19.1 Electric Circuits Electricity refers to the presence of electric current in wires, motors, light bulbs, and other devices. Electric current is similar to a current of water, but electric current flows in solid metal wires so it is not visible. Electric current can carry a lot of power.

19.1 Electric Circuits An electric circuit is something that provides a complete path through which electricity travels. Wires in electric circuits are similar in some ways to pipes and hoses that carry water.

19.1 Electric Circuits When drawing a circuit diagram, symbols are used to represent each part of the circuit. These electrical symbols are quicker and easier to draw than realistic pictures of the components.

19.2 Current and voltage Electric current is measured in units called amperes, or amps (A) for short. One amp is a flow of a certain quantity of electricity in one second. The amount of electric current entering a circuit always equals the amount exiting the circuit.

19.2 Current and voltage Conventional current was proposed by Ben Franklin in the 1700’s. Scientists later discovered that the particles that carry electricity in a wire actually travel from negative to positive.

19.2 Voltage Voltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy. Voltage is measured in volts (V).

19.2 Voltage Since 1 joule per second is a watt (power), you can interpret voltage as measuring the available electrical power per amp of current that flows.

19.2 Voltage The positive end of a 1.5 volt battery is 1.5 volts higher than the negative end. If you connect batteries positive-to-negative, each battery adds 1.5 volts to the total. Three batteries make 4.5 volts. Each unit of current coming out of the positive end of the three-battery stack has 4.5 joules of energy.

19.2 Current and voltage A battery uses chemical energy to create a voltage difference between its two terminals. In a battery, chemical reactions provide the energy to pump the current from low voltage to high voltage.

19.2 What does a battery do? A battery uses chemical energy to move charges. If you connect a circuit with a battery the charges flow out of the battery carrying energy.

19.2 Current is a flow of charge

19.3 Electrical resistance Resistance measures how difficult it is for current to flow.

19.3 Electrical Resistance The total amount of electrical resistance in a circuit determines the amount of current that in the circuit for a given voltage. The more resistance the circuit has, the less current that flows.

19.3 The ohm Resistance is measured in ohms (W). One ohm is the resistance when a voltage of 1 volt is applied with a current of 1 amp.

19.3 Ohm's law German physicist Georg Ohm experimented with circuits to find an exact mathematical relationship between voltage, current and resistance. Ohm's law can be used to predict any one of the three variable if given the other two.

19.3 The resistance of electrical devices The resistance of electrical devices ranges from very small (0.001 Ω) to very large (10×106 Ω). Each device is designed with a resistance that allows the right amount of current to flow when connected to the voltage the device was designed for.

19.3 Electrical Conductivity The electrical conductivity describes a material’s ability to pass electric current.

19.3 Conductors and insulators A material such as copper is called a conductor because it can conduct, or carry, electric current. Materials that insulate against (or block) the flow of current are classified as electrical insulators. Some materials are neither conductors nor insulators. These materials are named semiconductors.

19.3 Resistors Electrical components called resistors can be used to control current. Resistors have striped color codes to record their "values" (writing on them is difficult).

19.3 Potentiometers Potentiometers are a type of "variable" resistor that can change from low to high. They are wired so that as you turn the knob, it changes the distance the current has to flow.

Application: Hybrid Gas/Electric Cars