CHAPTER 3: Electromagnetic Effects

By the end of this chapter, you will be able to:

1. investigate the behaviour of magnets and magnetic fields.
2. understand that a current carrying conductor produces a magnetic field that can be detected.
3. understand the application of electromagnets in devices such as motors, bells and generators.
4. understand the difference between a.c and d.c. know how a.c and d.c can be interconverted using rectifiers and inverters.
5. understand the action and applications of transformers.

Keywords

conductor

current carrying conductor

generators

magnetic field

motor

transformers

3.1: Introduction

In S.2, you learnt about magnets and their properties. You also learnt about the different methods of making magnets. One such method is to use electricity to make magnets. This means that magnetism and electricity are related in some way. Did you know that magnets can also be used to make electricity? In this chapter, you will be able to understand the relationship between magnetism and electricity and how this can be applied in different appliances.

3.2: Magnetic Fields

A magnetic field is a region around a magnet where a magnetic force of attraction or repulsion can be felt by another magnetic substance. Magnetic fields are represented by magnetic field lines, which indicate the path and direction, which a small North-pole would follow freely under the influence of the field. Magnetic field lines originate from a North-pole and terminate at a South-pole.

DID YOU KNOW?

Did you know that the Earth behaves as a magnet by attracting objects towards itself? 3.2.1: Drawing Magnetic Field Lines

Figure 3.1: Magnetic field lines around a bar magnet

From the diagram in Figure 3.1, note that the magnetic field lines:

1. have arrows on them to show the direction of the field at any point.

2. come out of N (north-pole) and go into S (south-pole).

3. are more concentrated at the poles.

DID YOU KNOW?

Did you know that magnetic field is strongest at the poles, where the field lines are most concentrated? The magnetic field line patterns between two magnetic poles are shown in Figure 3.2.

Figure 3.2: Magnetic field lines between two magnetic poles

Note the different magnetic field line patterns seen when two like poles are used and when two unlike poles are used.

Research 3.1

As an individual, search on the internet for and watch videos on magnet field lines formed by different shapes of a magnet. Basing on what you have watched, draw magnetic fields due to different shapes of a magnet.

3.3: Magnetic Field of a Current Carrying Wire/ Conductor

Activity 3.1 Identifying magnetic field on a conductor carrying current

Key question: Sketch the magnetic field lines around current carrying conductors?

What you need

A bare wire carrying current (make sure the Iron fillings current is from an electric cell not mains current) • A piece of paper

What to do

1. Pass a bare wire carrying current through a horizontal piece of paper as shown in Figure 3.3. Piece of paper Current carrying conductor Figure 3.3:

Figure 3.3: Paper perpendicular to a current carrying conductor

2. Sprinkle iron filings onto the paper.

3. Gently tap the paper to spread out the filings.

4. Observe and record the results on how the iron filings distribute themselves around the conductor.

3.3.1: Direction of the Magnetic Field due to a Current Carrying Conductor

Activity 3.2 Determining the direction of magnetic field of a current carrying conductor

Key question:

Sketch the magnetic field due to a current carrying conductor for two opposite directions of current.

What you need

Any wire or a straight conductor (but not with current) or a pen or a pencil

NOTE: In this activity, you must just assume that current is flowing through the conductor but when the conductor is actually not carrying current.

What to do

1. Grip (hold firmly) the conductor as shown in Figure 3.4 using your right hand. Electric current

Figure 3.4: The right hand grip rule

2. Assuming that electric current is flowing through the conductor in the direction indicated in Figure 3.4, in what direction is the thumb pointing?

3. In what direction are the fingers coiling, clockwise or anti-clockwise?

4. Using the convention that represents current flowing out of the paper as and that the convention that represents current flowing into the paper as, draw on a paper the direction of coiling of your fingers around a current carrying conductor. Indicate the direction of coiling of the fingers with arrows. 5. Deduce what the direction of coiling of your fingers signify.

6. Reverse the assumed direction of current flow through the conductor and repeat procedures 2 to 5.

7. What scientific conclusion can you draw from this activity?

What to do

1. Tie two connecting wires at two separate points A and B on the straight metal conductor. Make sure that the ends of the connecting wires tied to the conductor are bare, without any insulation.

2. Suspend the metal conductor from the clamp using the connecting wires and connect the ends of the wires to a circuit as shown in Figure 3.6.

Figure 3.6: Investigating the direction of the force on a current carrying conductor in a magnetic field

3. Move the horseshoe magnet until the metal conductor is between its poles. Close switch K and observe.Reverse the poles of the horseshoe magnet and repeat procedures 3 and

4. What scientific conclusion can you draw?

The direction of the force on a current carrying conductor in a magnetic field is always right angles to the plane containing both the conductor and the magnetic field. You can use the first finger, second finger and the thumb of your left hand to determine the direction of the force on a current carrying conductor in a magnetic field as shown in Figure 3.7.

Figure 3.7: Determining the direction on a current carrying conductor

You must ensure that the first finger, second finger and the thumb of your left hand are at right angles to each other. Let your first finger point in the direction of the magnetic field and the second finger point in the direction of electric current Then your thumb will point in the direction of current flow.

DID YOU KNOW?

Did you know that Fleming’s left-hand rule states that if the first finger, the second finger and the thumb of the left hand are placed at right angles to each other with the first finger pointing in the direction of the magnetic field and the second finger pointing in the force on the conductor? the direction of current flow, then the thumb points in the direction of force on a carrying conductor

EXERCISE 3.1 With reference to Figure 3.7, what physical quantities are represented by the letters F, B and I?

ASSIGNMENT 3.1 Carry out experimental activities to investigate the factors that determine the magnitude of the current conductor in a magnetic field.

3.5: The Electric Motor

An electric motor is an electric device which converts electrical energy into mechanical energy. It applies the principle of force on a current-carrying conductor in a magnetic field. The motor consists of a coil that is free to rotate about an in-axis, in a magnetic field created by two poles of permanent magnets as shown in Figure 3.8

Figure 3.8: A simple motor

When electric current is made to flow through the coil, the coil becomes a current- carrying conductor in a magnetic field. Therefore, the coil experiences forces, F, on opposite sides as shown in Figure 3.8. These two forces are equal and opposite. They cause the coil to rotate about an axis through the centre of the coil. The axis of rotation of the coil is connected to a system of gears, which transfer the rotational motion to run other equipments.

ASSIGNMENT 3.2

1. Look at common electrical appliances in your community and identify those that need an electric motor for their operations.

2. Describe how the electric motor is used in the operations of such devices identified in (1) above.

3.6: Electromagnetic Induction

In the previous section of this chapter, you learnt that when electric current is flowing in a conductor, it creates a magnetic field around the conductor. Likewise, when a conductor is made to move through a magnetic field such that it cuts across magnetic field lines, an electric current is induced in the conductor. The process of producing magnetic fields from an electric current or producing an electric current from a changing magnetic field is called electro-magnetic induction.

DID YOU KNOW?

Did you know that electric devices in which temporary magnets are produced by electric currents are called electro-magnets?

Activity 3.4 Making an electro-magnet

Key question: How can you make an electro-magnet?

What you need

A copper wire A nail What to do A dry cell and Staple wires

1. Connect the ends of the wire tightly to the terminals of the dry cell as shown in Figure 3.9.

Figure 3.9: Making an electro-magnet

2. Make sure that the wire is tightly connected to the positive and negative terminals of the cell as shown in Figure 3.9.

3. Bring your staple wires near the nail.

4. What do you observe?

5. Remove the staple wires from the nail.

6. Reduce the number of windings on the nail to about half the original number and repeat procedures 3 to 5.

7. Disconnect the wire from the terminals of the cell and repeat procedures 3 to 5.

8. What general conclusion can you draw basing on your observations?

3.6.1: Some Applications of Electromagnets

Electric bell

Project work Act

1. Study the picture shown in Figure 3.10. Identify the materials used and search for these materials. Hence, design and construct an electric bell. Test it for its functionality and present it to the rest of the class.

Figure 3.10: Making an electric bell

2. Explain to the class how the electric bell works.

ASSIGNMENT 3.3

Read and make short notes on the application of electromagnets in:

(i) Loudspeakers.

(ii) Relay telephone receivers.

3.7: Generators

DID YOU KNOW? Did you know that an electric generator converts mechanical energy into electrical energy?

Activity 3.5 Creating current by electro-magnetic induction

Key question: How do generators produce electricity?

What you need • 10 m of copper wire (SWG 30) • A dry cell A milliammeter/Voltmeter Connecting wires A magnet What to do

1. Wind the copper wire provided on the dry cell so that it makes several turns.

2. After making a coil with many turns (you can count them), remove it from the dry cell.

3. Connect the ends of your coil to the terminals of the milliammeter (voltmeter) using connecting wires as shown in Figure 3.11.

Figure 3.11: Creating current by electro-magnetic Induction

4. Move the magnet to and fro inside the coil as you observe what happens to the pointer of the milliammeter. (If there is no change, increase the speed of movement of the magnet in and out of the coil).

5. Note down what you observe on the milliammeter.

6. Generate a scientific deduction using your observations.

In 1831, Michael Faraday invented the first electric current by moving a conductor through a magnetic field. Faraday noted that an e.m.f is induced in a:

a) Conductor if the conductor cuts through magnetic field lines.

b) Coil if the magnetic flux through the coil is changing. Faraday’s observations are summarised in a statement commonly referred to as Faraday’s law, which states that:

“The magnitude of the e.m.f induced in a coil is directly proportional to the rate of change of magnetic flux density through the coil”.

Project work Design and perform simple experiments to verify the observations made by Michael Faraday.

EXERCISE 3.2

Figure 3.12: Commonly used generators in Uganda

Look at the generators shown in Figure 3.12.

1. Describe how the generator is able to generate electricity.

2. What type of fuel is poured in the tanks shown on the top of the generators?

3. What is the purpose of this fuel in the generator?

4. For what purpose do most people in your community use electricity?

5. Mention some of the other sources of electricity that the people in your community use.

3.7.1: Types of Generators

There are two types of generators namely, a.c generators and d.c generators.

1. a.c generators produce electric currents whose direction of flow and magnitud vary continuously with time. a.c stands for alternating current and its variation against time is shown in Figure 3.13.

Figure 3.13: An a.c plotted against time

2. The electric currents produced by d.c generators flow in the same direction a all times but the magnitude of the current changes continuously with time. d. stands for direct current and its variation against time is shown in Figure 3.14

ASSIGNMENT 3.4

Search on the internet for a short video clip showing how the a.c generator works and how the d.c generator works. Hence, make short notes on how the generators work.

3.7.3: Rectifiers and Inverters

Rectifiers are electric devices that convert alternating current (a.c) into direct current (d.c). They are made of semi-conductor elements e.g. diodes. Various combinations of the diodes can result in half rectification or full rectification of alternating currents. Inverters are electronic devices that are used to convert direct current (d.c) into alternating current (a.c).

ASSIGNMENT 3.5

With reference to available textbooks or the internet, research on the different types of rectification and make short notes.

ASSIGNMENT 3.6

1. List all the electrical appliances that you know, which are used at home, school, hospital or industry and classify them according to whether they operate on a.c or d.c.

2. What are the advantages of a.c over d.c and vice versa?

3.8: The Transformer

A transformer is an electric device used to step-up or step-down voltages in an electric circuit. Step-up means a transformer can increase a low voltage to a high voltage. While step-down means a transformer can reduce a high voltage to a low voltage.

Figure 3.16: A transformer

DID YOU KNOW? Did you know that most electrical devices have got transformers in their electrical circuits?

Research 3.2

Using the internet or textbooks, find out other uses of transformers. 3.8.1: Components of a Transformer The basic parts of a transformer are core, the primary winding and secondary winding as shown in Figure 3.17.

The core is a magnetic material onto which the primary and secondary coils are wound. The primary winding is the coil on which the input voltage is connected while the secondary winding is the coil from which the output of the transformer is connected. The turns-ratio of a transformer is the ratio of the number of turns in the secondary winding to the number of turns in the primary winding i.e. Turns Ng ratio = N

3.8.2: Types of Transformers

There are two types of transformers namely: step-up transformers and step-down transformers. A transformer steps up or steps down voltage according to its turns- ratio. Therefore, in a step-up transformer, the number of turns in the secondary winding is greater than the number of turns in the primary winding whereas in step-down transformers, the number of turns in the secondary winding is less than the number of windings in the primary windings as shown in Figure 3.18.

ASSIGNMENT 3.7

Search in available textbooks or on the internet and make simple notes on the principle of operation of a transformer.

EXAMPLE 3.1

1. A transformer has 600 turns of the primary winding and 20 turns of the secondary winding. Determine the secondary voltage if the secondary circuit is open and the primary voltage is 140 V.

2. A transformer has a primary coil with 1600 loops and a secondary coil with 1000 loops. If the current in the primary coil is 6 amperes, then, what is the current in the secondary coil?

3.8.3: Efficiency of a Transformer

DID YOU KNOW?

Did you know that the efficiency of the transformer is defined by: Efficiency (m): output power = input power × 100% In current electricity, electric power is equal to VI, where V is voltage and I is current. Therefore, Efficiency(n) = Where Is Vs x 100% IV P p current in secondary coil voltage in secondary coil current in primary coil voltage in primary coil

EXERCISE 3.3

Calculate the power in the primary coil and secondary coil, given that the current flowing in primary coil is 0.8 A and the voltage is 480 V while in the secondary coil the current being 8 A, and voltage is 48 V.

(i) What type of transformer is this?

(ii) Find the turns-ratio of the transformer.

(iii) What is the efficiency of the transformer?

(iv) If the transformer was to be 80 eificient, determine the current and voltage at the output terminal.