron block and a spring keeping the two contacts apart
- when the low voltage circuit is switched on, the electromagnet is switched on and produces a magnetic field
- the iron block is attracted to the electromagnet, making the metal contacts touch and closing the high voltage circuit, turning it on
- once the low voltage circuit is turned off, there is no magnetic field and the spring breaks the high voltage circuit
Describe how an electric doorbell works
- in the circuit, an iron contact connected to the clapper that hits the bell is kept in place to complete the circuit by a spring, and there is also an electromagnet
- when the buzzer is pressed, the circuit closes and current flows through the circuit
- the current activates the electromagnet
- the electromagnet now produces a magnetic field and attracts the iron contact
- this makes the clapper hit the bell, and it sounds
- however, this breaks the circuit, so the electromagnet turns off and the field goes away
- the spring then pulls the iron contact back into place completing the circuit again
- the cycle repeats very quickly until the buzzer is released
How does a circuit breaker work ?
- electromagnet in series with switch held closed by a spring
- when current is too large, electromagnet activated and switch is pulled open by the magnetic field
- stays open, breaking the circuit until reset manually
What is the motor effect ?
when an upwards force acts on a wire with a current through it whilst it is in a magnetic field
- in other words, the force acting on a wire produced by the interaction of the wire’s magnetic field and another
What can increase the force produced in the motor effect ?
- longer conducting wire
- stronger magnet used
- more current passed through the wire
- making sure the wire is perpendicular to the magnetic field
[] if parallel instead, the force is zero
Give the equation for calculating the size of the force produced in the motor effect (with units)
force (N) = magnetic flux density (T) x current (A) x length of conductor (m)
NOTE: only applies to a wire perpendicular to the magnetic field
What is the magnetic flux density of a magnetic field ?
measure of the strength of a magnetic field in the unit tesla
Describe how to determine the direction of the force experienced by a wire in the motor effect
Fleming’s left hand rule
- first finger = F for magnetic FIELD direction
- second finger = C for CURRENT
- thumb = M for MOVEMENT/direction of force experienced
hand should be in a scissor shape
Describe how the motor effect is used by electric motors
- there is a rectangular coil of insulated wire with a current through it in a magnetic field
- coil connected to battery as is fixed to split ring commutator connected to two metal/graphite brushes connected to the circuit
- when current passed through coil forces act in opposite directions on either side of the rectangle because current runs in opposite directions on either side
[] there is now a moment on either side
[] so it spins - when the coil rotates 90 degrees (one half-turn), the split in the metal ring breaks the circuit momentarily
- the coil keeps rotating due to momentum
- when the ring again contacts the brushes and current passes through, the current is reversed
- the split-ring commutator reverses the current and forces the coil to keep turning in the same direction
Describe how the motor effect is used in moving-coil loudspeakers and headphones
- inside the speaker, there is a cone, with a conducting wire wrapped around the thin, tubular end towards the inside
[] this coil of wire is connected to an AC electrical supply
[] inside the tubular end of the cone, there is the middle protrusion of an E-shaped permanent magnet (the NORTH pole) - magnetic field generated when current passes through coil of wire and interacts with permanent magnet’s magnetic field
- attraction/repulsion of both magnetic fields on one another create resultant force causing the cone to move
- when current reverses direction due to AC, direction of force on cone also reverses
- cone moves in and out
- these vibrations create soundwaves
- by changing AC supply frequency, can change frequency of vibration of cone
[] higher freq. = higher pitch sound
[] lower freq. = lower pitched sound - increasing size of current supplied increases amplitude of soundwaves, thus increasing the volume
