Electric Current and Circuits

Electric Current and Circuits

Even more of the world explained

What is current?

• Current is the flow of electric charge• Remember, in a circuit (or most anywhere

else), it is the electrons that move.• So for an electric circuit, it is the flow of

electrons that is the current.• Current flows because there is a

VOLTAGE• You can think of voltage as the pressure in

the circuit that makes the electrons move

A picture is worth a thousand words

• K, pull out your books and open to page 532 (figure 34.1).

• In the picture on the left, water would simply flow to the right side of the container to equal itself out

• But if there is a pump that takes water from the right and puts it on the left, you can maintain a height difference between the sides.

Current flow• Current flows

because the outermost electrons of the atoms in a conductor are “free”

• This means they can wander away from their atoms easily

• They move around the other atoms in the conductor as they generally flow in the same overall direction http://www.powerworks.com.au/images/atomicflow.gif

Check for understanding

• Once again with the volunteers

Voltage Sources

• Current will not flow if there is no voltage source• Voltage provides the “pressure” for current to

flow in a circuit• So if you just have a plain loop of wire, even

though there are electrons in it, they will not flow as current

• But if you put a battery into the circuit, current will flow

• A battery is what acts as the “pump” that we saw in figure 34.1

What creates voltages?

• Batteries create voltages by using chemicals• You create a voltage by shuffling your feet

across the carpet and building up extra electrons on your body

• Clouds create voltage with respect to the ground by having much more negative charge at the bottom of the cloud than the ground has.

• Voltages all come from a PHSYICAL SEPARATION OF CHARGES

Man, the Internet is great…

• One instructive example of the nature of voltage is the fact that a bird can sit on a high-voltage wire without harm, since both of its feet are at the same voltage. You can also see that the bird is not "grounded" -- you will not be shocked by touching a high voltage if there is no path for the current to reach the Earth or a different voltage point. Typically if you touch a 120 volt circuit with one hand, you can escape serious shock if you have insulating shoes which prevent a low-resistance path to ground. This fact has led to the common "hand-in-the-pocket" practice for engineers and electrical workers. If you keep one hand in your pocket when touching a circuit which might provide a shock, you are less likely to have the kind of path to ground which will result in a serious shock.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html#c1

Check for Understanding

• Once again with the volunteers

Electrical resistance• Resistance is what it

sounds like: it resists the flow of electricity

• Electricity flows more easily through some materials than others

• Also, the skinnier the wire, the greater the resistance.

• Specially designed resistors (right) are used in electric circuits

http://blog.makezine.com/img413_204-1.jpg

Ohm’s Law

• Ohm’s law is a relationship among voltage, current and resistance:

• V = IR

• V = voltage

• I = current (don’t ask my why it isn’t “c”, because I don’t know)

• R = resistance

Exploring Ohm’s Law

• If a 12 V battery is connected to a 24 Ohm resistor, what is the current flowing in the circuit?

• If Voltage is held constant and you lower the resistance, what happens to the current?

• If Voltage is held constant and you raise the resistance, what happens to the current?

Series versus Parallel

• There are two basic types of circuits: series and parallel

• In a series circuit, there are no branches. All the current has to go through all the elements

• In a parallel circuit, there are branch points and the current splits.

Examples of Each

SeriesParallel

Series and Parallel

• In a series circuit, the CURRENT stays the same the whole time and the VOLTAGE changes after each resistor

• In a parallel circuit, the VOLTAGE stays the same in each branch and the current splits up

Check for Understanding

• Once again with the volunteers

Your body’s resistance: Don’t write this all down

• There are a lot of factors involved and not every person has the same electrical resistance.  For instance, men tend to have lower resistance than women.  Just like for the resistors used in electronics, the resistance of a person’s arm depends on the arm’s length and diameter.  Resistance goes up with length and down with diameter.  Since men tend to have thicker arms and legs (more muscle), they usually have lower resistance.  (An implication of this is that the lethal current for men is higher than that for women.)  A rough value for the internal resistance of the human body is 300-1,000 Ohms.  Naturally, the resistance also depends on the path that electricity takes through the body - if the electricity goes in the left hand and out the right foot, then the resistance will be much higher than if it goes in and out of adjacent fingers. 

Within the body, the tissues with the greatest resistance are bone and fat - nerves and muscle have the least resistance.  That said, the majority of the body’s resistance is in the skin - the dead, dry cells of the epidermis (the skin’s outer layer) are very poor conductors.  Depending on the person, the resistance of dry skin is usually between 1,000-100,000 Ohms.  The skin’s resistance is much lower if it is wet or burnt/blistered.  This means that when a person is electrocuted in real life, the body’s resistance drops as the skin is burned.  To determine a person’s total resistance, just add together the resistance of each part of the body - remember that the electricity must pass through the skin twice (on the way in and on the way out), so the total resistance is:

        Rtotal = Rskin(in) + Rinternal + Rskin(out)

http://van.physics.illinois.edu/qa/listing.php?id=6793

Now let’s lighten the mood some more

Current Effect If body 100K Ohms

If body 1000 Ohms

1 mA Tingling 100 V 1 V

5 mA Max harmless

500 V 5V

10-20 mA Can’t let go 1000V 10V

100-300 mA Heart stops 10,000V 100V

6 amps defibrillator 600kV 6,000V

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html#c3

Direct Current vs. Alternating Current

• Direct current is the same value all the time and only goes in one direction around the circuit

• Alternating current changes value all the time and reverses direction at 60 times/sec

http://www.physics4kids.com/files/art/elec_current3_240x180.gif

A note on Ground

• “Ground” has a special meaning for electricity.• Ground always has zero voltage. So if

something has a high voltage and is then connected to ground, current will flow to ground

• There is a ‘ground’ in a circuit and also the literal ground outside. Both are the same for this explanation of voltage.

• If you provide a path between a high voltage source and the physical ground, the current will pass through your body to get to the ground.

Wrapping it all up: Water analogy

• Let’s say you have water flowing in a series of pipes.

• The amount of water flowing past a certain point per second is the CURRENT

• The water pressure in the pipes (provided by a water pump) is like the VOLTAGE

• The difficulty water has going through a pipe (clogged, skinny pipe, whatever) is the RESISTANCE