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BackgroundIn the previous topics we found out that it was possible to make a magnet using electric current. We came encountered this concept during magnetization using electrical method. This implies that there is a relationship between electricity and magnetism. In this topic we shall learn more about this relationship.
By the end of this topic, learners should be able to:
- Describe experiments to determine the direction of magnetic field around a current carrying conductor
- Construct a simple electromagnet
- State the factors that affect an electromagnet
- State the factors affecting force on a current carrying conductor in a magnetic field
- Explain the working of a simple electric motor and electric bell
The relationship between electricity and magnetism has long been established, beginning with the work of early scientists such as Michael Faraday. This relationship is today applied in many areas such as industries, generation of electricity, magnetic picking, loading and unloading.
Magnetic effect around a current-carrying conductor
To demonstrate the magnetic effect around a current-carrying conductor
Play the video clip and observe the effect of holding a current-carrying conductor near a plotting compass.
When the switch is open, and the plotting compass is placed below the wire, the needle deflects and tends to be perpendicular to the wire. The direction of deflection reverses with the direction of current. This observation is an evidence that a wire carrying an electric current sets up a magnetic field around itself. The direction of this field depends on the direction of current.
The experiment with the compass needle may be repeated by placing the compass needle above the wire, without changing the direction of current. In this animation, drag the compass needle to the position marked Y so that it is directly above the wire. Close the switch and observe the direction of the needle in relation to the current. What is the effect on the direction of the needle, of moving the compass needle from below the wire to the top?
Direction of magnetic field around a current-carrying conductor
Activity 4.2: To determine the direction of a magnetic field around a current-carrying conductor
Double click on the ON switch to observe the alignment of the compass needles when cell A is inserted and again when it is replaced by cell B.
A piece of soft iron placed in a solenoid constitutes an electromagnet. The piece of iron becomes magnetized and therefore attracts iron filings when current flows through the solenoid.
Activity 4.3: Factors affecting the strength of an electromagnet
In this activity, you will assemble the circuit and proceed to investigate factors that affect the strength of an electromagnet. Observe, in particular (a) the effect of soft iron core (b) increasing current and (c) using more turns of the coil.
Fig. 4.2: Electromagnet
Observations: With only one cell inserted, the solenoid attracts fewer iron filings, compared to two cells, which drive a larger current.
A solenoid with more turns attracts more iron filings than one with fewer turns. Also, the presence of soft iron drastically increases the amount of filings picked.
Conclusion:The following conclusions can be made from the observations. Strength of an electromagnet increases with:
- the amount of current
- number of turns in the solenoid and
- presence of soft iron core.
Force on a current-carrying conductor
Activity 4.4: Investigating force on a current-carrying conductor
In the set-up in Fig. 4.4, two stiff brass rods X and Y are mounted parallel to each other on a plastic (insulator) support. Another brass rod AB is mounted across X and Y. A strong horse-shoe magnet is placed such that AB is between the poles. Observe the effect on the rod AB of closing the switch, reducing current (by increasing resistance), aligning the magnetic field with the rod, reversing the direction of current, and reversing the poles of the magnet.
Observations: From the experiments, it is observed that:
- The brass rod AB experiences a force and rolls along the rods XY when current flows through it.
- When either the direction of the field or current is reversed, the direction of movement of AB also changes.
- When the amount of current is increased, the rod moves faster.
- When magnetic field is aligned with rod AB, the rod remains at the same position.
1. A conductor carrying current in a magnetic field experiences a force and if the conductor is free to move, it does.
2. The size of the force depends on the amount of current.
3. The direction of the force depends on
- the direction of the magnetic field
- the direction of current
4. For a stronger magnet, the force is larger.
The direction of force on the conductor can be predicted by Fleming's Left Hand Rule which states as follows.
If the left hand is held with the thuMb, the First finger and the seCond finger mutually at right angles, so that the First finger points in the direction of the magnetic Field and the SeCond finger in the direction of Current, then the thuMb points in the direction of Motion. This rule is illustrated in Fig. 4.6.
Fig. 4.6: Fleming's Left Hand Rule
Applications of magnetic effect of electric current
(a) Electric bell
Electric bell is one of the important common applications of magnetic effect of an electric current.
Activity 4.5: To study the working of an electric bell
Click on the switch to close it and observe the working of an electric bell. ANIMATION Ph2-400500textANIM1)
Explanation: When the switch is closed, current flows. The soft iron core gets magnetized and attracts the soft iron armature. The iron armature moves with the hammer, which then hits the gong. Meanwhile, this creates a gap at the contact, X, causing the circuit to break. This causes current to stop flowing. The soft iron core loses its magnetism and releases the soft iron armature, which then springs back to remake contact and complete the circuit again. The process is repeated until the switch is opened.
(b) Electric motor
Electric motor is used in many devices such as electric drill, dryers, electric fan and a host of other electric machines.