Electromagnetic Induction (O level Physics)

Posted on Monday, September 26, 2016 by Mysterious QUIZ!

ELECTROMAGNETIC INDUCTION!

Electromagnetic Induction:

When a magnet is moved into and out of the solenoid, magnetic flux is being cut by the coil. The cutting of magnetic flux by the wire coil induces an e.m.f in the wire. When the solenoid is connected to a closed circuit, the induced current will flow through the circuit.

The direction of the induced current and the magnitude of the induced e.m.f due to the cutting of the magnetic flux can be determined from Lenz's Law and Faraday's Law.

Faraday’s Law:

Faraday’s law states that whenever flux linking to a conductor changes, an e.m.f. is induced in the conductor, perpendicular to flux and direction of motion. The rate of change of flux is proportional to the e.m.f. induced. These are the really scientific terms for the law. What is flux anyway? Flux is basically the amount of magnetic field passing through a given surface.

Lenz’s Law:

The direction of the induced e.m.f., and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it.

The diagram shows that when a magnet is moved into the solenoid, the galvanometer deflects, which means there must be current flowing. Presence of current means that an induced e.m.f. is generated in the circuit which drives a current round the closed circuit.

If the magnet remains stationary, there won’t be any current passing and no deflection in galvanometer.

A.c. generator:

The a.c. generator transforms mechanical energy into electrical energy, hence it generates electricity. Generator can be modified to an a.c generator by replacing its commutators with two (separate) slip rings. The two slip rings rotate in tandem with the armature (the rectangular coil). Carbon brushes connect the armature to the external circuit. The armature is initially at the vertical position. No magnetic flux is cut and hence no induced current exists.

When the armature rotates, the change in magnetic flux increases and the induced current increases until its maximum value at the horizontal position. The direction of the induced current can be determined from Fleming's Right Hand rule. Fleming's Right-Hand Rule is used to determine the direction of the induced current that flows from the wire when there is relative motion with respect to the magnetic field.

As the armature continues on its rotation, the change in magnetic flux decreases until at the vertical position, no induced current exists.

Now upon reaching the horizontal position again, the induced current is maximum, but the direction of the induced current flowing through the external circuit is reversed.

The direction of the induced current (which flows through the external circuit) keeps changing depending on the orientation of the armature. This induced current is also known as alternating current. The current is positive (+) in one direction and negative in the other (-). The smooth rings play an important role in the generation of alternating current.

The diagram shows the front view of the armature (A and B shown in the previous diagram). This graph is for one revolution of the coil.

The frequency f of the rotation is related to its period T by the equation:

f = 1 / T

From this equation, we can see that doubling the frequency f means halving the period T.

To increase the induced e.m.f. of an a.c. generator, we can

● increase the number of turns on the coil,

● increase the frequency of rotation of coil,

● use stronger permanent magnet,

● use a soft-iron core.

Transformers:

What is transformer? A transformer is a device that is used to raise or lower down the potential difference of an alternating current. It either increases or decreases the p.d. of an a.c. supply. This is how it looks like:

It consists of the primary coil, the core and the secondary coil.

● The primary circuit is the circuit that connected to the input energy source. The current, potential difference and coil (winding) in the primary circuit are called the primary current (Ip), primary potential difference (Vp) and primary coil respectively.

● The core is the ferromagnetic metal wound by the primary and secondary coil. The function of the core is to transfer the changing magnetic flux from the primary coil to the secondary coil.

● The secondary circuit is the circuit that connected to the output of the transformer. The current, potential difference and coil (winding) in the secondary circuit are called the secondary current (Is), secondary potential difference (Vs) and secondary coil respectively.

Now this is how a transformer works:

1. A transformer consists of a primary coil and a secondary coil wound on a soft iron core.

2. When an alternating current flows in the primary coil, a changing magnetic flux is generated around the primary coil.

3. The changing magnetic flux is transferred to the secondary coil through the iron core.

4. The changing magnetic flux is cut by the secondary coil, hence induces an e.m.f. in the secondary coil.

5. The magnitude of the output voltage can be controlled by the ratio of the number of primary coil and secondary coil.

In a step-up transformer, the e.m.f. of the secondary coil is greater than the e.m.f. of the primary coil. Similarly, in a step-down transformer, the e.m.f. of the secondary coil is less than the e.m.f. of the primary coil. It can be shown that:

Vs / Vp = Ns / Np

where Vs is the secondary output voltage, Vp is the primary input voltage, Ns are the number of turns in the secondary coil and Np are the number of turns in the primary coil.

Ns / Np is called the turns ratio.

Power transfer in a transformer:

Here we’ll consider an ideal transformer that is 100 % efficient, so power of the primary coil is fully transferred to the secondary coil. Hence, the Principal of Conservation of energy is applied, from where the power in the primary coil = power in the secondary coil.

We know the formula P = VI, right?

So we can say that:

(Ip) (Vp) = (Is) (Vs)

where Ip is current in the primary coil, Is is current in the primary coil, Vp is the primary input voltage and Vs is the secondary output voltage.

Now coming to a non-ideal transformer, there will always be power loss, i.e. the efficiency is less than 100 %.

Efficiency = (Output power / Input power ) x 100%

Converting a.c. to d.c.:

We know that the electricity supplied to our homes is in the form of a.c.. But we also know that many appliances require d.c.. So how is a.c. converted into d.c.?

The use of diode is the solution to this! A diode is a semiconductor device that allows a current to flow easily in one direction only. Simple.

When the diode is connected as in Figure A above, where the anode wire is connected to the positive pole and the cathode connected to the negative pole of battery, we say that forward biased diode. A diode will only conduct electricity (turn on a light) when given forward bias.

When a diode is connected with reversed polarity as shown in Figure B, where the cathode wire is connected to the positive pole and the anode connected to the negative pole of battery, we say that reverse biased diode . A diode will not conduct electricity (turn off a light) when given reverse bias.

Half-wave rectification:

Electrical current is supplied to the circuit is an alternating current generated by a transformer. During the positive half cycle of AC, diodes are forward biased so current can flow. Current that flows through the diode to the load (RL) and back toward the transformer. Then the negative AC half cycle, diode does not conduct electric current, because given the reverse bias.

Waveform of the current, then, is as shown below:

Full-wave Rectification:

This is the circuit used for full-wave rectification. The four diodes are connected in series. During the positive half cycle of the input voltage, diodes D1 and D2 will conduct, while D3 and D4 will remain off. The current will take the path ABDEF.

Now during the negative half cycle of the input voltage, diodes D1 and D2 will remain off. The current will follow the path FECDBA.

Cathode-Ray Oscilloscope:

Shown in the diagram is a simple cathode ray oscilloscope.

The device consists mainly of a vacuum tube which contains a cathode , anode , grid , X & Y-plates, and a fluorescent screen . When the cathode is heated (by applying a small potential difference across its terminals), it emits electrons (this process is called thermionic emission). Having a potential difference between the cathode and the anode (electrodes), accelerate the emitted electrons towards the anode, forming an electron beam, which passes to fall on the screen.

When the fast electron beam strikes the fluorescent screen, a bright visible spot is produced. The grid, which is situated between the electrodes, controls the amount of electrons passing through it there by controlling the intensity of the electron beam. The X & Y-plates, are responsible for deflecting the electron beam horizontally and vertically.

The front panel of the CRO looks like this:

The Y-gain of the CRO amplifies the Y-deflection. Amplifying circuits are built into the CRO so that small input voltages are amplified before they are applied to the Y-plates.

Time-base controls the speed at which the electron beam sweeps across the screen horizontally from left to right.

The settings of the CRO are set, for example time base is set at 0.5 ms/div (0.5 millisecond per division) and the Y-gain is set at 1 V/div (1 volt per division).

And this is how the trace will look like:

Here you can be asked to find the frequency and the peak voltage.

We know that frequency f is f = 1 / T.

How many divisions across the x-axis is the wave covering? It’s almost five.

So T = 0.5 * 5 = 2.5 ms

For frequency, we need to change it to seconds.

Now for the peak voltage, the wave is covering two divisions, so it will be V = 1 * 2 = 2V.

Task: Find more questions on CRO for practice.

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