Introduction
A bar magnet produces magnetic fields which are believed to follow prescribed magnetic lines. In the previous experiment, the application of the magnetic field in the operation of a transformer was observed. In the transformer, two coils which are electrically isolated develop a magnetic flux in the primary coil. The magnetic flux, in turn, induces an electromagnetic force (emf) on the secondary coil. The emf was registered as a voltage in the galvanometer. However, in the experiment, the magnets were stationary and only the magnetic flux moved between the coils. In the current experiment, I seek to establish the behavior of the coil when the magnet is moved inside the coil (solenoid) and in the absence of an electric current or power source. The phenomenon where an emf is induced due to either the movement of the magnet or magnetic flux is referred to as the electromagnetic induction. According to Magnet Academy, the phenomenon was first discovered by Michael Faraday in 1831. The finding by Faraday was very important as it was the experiment to prove that electricity could form from magnetism. The experiment includes a solenoid or coil, a bar magnet, and a galvanometer. The experiment follows the First Law of Faraday’s electromagnetic induction which states that “whenever a conductor is placed in a varying magnetic field emf are induced which is called induced emf, if the conductor circuit is in a closed circuit, a current is also induced which is referred to as the induced current” (DAE Notes). The emf induced is described by the equation:
emf=-N ∆BA/Δt
Where; N = number of turns
B = External magnetic field
A = Area of the coil
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The negative sign indicates that the voltage is induced as a negative of the rate of change of magnetic flux. In a closed circuit, the current changes direction relative to the inward or outward direction of the bar magnet. There are various factors which determine the strength of the current induced in the Faraday’s coil. The factors include the speed at which the bar magnet is moved past the coil, the strength of the bar magnet, and the number of turns of the coil. The objective of the current experiment is to generate a voltage in a voltage by moving a bar magnet inside the coil. Furthermore, the experiment will seek to maximize the power or voltage obtained from the coil and magnet combination.
Material
A strong bar magnet with inscribed polarity
Galvanometer or voltmeter
A pre-coiled wire with 10, 20, 30, 40 and 50 turns.
Connecting wires
Procedure
The circuit was initially connected according to the set-up shown in Figure 1 shown below. The 30 turns coil was initially chosen.
Secondly, the magnet was slid inside the coil as shown in Figure 1 from the end of S Pole. The strength and sign of the induced emf were measured and recorded. The direction of the bar magnet was then changed such that it slid the coil on the N pole. The strength and the sign (polarity) of the emf were measured and recorded. The magnet was then slid up and down repeatedly inside the coil and the behavior of the galvanometer observed. The 30 turn coil was then replaced with the 10 turns coil and two measurements of the induced emf were measured. This was repeated for the 20 turns, 40 turns, 50 turns coils. All the measurements were recorded. The measurements were used to plot a graph of emf vs turns.
Results
Magnet moved through coil in the S pole direction
Going inside the coil: –
Maximum voltage: 0.007V
Polarity (+/-): Positive
Going out of the coil:
Maximum voltage: 0.007V
Polarity (+/-): Negative
Magnet moved through coil in the N pole direction
Going inside the coil:-
Maximum voltage: 0.007V
Polarity (+/-): Negative
Going out of the coil:-
Maximum voltage: 0.007V
Polarity (+/-): Positive
Discussion
At 30 turns, the strength of the magnet was 0.007V. It was noted, however, that the direction of the magnet determined the polarity of the voltage. When the magnet entered the coil through S pole, then a positive polarity was obtained. Similarly, a negative polarity was obtained when removing the magnet. The polarities changed when the magnet entered the coil through the N pole. The change of polarity partly confirms First Faraday’s Law of electromagnetic induction. According to Faradays’s law, the negative sign indicates that the voltage is induced as a negative of the rate of change of magnetic flux. The graph of emf against the number of turns indicates that the induced voltage is directly proportional to the number of turns in the coil (Byju’s). Therefore, as the number of coil increases, then the induced voltage increases. As such, it is evident that the increasing number of turns can be used to maximize the induced voltage. Another aspect (not measured in this experiment) that can maximize the induced voltage is increasing the strength of the magnet. Another factor that can maximize the induced voltage is the speed of passing the magnet over the coil. In a more sophisticated experiment, the relationship between the induced voltage and the speed of passing the magnet over the coil or the strength of the magnet can be demonstrated.
- Byju’s. Electromagnetic Induction- Definition | Principle | Examples. https://byjus.com/physics/electromagnetic-induction/. Accessed 6 Dec. 2017.
- Chabot Space & Science Center. Magnet, Coil, and Meter: Generating Electricity. http://www.chabotspace.org/assets/BillsClimateLab/Electricity%20Lab%20-%20Magnet%20and%20Coil.pdf
- DAE Notes. Faraday’s law’s of Electromagnetic Induction. http://www.daenotes.com/electronics/basic-electronics/faraday-laws-of-electromagnetic-induction
- Magnet Academy. Electromagnetic Induction – MagLab. https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/electromagnetic-induction. Accessed 6 Dec. 2017.
