Section 11 - Magnetic Fields Flashcards
When a magnetic field is perpendicular to a current-carrying wire, does the wire feel a force?
Yes, the magnitude of the force is BIL
Flemming’s left hand rule for motors represents what properties on what fingers?
Thumb - Thrust/Force
First dinger - Field (Magnetic)
SeCond finger - Current
What is magnetic flux density (B)?
Flux density measured in Tesla (T0 or Webers/metres^2, is the flux per metre^2
A charged particle moving through a field feels a force when it is travelling along the field lines or perpendicular to them?
Perpendicular to the field
Is the force applied to the particles applied perpendicular to the particles motion or in one direction?
Perpendicular to its motion, causing it to move in circular motion
Which fields do cyclotrons use?
An electric field and a magnetic field
How does a Cyclotron work and what’s the electric and magnetic field’s purpose in a cyclotron?
- A cyclotron is made up of 2 semicircular elecrodes called “Dees” with a magnetic field applied perpendicular to the Dees and an alternating pd applied between the Dees
- Each Dee is a metal electrode with opposite charges, this creates an electric field in the gap between the 2 Dees - this is what accelerates the particles
- The magnetic field causes the particles to move in a circular motion, which allows it to gain speed whilst minimising space - as they speed up the radius of their motion increases, until it breaks free tangential to one of the Dees
What is flux linkage?
NΦ = The number of turns cutting the flux one time
What is Faraday’s Law?
The induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage
What is Lenz’s law?
The direction of the induced e.m.f. is such as to oppose the change that induces it
What happens when you move a straight conductor through a magnetic field?
- The electrons experience a force pushing them to one end of the conductor creating an emf across the conductor
- The rod obeys Faraday’s law, it is changing flux as it moves through the field hence and e.m.f. is induced
Describe how one would use an oscilloscope
- Oscilloscopes are used to displace AC waves, the x axis is called the time base and shows how long it takes the wave to move 1 division and the y axis shows how much PD is needed to move the way up 1 division
- Using this we find the peak voltage, time period and frequency
How does a transformer work?
- A primary coil wrapped around an iron core with an alternating p.d. creates an alternating magentic field, this magnetic field induces an e.m.f. in a secondary coil also wrapped around the core
- This creates a current in the secondary coil
What kind of current is produced by a transformer and why?
- An alternating current
- An e.m.f. is induced by a changing magnetic field, hence the e.m.f. induced is alternating producing an alternating current
Why are transformers used?
- By changing the number of coils, the transformers can be used to increase the voltage and reduce current when transporting power, with minimal energy loses
- The voltage is then dropped again locally to ensure safe usage in households
In a step-up transformer, does the secondary coil or primary coil have more coils?
- The secondary coil
- Step-up transformers increase the voltage, hence more coils need to be in the secondary coil for a larger p.d.
What is an eddy current?
- As the primary coil’s magnetic field induces e.m.f. in the secondary coil, it also induces e.m.f. and hence mini currents within the iron core
- These are known as eddy currents
Why are eddy currents a problem?
- By Lenz’s law the e.m.f. created and its field opposes that of the primary coil
- This causes energy loss via resistive heating of the iron core by the eddy currents, which reduces efficiency
How can you reduce eddy current losses?
- Use a laminated core
- Thin sheets of iron with an electrical insulator in between, which reduces the eddy currents circuit
