6.3 Electromagnetism Flashcards
What are magnetic field lines and what are they represented by?
-Magnetic field: the region around a permanent magnet or a moving charge(electric current) where a force will act on another magnetic material or charged particle
-Magnetic fields are represented by magnetic field lines they show the direction and shape, the direction of a field line shows the direction a plotting compass would point at that point.
How do we determine the magnetic field of a straight current-carrying wire and what is it?
-You can determine the direction of the field lines using the right hand thumb rule
-The magnetic field around a straight current-carrying wire is concentric circles around the wire
How does coiling a single current carrying wire help increase magnetic field strength?
-Due to the right hand thumb rule one coil will have an anticlockwise direction and the other will have a clockwise so they are both going in the same direction, the vector addition of the fields therefore leads to an increase in strength of magnetic field in the coil
What is a solenoid and describe the magnetic field associated with a solenoid?
-A solenoid is a long coil of current carrying wire
-Inside the solenoid, the field is strongest and very uniform, but as we move outside the field is very similar to the magnetic field produced by a bar magnet
-If a cylinder of iron or another magnetic material is placed inside the magnetic field is increased considerably
True or False: the geographic north pole is also the magnetic north pole
False
What is magnetic flux and magnetic flux density?
-Magnetic flux (Φ) is the product of magnetic flux density and the area, it is given by the equation(in picture) where the angle is the angle between the field lines and normal to the surface, measured in webers (Wb)
-Magnetic flux density is a measure of strength of the magnetic field, its units are teslas and (equation in picture)
What is the motor effect?
-When a current carrying wire is placed in a uniform magnetic field, it experiences a force known as a catapult force due to the interaction between the magnetic fields
-This concept can be used to make a motor spin
What do the fingers represent in flemmings left hand rule?
-Thumb-Force
-First finger- magnetic field direction
-Second finger- direction of current
What is the force exerted on a current carrying wire in a magnet field directly proportional to?
- Current flowing in the wire
2.The length of the wire that is within the magnetic field - Magnetic flux density
- The sinθ of the angle between the wire and field lines
How do you carry out an experiment to investigate the magnetic flux density between the poles of magnets?
- Set up apparatus as shown
2.Once the current begins a force will be exerted on the wire, the wire will exert an equal and opposite force causing the magnet to change weight - This gives you the size of the force, f=mg and use F=BIL to find the magnetic flux density
- The current is increased in increments and magnetic flux density is found for each
What is the equation for the force on a charged particle in a uniform magnetic field?
F=BQv
where
F-force
B-magnetic flux density
Q-charge of particle
v-velocity of charged particle
Can you use flemming left hand rule to predict the direction of the force acting on a charged particle in a magnetic field?
Yes
-Thumb=force
-First finger= direction of magnetic field
-Second finger= direction charged particle is moving( however if charge is negactive direction flips)
Why does a charged particle in a uniform magnetic field have circular motion?
-The force acting on the particle will always be perpendicular to its motion (flemmings left hand rule), so this produces a centripetal force and the charge will have circular motion
-Also because the force is always at right angles no net force will ever be in the direction the particle is moving so it will move at a constant speed.
What will increasing the velocity and increasing the charge of a charged particle in a uniform magnetic field travelling in a circular path do?
-Increasing the velocity will increase the radius of its movement
-Increasing the charge will decrease the radius of its movement (both shown in equation)
What is a velocity selector?
-Velocity selector: a device that uses perpendicular magnetic and electric fields to select the charged particles travelling at a specific velocity, as they leave the region of fields undeflected
What is the definition of electromagnetic induction and induced e.m.f?
-Electromagnetic induction: the process of inducing an e.m.f in a conductor when there is a change in magnetic flux linkage across the conductor
-Induced e.m.f: the e.m.f produced by electromagnetic induction
What will happen if you move a magnet in and out of a coil of wire and leave it stationary in the coil of wire?
-Moving a magnet into a coil of wire will induce a current in the wire and out will cause an induced current in the opposite direction, flipping the poles of the magnet also reverses the direction of the induced current
-Leaving the magnet stationary will not induce a current
What did faraday conclude in his experiments with electromagnetic induction?
-Faraday concluded that in order for an electric current to be induced in a wire, there must be a change in magnetic flux in the region around the circuit
-This can be done with relative motion between the magnet or wire or a change in magnetic field
What is the equation and symbol for magnetic flux and what is the equation and units for magnetic flux linkage?
Φ=BAcosθ
-Φ: magnetic flux
-Magnetic flux linkage is measured in weber turns Wb
What is Faraday’s law and the equation?
-Faradays law of electromagnetic induction states that the magnitude of the induced e.m.f is equal to the rate of change of flux linkage
What does Lenz’s law state?
-Lenz’s law states that the direction of any induced e.m.f or induced current is in a direction that opposes the flux change that causes it.
What do each of the symbols mean in this equation
ϵ- induced e.m.f
N-number of coils
Φ-magnetic flux
t-time
How is a seach coil used to measure the strength of the magnetic field?
-A search coil is a small flat coil with a known number of coils
-It can be placed into a magnetic field with a circuit attached containing a resistor and an ammeter or galvanometer
-The equations Q=It and Q=BAN/R can then be used to work out magnetic field strength
What are the definitions of a generator and an alternating current?
-Generator: a device used to generate electricity, the rotation of a coil within a magnetic field produces a constantly changing flux linkage through the coil. This produces a constantly changing induced e.m.f in the coil
-Alternating current: is electrical current that reverses its direction with a constant frequency
If a rectangular coil is rotated in a magnetic field at a rate of 50 rotations per second what is the frequency of the alternating current produced?
50 Hz
Where do the conduction electrons drift in an alternating current and a direct current?
-In an A.C the conduction electrons drift back and forth about their mean positions
-In a DC the conduction electrons drift along the wire in one direction
What is four main components of a a.c. generator?
-A permanent magnet, a rectangular coil, slip rings and brushes
-The slips rings rotate with the coil and are connect to brushes which allow electrical contact from the slip rings to an external contact
Describe the stages of operation of an A.C generator?
- In the vertical position (0°), there is no induced e.m.f as the rate of change of flux linkage is zero
- As the coil rotates through 90°, the rate of change of flux linkage reaches a maximum value and at 90° e.m.f generate reaches a maximum
- As the coil rotates to vertical again, the rate at which flux changes decreases and induced e.m.f falls to zero at 180°
4.The process then repeats from 180-360° but the current induced will be in the opposite direction
What is a transformer and what is its structure?
-A tranformer is a device that can either increase or decrease the size of alternating voltage.
-A simple transformer consists of two coils of wire on a core of magnetised iron, one coil is connected to an alternating voltage supply (primary coil) and one is connected to the output (secondary coil)
How does a transformer work?
- An alternating current in the primary coil causes the iron core to be magnetised and then remagnetised in the other direction (alternating current)
- This rapidly changing magnetic flux leads to a rapidly changing magnetic flux in the secondary coil
- An alternating current of the same frequency is induced in the secondary coil
-This is called the transformer effect, varying the number of turns in the secondary and primary coil allow electrical supplies to be changed from low voltage, high current to high voltage, low current.
Why are transformers necessary?
-At high currents more electrical energy is dissipated by heating, the power disipated in a wire with R is P=I^2R so the power lost by heating is proportional to the square of the current
-So when electrical energy is transmitted over long distances transformers step up the voltage therefore decreasing the current
What is the equation for transformers?
What do transformers used in the electricity supply usually have an efficiency of?
More than 99%
