17 Motor Effect & Electromagnetic Induction

Question 1

1. In a speaker, there is a current carrying coil
2. 90º to a magnetic field
3. Due to LHR there is a force on the coil
4. The coil is attached to the cone - therefore the cone moves in one direction

If the current in a speaker is a.c, describe the movement of the coil and cone and how this creates a sound wave.

Answer 1

1. The alternating current means the current in the coil changes direction continuously.
2. This means the force on the coil/cone changes direction continuously - cone moves in and out (vibrates)
3. This creates a sound wave (compression wave) in front of the cone at the same frequency as the alernating current.

Question 2

Describe the left hand rule (LHR)

Answer 2

First Finger - Field

Second Finger - Current

Thumb - Movement

Question 3

When a magnet is moved in and out of a coil, the magnetic field lines of the magnet cut the coil at 90º , which induces an a.c. current.

State three way in which you could increase the current induced?

Answer 3

1. More turns in the coil
2. Stronger magnet
3. Move the magnet faster

Question 4

Explain why a current is induced in the wire.

Answer 4

1. Wire is moved downward (thuMb- MOVEMENT)
2. 90º to a magnetic field (First finger- FIELD)
3. Therefore a current is induced in the wire (seCond finger- CURRENT)

Question 5

The current flows into the left brush making contact with the left side of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward.

The coil rotates clockwise.

What would happen if the battery was reversed?

Answer 5

The direction of the current is reversed AND the coil will rotate anticlockwise

Question 6

Draw the magnetic field around a solenoid.

Answer 6

Question 7

Use the LHR to explain why the wire moves to the left.

Answer 7

1. Current carrying wire (seCond finger- CURRENT)
2. 90º to magnetic field (First finger field)
3. Using LHR this creates a force to the left- wire moves left (thuMb movement)

Question 8

How do you use the right hand grip rule to work out which end of a solenoid is the north end?

Answer 8

Fingers- wrap your fingers in the direction of the current

Thumb- will point to the North end of the solenoid

Question 9

Explain why a current is induced in the coil

Answer 9

1. Magnet is moved into the solenoid (thuMb- MOVEMENT)
2. Magnetic field lines around magnet cut the coil at 90º (First finger- FIELD)
3. Therefore a current is induced in the coil (seCond finger- CURRENT)

Question 10

Using the transformer equation below. Describe what would happen to the voltage across a transformer if the number of turns in the secondary coil is three times larger than the number of turns in the primary.

Answer 10

If no. of turns triples, the voltage triples.

Voltage in the seconday coil will be three times greater than the voltage in the primary

Step-up transformer

Question 11

How do we use dot and crosses to represent the direction of a mangetic field or current?

Answer 11

dot means out of the page

cross means into the page

Question 12

When a magnet is moved in and our of a coil, a current is induced in the coil. What is this phenomenon called?

Answer 12

Electromagnetic induction

Question 13

Complete the explanation of why the comutator allows the motor to rotate

1. There is a current carrying coil
2. 90º to a magnetic field
3. Using LHR- left side of coil experiences an upward force.
4. Using LHR- current is travelling the opposite direction on the right side of coil- it experiences a downward force
5. Coil rotates clockwise

Answer 13

6. When coil is at 90º, the current in the coil reverses due to the commutator and brushes.
7. Current continues to travel IN on the left and OUT on the right

8 Using LHR- left side of coil still experiences an upward force and the right hand side still experiences a downward force

9. Coil continues to rotate clockwise

Question 14

When the magnet is moved into the coil, a current is induced and the needle on the ammeter moves to the left.

Describe what happens when the magnet is pulled out of the coil.

Answer 14

The needle with move to the right

Reversing the direction of the magnet, reverses the induced current

Question 15

What is a step-down transformer?

Answer 15

no. of primary turns > no. of seconday turns

primary voltage > secondary voltage

Question 16

How would you demonstrate the direction of the magnetic field around a single wire?

Answer 16

Place compasses around the wire

The compass will point in the direction of the magnetic field

Question 17

Why is voltage stepped up using transformers before transmitting long distances?

Answer 17

If current is reduced then the power loss along the transmission lines is reduced.

Less heat loss along the wire

Question 18

Describe the force on the particle below

Answer 18

No force - as particle is moving parallel to the field (it must move 90º to the field)

Question 19

What can be done to increase the magnetic field around a solenoid?

Answer 19

1- more turns

2- increase the current

3- add an iron core

Question 20

When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.

What would happen if the wire is moved more quickly?

Answer 20

A larger current would be induced

Question 21

Draw the shape of a magnetic field around a coil.

Answer 21

Question 22

How do you determine the direction of a magnetic field around a wire?

Answer 22

Use the right hand grip rule

Thumb- direction of current

Fingers- wrap in direction of magnetic field (clock wise or anti-clockwise)

Question 23

Explain why a current is induced in a wire when it it is moved downward through a magnetic field

Answer 23

1. Wire is moved downward (thuMb - MOVEMENT)
2. The wire cuts the magnetic field lines at 90º - First finger- FIELD)
3. This induces a current in the wire- (seCond finger- CURRENT)

Question 24

Electromagets are used in electric bells.

• When the button is pushed, a current flows in the electromagnet -This creates a magnetic field around the electromagnet
• The electromagnet attracts the soft iron armature which moves to the right
• This causes the hammer to hit the bell.

What happens next?

Answer 24

The moment the iron armature moves to the right this creates a _break in the circuit- t_he circuit is no longer complete.

The current drops to zero and the magnetic field around the electromagnet disappears.

The iron armature is no longer attracted and springs back to the left

The armature completes the circuit again and the whole cycle repeats alowing the bell to repeatedly sound.

Question 25

Why is iron a better core to use in an electromagnet than steel?

Answer 25

**Iron is magnetically soft**- the domains in iron line up easily - it can _magnetise and demagentise easily_ **Steel is magnetically hard**- the domains do not line up easily- it _cannot magnetise easily_ and it can be used to make permananet magnets becaouse it also _does not easily demagnetise_

Question 26

When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire. ## Footnote **What would happen if the strength of the magnets were increased?**

Answer 26

A **large current** would be induced

Question 27

How is the primary voltage, seconday voltage, no. of primary turns, no. of secondary turns related?

Answer 27

Question 28

Explain why a current is induced in the coil when the magnet is pushed into the coil

Answer 28

1. The magnet is moving (thuMb- MOVEMENT) 2. The magnetic field lines of the magnet are cutting the wires of the coil at 90º - (First finger- FIELD) 3. This induces a current in the coil - (seCond finger- CURRENT)

Question 29

What happens to the magnetic field around a wire if more wires are added side by side- How can you tell from the image that the magnetic field is stronger?

Answer 29

More wire- stronger field The closer the magnetic field lines the stronger the field. iron filing will line up more easily and more closely together in a stronger magnetic field.

Question 30

Explain why a current is induced in the wire.

Answer 30

1. Wire is moved downward (thuMb- MOVEMENT) 2. 90º to a magnetic field (First finger- FIELD) 3. Due to LHR a current is induced in the wire (seCond finger- CURRENT)

Question 31

The current flows into the left brush making contact with the left of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward. Why is the force on the right side of the coil downward?

Answer 31

The _direction of the current is reversed_ or out of the page on the right side of the coil. Therefore the direction of the force is in the opposite direction OR downward The coil rotates clockwise

Question 32

What happens to the direction of the magnetic field around a magnet if the direction of the current it reversed?

Answer 32

The direction of the magnetic field is reversed.

Question 33

When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire. ## Footnote **What would happen if the number of turns in the wire was increased?**

Answer 33

A **large current** would be induced

Question 34

What happens to the magnetic field around a solenoid if you add an iron core (magnetically soft)

Answer 34

Stronger magnetic field

Question 35

When a wire is moved down through a magnetic field, the wires cut the magnetic field lines at 90º and this induces a current in the wire which moves the ammeter needle to the **left.** ## Footnote **What happens when the wire is pulled up and out of the magnetic field?**

Answer 35

The needle will move to the **right** Reversing the direction of the movement of the wire will reverse the direction of the induced current.

Question 36

How does an a.c current in the primary coil **induce** a current/voltage in the secondary coil of a transformer?

Answer 36

1. The alternating current in the primary current creates a changing magnetic field around the primary coil 2. This continuously cuts through the secondary coil 3. inducing an alternating current in the secondary coil

Question 37

If a vertical wire has the current upward in the wire, work out the direction of the magnetic field using your Right Hand Grip Rule

Answer 37

Thumb- up and in direction of current Finger wrap in direction of magnetic field.

Question 38

Describe the parts of a transformer

Answer 38

1. laminated iron core 2. primary coil N₁ 3. secondary coil N₂ 4. primary alternating voltage V₁ 5. secondary alternaing voltage V₂

Question 39

Describe the shape of the magnetic field around a wire

Answer 39

Circular field shape field lines getting futher apart- field getting weaker as you move away from the wire

Question 40

Describe why the iron core must be laminated in a transformer.

Answer 40

A laminated iron core _reduces the eddy currents_ in the iron core and makes the transformer _more efficient - less heat loss_ in the core.

Question 41

How would you construct a simple electromagnet?

Answer 41

1) wrap a wire around an iron nail 2) connect a cell across the wire

Question 42

What is a step-up transformer?

Answer 42

no. of secondary turns \> no. of primary turns secondary voltage \> primary voltage

Question 43

The current flows into the left brush making contact with the left of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward. The coil rotates clockwise. What would happen if the magnetic poles were reversed

Answer 43

The _direction of the field reverses, and therefore the direction of the force reverses-_ the coil rotates anticlockwise

Question 44

Answer 44

7. No force at all Note: Particle is moving parallel to the field- no force.

Question 45

Transformers are nearly 100% efficient. What does this mean?

Answer 45

Power in = power out I_pV_p=I_sP_s

Question 46

What sort of transformer must be used after electricity has been transmitted across the country and before it is used in homes and factories?

Answer 46

step-down The voltages transmitted through the power lines are around 400 000V! this must be stepped down to a safe level (240V) for homes.