sace stage 1 conceptual physics electric current

SACE Stage 1 Conceptual SACE Stage 1 Conceptual PhysicsPhysics

Electric CurrentElectric Current

Flow of ChargeFlow of Charge

Voltage is an electrical pressure that can Voltage is an electrical pressure that can produce a flow of charge or an produce a flow of charge or an electric electric currentcurrent within a conductor. within a conductor.

The flow is restrained by the resistance it The flow is restrained by the resistance it encounters.encounters.


When the flow takes place in one When the flow takes place in one direction, it is called direction, it is called direct currentdirect current (dc). (dc).

When the flow of charge is to and fro, it is When the flow of charge is to and fro, it is called called alternating currentalternating current (ac). (ac).


Heat flows through a conductor when one Heat flows through a conductor when one end is warmer than the other end.end is warmer than the other end.

The heat stops flowing when the ends The heat stops flowing when the ends have the same temperaturehave the same temperature


Similarly, electricity flow in a conductor Similarly, electricity flow in a conductor when the ends have different potential (when the ends have different potential (a a potential differencepotential difference).).

The electrons will keep flowing until the The electrons will keep flowing until the ends are at the same potential.ends are at the same potential.


To sustain a flow of charge in a conductor, To sustain a flow of charge in a conductor, there must be some arrangement where there must be some arrangement where the potential difference can be maintained.the potential difference can be maintained.

This is like maintaining water flow from a This is like maintaining water flow from a higher reservoir to a lower reservoir. higher reservoir to a lower reservoir.


Water flows from the reservoir of high Water flows from the reservoir of high pressure to the reservoir of low pressure pressure to the reservoir of low pressure until the pressure is equal. The picture on until the pressure is equal. The picture on the right shows that water will continue to the right shows that water will continue to flow due to the pump which is maintaining flow due to the pump which is maintaining the pressure difference.the pressure difference.


The flow of charge in a conductor.The flow of charge in a conductor. In solid conductors it is the electrons that In solid conductors it is the electrons that

carry the charge through the circuit.carry the charge through the circuit.This happens because these electrons This happens because these electrons

(conduction electrons) are free to move (conduction electrons) are free to move throughout the atomic network.throughout the atomic network.


Electric Current is measured in amperes Electric Current is measured in amperes (A).(A).

An ampere is the flow of one coulomb of An ampere is the flow of one coulomb of charge per second.charge per second.

1 Coulomb = 6.25 billion electrons

Voltage SourcesVoltage Sources

Charges do not flow unless there is a Charges do not flow unless there is a potential difference.potential difference.

A sustained current needs a suitable A sustained current needs a suitable “electric pump” to provide a sustained “electric pump” to provide a sustained potential difference.potential difference.

Something that provides a potential Something that provides a potential difference is known as a voltage source.difference is known as a voltage source.

Voltage SourcesVoltage Sources

The potential energy per coulomb of The potential energy per coulomb of charge available to electrons moving charge available to electrons moving between the terminals is known as the between the terminals is known as the electromotive forceelectromotive force or or emfemf..

Electrical ResistanceElectrical Resistance

Similar to the rate of water flow in a hose, Similar to the rate of water flow in a hose, the harder the water find it to move the harder the water find it to move through the hose, the less water flows.through the hose, the less water flows.

Electrical ResistanceElectrical Resistance

In a wire, a number of factors apply,In a wire, a number of factors apply,Thickness of wireThickness of wireType of wireType of wireVoltage sourceVoltage source

Units of resistanceUnits of resistance is the ohm ( is the ohm ().).

Ohm’s LawOhm’s Law

Ohm discovered that the amount of current in a Ohm discovered that the amount of current in a circuit is directly proportional to the voltage circuit is directly proportional to the voltage impressed across the circuit, and is inversely impressed across the circuit, and is inversely proportional to the resistance of the circuit.proportional to the resistance of the circuit.

R

v

resistance

voltage(I)current


Q – What is the resistance of an electric Q – What is the resistance of an electric frying pan that draws 12 amperes of frying pan that draws 12 amperes of current when connected to a 120 volt current when connected to a 120 volt circuit?circuit?



A – The resistance is 10 ohms. (why?)A – The resistance is 10 ohms. (why?)



A – The resistance is 10 ohms. (why?)A – The resistance is 10 ohms. (why?)

ohmsI

VR 12

12

120


Q – How much current is drawn by a lamp Q – How much current is drawn by a lamp that has a resistance of 100 ohms when a that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?voltage of 50 volts is impressed across it?



A – The current is 0.5 ampere. (Why?)A – The current is 0.5 ampere. (Why?)



A – The current is 0.5 ampere. (Why?)A – The current is 0.5 ampere. (Why?)

AR

VI 5.0

100

50

Ohm’s Law and Electric ShockOhm’s Law and Electric Shock

What causes electric shock?What causes electric shock?

Current or VoltageCurrent or Voltage


Current causes the electric shock.Current causes the electric shock.

You can be exposed to a large voltage but You can be exposed to a large voltage but if the resistance is very high, not much if the resistance is very high, not much current will pass through you and you will current will pass through you and you will be safe.be safe.


The resistance of your body depends on The resistance of your body depends on its condition. its condition.

If your are soaked in salt water you will If your are soaked in salt water you will have a resistance of about 100 ohms.have a resistance of about 100 ohms.

If your skin is very dry, about 500,000 If your skin is very dry, about 500,000 ohms.ohms.


Q – If the resistance of your body were Q – If the resistance of your body were 100, 000 ohms, how much current would 100, 000 ohms, how much current would be produced in your body if you touched be produced in your body if you touched the terminals of 12V battery?the terminals of 12V battery?


Q – If the resistance of your body were Q – If the resistance of your body were 100, 000 ohms, how much current would 100, 000 ohms, how much current would be produced in your body if you touched be produced in your body if you touched the terminals of 12V battery?the terminals of 12V battery?

A – The current in your body would be A – The current in your body would be 0.00012 A. (Why ?)0.00012 A. (Why ?)


Q – If the resistance of your body were 100, Q – If the resistance of your body were 100, 000 ohms, how much current would be 000 ohms, how much current would be produced in your body if you touched the produced in your body if you touched the terminals of 12V battery?terminals of 12V battery?

A – The current in your body would be A – The current in your body would be 0.00012 A. (Why ?)0.00012 A. (Why ?)

AR

VI 00012.0

000,100

12


Q – If your skin were very moist so that Q – If your skin were very moist so that you resistance was only 1000 ohms, and you resistance was only 1000 ohms, and you touched the terminals of a 24V battery, you touched the terminals of a 24V battery, how much current would you draw?how much current would you draw?



A – 0.024A (A very dangerous current) A – 0.024A (A very dangerous current) (Why?)(Why?)



A – 0.024A (A very dangerous current) A – 0.024A (A very dangerous current) (Why?)(Why?)

AR

VI 024.0

1000

24

DC or AC?DC or AC?

DC – Direct current, electrons flow in one DC – Direct current, electrons flow in one directiondirection

AC – Electrons flow backwards and AC – Electrons flow backwards and forwards.forwards.

DC or ACDC or AC

Can convert AC to DC by the use of a Can convert AC to DC by the use of a diode in a electric current.diode in a electric current.

A diode acts as a one way valve, allowing A diode acts as a one way valve, allowing electrons to only pass through in one electrons to only pass through in one direction.direction.

DC or ACDC or AC

The instantaneous speed of electrons is The instantaneous speed of electrons is close to the speed of light. But this motion close to the speed of light. But this motion is in random directions.is in random directions.

The actual net speed of the current is quite The actual net speed of the current is quite slow as the electrons arte gently nudged slow as the electrons arte gently nudged through he wire due to the electric field set through he wire due to the electric field set up inside the wire.up inside the wire.

Electric PowerElectric Power

The rate at which electrical energy is The rate at which electrical energy is converted into another form such as converted into another form such as mechanical energy.mechanical energy.

Electric Power = current x voltageElectric Power = current x voltage


Q – How much power is used by a Q – How much power is used by a calculator that operates on 8V and 0.1 calculator that operates on 8V and 0.1 amps? If it is used for 1 hour, how much amps? If it is used for 1 hour, how much energy does it use?energy does it use?


Q – How much power is used by a Q – How much power is used by a calculator that operates on 8V and 0.1 calculator that operates on 8V and 0.1 amps? If it is used for 1 hour, how much amps? If it is used for 1 hour, how much energy does it use?energy does it use?

(J) hrs- watt0.81hr0.8WtPE

:Energy

0.8W8V0.1AVI P

:Power

sace stage 1 conceptual physics electric current

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