Week 6

In order for you to gain the most value from your qualification and to prepare you for your assessment and the industry, make sure you complete all of the online and SDL tasks.

What we're covering:

• varying the magnetic field
• slip rings
• sine waves

Begin this topic by reading about Electric Circuits on pages 1-3 at the BBC Bitesize site.

Electric Circuits

AC or DC generators apply the same principle to generate electric current - current will be induced in a conductor inside a magnetic field when there is a relative motion between the conductor and electric field.

There are two ways to vary the magnetic field acting on the conductors - either by rotating the magnetic field around a stationary conductor or spinning the conductor inside a stationary magnetic field. In both cases, an electric current is induced in the conductor.

The AC generator is designed to generate alternating current with a frequency of 50 or 60 Hz. The current induced in the armature is supplied to the circuit using slip rings.

The slip rings are two separate conducting rings connected to each terminal of the armature. They maintain constant contact with the same sides of the coil.

Slip rings supply current to the circuit through a sliding contact called brushes. If we connect two terminals of an external load with these two brushes, we will get an alternating current in the load. Brushes make continuous contact between the external circuit and the slip rings.

Note - the rings always rotate with the armature while the brushes remain stationary.

This video gives a very straightforward explanation of the working principle of AC generator. Watch it a couple of times if necessary to ensure you fully understand the mechanism. Working Principle of AC Generator

Developing the Sine Wave

As the armature rotates, each half cuts across the magnetic lines of force at the same speed. Thus, the strength of the voltage induced in one side of the armature is always the same as the strength of the voltage induced in the other side of the armature. Each half of the armature cuts the magnetic lines of force in a different direction. For example, as the armature rotates in the clockwise direction, the lower half of the coil cuts the magnetic lines of force from the bottom up to the left, while the top half of the coil cuts the magnetic lines of force from the top down to the right. Therefore, the voltage induced in one side of the coil is opposite to the voltage induced in the other side of the coil. The voltage in the lower half of the coil enables current to flow in one direction, and the voltage in the upper half enables current to flow in the opposite direction.

However, since the two halves of the coil are connected in a closed loop, the voltages add to each other. The result is that the total voltage of a full rotation of the armature is twice the voltage of each coil half. This total voltage is obtained at the brushes connected to the slip rings and may be applied to an external circuit.

This diagram shows four different positions of the coil in a simple AC generator, and the potential difference produced at each position.

Self-directed Learning

Look at the glossary page in the menu and download the worksheet. You should make sure you know the words listed in the glossary.

Complete and submit any outstanding assessment work.

What we're covering:

• impedance, capacitance and inductance

In a DC circuit the opposition to current flow is called resistance (R) and measured in ohms, Ω.
In an AC circuit, we talk about impedance (symbol Z) which is a combination of both resistance and reactance. Impedance is similar to resistance, but it also takes capacitance and inductance into account.

• Resistance (R) is the slowing of current due to effects of the material and shape of the component. This effect is largest in resistors, but all components have some resistance.
• Reactance (X) is the slowing of current due to electric and magnetic fields opposing changes in the current or voltage. This is most significant for capacitators and inductors.

The magnitude of the impedance Z of a circuit is equal to the maximum value of the potential difference, or voltage, V (in volts) across the circuit, divided by the maximum value of the current I (measured in amperes) through the circuit, or simply:

Impedance is measured in ohms, Ω.

Simple AC circuits involving nothing more than an AC power source and resistance, obey the same simple laws and rules as DC circuits. However, AC circuit measurements and calculations become very complicated when we factor in inductance and capacitance.

Measurements of AC voltage and current must be expressed in the same terms (peak, peak-to-peak, average, or RMS). For instance, if the source voltage is given in peak AC volts, then all currents and voltages subsequently calculated must also use peak units, but if the source voltage is given in AC RMS volts, then all calculated currents and voltages should also be in AC RMS units. This holds true for any calculation based on Ohm’s Laws, Kirchhoff’s Laws, etc.

Unless otherwise stated, all values of voltage and current in AC circuits are generally assumed to be RMS rather than peak, average, or peak-to-peak.

This video looks at what happens when a coil is connected to a DC supply and then to an AC supply.

AC Theory: The Mystery of the Coil

Exercise 24

When you have completed the questions below, email or message your tutor to find out how well you did.

1. In this AC circuit, the resistor offers 300 Ω of resistance and the inductor offers 400 Ω of reactance. Together their series opposition to alternating current results in a current of 10 mA from the 5 V source.

   How many ohms of opposition does the series combination of resistor and inductor offer?
   What name do we give this quantity? What is its symbol?

2. In this AC circuit, the resistor offers 3 kΩ of resistance and the capacitor offers 4 kΩ of reactance. Together their series opposition to alternating current results in a current of 1 mA from the 5 V source.

   How many ohms of opposition does the series combination of resistor and capacitor offer?
   What name do we give this quantity? What is its symbol?

3. With reference to the circuit shown answer the following questions:
   • What is the reactance XL of inductor L?
   • What is the circuit impedance Z?
   • What is the supply Current IS?
   • What is the voltage VR across the resistor R?
   

Self-Directed Learning

Look at the glossary page in the menu and download the worksheet. You should make sure you know the words listed in the glossary.

Complete and submit any outstanding assessment work.

Glossary

The following worksheet contains a list of words taken from this resource. To help improve your understanding, research their meanings or find the definitions in the workbook. There is space to write the definitions. You could also look up how to say the word – most online dictionaries will demonstrate this.

Glossary worksheet.

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