Current Electricity

Parallel Circuit Series Circuit

What You Will Learn Transfer of energy in circuits. Conversion of energy. Electric Current – Conventional vs. Flow of

Electrons Resistance and Ohm’s Law Basic Circuits

What You Already Know You flip a switch to turn on a light, TV or

computer. To turn on the car, you turn the ignition switch. MP3 players, cell phones and flashlights have

on/off switches and use batteries. In each of these cases, you have a closed circuit in

which electricity flows.

What You Already Know Charge by Conduction – The process by

which electrons are transferred from one object to another because of differences in excess number of electrons on one surface compared to the other.

What You Already Know - Electric Potential the Electric Potential Difference is equal to the Work

required to move a test charge in an electric field divided by the magnitude of the test charge.

Vtotal = W/qo = Fd/qo = Ed

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UniformElectricField

qo

A

F = qoE

B

F is constant since the electric field is constant from one plate to the other.

Creating a Circuit

Two equal and oppositely charged plates

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What would happen if a conductor was connected to both plates?

Plate with excess number of electrons

Plate with deficiency of electrons

Creating a Circuit

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Electron Flow

Conventional Current Flow

The electrons would flow from the negatively charged plate to the positively charged plate until the amount of charge was the same for both plates and the wire.

How do we maintain the flow of current?

+-+-+-+-+-+-+-+

+-+-+_+-+-+-+-+

+ - + - + -+ - + - + - + - + - + - + - + - + -

Creating a Circuit

Electron Flow

Electron FlowCircuit: • A closed loop in which electric current can flow. • It generally includes a device such as a light bulb that reduces the electric potential energy.• It also includes a device to increase potential energy (Charge Pump).

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Charge pump• Battery•Generator

•Gas/Oil •Nuclear•Hydro•Wind•Tidal•Solar

What is Current? Current is the rate of flow of charge.

I = q/t = 1 Coulomb/second = 1 Ampere (A)

Conventional Current = flow of positive charge. (Note that positive charges do NOT flow in metallic conductors.)

Electron flow is simply theflow of electrons.

Electron Flow

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Conventional Current

Electron FlowElectron Flow

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Conventional CurrentConventional Current

Ohm’s Law German Georg Simon Ohm discovered that the

ratio of the potential difference to current is a constant for a given conductor.

R = V/IWhere:

R = Resistance in Ohms ()

V = Electric Potential in Volts (V)

I = Current in Amperes (A)

Resistance is the hindrance to the flow of charge. Most metallic conductors obey Ohm’s Law.

Ohm’s Law The resistance (R) represents the slope (m) of a

curve where V is plotted against I. What is R? For Ohmic materials, the curve is a straight line.

m = R = V/I

Non-Ohmice.g. light bulb

10

Examples: Ohm’s Law

How much current flows through a 12 flashlight bulb operating at 3.0 volts?

What is the voltage drop in a 5 resistor that has 2 amperes of current running through it?

What is the resistance of a heating element in a toaster operating at 120 volts with a current flow of 2 amperes?

What causes resistance? E-field in conductor (resistor) is provided by a

battery or voltage source. Charges (electrons) are put in motion due to influences

of the electric field, but scatter in a very short time from things that get in the way

defects, lattice vibrations (phonons), etc The more collisions, the greater the resistance and the

fewer the collisions, the less the resistance. Imagine trying to run down the hallway in

between periods versus running down the hallway during the period when there is nobody in them. The latter would be much easier.

How fast do the electrons travel? A simple observation would tell an

observer that the flow of electricity appears to be instantaneous when flipping on a light switch.

Does that mean the electrons travel at the speed of light?

Drift Velocity When an electric field is applied to a conductor, it will set

the electrons in motion in an overall direction opposite the applied field.

While the electric field travels at nearly the speed of light, the overall drift speed of the electron from one end of the conductor to the other is quite slow and random in direction due to collisions.

For a 20A circuit in your home with 1A of current flow, the electrons would only travel 0.08 m (8 cm) in 1 hour!

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Resistivity & Resistance Resistivity is a measure of the conductive ability of the

material. Resistivity is an intrinsic (natural) property of a material. The higher the resistivity, the higher the resistance and

vice versa. For a conductor of length L (m) and cross-sectional area A

(m2), the resistance can be determined by:

R = L/AWhere

= resistivity (•m)L = length of the conductorA = Cross-Sectional Area

Ex.: Resistance & Resistivity1. What would happen to the resistance in a wire if the length

were increased?

A. It would decrease.B. It would increase.C. It would remain the same.

2. What would happen to the resistance in a wire if the cross-sectional area were increased?

A. It would decrease.B. It would increase.C. It would remain the same.

3. What would happen to the resistivity the length were increased?

A. It would decrease.B. It would increase.C. It would remain the same.

A

LR

Low Resistance vs. High Resistance

To Summarize: Short fat wires make good conductors.

While long skinny wires make poor conductors.

I

I

Short & Fat = Low Resistance

Long & Skinny = High Resistance

Resistance vs. Length and Resistance vs. X-Sectional Area

What is the relationship between:

Resistance and Length?

Resistance and X-Sectional Area?

Length X-Sectional Area

A

LR

Resistivity vs. Temperature

0

2.5

5

7.5

10

- 200 400 600 800 1,000 1,200 1,400

Temperature (K)

Res

isti

vity

(10

-8

m)

Note: The Resistivity is zero at 0 K, therefore, the resistance is also zero.

Power

Power = Rate at which work is done where:

P = VI

P = 1 Joule/second = 1 WattP = VI = (1 Volt)•(1 Ampere) = 1 Watt

V = W/q = 1 Joule/Coulomb I = q/t = 1 Coulomb/second

Since V = IR and I = V/R:P = IRI = I2RP = V•V/R = V2/R

Example (Power)

What is the power rating of a lightbulb in circuit where the current is 0.50 A and the voltage is 120V?

P = VI P = 120 V•0.50 A P = 60 VA = 60 W

Power vs. Current and Power vs. Voltage (Ohmic Materials)

What is the relationship between: power and current? Power and voltage?

Current Voltage

P = I2R P = V2/R

Energy Since power is the rate at which

work is done the amount of energy required to complete a task is as follows:

Total Energy = Power x timeW = Pt

Example (Energy)

How much energy is consumed by a lightbulb operating in circuit where the current is 0.50 A and the voltage is 120V for 1 hour?

W = VIt W = 120 V•0.50 A•3600 s W = 216,000 JW = 216 kJ

Key Ideas A circuit is a closed path where current can flow. Current is the flow of charge. Resistance is the hindrance to the flow of charge. Ohm’s Law = voltage to current ratio (V/I) = Resistance. Resistivity is an intrinsic property of a material that is

proportional the the resistance. An increase in length of a conductor will increase

resistance. An increase in cross-sectional area of a conductor will

decrease resistance. Power equals the rate at work is done and is represented

electrically by P = IV.