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

Dave ShattuckUniversity of Houston

© Brooks/Cole Publishing Co.

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 1What 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 voltage

• Hydraulic analogies to current and 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.


© Brooks/Cole Publishing Co.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 1018 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.


© Brooks/Cole Publishing Co. Hydraulic Analogy:Voltage and Current

height ~ voltageflow rate ~ current



Hydraulic Analogy With Two Paths

This diagram is intended toshow a water pipe that

breaks into two parts andthen combines again. Thesize of the blue arrows are

intended to reflect theamount of water flow at

that point.

Two Pipes AnalogyWater 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.

Flow rate in thesmaller pipe

is less than it isin the

larger pipe.

Like flow rate,current is path

dependent.



…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

R

vR

i +

R

viR -


© Brooks/Cole Publishing Co. Hydraulic Analogy:Voltage and Current

height ~ voltageflow rate ~ current



GRADE Camp

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Some slides adapted from lectures by D. Shattuck & L. Trombetta

Documents

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