Sang Lee

Lab 35

4/22/14

Title: Electromagnetic Induction

Objective: To characterize how the electromagnetic induction occurs in solenoids, and how two solenoids together can form a transformer, a device for transferring electromagnetic energy from one circuit to another. We will also see how the presence of an iron core within the solenoid greatly enhances the magnetic flux through the solenoid, and thus increases the emf generated.

Procedure:

observe the effect of moving a permanent magnet near the core of a passive solenoid choose a solenoid from the kit and connect the two inputs to the inputs of the

galvanometer The galvanometer is nothing more than a sensitive current measuring device. Note that

there is no power supplied in this setup. Move the permanent magnet in and out of the solenoid. Note the deflection direction.

What happens if you reverse the orientation of the permanent magnet? Why? How does the number of windings in the coil affect the magnitude of the deflection How does the speed of movement of the permanent magnet affect the magnitude of the

deflection What happens when you leave the permanent magnet stationary inside the core of the

solenoid The setup for Part II is a little different; instead of generating the magnetic flux using a

permanent magnet as we did above, we will use an electromagnet Secondary coild will be placed nearby, and because magnetic flux is passing through it,

and emf will be generated. We will use a function generator as an AC power supply. Set the frequency of the

function generator to 60 Hz (the frequency as regular utility outlets Set the output voltage to about 6 VAC (note this as an open circuit voltage, when the

circuit is closed, this value will decrease). Position two 400 turn coils as in Scheme A of Figure 2. Attach your function generator

across the primary along with a multimeter to measure the input voltage. Also place a multimeter across the secondary to measure the output voltage (emf). Record the input and output voltage for Scheme A. Record the input and output values for Schemes B-E Set up your transformer as in Scheme D again with the 400 turn coil as primary.

Using this square core arrangement, measure the output to input voltage ratio for each of the possible choices of secondary coils in the set provided (200, 400, 800, 1600 and 3200 turns).

Plot this ratio vs. the ratio the number of turns in the secondary to that in the primary Set up your transformer with the two 400 turn coils with the E-shaped cores as in Scheme

E above. Using the resistance box, complete the circuit on the secondary side using a 10 Ohm resistance (so that current can flow on the secondary side).

Set up the multimeters on the secondary side with one measuring AC current and the other AC voltage

Measure the voltage output and current output with several different coils as secondary: the 400, 800, 1600, and the 3200.

Data:

Primary and Secondary are 400 coil solenoids

Input (V)

Output (V)

Scheme A

0.268 0.01

Scheme B

0.535 0.268

Scheme C

1.032 0.35

Scheme D

2.1 1.42

Scheme E

3.47 3.08

Primary is 400 coil solenoid

Solenoid Input (V) Output (V) Output/Input Secondary/Primary200 2.00 0.634 0.317 0.5400 2.1 1.42 0.676190476 1800 2.79 2.08 0.745519713 2

1600 5.4 2.02 0.374074074 43200 18.93 2.99 0.157950343 8

Primary:400

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80123456789

f(x) = − 7.90455361740611 x + 6.68982869022262R² = 0.415126828408006

Ratio of Output/Input vs. Ratio Turns in Secondary/Turns in Primary

Ratio of Turns in the Secondary to Turns in the Primary

Ratio

of O

utpu

t to

Inpu

t Vol

tage

The results we got were bad because the graph is skewed. The expected relationship between the two ratios is that with increased voltage on the secondary compared to the primary, the output voltage to input voltage ratio should increase. Therefore, there should be a linear relationship with a positive slope to the right.

Secondary Solenoid Current (A) Voltage (V)400 .057 .598800 .041 .429

1600 .020 .2183200 .010 .113

Questions:

Part I.

When the magnet was put in the solenoid core, the reader on the galvanometer went to the right. When the magnet was pulled up out of the solenoid core, the reader on the galvanometer went to the left. This happened because the bar magnet has inherent dipoles and how it affects the magnetic field around the solenoid. Moving the magnet in and out of the solenoid induces changes in the solenoid's magnetic field.

Deflection increased with increasing number of windings, since the greater number of coils, the greater magnetism is generated.

Because higher number of coils means greater magnetism generated, deflection increases with increasing number of windings. A proportional comparison.

As the speed of the movement of the permanent increases (faster), the greater the magnitude of the deflection

When you leave the permanent magnet stationary inside the core of the solenoid, the galvanometer reads 0.

Part II

By recording the input and the output for Scheme A and then moving the coils further apart, the output voltage went to zero.

The magnetic field in the primary induces a magnetic field in the secondary. The magnetic field in the primary does not fully transfer to the secondary.

The core arrangement of Scheme E provided the highest output because it has the biggest iron core where the magnetic field can flow through to.

After running current through the primary for a few minutes, it might heat up because the wires have a small resistance value within them. The primary coil receives the most current and will thus heat up before the secondary coil will.

Ns/Np = εs/εp A step-down transformer :Ns/Np ratio of less than one. A step up transformer: Ns/Np ratio of more than one. It is better to leave a cell phone charger unplugged when not in use because even without

a phone plugged in, it still uses some power when plugged in. This continuous use will wear down and shorten the lifespan of the transformer in the charger.

% Difference = Abs(measured-ideal)/(ideal)*100 Percent Loss Scheme A = Abs(0.01-.268)/(.268)*100 = 96.3% Percent Loss Scheme B= 49.9% Percent Loss Scheme C= 66.1% Percent Loss Scheme D=32.4% Percent Loss Scheme E= 11.2% Scheme E is the most efficient and had the smallest percent error. This may be due to the

solenoids being in close proximity and the iron core conducting the magnetic field is the largest.

Current as we increase the amount of stepping of our transformer decreases because power remains the same. For the same amount of power with increased voltage, current must decrease. A greater change in magnetic flux goes through the primary that later gets diminished through losses during transfer to the secondary, which is why it heats up first.

The primary voltage increased from 1.081 V to 2.14V once the secondary was disconnected because back emf was reduced. Back emf is the electropotential force that opposes the current that induces it. Back emf is never larger than the original emf so it only reduces the primary voltage partially.

Conclusion:

Overall, this lab was interesting to look at. One thing that I specifically learned was that it is possible for current to flow in a wire without a power generator or supply (as learned previously). This is possible by for current to flow just by waving a magnet near the wires. As to say step-down transformers have fewer turns in the secondary coil while step-up transformers have more turns in the secondary coil. An incomplete transfer between the primary and secondary coil can reduce magnetic field in the secondary coil. Doing the lab was boring at times, but it was still a good experience.