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PHYSICS CHAPTER 7- Electromagnetism
Fichas del conjunto (51)
- 1Where are the magnetic forces strongest on a magnet?At the poles (north and south)
- 2Like poles .................... and unlike poles ........................Repel
Attract - 3What is a magentic field?The region around a magnet where a force acts on another magnet or on a magnetic material
- 4Which 2 ways can you use to reveal the magentic field of a magnet?○ Plotting compasses- Put the magnet on a piece of paper and draw around it. Place the plotting compass near the north pole of the magnet. Draw a dot by the head of the arrow on the compass. Move the compass so that the tail of the pointer is by the dot you've just draw. Draw another dot by the head. Repeat, moving compass until you reach south pole- draw smooth curve to join points. Repeat for slightly different starting points near magnet.
○ Iron filings- scatter iron filings around magnet. Shape of magnet reveals shape of field lines - 5What are some magnetic materials? (4)○ Iron
○ Some steels
○ Nickel
○ Cobalt - 6What is an induced magnet?A material that becomes a magnet when it is placed in a magnetic field
- 7What happens when the induced magnet is removed from the magnetic field?It loses most or all of its magnetism quickly
- 8The force between a magnet and a magnetic field is always .........................Attractive
- 9Why does iron only make a temporary magnet?Because it is very easily magnetised but also just as easily demagnetised- loses its force when the magnetism when the magnetising force is removed
- 10Magnetic field lines never ...........................Meet or cross
- 11What direction is a magentic field line?North to south
- 12What is a compass?A small bar magnet. One end is a north pole and one end is a south pole. The south pole of the compass needle is attracted to a magnetic north pole
- 13How do we know that the Earth has a magnetic field?Because a compass needle points north
- 14The geographic north pole is the magnetic ............................................... and the geographic south pole is the magnetic .............................................○ South pole
○ North pole - 15The closer the field lines, the ............................... the magnetic fieldStronger
- 16What happens when a current flows through a wire?The current creates a magnetic field around the wire
- 17The strength of the magnetic field .................................... as the distance increasesDecreases
- 18What is the right-hand grip rule?
- 19What is a solenoid?A long straight coil of wire
- 20How does the magnetic field around a loop of wire look like?
- 21How does the magnetic field around a solenoid look like?
- 22What is the motor effect?When a straight wire with a current passing through it is placed between the poles of a magnet, the two magnetic fields combine making the resultant magnetic field stronger in one area and weaker in another area. This produces a force on the wire which makes it move.
- 23What is Fleming's left-hand rule?States that motion, magnetic field and current are perpendicular to one another and allows us to predict the direction of the force on the wire
- 24How can you increase the force on a wire?• Increase no. of turns in coil of wire
• Increase area of wire
• Increase length of wire
• Increase current passing through wire
• Wrapping wire around an iron core - 25What direction does convectional current travel in?Positive to negative
- 26How does an electric bell work?When the switch is pressed and the circuit is closed, the electromagnet is turned on and the iron contact slip is attracted to it, so the hammer hits the gong. However when the armature moves forward there is a break in the circuit so current stops flowing and the iron core is demagnetised so the armature moves back to its original position. This means that the circuit is once again complete and the iron core is magnetised and the armature is attracted to it and the process repeats. This repeating process occurs as long as the switch is pressed closed.
- 27What is a relay?A magnetic switch which requires a small current to switch on a much larger current
- 28Relay diagram
- 29Uses of relays (3)○ In bells
○ In a scrapyard to lift up metal
○ The starter motor in a car - 30What is magnetic flux density?The strength of a magnetic field (i.e. how close the lines of force are)
- 31Equation for magnetic flux densityF= B I L
Size of the force, F on a wire (in newtons, N)= Magnetic Flux Density, B (in teslas, T) x Current, I (in A) x Length of the wire, L (in m) - 32How do d.c. motors work?When a current passes through the coil, which is placed in between the poles of a magnet, there is a force on each side of the coil. Fleming's left-hand rule shows that these forces are opposite, causing the coil to rotate clockwise- one side moves up and the other down. The d.c. motor has a split-ring commutator, which have a gap down the middle. When the wire reaches the gap, there is no current flowing through the circuit anymore. However because of momentum and the because the coil was already rotating clockwise it continues in this direction, until the coil touches the commutator and current flows through the circuit once more, restarting this process and causing the coil to spin in the same direction, instead of reversing direction when it becomes vertical. The carbon brushes allow it to not be tangled up.
- 33How do a.c. motors work?Instead of a split-ring commutator, there is a slip ring, which ensures that the coil does not get tangled up. When the coil has gone through half a rotation, the direction of the current reverses. However because the coil has already turned, the coil continues moving in the same direction. This produces continues rotation of the coil.
- 34What are longitudinal waves?
- 35How does the moving-coil loudspeaker work?A moving-coil loudspeaker has a permanent magnet and an electromagnet. The magnet is fixed and the coil of the electromagnet can move backwards and forwards. he coil is attached to he cone (usually paper or plastic) and the outer edge of the cone is fixed and doesn't move. The magnet produces a strong radial magnetic field. This is at right angles to the wire in the coil. A signal in the form of a current that varies flows through the coil. A change in the current in the coil causes a change in the magnetic force acting on the coil. This changing force causes the coil to move in and out, in turn causing the paper cone to also move, causing vibrations in the air which are interpreted as sound.
- 36What is the biggest difference between headphones and loudspeakers?Size: loudspeaker needs to carry sound to a whole room but headphones only have to move the volume of air inside your ear canal
- 37What is the generator effect?A changing
magnetic field near a conductor (either through relative motion of a magnet and the conductor or
through an increasing or decreasing magnetic field strength) produces an electric current in the conductor. If the wire remains stationary, there is no current. - 38What is Lenz's law?The induced current is always in a
direction such that its magnetic field opposes the changing field that created it- the magnetic field of the coil OPPOSES the motion of the original magnet - 39How can you increase the induced potential difference?○ Moving the wire or magnet faster
○ Using a stronger magnet
○ Replacing the wire with a coil - 40What is a galvanometer?An instrument for detecting and measuring small electric currents
- 41How are electricity and magnetism interrelated?Movement of electrical charges creates magnetic fields, while changes in magnetic fields can generate electricity
- 42What is a dynamo and how does it work?It is used to generate a direct current by moving a coil in a magnetic field. As the coil turns, it cuts through the magnetic field lines, This induces a potential difference between the ends of the coil. This causes an induced current to pass through the coil and through the circuit the coil is part of.
- 43What is an alternator and how does it work?An alternator uses the generator effect. As the coil rotates, cutting magnetic field lines, it generates a potential difference between the ends of the coil. The potential difference is greatest when the coil is horizontal and zero when it is vertical, as no field lines are being cut. As the coil continues to rotate, the side coming up is now going down, so the induced potential difference is negative. This is alternating current.
- 44What is the purpose of a transformer?It changes the size of an alternating potential difference
- 45How do transformers work?A transformer consists of 2 wires wound separately on an iron core. An alternating potential difference applied to the primary coil produces an alternating current in the primary coil. This produces an alternating magnetic field in the iron core. The alternating magnetic field in the iron core induces a potential difference in the secondary coil. This induces alternating current in the circuit connected to the secondary coil.
- 46What is a step-up transformer?A transformer that increases the potential difference- it has more turns in the secondary coil than on the primary coil
- 47What is a step-down transformer?A transformer that decreases the potential difference- it has more turns in the primary coil than on the secondary coil
- 48Turns ratio equationVp = voltage input to primary
Vs = voltage output to secondary
Np = no. of turns in primary
Ns = no. of turns in secondary
- 49Power input and output equationI (in) x V (in) = I (out) x V (out)
I (s) x V (s) = I (p) x V (p) - 50What is switch mode?It means that the transformer uses almost no energy (i.e. electric current) when the output is connected to anything (when there is n load)/ Switch mode transformers are smaller and lighter.
- 51What is the frequency range of switch mode transformers?50Hz - 200Hz
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- Year 11 Physics Fichas sobre PHYSICS CHAPTER 7- Electromagnetism, creado por UnknownSnowFlake 123 el 08/03/2018.
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- UnknownSnowFlake 123
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