Current Electricity

Explore how a circuit works

• Follow the instructions on the handout to investigate how an electrical circuit works.

• You will find 2 ways to light the bulb and 2 2 ways that do not light the bulb.

• You will sketch the ways that work and the ways that do not work.

• You will use symbols to represent the parts of the circuit.

Objectives

• Explain how electrical energy is supplied to devices in a circuit.

• Use electrical symbols to draw simple circuit diagrams.

• Distinguish between open and closed circuits.

Which of these circuits will light?

A B C D E

F G H I J

Y NN N Y

N

N

NYY

The source of the electrons moving in the circuit is NOT the battery or the wall outlet! The free electrons are contained within the wire itself.An individual electron does not actually travel all the way around a circuit.One electron bumps into the next that bumps into the next that bumps into the next …..It is the ENERGY that gets transferred all the way around the circuit.You buy energy, not electrons from the electric company!

Flow of Charge

• Charge flows when there is a potential difference (voltage).

• Voltage pushes current like the force of water in a water tower due to its height pushes the water to flow from high to low.

• Current flows through wire/conductors like water flows through pipes.

• Resistance to flow of electrons depends on the width and length of wire like water is restricted by the size of pipe

Current

CURRENT: a flow of charged particles (electrons) through a conductor

Current, I, is measured in amperes, A, or “amps”.

timecharge

I Andre Ampere

Example:

What is the current, I, if 8 C of charge passes through a wire in 2 seconds?

I = q / t

I = 4 amps

The Damage Caused by Electric Shock

1 mA = 0.001 A

Mild shock can be felt

5 mA = 0.005 A

Shock is painful

15 mA = 0.015 A

Muscle control is lost

100 mA = 0.1 A

Death can occur

60W light bulb - 0.5 AStarter motor – 210 AClothes dryer – 18 AIron- 3 A

Voltage, VSI unit is Volt, V

• Voltage is like electric “pressure” provided by batteries (dry cells, wet cells) and generators.

• Batteries provide energy from chemical reactions.

• Generators provide energy from the conversion of mechanical energy to electrical energy using rotating electromagnets.

Electrical Resistance, RSI unit is Ohm,

• Resistance in a conductor restricts the flow of charge in a conductor. It depends on the materials in the wire and the size/shape of the wire.

• Different conductors have different conductivity.• Thick wires have less resistance than thin wires.• Longer wires have more resistance than short

wires.

Finding Current:What is the current through a 30 Ω resistance that has

a potential difference of 120 V?

• Given:

• V = 120 V

• I = ?

• R = 30 Ω

• I = V/ R

= 120 V/ 30 Ω

= 4 A

Finding Voltage: How much voltage is required to make 2 amperes flow

through a resistance of 8 ohms?

• Given:

• V = ?

•I = 2 A

• R = 8Ω

V = I·R = 2A·8Ω

= 16 V

V

I R

Finding Resistance: Calculate the resistance of the filament in a light bulb that carries 0.4 A when 3.0 V is impressed across it.

• Given:

• V = 3.0 V•I = 0.4 A•R = ?

R = V/I = 3.0V ÷ 0.4A = 7.5Ω

V

I R

Click Click here and and here to run to runsimulations of Ohm’s Law.simulations of Ohm’s Law.

Many circuit elements do notMany circuit elements do notobey Ohm’s Law. Resistorsobey Ohm’s Law. Resistorsthat get hot, like light bulbsthat get hot, like light bulbsand heating elements, do notand heating elements, do notkeep a constant resistance. keep a constant resistance. Resistance generally Resistance generally increases as increases as objects become hotter.objects become hotter.