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Electric Circuits. Flash activity (these activities are not editable). Virtual experiment. Teacher’s notes included in the Notes Page. Accompanying worksheet. For more detailed instructions, see the Getting Started presentation. What is an electric circuit?.
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Flash activity (these activities are not editable) Virtual experiment Teacher’s notes included in the Notes Page Accompanying worksheet For more detailed instructions, see the Getting Started presentation
What is an electric circuit? At the beginning of the lesson, your teacher had to press a switch to turn the interactive whiteboard on. Pressing the switch completed an electric circuit. Look around you now. What objects can you see that contain electric circuits? An electric circuit is made up of the following: • a source of electricity, such as a battery • a conducting material, usually a wire, through which electricity can flow • a component to use some of the electricity, such as a light bulb • a switch to break the circuit.
Drawing circuit diagrams Electricians, architects and engineers may need to draw diagrams of electric circuits. To make diagrams easier to understand, electric circuits can be drawn using symbols to represent the components. Wires are shown as straight lines.
What is current? Current is a measure of how much electric charge flows through a circuit. The more charge that flows, the larger the current. Current is measured using an ammeter, which must be connected in series in the circuit. The units for current are amps (A). When two or more circuit components are connected in series, the same current flows through each component.
What is voltage? Voltage is a measure of how much energy the electrons are carrying around the circuit. Voltage is measured using a voltmeter, which must be connected in parallel with a component in the circuit. The units for voltage are volts (V). When two or more circuit components are connected in series, the total voltage of the supply is shared between them.
Case study: electricity and the heart Most of the body is controlled by electrical impulses from the brain or different organs. A small area within the heart, called the sinoatrial node (SAN), controls its beating. Sometimes the rhythm of the heart can be disrupted – beating too quickly or in an uncontrolled way. This can lead to death if a normal rhythm isn’t established quickly. A defibrillator can be used to shock the heart into re-establishing a normal rhythm. Unsynchronized current flows from the defibrillator into the heart, causing it to momentarily stop beating. It can then restart with a more controlled rhythm.
What is a resistor? A resistor is a component designed to reduce the current. Resistors are often used with components that require less current than is being supplied to the whole circuit. Most electronic circuits need to have resistors to work properly. Resistors can be produced that reduce the current by different amounts.
Case study: the heating effect of resistance High resistance in a wire can produce heat. Electricity is transmitted across the country with a very high voltage and a low current. This reduces the resistance in the wire and the heat produced. Resistance is important in a filament light bulb. The filament is made from a very thin, long piece of high resistance wire. The resistance is so high that when electricity flows through the wire, it glows ‘white hot’ and emits light.
voltage = current × resistance V = I × R Ohm’s law Ohm’s law is a formula that links voltage, current and resistance. Resistance is measured in ohms (). Depending on the values given, the formula can be used to calculate voltage, current and resistance in a circuit. What two other equations could you write from this Ohm’s law formula triangle?
What does Ohm’s law mean? These three equations can be derived from the Ohm’s law formula triangle. But what do they actually mean? If the voltage increases, then either the current flowing through a material or the resistance will also be increased. • V = I × R The resistance of a material is constant for a given temperature, which means that current and voltage must be proportional. • R = V/I For a low resistance material, more current is allowed to flow for a given voltage. For a high resistance material, less current will flow at the same voltage. • I = V/R
Using Ohm’s law Ohm’s law can be used to calculate the resistance of a component in a circuit. It is difficult to measure the resistance of a component in a circuit directly, so it is usually done by measuring the voltage and current through the component instead. Which equation would you use to calculate resistance? R = V/I Which instrument would be used to measure the voltage? voltmeter Which instrument would be used to measure the current? ammeter
Case study: dimmer switches Dimmer switches provide control over the brightness of a light bulb. Dimmer switches used to contain variable resistors. These are similar to normal resistors, but the level of resistance can be changed. Changing the resistance changed the amount of current passing through the bulb. However, this produces a lot of heat and so these dimmer switches were dangerous. New dimmer switches switch the light circuit on and off very rapidly to reduce the total amount of current flowing. The circuit is automatically switched off and on at different voltage values during alternating current cycles.
You are an electrical engineer and have been asked to train some new employees. Prepare a handout about Ohm’s law and electric circuits. You should include the following: • a brief description of how a voltmeter and ammeter are used in a circuit to measure voltage and current • a diagram of a series circuit showing how to measure the current and voltage across the battery and another component • an explanation of how to use Ohm’s law to calculate resistance, current and voltage, with worked examples.
