drift speed when you connect a circuit, current flows in every part of the circuit instantaneously...

Circuits

Drift speedWhen you connect a circuit, current flows IN EVERY PART of the circuit instantaneously (near the speed of light).

• ALL free electrons in the circuit start moving the moment the circuit is connected / the moment the electric field is applied.• Batteries create potential difference, they do NOT supply electrons; the electrons come from the wire.



Electrons move very fast (106 ms-1), but are not instantaneous.




Because electrons bounce around due to collisions with atomsin the wire, the average speed of electrons opposite the flow of current – known as drift speed – is VERY slow (0.01 cm/s)




Because electrons bounce around due to collisions with atomsin the wire, the average speed of electrons opposite the flow of current – known as drift speed – is VERY slow (0.01 cm/s)

This means it can take an electron 3 hours to travel through 1 m of wire!

it’s not even a snail’s pace!!!!!

Drift speedTurn & Talk with your table partner

…The scholar with the bigger shoe sizeDefine drift speed

… The scholar with the smaller shoe sizeRespond to the following:“Lights turn on so quickly after you flip the

switch because electrons move very quickly.”

Direct Current (DC) electric circuits

• a circuit containing a battery is a DC circuit• in a DC circuit the current always flows in the same

direction. • Current flows from + to –• Electrons flow the opposite

direction

Duracell

+

historic explanationclick me

http://www.mste.uiuc.edu/murphy/HoleFlow/ElectricFluid.html

• Power outlets supply AC current• In an AC circuit the current reverses at a frequency of 50

Hz (US) or 60 Hz (Europe)

_ +

AC movement of electrons in a wire

Alternating Current (AC) Circuits

!! the source of electrons are the free electrons in wire itself !!

When you are shocked by AC current, the electrons that make the current come from your body. The current makes the

electrons in your body vibrate.

curr

ent

time

AC

curr

ent

time

DC

How does the voltage and current change in time?

In DC, current and voltage do not change direction over time

the actual voltage in a 120-V AC circuit varies between +170V and -170V peaks.

Why do power plants produce AC?

• AC can be much more easily stepped up or down to different voltages through use of a transformer.

• Low voltage is good in a house because it is safer. • High voltage is useful for transmitting current over

large distances – thinner wires can be used and there is less loss to heat.

This means that AC is cheaper to transmit and the plants can be farther from the users.

AC vs DC

Turn and talk with your table partner …

… the scholar who has a birthday earlier in the monthIdentify 3 differences between AC and DC

… the scholar who has a birthday later in the monthDescribe two benefits of transmitting high voltage current

In the mid-nineteenth century, G.R. Kirchoff (1824-1887) stated two simple rules using the laws of conservation of energy and charge to help in the analysis of direct current circuits.

These rules are called Kirchoff’s rules.

‘The sum of the currents flowing into a point in a circuit equals the sum of the currents flowing out at that point’.

1. Junction rule – conservation of charge.

I1 + I2 = I3 + I4 + I5

Kirchoff’s rules



I1 + I2 = I3 + I4 + I5

Kirchoff’s rules

This is a slight alteration to our ‘current is the same everywhere rule’ that we used before.

It means that current can be split (or combined) at junctures.



I1 + I2 = I3 + I4 + I5

2. Loop rule – conservation of energy principle

‘In a closed loop, the sum of the emfs equals the sum of the potential drops’.

V = V1 + V2 + V3

Kirchoff’s rules



I1 + I2 = I3 + I4 + I5

2. Loop rule – conservation of energy principle

‘In a closed loop, the sum of the emfs equals the sum of the potential drops’.

V = V1 + V2 + V3

Kirchoff’s rules

Emf = electromotive force = the potential difference supplied by a battery

This is exactly the same as our rule that voltage is ‘used up’ across a circuit.

Circuit Diagrams

Common symbols

Branching points in the wire are called nodes

Circuit Diagrams You Do: Draw the schematic of the circuit

Exit TicketTrue / False

1. When a battery no longer works, it is out of charge and must be recharged before it can work again.

2. The amount of charge that exits a light bulb is less than the amount that enters the light bulb.

3. Draw the circuit schematic

4. Batteries use ____________ current

5. What is the major difference between AC and DC current?

EC What is the major advantage for using AC current?

drift speed when you connect a circuit, current flows in every part of the circuit instantaneously...

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