Class practical

Displaying slow AC on an oscilloscope.

Apparatus and Materials

For each student group

• Low frequency AC generator with battery – see technical note
• Leads, 4 mm, 4

• Oscilloscope

Health & Safety and Technical Notes

A low frequency AC generator consists of a coil of resistance wire with a rotating pair of contacts. A smooth low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.

Two or three (fresh) 1.5 V cells will probably do, but it would be wise to disconnect them as soon as the experiment is over.

The low frequency generator works well. If it gets dirty, a few drops of thin oil on the brushes will ensure continuous contact.

A signal generator capable of giving low frequency (0.1 Hz) output is a suitable alternative.

Procedure

1. Set the AC-DC switch on the oscilloscope to DC, the time-base to off, and the Y-gain to about 1.
2. Connect 2 - 4 volts across the low frequency AC generator. Connect the output terminals to the input of the oscilloscope.
3. Turn the generator by hand to show the spot moving up and down.
4. Switch on the time-base to its slowest speed on range 1\. Turn the generator as steadily as possible. Turn up the time-base speed and repeat this, increasing the rate at which the generator is turned.

Teaching Notes

Rotating the generator at a steady speed results in a sinusoidal output voltage. (This is an example of the link between circular motion and simple harmonic motion.)

This experiment was safety-tested in July 2007

A video showing how to use an oscilloscope:

A video showing how to use a signal generator:

 

Demonstration

Displaying slow AC using a voltmeter. This is an impressive teacher demonstration if performed using large-sized meters.

Apparatus and Materials

• Low frequency AC generator with battery – see technical note
• Voltmeter (± 5 V), DC
• Voltmeter, AC
• Power supply, low voltage, variable (for motor)
• Electric motor, small
• Rubber band
• Leads, 4 mm, 6

• Oscilloscope

Health & Safety and Technical Notes

Read our standard health & safety guidance

A low frequency AC generator consists of a coil of resistive wire, with a rotating pair of contacts. A low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.

Procedure

1. Connect the battery to the DC terminals of the generator, and the voltmeter to the AC terminals. Set the pointer of the voltmeter centrally.

2. Turn the generator faster and faster, so that the amplitude of the pointer's movement gets less and less. Then replace the DC meter with an AC meter.

3. Use a motor to drive the generator, using an elastic band as the driving belt. The low-voltage variable power supply can be used to drive the motor, first at low speed and then at high speed.

4. Show how the output from the AC generator, driven by the small motor, can be shown on the oscilloscope.

Teaching Notes

The generator is turned slowly so that the meter's pointer can be seen oscillating in time to the motor rotation. When the motor turns the generator too quickly, the pointer on the meter is unable to keep up. This is the time to use an AC meter.

This experiment was safety-tested in October 2006

Class practical

This experiment shows that current and voltage are in phase for an ohmic resistor.

Apparatus and Materials

• Low frequency AC generator with battery – see technical note
• Voltmeter (± 5 V), DC
• Milliammeter, DC
• 1 kΩ. resistor (0.5 W)
• Leads, 4 mm, 7
• Crocodile clips, 2

• Oscilloscope

• Diode IN4001 (OPTIONAL)

Health & Safety and Technical Notes

A low frequency AC generator consists of a coil of resistance wire, with a rotating pair of contacts. A smooth low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.

Read our standard health & safety guidance

Procedure

1. Connect 5 V across the input of the low frequency AC generator. Connect the output terminals to the DC voltmeter. The pointer of the meter should be set centrally (for this reason a 2.5-0-2.5 volt dial is ideal, but not essential).
2. Connect the output from the generator in series through the milliammeter and the 1 kΩ. resistor. Crocodile clips can be used for connecting the resistor. Care should be taken to ensure that the meters are connected with the same polarity.
3. Turn the generator by hand, at about 1 turn per 5 seconds, and observe the voltage and current phases. Then speed up the generator a little; the phases remain the same.

4. Replace the voltmeter with the oscilloscope. The time-base of the oscilloscope should be switched off, the AC-DC switch put in the DC position, the gain set to 5 div./volt, and the Y-shift adjusted so that the spot is in the centre when the galvanometer reads zero.
5. Turn the generator by hand at speeds of 1/2 to 1 rev/second and observe the phases.

Teaching Notes

• This demonstration with a resistor in series with an ammeter and the slow AC generator shows that the phase of the current is the same as the phase of the supply voltage.
• In step 4 it is easier to see any phase difference if the face of the ammeter is put side by side with the oscilloscope screen. The ammeter should be turned so that the needle moves up and down parallel to the spot.
• A diode can be connected in series with the resistor; this will show that, even though the voltage across the resistor varies through a full AC cycle, the current is always positive.
• Increasing the speed of rotation of the generator shows the effect of the inertia of the galvanometer needle compared with that of the electron beam.

This experiment was safety-tested in October 2006

• A video showing how to use an oscilloscope:

Demonstration

This experiment shows that, for a capacitor, there is a phase difference between current and voltage.

Apparatus and Materials

• Low frequency AC generator with battery – see technical note
• Voltmeter (5 volt), DC
• Milliammeter, DC
• Electrolytic capacitor (100 µF, 50 V)
• Leads, 4 mm, 7
• Crocodile clips, 2

• Oscilloscope

Health & Safety and Technical Notes

Electrolytic capacitors should normally be used only with direct voltages with the correct polarity. However, they work well in these experiments with very low frequency AC. What matters is the allowable ripple current; the currents in this circuit are much lower than the ripple current specified.

A low frequency AC generator consists of a coil of resistance wire, with a rotating pair of contacts. A smooth low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.

Read our standard health & safety guidance

Procedure

1. Connect 4.5 V across the input of the low frequency AC generator. Connect the output terminals to the DC voltmeter. The pointer of the meter should be set centrally (for this reason a 2.5-0-2.5 volt dial is ideal, but not essential).
2. Connect the output from the generator in series through the milliammeter and the 100 μF capacitor. Crocodile clips can be used for connecting the capacitor. Care should be taken that the meters are connected with the same polarity.
3. Turn the generator by hand at about 1 turn per 5 seconds and observe the voltage and current phases. Then speed up the generator a little; the phase difference remains the same.

4. Replace the voltmeter with the oscilloscope. The time-base of the oscilloscope should be switched off, the AC-DC switch put in the DC position, the gain set to 5 div./volt, and the Y-shift adjusted so that the spot is in the centre when the galvanometer reads zero.

Teaching Notes

• In step 2, the generator and a capacitor are in series with an ammeter. This enables students to observe the phase difference between the current in the circuit and the voltage on the generator. The current to the capacitor leads the voltage by quarter of a cycle or 90°.
• In step 4, the phase difference is easier to see if the meter is held on its side near to the oscilloscope - this has its time-base turned off.
• An electric current does not flow through a capacitor. Charge flows onto one capacitor plate, and the electrostatic force repels charge off the other plate and on round the circuit back to the generator.

This experiment was safety-tested in January 2007

• A video showing how to use an oscilloscope:

Demonstration

A good way to show phase differences in capacitive and inductive circuits.

Apparatus and Materials

• Inductor, e.g. 12 000 turn coil on laminated core
• Electrolytic capacitor (500 µF, 15V), see technical note 1
• Low frequency AC generator with battery, see technical note 2
• DC milliammeter (±2.5 mA.)
• DC voltmeter (±5 V)
• Leads, 4 mm, 7
• Crocodile clips, 2

Health & Safety and Technical Notes

• Electrolytic capacitors should normally be used only on direct voltages of the correct polarity. However, they may work well in these slow AC experiments. The lack of any direct polarizing voltage may cause some deterioration of the dielectric in these capacitors. It is a wise precaution to form the plates before and after use. This is done by connecting the capacitors to DC with a voltage less than or equal to the working voltage of the capacitor and of the correct polarity.
• A low frequency AC generator consists of a coil of resistance wire, with a rotating pair of contacts. A smooth low-voltage DC supply is connected across the coil of the generator. The metal brushes rotate in contact with the coil and are connected to the AC output terminals, giving an alternating output.

Procedure

1. Connect 3 V, DC across the input of the low frequency AC generator. Connect the voltmeter across the output. Put the output also through the milliammeter in series with the 500 μF capacitor. Set the pointers of the meters to their centre positions.
2. Turn the generator slowly and show the phase difference.
3. Repeat with the inductor in place of the capacitor. Observe the phase difference.

Teaching Notes

• This demonstration shows that the current to a capacitor leads the supply voltage by 90 degrees. The current through the inductor lags behind the supply voltage. If it is a very large inductor, then this lag will be about 90 degrees. However all inductors have some resistance and the lag may well be less than 90 degrees because the effects of the resistance and the inductance interact.
• A circuit with resistance, capacitance and inductance could be set up for demonstration, but the phase difference would then be a complicated vector sum.
• With a resistor, both V and I are positive at the same time and negative at the same time, and so the power at any instant is always positive, Vrms x I rms. With a capacitor or an inductor, the current and voltage are not in phase, and in extreme cases the power might be zero. The energy just surges in and out of the magnetic field of the inductor, or piles up in the field between the plates of the capacitor and then comes pouring back again. An inductor and capacitor connected in parallel will become an oscillator. If interested, look at a tuned circuit such as in a radio.

This experiment was safety-tested in January 2007