GRADE Camp Introduction to Voltage and Current Some slides adapted from lectures by D. Shattuck & L. Trombetta

Voltage and Current Motivation: - Many fundamental EE principals are used in circuit theory. - It will give us a “feel” for an important part of your robot project.

Voltage and Current What is... Charge -- Coulombs? Current -- Coulombs/Sec Voltage – Joule/Coulomb Ohm’s Law – R = v/i (Ohms) Power – P = i*v (Watts) Energy – w = P*t (Joules)

Module 1 – Part 1 What are Current and Voltage? Modified from Dr. Dave Shattuck, Dynamic Presentation of Key Concepts, modules for circuit theory self-study.

Overview In this part we will cover: Definitions of current and voltagecurrentvoltage Hydraulic analogies to current and voltagecurrent voltage

Currents and the Ampere Current is the net flow of charges, per time, past an arbitrary “plane” in some kind of electrical device. The unit of current is the Ampere, which is a flow of 1 Coulomb of charge per second, i.e.: 1[A] = 1[Coul/sec]

Hydraulic Analogy for Current It is often useful to think in terms of hydraulic analogies. The analogy here is that current is analogous to the flow rate of water: Charges going past a plane per time – is analogous to – volume of water going past a plane in a pipe per time.

Water flow  Current So, if we put a plane (a screen, say) across a water pipe, and measure the volume of water that moves past that plane in a second, we get the flow rate. In a similar way, current is the number of positive charges moving past a plane in a current-carrying device (a wire, say) in a second. The number of charges per second passing the plane for each Ampere of current flow is called a Coulomb, which is about 6.24 x electron charges. Animated graphic provided by David Warne, student in UH ECE Dept.

Voltage and the Volt When we move a charge in the presence of other charges, energy is transferred. Voltage is the change in potential energy as we move between two points; it is a potential difference. The unit of voltage is the Volt. A Volt is defined as a Joule per Coulomb. One Joule of energy is lost from an electric system when a Coulomb of positive charges moves from one potential to another potential that is one Volt lower.

Hydraulic Analogy for Voltage Hydraulic analogy: voltage is analogous to height. In a gravitational field, the higher that water is, the more potential energy it has. The voltage between two points – is analogous to – the change in height between two points, in a pipe.

Hydraulic Analogy: Voltage and Current height ~ voltage flow rate ~ current

Hydraulic Analogy With Two Paths Water is flowing through the pipes. There is a height difference across these pipes. We can extend this analogy to current through and voltage across an electric device…

Current Through… If we have two pipes connecting two points, the flow rate through one pipe can be different from the flow rate through the other. The flow rate depends on the path.

…Voltage Across No matter which path you follow, the height is the same across those two points. The height does not depend on the path

Resistors A resistor is a two terminal circuit element that has a constant ratio of the voltage across its terminals to the current through its terminals. The value of the ratio of voltage to current is the defining characteristic of the resistor. In many cases a light bulb can be modeled with a resistor.

A resistor obeys the expression where R is the resistance. If something obeys this expression, we can think of it, and model it, as a resistor. This expression is called Ohm’s Law. The unit ([Ohm] or [  ]) is named for Ohm, and is equal to a [Volt/Ampere]. Resistors – Definition and Units To a first-order approximation, the body can modeled as a resistor. Our goal will be to avoid applying large voltages across our bodies, because it results in large currents through our body. This is not good. + R v iRiR -

Hydraulic Analogy: Voltage and Current height ~ voltage flow rate ~ current

GRADE Camp END NOW Some slides adapted from lectures by D. Shattuck & L. Trombetta