the motor effect (topic 7)

Question 1

describe how a student could construct an electromagnet (3)

Answer 1

• current carrying wire
• wrapped into a coil
• around an iron core

Question 2

describe two ways in which the student could make the electromagnet stronger (2)

Answer 2

• increase the number of turns on the coil
• increase the current through the coil

Question 3

explain what is meant by a D.C. power supply (2)

Answer 3

• direct current
• current continuously flows in the same direction

Question 4

describe two ways the student could increase the number of pins picked up at 1.2 A (2)

Answer 4

• increase the number of turns on the coil
• wrap the wire around an iron core

Question 5

describe how the number of pins picked up would vary if an A.C. power supply was used (3)

Answer 5

• alternating current (A.C.) changes direction every half cycle
• magnetic field is weaker
• fewer pins picked up

Question 6

explain a method the student could follow to identify the shape of the magnetic field around the wire (2)

Answer 6

• sprinkle iron filings on paper around the wire
• tap paper to align iron filings with the magnetic field

Question 7

Answer 7

Question 8

Answer 8

B

Question 9

explain how the electromagnet keeps the door open (3)

Answer 9

• a current through the electromagnet causes it to set up a strong magnetic field
• iron plate is magnetic
• the metal plate is attracted to the magnetic field of the electromagnet

Question 10

describe what happens to the door when the switch is pressed (3)

Answer 10

• when the button is pressed, no current can flow in the circuit
• magnetic field strength of the electromagnet rapidly decreases, no magnetic field
• iron plate loses magnetism and is no longer attracted to the electromagnet allowing door to swing closed

Question 11

Answer 11

Question 12

state one advantage of using an electromagnet over a bar magnet (2)

Answer 12

• the strength of the electromagnet can be varied
• the electromagnet can be turned on/off

Question 13

sketch out the construction of an electromagnet

Answer 13

Question 14

Answer 14

• when the switch is closed a current flows through the electromagnet
• this causes the iron armature to be attracted to the electromagnet causing the striker to hit the gong
• as the armature moves towards the electromagnet, the contacts are separated, breaking the circuit and no current is able to flow
• the spring pulls the armature back to its original position and the contacts touch once again
• the process repeats until the switch is opened

Question 15

why is it important to keep the current flowing through the electromagnet fixed during each trial (1)

Answer 15

if both number of turns and current are increased at the same time, there will be no way to tell which change caused the effect in each trial

Question 16

how does a dynamo work (3)

Answer 16

• a split ring commutator changes the coil connections every half turn
• as the induced potential difference is about to change direction, the connections are reversed
• this means that the current to the external circuit always flows in the same direction