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 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.

Current and Voltage How does current move through a circuit?

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.

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. Today, we still use Franklin’s definition.

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). A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.

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.

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.

19.3 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 Calculate current 1) You are asked for the current. 2) You are given the voltage and resistance. 3) Ohm’s law relates current, voltage, and resistance 4) Solve: I = (1.5 V) ÷ (2 Ω) = 0.75 A The light bulb draws 0.75 amps of electric current. A light bulb with a resistance of 2 ohms is connected in a circuit that has a single 1.5-volt battery. Calculate the current that flows in the circuit. Assume the wires have zero resistance.

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.