Download 12.6 Electrical Circuits

Physics
(14 - 16)
Electrical Circuits
Varying Resistance
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Current
Electricity effectively travels at the speed of light in a conductor
(186,000 miles/second) but slower in an insulating material.
An electric current is a flow of charge carried by tiny particles
known as electrons.
Movement of charge is
caused by the force from a
voltage supply such as a
battery or mains supply
acting on electrons which
are free to move.
Note: The electrons flow in the opposite direction to that of the
current.
Current
Electricity effectively travels at the speed of light in a conductor
(186,000 miles/second) but slower in an insulating material.
An electric current is a flow of charge carried by tiny particles
known as electrons.
Movement of charge is
caused by the force from a
voltage supply such as a
battery or mains supply
acting on electrons which
are free to move.
Note: The electrons flow in the opposite direction to that of the
current.
Current
Electric current is measured in units called amps (A).
The electric current is not used up by the components in a circuit
but it transfers energy from the voltage source to the various
components making up the circuit.
Electric current is measured using an ammeter.
Current
Electric current is measured in units called amps (A).
The electric current is not used up by the components in a circuit
but it transfers energy from the voltage source to the various
components making up the circuit.
Electric current is measured using an ammeter.
The ammeter is placed in series within the circuit.
Voltage
Voltage is supplied by a battery, cell or power supply and is
measured in volts (V).
The voltage of a power supply
(converting other forms of energy
into electrical) is an electromotive
force (emf).
The voltage across a circuit
component, e.g. a bulb converting
electrical energy into heat and light
is a potential difference (p.d.).
Potential difference is also measured in volts (V).
The potential difference is a measure of the energy carried by the
charge (current) flowing within the circuit.
Voltage
Within an electrical circuit, the difference in electrical potentials
across different components drops as each component converts
electrical energy into other forms of energy.
Potential difference is measured using a voltmeter.
The voltmeter is placed in parallel to the circuit.
Voltage
Within an electrical circuit, the difference in electrical potentials
across different components drops as each component converts
electrical energy into other forms of energy.
Potential difference is measured using a voltmeter.
The voltmeter is placed in parallel to the circuit.
Note: the switch does not need to be closed to take a voltage
reading across a battery.
Resistance
Resistance is the property of a material which restricts the flow of
electricity. Energy is converted into other forms (e.g. light, heat),
as the voltage across the component drives the current through it.
The unit of resistance is the Ohm (Ω).
What causes resistance?
As an electric current flows, charged particles called electrons
move through a conductor. These moving electrons collide with the
atoms of the conductor, making it more difficult for the current to
flow. This causes the resistance.
What factors affect resistance?
Longer wires increases resistance. The electrons collide with
atoms more often in a long wire than in a short wire.
Thin wires increases resistance. A thin wire has fewer electrons to
carry the current than a thick wire.
Georg Ohm
In 1826, based on his work on conduction
in metal wires, Georg Ohm formulated a
law relating to the current passing through
a wire, and the potential difference
(voltage) applied.
He found that for a fixed temperature,
the current flowing through a resistor
is directly proportional to the potential
difference across the resistor.
V=IxR
Georg Simon Ohm
(1789 - 1854)
This is known as Ohm’s Law.
Ohm also introduced the SI unit of resistance, as the Ohm (Ω).
Ohm’s Law
Using Ohm’s Law, the resistance of any component can be
calculated by measuring the current in the circuit, and the
potential difference across the component.
Ohm’s Law
Using Ohm’s Law, the resistance of any component can be
calculated by measuring the current in the circuit, and the
potential difference across the component.
Ohm’s Law
Ohm’s Law also allows you, given any two values, to calculate any
one of the other three values within the triangle.
A resistor is placed in a circuit
with a potential differences of 12
volts and a current flow of 0.1A.
What is the resistance?
R= V
I
R = 12V
0.1A
R = 120Ω
The resistance is 120Ω
Ohm’s Law
Ohm’s Law also allows you, given any two values, to calculate any
one of the other three values within the triangle.
A 10Ω resistor is placed in a
circuit with a potential
difference of 5 volts.
What is the current?
I = V
R
I = 5V
10Ω
I = 0.5A
The current is 0.5A
Ohm’s Law
Ohm’s Law also allows you, given any two values, to calculate any
one of the other three values within the triangle.
A 20Ω resistor is placed in a
circuit with a current of 0.1 A.
What is the voltage across
the resistor?
V = IxR
V = 0.1A x 20Ω
V = 2V
The voltage is 2V
Electron Flow in a Circuit
To appreciate how a flow of electrons can carry an electric charge,
a simple electric circuit can be compared with a domestic central
heating system.
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Ohm’s Law
Drag each of these resistors into the circuit to find out how they
affect the current flowing in the circuit.
From the results you should see that:
For a fixed potential difference within the circuit, the greater the
resistance, the smaller the current.
Current v Potential Difference Graphs
Current against potential difference graphs can be used to show
how the current varies through a given component, dependant
upon the potential difference.
.
Thermistor
The resistance of a thermistor is dependant upon the temperature.
Turn up the temperature on the thermistor (drag the red slider on the
thermometer) and see how it affects the resistance in the circuit.
(If the current goes down, the resistance has gone up.)
A thermistor’s resistance decreases as the temperature increases.
Light-dependant Resistor (LDR)
The resistance of a light-dependant resistor (LDR) is dependant
upon the levels of light.
Turn up the light on the LDR and see how it affects the resistance in
the circuit. (If the current goes down, the resistance has gone up.)
The LDR’s resistance decreases as the light intensity increases.
End of Show
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