Voltage, Current, Resistance and Power

Voltage

Voltage is the energy that moves the electrons through an electric circuit.

Voltage is measured in J/C. In an electric circuit the voltage may be

provided by a battery, a voltage source or a potentiometer.

Cell or BatteryLonger Line indicates

positive

Direct Current Voltage Source

Potentiometer

Electric Current

Any flow of charge is called an electric current.

Current is measured in Amperes (Amps or A). 1 Amp = 1 C/s In electric circuits, we assume it is positive charges flowing

through the circuit (conventional current). In reality, it is the electrons that flow. However in gases and

liquids both positive and negative charges can flow.

tΔ

QΔ=I tΔI=QΔ

Electrostatics & Circuits Example 19: Electric Current

A steady current of 4.7 A exists in a wire for 180-s.

a) How much total charge passed by a given point in the circuit during this time?

b) How many electrons was this?The charge on one electron is 1.60 X 10-19-C {We can ignore the sign}

Electric Circuits

All voltage sources have a positive “pole” and a negative “pole”.

A battery produces current by repelling the electrons with the negative pole and attracting electrons with the positive pole.

Current will only be produced if there is a closed path (circuit) for the electrons to flow from the negative pole to the positive pole.

+ -

Conventional Current

I

When there is no closed path, a circuit for the electrons to

follow from the negative pole of the

voltage source to the positive pole of the

voltage source, current will NOT

flow!!!!

But once the path is closed the electrons are repelled by the negative pole of the voltage source and

attracted to the positive pole of the

voltage source.

Sorry, I didn’t know.

Electrostatics & Circuits Example 20: An Electric Circuit

Which of the set-ups below will cause the bulb to light?

No closed path from the negative pole of the battery, through the bulb, to the positive pole of the battery.

Now there is a path from the negative pole of the battery, through the bulb and then to the positive pole.

THE BULB LIGHTS UP!!!!!

Resistors & Resistance

A property of all conductors is their ability to hinder the flow of electric current.

This property is called resistance, R.

The units for resistance are ohms, Ω. 1 Ω = 1 V/A

I

VR

Resistor

OR

Ohm’s Law

Georg Simon Ohm discovered that the resistance, R, was constant in many conductors.

This is known as Ohm’s Law. Materials that obey Ohm’s Law are called

ohmic. Materials that don’t obey Ohm’s Law are called

nonohmic.

IRV

Electrostatics & Circuits Example 21: Ohm's Law

A light bulb has 120-V across it and carries 0.012-A.

a) What is the resistance of the bulb.

b) Find the current through the bulb if the voltage across it is now 80-V.

c) What voltage would be required to push 1.0-A through the bulb?

Electrical Power

VIP

RIP 2

Electical Power is measured in Watts

If the electrical Device is an Ohmic Resistor:

ORRVP

2

Electrostatics & Circuits Example 22: Electrical Power

A light bulb has 110-V across it and a current of 0.5-A through it

a)What is the power of this bulb?

b)What is the resistance of this bulb?