\Section 7 part 3 - Magnetic fields and EM Induction Flashcards
Define a magnetic field
A force field surrounding a magnet or current carrying wire which acts on any other magnet or current carrying wire placed in the field
Where is the magnetic field of a bar magnet strongest?
At the ends (poles)
What are magnetic field lines?
Lines of force
What do the arrows on magnetic field lines represent?
The direction a small free north pole would move if placed in the field
Can field lines cross?
No, they show the resultant force
What is a neutral point, and how are they created?
A point where no force is experienced, and created when like poles are near to each other
What do current carrying wires produce?
A magnetic field
Define motor effect
A current carrying wire when placed at a non-zero angle to the lines of force of an external magnetic field feels a force due to a field. This effect is the motor effect
Which direction does the force act in the motor effect?
Perpendicular to the current and lines of force
What does the force experienced by an object in a magnetic field depend on?
The current, strength of magnetic field, length of wire, angle between lines of force and current direction
When is the force experienced by a wire in a magnetic field strongest and weakest?
The force is max when the wire is at right angles to the magnetic field
The force is 0 when the wire is parallel to the magnetic field
Define magnetic flux density
The strength of the magnetic field - the force per unit length per unit current-carrying conductor at right angles to the magnetic field lines
State the components and the conditions for the equation F=BIL
F = Force, N B = Magnetic flux density, T I = Current, A L = Length, m B and I must be at right angles for the equation to apply
What is the equation for torque on a coil at an angle to the magnetic field lines?
T = BIANcostheta, or BILdN
State the components for the equation T = BIANcostheta
T = Torque, Nm B = Magnetic field density, T A = Area, M^2 N = Number of turns in wire costheta = Angle between coil and field lines
Why do electron beams move in a circular path
The direction of the force on each electron is perpendicular to the direction of motion of the electron
Which direction do stationary charges in a magnetic field experience a force relative to?
They don’t experience a force
What is the equation for Hall voltage?
V = Bvd
Can magnetic fields do work on charged particles?
No, as the force is always perpendicular to the velocity. The direction of motion is changed by the force, but not the motion
How much does the magnetic field change the kinetic energy of a charged particle?
It cannot
What is the equation for the radius of a charged particle path?
r = mv/BQ
Define the motor effect
A current carrying wire with its associated magnetic field will experience a motor effect if placed at a non-zero angle in a magnetic field
When is an emf induced?
When a wire cuts through the field lines of a magnet, changing the magnetic flux through the coil
How can an induced emf be increased?
- Increasing the speed of the rate of cutting of field lines, as this increases the rate of change of flux
- Using a magnet with stronger field lines
- Making the wire into a coil
Why is no emf induced if the wire is parallel to the magnetic field lines?
Because the wire is not cutting any field lines, so there is no change in flux and therefore no induced emf
What does an electric current do in a circuit?
Transfer energy from the source of the emf to the other components
Define emf
Energy transferred per unit charge
Define current
Rate of flow of charge
When do you use Fleming’s left hand rule and the Dynamo rule?
Fleming’s Left hand rule for motors
Dynamo rule for generators
When is a magnetic field produced in or around a coil?
When the coil is connected to a battery and a current is passed through it
State Lenz’s law
The direction of the induced current is always such as to oppose the change that causes the current
What is magnetic flux density?
The magnetic field strength, B in T
What is magnetic flux?
The total amount of magnetic field passing through an area, Phi in Tm^2 or Wb
What is magnetic flux linkage?
The total amount of magnetic field linked by a coil, N phi in Wb turns
Describe the magnetic flux linkage at 0, 90, 180 and 270 degrees compared to the magnetic field lines
0 - 0 flux linkage
90 - BAN flux linkage
180 - 0 flux linkage
270 - -BAN flux linkage
State Faraday’s Law of electromagnetic induction
The induced emf in a circuit is equal to the rate of change of flux linkage through the circuit
What is the equation you are not given for induced emf?
emf = BLv
Define back emf
emf induced which acts against the pd applied to the motor due to Lenz’s law
What is the relation between induced emf, speed of rotation, and current of a motor?
Induced emf is proportional to the speed of rotation, and the current changes as the motor speed changes
What is the size of the current at high and low speeds of rotation:?
Current is high at low speeds because induced emf is small
Current is low at high speeds because induced emf is high
What is the equation for electrical power supplied by the source? (words and symbols)
IV = Ie + I^2R
Electrical power supplied = electrical power transferred to mechanical power + electrical power wasted due to circuit resistance
What is an alternating current?
A current that repeatedly reverses its direction
What is the frequency of mains electricity?
50Hz
What is the peak pd in a mains circuit?
325v
What is the root mean square value for an alternating current?
The value of direct current that would give the same heating effect as the alternating current in the same resistor
What do transformers do?
Change an alternating pd to a different peak value
Describe how an alternating emf is induced in the secondary coil
The primary coil is connected to an alternating pd, which produces an alternating magnetic field in the laminated soft iron core. This field passes through the secondary coil, so the changing magnetic field in the secondary coil induces an emf
What is a step up transformer?
One where there are more turns on the secondary coil than the primary coil, so the voltage is stepped up and the current is stepped down compared with the primary coil
What is a step down transformer?
One where there are fewer turns on the secondary coil than the primary coil, so the voltage is stepped down and the current is stepped up compared with the primary coil
Describe the 3 things that make transformers almost 100% efficient?
- Low resistance windings to reduce power wasted due to the heating effect of the current
- Laminated core to reduce eddy currents so the magnetic flux is high as possible. This also reduces the heating effect of induced currents in the core
- A soft iron core is easily magnetised and demagnetised which reduces power wasted through repeated magnetisation and demagnetisation
What is the national grid?
A network of transformers and cables, underground and on pylons which covers all regions of the UK
What voltage and frequency do National Grid power stations generate?
25kV and 50Hz
How can you make the transmission of electrical power at long distances higher and why?
Use higher voltage because the current needed to deliver a certain amount of power is reduced. Wasted power due to heating effect of the current is reduced
What does an AC generator consist of?
A spinning coil in a magnetic field with a slip ring/brush arrangement
Why do you use a slip ring/brush arrangement in an AC generator?
To allow electrical connections to be maintained as the coil spins. As the coil spins the flux linkage changes continually
What is the difference between a DC and AC generator components?
DC - Split ring commutator
AC - Slip ring/brush arrangement
What voltage is the national grid stepped up to?
Voltages of 400kV or more for long distance
