20 — electromagnetism (DC motor)

Question 1

Motor

Answer 1

Converts electrical energy into mechanical energy

Question 2

Current and deflection of galvanometer

Answer 2

1. When no current flows thru wire, no deflection seen
2. When current flows, compass placed below wire shows deflection to West
3. When large current flows in wire, compass placed below shows a larger deflection to the west
4. When a current flows in wire in the same direction, compass when placed ABOVE the wire shows a deflection to the East
5. When current flows in wire in opposite direction, compass when placed above the wire shows a deflection to the West.

Question 3

How to increase strength of magnetic field

Answer 3

1. Increasing current flowing thru the solenoid -> magnetic field stronger -> alignment of domains in 1 direction

2.increasing the number of turns per unit length of the solenoid
3. Placing a soft iron core within the solenoid

Question 4

Function of the split ring commutator

Answer 4

Changes current direction every half a turn so that the coil continues to rotate in 1 direction only

When the coil rotates 90dg and reaches the vertical position, the contact betw the carbon brushes and the commutators r broken. No current flows thru the coil. Because of its inertia, the coil keeps rotating until the commutators r in contact w the carbon brush.

Question 5

Carbon brushes function

Answer 5

Provides electrical contact for current to flow

Question 6

State the effect on the magnetic field lines of having a larger current in the solenoid. [1]

Answer 6

There will be an increase in the number of field lines and field lines will be more closely spaced together.

Question 7

Describe 2 ways in which the magnetic field at P differs from at Q. [2]

Answer 7

1. Direction of magnetic field is opposite
2. Strength of magnetic field is weaker at Q than P.

Question 8

Explain why the 2 parallel lightning rods attract each other when lightning strikes. [2]

Answer 8

When a current flows thru the rod, a magnetic field will be generated around the rod. When 2 current carrying rods r side by side, magnetic field of 1 rod and current in the other will produce a force pulling it to the other rod, attracting the rods tgt.

Question 9

Explain how the circuit breaker breaks the circuit. [3]

Answer 9

When a large current flows thru the circuit breaker, the soft iron core is magnetised strongly enough to exert a downward attractive force on the iron arm, causing the contacts to separate thus an open circuit results, preventing further current flow.

Question 10

Suggest the purpose of the springy metal in the circuit breaker. [1]

Answer 10

Springy metal resting on iron arm provides the resistive force to push the iron arm down, producing a moments about the pivot for lower contact to be attracted to upper contact.
It allows for adjustments to be made to change the current value that can break the circuit and the reset of the circuit breaker by pushing the right side of the iron arm under the springy metal.

Question 11

The coil is replaced with another solenoid with more turns of wire in the circuit breaker. State and explain what effect this has on the current needed to break the circuit. [1]

Answer 11

A coil containing more turns will cause the iron core to magnetise more strongly with the same amount of current. Using this coil will allow a smaller current to sufficiently magnetise the iron core to break the circuit.