Download Electric Current File

SACE Stage 1 Conceptual
Physics
Electric Current
Flow of Charge
Voltage is an electrical pressure that can
produce a flow of charge or an electric
current within a conductor.
 The flow is restrained by the resistance it
encounters.

Flow of Charge
When the flow takes place in one
direction, it is called direct current (dc).
 When the flow of charge is to and fro, it is
called alternating current (ac).

Flow of Charge
Heat flows through a conductor when one
end is warmer than the other end.
 The heat stops flowing when the ends
have the same temperature

Flow of Charge
Similarly, electricity flow in a conductor
when the ends have different potential (a
potential difference).
 The electrons will keep flowing until the
ends are at the same potential.

Flow of Charge
To sustain a flow of charge in a conductor,
there must be some arrangement where
the potential difference can be maintained.
 This is like maintaining water flow from a
higher reservoir to a lower reservoir.

Flow of Charge

Water flows from the reservoir of high
pressure to the reservoir of low pressure
until the pressure is equal. The picture on
the right shows that water will continue to
flow due to the pump which is maintaining
the pressure difference.
Electric Current
The flow of charge in a conductor.
 In solid conductors it is the electrons that
carry the charge through the circuit.
 This happens because these electrons
(conduction electrons) are free to move
throughout the atomic network.

Electric Current
Electric Current is measured in amperes
(A).
 An ampere is the flow of one coulomb of
charge per second.

1 Coulomb = 6.25 billion electrons
Voltage Sources
Charges do not flow unless there is a
potential difference.
 A sustained current needs a suitable
“electric pump” to provide a sustained
potential difference.
 Something that provides a potential
difference is known as a voltage source.

Voltage Sources

The potential energy per coulomb of
charge available to electrons moving
between the terminals is known as the
electromotive force or emf.
Electrical Resistance

Similar to the rate of water flow in a hose,
the harder the water find it to move
through the hose, the less water flows.
Electrical Resistance

In a wire, a number of factors apply,
Thickness of wire
 Type of wire
 Voltage source


Units of resistance
is the ohm ().
Ohm’s Law

Ohm discovered that the amount of current in a
circuit is directly proportional to the voltage
impressed across the circuit, and is inversely
proportional to the resistance of the circuit.
voltage
v
current (I) 

resistance R
Ohm’s Law

Q – What is the resistance of an electric
frying pan that draws 12 amperes of
current when connected to a 120 volt
circuit?
Ohm’s Law
Q – What is the resistance of an electric
frying pan that draws 12 amperes of
current when connected to a 120 volt
circuit?
 A – The resistance is 10 ohms. (why?)

Ohm’s Law
Q – What is the resistance of an electric
frying pan that draws 12 amperes of
current when connected to a 120 volt
circuit?
 A – The resistance is 10 ohms. (why?)

V 120
R 
 12ohms
I
12
Ohm’s Law

Q – How much current is drawn by a lamp
that has a resistance of 100 ohms when a
voltage of 50 volts is impressed across it?
Ohm’s Law
Q – How much current is drawn by a lamp
that has a resistance of 100 ohms when a
voltage of 50 volts is impressed across it?
 A – The current is 0.5 ampere. (Why?)

Ohm’s Law
Q – How much current is drawn by a lamp
that has a resistance of 100 ohms when a
voltage of 50 volts is impressed across it?
 A – The current is 0.5 ampere. (Why?)

V
50
I 
 0.5 A
R 100
Ohm’s Law and Electric Shock

What causes electric shock?
Current or Voltage
Ohm’s Law and Electric Shock

Current causes the electric shock.

You can be exposed to a large voltage but
if the resistance is very high, not much
current will pass through you and you will
be safe.
Ohm’s Law and Electric Shock
The resistance of your body depends on
its condition.
 If your are soaked in salt water you will
have a resistance of about 100 ohms.
 If your skin is very dry, about 500,000
ohms.

Ohm’s Law and Electric Shock
Ohm’s Law and Electric Shock

Q – If the resistance of your body were
100, 000 ohms, how much current would
be produced in your body if you touched
the terminals of 12V battery?
Ohm’s Law and Electric Shock
Q – If the resistance of your body were
100, 000 ohms, how much current would
be produced in your body if you touched
the terminals of 12V battery?
 A – The current in your body would be
0.00012 A. (Why ?)

Ohm’s Law and Electric Shock
Q – If the resistance of your body were 100,
000 ohms, how much current would be
produced in your body if you touched the
terminals of 12V battery?
 A – The current in your body would be
0.00012 A. (Why ?)

V
12
I 
 0.00012 A
R 100,000
Ohm’s Law and Electric Shock

Q – If your skin were very moist so that
you resistance was only 1000 ohms, and
you touched the terminals of a 24V battery,
how much current would you draw?
Ohm’s Law and Electric Shock
Q – If your skin were very moist so that
you resistance was only 1000 ohms, and
you touched the terminals of a 24V battery,
how much current would you draw?
 A – 0.024A (A very dangerous current)
(Why?)

Ohm’s Law and Electric Shock
Q – If your skin were very moist so that
you resistance was only 1000 ohms, and
you touched the terminals of a 24V battery,
how much current would you draw?
 A – 0.024A (A very dangerous current)
(Why?)

V
24
I 
 0.024 A
R 1000
DC or AC?
DC – Direct current, electrons flow in one
direction
 AC – Electrons flow backwards and
forwards.

DC or AC

Can convert AC to DC by the use of a
diode in a electric current.

A diode acts as a one way valve, allowing
electrons to only pass through in one
direction.
DC or AC

The instantaneous speed of electrons is
close to the speed of light. But this motion
is in random directions.

The actual net speed of the current is quite
slow as the electrons arte gently nudged
through he wire due to the electric field set
up inside the wire.
Electric Power

The rate at which electrical energy is
converted into another form such as
mechanical energy.

Electric Power = current x voltage
Electric Power

Q – How much power is used by a
calculator that operates on 8V and 0.1
amps? If it is used for 1 hour, how much
energy does it use?
Electric Power

Q – How much power is used by a
calculator that operates on 8V and 0.1
amps? If it is used for 1 hour, how much
energy does it use?
Power :
P  I  V  0.1A  8V  0.8W
Energy :
E  P  t  0.8W 1hr  0.8 watt - hrs (J)