Chapter 4: Electricity and Magnetism {Pt 3}

Question 1

How is an emf produced in a conductor?

Answer 1

By moving it through a magnetic field because the magnetic lines of force are applying a force on the free electrons in the conductor and causing them to move.

Question 2

What is induction?

Answer 2

The process of generating a current in a conductor by placing the conductor in a changing magnetic field.

This is because there is no physical connection between the conductor and the magnet

The emf is said to be induced in the conductor by the magnetic field.

Question 3

How do you demonstrate electromagnetic induction?

Answer 3

Electromagnetic induction requires motion between the magnet and the coil to produce a voltage

1. Connect a solenoid with an ammeter
2. Move a magnet in and out of the solenoid
3. Observe the ammeter reading when the magnet moving in and out of the solenoid.

Conclusion:
1. A current flowing in both directions can be observed
2. This means that an emf is induced in the coil
3. Moving the magnet in one direction presents a positive vale
4. It also shows a negative value when moving the magnet in the reverse
5. No voltage is induced when the magnet is still
6. To have an induced emf, a coil must cut through the magnetic field lines or a magnet must cut through the magnetic field lines of a coil

Question 4

What does Lenz’s law state?

Answer 4

The direction of the induced emf drives current around a wire loop to always oppose the change in magnetic flux that causes the emf.

Question 5

What are the factors that affect the magnitude of an induced emf?

Answer 5

The rate/speed in which the magnet moves
The strength of the magnet
The number of coils of the solenoid

Question 6

What happens when a magnet and its field are moved through a coiled conductor?

Answer 6

A DC voltage with a specific polarity is produced.

The polarity of this voltage depends on the direction in which the magnet is moved and the position of the north and south poles of the magnetic field.

The direction of current flow is a function of the polarity of the voltage induced into the conductor.

Question 7

How can we determine the direction of current flow within the conductor?

Answer 7

Using the left-hand rule.
The thumb –> direction of the thrust on the conductor
The fore finger —> the direction of the magnetic field.
The centre finger —> the direction of the current

Question 8

What is a.c. current?

Answer 8

An electric current in which the electrons flow back and forth, changing direction at regular intervals many times in one second.

The graph for an a.c. current : a sine wave

Question 9

What does a simple a.c. generator consist of?

Answer 9

A rectangular coil between the poles of a C-shaped magnet.
The ends of the coil are joined to two slip rings on the axle and against which carbon brushes press.
When the coil is rotated, it cuts the field lines, and a voltage is induced in it.

Question 10

Analyse the above graph

Answer 10

As the coil moves through the vertical position with ab uppermost, and no cutting occurs. The induced voltage is zero

During 1st quarter rotation, the p.d. increases to a maximum when the coil is horizontal. Sides ab and dc are then cutting the lines at the greatest rate

In the 2nd quarter rotation, the p.d. decreases again and is zero when the coil is vertical with dc uppermost. After this, the direction of the p.d. reverses because during the next half-rotation the motion of ab is directed upwards and dc downwards.

An alternating voltage is generated which acts first in one direction and then the other; it causes alternating current to flow in a circuit connected to the brushes.

Question 11

What does the magnetic field around a straight wire consist of?

Answer 11

Concentric circles round the center of the wire.

The magnetic field is stronger near the wire.

Increasing the current of the wire increases the magnetic field strength

These are at right angles to the direction in which the electric current flows

The direction of the magnetic field is reversed if the direction of the electric current is reversed

The direction of the magnetic field due to the current carrying wire can be determined by the right hand rule.

Question 12

What is a solenoid?

Answer 12

A long coil of wire

Question 13

How does the strength of the field diminish?

Answer 13

With distance from the solenoid

Question 14

How can the strength of the magnetic field be increased?

Answer 14

By increasing the current in the coil
By increasing the number of coils in the solenoid

Question 15

What does reversing the direction of the current do?

Answer 15

It reverses the direction of the magnetic field.

Question 16

How do you identify the pattern of the magnetic field in a wire?

Answer 16

1. Thread a wire through a piece of card held horizontally by a clamp stand
2. Sprinkle iron filings onto the card and then tap
3. The iron filings will show the shape of the field
4. Plotting compasses placed on the card settle along the field lines and show the direction of the field at different points
5. When the current direction is reversed, the compasses point in the opposite direction showing that the direction of the field reverses when the current reverses.

Question 17

How do you identify the pattern of the magnetic field in a solenoid?

Answer 17

Question 18

List the applications of magnetic effect of current.

Answer 18

1. Transformers
2. Relays
3. Motors
4. Ballasts in fluorescent lamps
5. Alarming systems
6. Circuit breakers
7. In storage devices like hard disks

Question 19

What is a relay?

Answer 19

An electromagnetic switch operated by a relatively small electric current that can turn on or off a much larger electric current.

Question 20

How does a relay work?

Answer 20

When the switch in the low current circuit is closed, it turns the electromagnet on which attracts the iron armature

The armature pivots and closes the switch contacts in the high current circuit.

When the low current switch is opened, the electromagnet stops pulling the armature and the high current circuit is broken again.

Question 21

Where are relays often used?

Answer 21

In cars for switching the starter mortor, because it draws a very big current.

Question 22

How do loudspeakers work?

Answer 22

Electrical signals from the receiver cause the current flowing in the coil to change.
A change in the current causes a change in the magnetic force acting on the coil.
In this way, the paper cone is made to move in and out.
The vibrations of the cone produce sound waves.

Question 23

How is the magnitude of magnetic field related to current in the wire?

Answer 23

The strength of a magnetic field is directly related to the current in the wire.

Increasing the magnitude of current of the wire increases the strength of magnetic field

Decreasing the magnitude of current of the wire decreases the strength of magnetic field

Reversing the direction of current reverses the direction of magnetic field.

Question 24

Tell me about the force on a current-carrying conductor.

Answer 24

Question 25

How does a dc motor work?

Answer 25

1. They use the turning effect to rotate.
2. When an electric current flows through the coil, the coil experiences a force and moves. One side moves up and the other side moves down
3. The direction of the current must be reversed every half-turn, otherwise the coil comes to a halt
4. This is acheived using a conducting ring split in two, called a split ring or ‘commutator’
5. The turning effect can be increased by:
   increasing the current
   using a stronger magnet
   increasing the number of turns on the coil
   increasing the area of the coil

Question 26

What is a transformer?

Answer 26

A static device which converts electrical power from one circuit to another without changing its frequency.

Question 27

What are the two types of transformers?

Answer 27

Step up transformer
Step down transformer

Question 28

What principles is the transformer based on?

Answer 28

1. An electrical current can produce a magnetic field
2. A changing magnetic field within a coil of wire induces a voltage across the ends of the coil.

Question 29

How does the transformer work?

Answer 29

1. Changing the current in the primary coil changes the magnetic flux that is developed
2. The changing magnetic flux induces a voltage in the secondary coil
3. A simple transformer has a soft iron or silicon steel core.
4. Both the core and the windings are insulated from each other
5. Winding connected to the main supply is called the primary and the winding connected to the load circuit is called the secondary
6. The coil connected to higher voltage is known as high voltage winding while the coil connected to low voltage is known as low voltage winding.

Question 30

Write the equation for the relationship between the number of coils and the voltage.

Answer 30

Question 31

Answer 31

Question 32

Answer 32

Question 33

What is the benefit of high voltage transmission?

Answer 33

High voltage transmission minimises the amount of power lost as electricity flows from one location to the next.

The higher the voltage, the lower the current.
The lower the current, the lower the resistance in the conductors.

When the resistance is low, energy losses are also low.

Question 34

How much power would be wasted in heating the cables in each case, given that 200 km of cable has a resistance of 10 ohms

Answer 34