Define electric field strength
Force per unit charge
E=F/q
How is the direction of an electric field indicated?
Using arrows pointing in the direction of the force applied on a positive charge.
How can the motion of charged particles in electric fields be calculated?
Using the projectile motion equations, replacing gravity with the acceleration due to the electric field. a=qE/m
What is electromotive force (emf)?
Energy per unit charge applied by an external source, measured in volts.
Describe the motion of a charged particle in a magnetic field.
Force is applied perpendicular to the motion of the particle by the right hand push rule. This causes circular motion with the centripetal force being F=qvB.
Decribe the right hand push rule.
Fingers in direction of the magnetic field
Thumb in direction of current (positive charge)
Palm pushes in direction of force
Describe the force between two parallel, current carrying wires.
The magnetic fields created by the wires both exert force on the other wire. If current flows the same direction, the wires are attracted. If opposite, wires are repelled.
What is magetic flux?
A measure of the total amount of magnetic field that passes through an area, measured in Webers.
𝛷 = 𝐵∥𝐴 = 𝐵𝐴cosθ
State Faraday’s Law
If a conductor experiences a change in magnetic flux, an emf is created and current induced.
Explain Lenz’s Law
A conductor will try to resist any changes in magnetic flux. It does this by creating its own current, which in turn creates a magnetic field to replace the one it has lost.
Describe the motor effect
A conductor that has a current moving through a magnetic field will experience force. F = lIBsinθ
What is the role of step-up transformers in power transmission?
Increase voltage to reduce current, minimising power losses over long distances.
Why are transformers not 100% efficient?
Energy losses due to heat, eddy currents, incomplete flux linkage.
Explain why a transformer doesn’t work for a constant DC current
A change in current in the primary coil is needed to create a changing magnetic flux in the secondary coil to create an emf
State some methods of increasing transformer effiency
Iron core to increase flux linkage
Laminated iron core to reduce eddy currents
Highly conductive wires such as copper
Cooling systems to reduce heat
Explain the role of the split-ring commutator in a DC motor
It allows the current to alternate direction in the armature. This is necessary to allow for full rotation, otherwise the direction of torque would change every time the armature rotated 180 degrees
What is back emf?
An emf induced in a coil that opposes the applied current.
State some methods for increasing the torque of a DC motor
Increasing the area inside the coils
Increasing magnetic field strength
Increasing the number of coils
Increasing the current
What determines the maximum angluar velocity of a DC motor?
The moving wires inside the magnetic field create back emf which increases as the motor spins faster. At high speeds, the back emf will cancel out the current, causing a loss in torque and the motor slows down, reducing the emf, so it can speed up again. This is called “slippage”
Why is AC used to power cities instead of DC?
AC allows the use of transformers which greatly increase the efficiency of energy transmission.
