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 and changes current direction at every half of coil’s turn to allow the coil to turn continuously in one direction.

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.

Question 12

DC motor

Answer 12

A device that converts electrical energy to mechanical energy

Question 13

Motor effect

Answer 13

Produced when a current carrying conductor is placed in a magnetic field

Question 14

Electromagnetic induction

Answer 14

A process through which an induced emf is produced in a conductor due to a changing magnetic field

Question 15

Magnetic flux

Answer 15

Magnetic field in a given area

Question 16

Transformer

Answer 16

A device that can change a high alternating voltage to a low alternating voltage or vice versa

Question 17

Lenz’s law

Answer 17

States that the direction of the induced emf and thus the induced current in a closed circuit is always such that its magnetic effect opposes the motion or change producing it.

Question 18

Faraday’s law

Answer 18

States that the magnitude of the induced emf in a circuit is directly proportionate to the rate of change of magnetic flux in the circuit.

Question 19

Function of soft iron core

Answer 19

Converge and concentrate magnetic field lines to increase strength of magnetic field in the coils. They also have low resistance as they are a soft ferromagnetic material, increasing efficiency of transformer by reducing energy lose, forming higher induced current.

Question 20

Why is a steel core not used in transformer but soft iron

Answer 20

Soft iron is a soft ferromagnetic material that is more permeable to Magnetic field lines and does not retain magnetism after magnet is removed.

Question 21

Ammeter vs centre zero galvanometer

Answer 21

Ammeter only tells current in 1 direction but centre zero galvanometer shows current direction + detect very small currents

Question 22

How to increase the strength of the induced current

Answer 22

1 speed of relative motion between the coil and the magnet is increased
2. Number of turns of the coil is increases
3. Strength of the magnet is increased.

Question 23

Purpose of slip rings

Answer 23

Prevent the coil from getting entangled and ensuring alternating current flows

Question 24

Why it is difficult to achieve a 100% efficient transformer

Answer 24

Some energy is lost in the form of heat produced by the current flowing in the coils which have resistance. (Hence thick wires used to reduce resistance of wires)

Some energy is lost due to the formation of eddy current produced in the iron core. (Hence soft iron core is usually laminated)

Question 25

Design features to improve efficiency of a transformer

Answer 25

Use of magnetic material for the core of the transformer

Use of laminated sheets of iron for the core

Question 26

What happens when the strength of the magnet is doubled

Answer 26

Induced emf doubles, frequency remains unchanged

Question 27

Why is an ac used

Answer 27

Transformer only works on ac

Question 28

Explain why the induced current is alternating

Answer 28

When the magnet rotates towards the coil, an induced current is produced in one direction by Lenzs’ law, and when it rotates away from the soft iron, an induced current is produced in the opposite direction. The induced currents in both directions are produced to oppose the change producing it by Lenzs law

Question 29

Voltage of the circuit usually stepped up before long distance transmission explain why

Answer 29

Voltage stepped up, current decreases by V=IR. By P=I2R, power loss in cable reduced.

Question 30

Explain how the transformer produces various output voltages that is different from the magnitude of the input voltage.

Answer 30

Connecting the output voltage at different points will vary the number of coils in the output. The number of turns in the secondary coil Yz is lower than that of primary coil thus experiences a smaller change in magnetic flux, resulting in a lower induced emf by Faraday’s law.

Question 31

Alternating voltage

Answer 31

Alternating voltage is voltage that changes direction of the polarity periodically.