Relating Voltage, Current and Resistance

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Brandon Pitts
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1 Relating Voltage, Current and Resistance Using Ohm s Law in a simple circuit. A Simple Circuit Consists of:! A voltage source often a battery! A load such as a bulb! Conductors arranged to complete a circuit the wires

2 Consider three quantities in a circuit:! Current: The rate at which electrons move through a single place.! Voltage (aka Potential ) Difference: The change in energy per electron as it travels between two points.! Resistance: A measure of the difficulty electrons have traveling between two points. Current! The current going into any point must equal the current going out.! In a simple loop, the current must be the same everywhere on the loop.! We will consider the current to be the electron motion. In this circuit, the electron (-) current flows counterclockwise. The red arrows indicate the direction of the conventional (+) current.

3 Current Amounts! Current is measured in units of Coulombs per second.! One C/s equals one Amp.! A device that measures current is called an ammeter. Example: A typical light bulb draws roughly one amp of current. A Simple Circuit is Like an Amusement Park Ride

4 A Simple Circuit is Like an Amusement Park Ride There are no branches. Every passenger (electron) takes the same path. Where it s slow, passengers (electrons) bunch together. Where it s fast, they are far apart. But at every point in the loop, the number of passengers per minute (current) is basically the same. Effects of Electricity Passing Through a Human Torso: 1 ma: Threshold of sensation ma: Painful shock. Person able to let go ma: Painful shock. Muscle control lost near point of contact ma: Extreme pain. Severe muscle contraction along current path. Breathing becomes difficult ma: Heart fibrillation ma: Severe burns. Heart stops beating for duration of shock.

5 A current events question: The circuit shown at the right has two points marked 1 and 2. Bulb A Which point has the most Battery current flowing through it? A. Point 1. B. Point 2. C. I 1 = I 2 Voltage! Casually speaking, voltage is the push that electrons have to get through a circuit.! In our circuits, batteries provide the voltage.! The terminals of a battery provide a constant voltage difference.

6 Voltage!!! Batteries can do work on electrons, moving them through the circuit. A voltage is a measure of the potential energy of the electrons as they move from place to place. The potential energy is used up (turned into other forms of energy) in the circuit by the time the current gets back to the battery. A Simple Circuit is Like an Amusement Park Ride The conveyor belt that lifts the passengers is like the battery that gives energy to the electrons. The downhill slopes are like the resistors, where the passengers (electrons) lose energy.

Resistance! Some materials allow current to flow easily, some only allow current to flow with difficulty.! Resistance is measured in units of Ohms (symbol: W).

7 Resistance! Some materials allow current to flow easily, some only allow current to flow with difficulty.! Resistance is measured in units of Ohms (symbol: W).! We usually treat our wires and batteries as having no resistance. ( Z e r o r e s i s t a n c e i s a g o o d a p p r o x i m a t i o n f o r w i r e s, b u t n o t f o r b a t t e r i e s! ) Resistance! A bulb, motor, heater or other load can be part of a circuit.! These devices ( loads ) use the electrons energy to function.! Because the electrons must do work, they have difficulty getting through the load, so we say the load has resistance.

8 Now you get to do something!! Work in your groups.! Complete the activity, Ohm s Law without Numbers! We ll come back together to talk about the results, then check what you ve learned with some questions. Filament in a Bulb! No voltage across it! No current through it! Does have resistance! Voltage applied to it! Current flows through it! Still has resistance

A question of current: The same current I 0 enters each of the two resistors. The resistors have unequal magnitudes, and R 2 > R 1. Which resistor has the most current leaving it? A. I 1 is greater.

9 A question of current: The same current I 0 enters each of the two resistors. The resistors have unequal magnitudes, and R 2 > R 1. Which resistor has the most current leaving it? A. I 1 is greater. B. I 2 is greater. C. I 1 = I 2 A Model for Electric Circuits! Electrons flow through the circuit (Current).! The battery pumps up the electrons, giving them energy (Voltage or Potential)! The load uses the energy of the electrons (Resistance). Water Flow Analog of Electrical Current

10 Which way does current flow? The zigzag line represents a resistor. The arrow represents the direction the current flows. What must be true about the potentials at A and B? A. The potential at A is higher than at B. B. The potential at B is higher than at A. C. The potential at A is the same as at B. A Question of Voltage: Suppose you have a simple circuit. If you replace the battery with one of higher voltage, what probably happens to the current through the bulb? A. Current decreases B. Current stays the same C. Current increases Circuit Applet

11 A Question of Resistance: Suppose you have a simple circuit. If you increase the resistance of the bulb, what probably happens to the current through it? A. Current decreases B. Current stays the same C. Current increases This Relationship is Summarized as Ohm s Law. Current = Voltage. Resistance

12 An Example Using Ohm s Law. Example: A 5 W bulb is connected to complete a circuit with a 10 V battery. The resulting current through the bulb is: I = V / R = (10 V) / (5 W) = 2 A Yet another question of current In the figure below, five different resistors are shown. They are labeled with the resistance of each and the potential at each end. Through which resistor will the magnitude of current flow be greatest?

13 What was this all about again?! Current, Voltage and Resistance! Ohm s Law: Voltage = Current x Resistance Q u e s t i o n s?

In this unit, we will examine the movement of electrons, which we call CURRENT ELECTRICITY.

In this unit, we will examine the movement of electrons, which we call CURRENT ELECTRICITY. Recall: Chemistry and the Atom! What are the 3 subatomic Where are they found in the particles? atom? What electric charges do they have? How was a positive ion created? How was a negative ion created?