Revision
of Year 8
New Terms
Electric
Circuits
A piece of wire contains many free electrons
which carry an electric charge.
These charges can be made to move along the
wire by giving them some energy.
When the charges move, an electric current
is produced. To produce an electric current, an electric circuit is required.
An electric circuit needs three essential items
-
a battery or power supply which provides the electrical
energy;
-
a conducting path (wires) along which the electrons
can move;
-
a load (light globe, heater, fan) in which the
electrical energy is converted into other forms of energy (light, heat,
motion).
Electric current (symbol I) is the rate at which
electric charge moves through a load. The unit of electric current is the
ampere (symbol A). Electric
current is measured with an ammeter placed in the circuit in , as
shown.
Electrons in the wires flow from the negative
terminal to the positive terminal. However, the conventional direction
of flow of current was defined as being from the positive terminal to the
negative before people knew about electrons, and so now we are stuck
with both systems.
The amount of energy
given to electrons by the battery is called the voltage of the battery.
This is sometimes called emf and is measured in volts (symbol V).
The amount of energy used by a load is also
called voltage and is measured in volts. This is sometimes called
pd or potential difference across the load. Voltage is measured with
a voltmeter placed across the battery or load, in parallel, as
shown.
Remember, energy is transferred
to the electrons as they pass through the power supply or battery, and
transferred to the load components
as the electrons pass through them.
The amount of energy transferred to the electrons
is equal to the amount transferred to the load components
by the electrons.
Crudely put, Volts in = volts out.
The available energy is shared among all the
load components, depending on their resistance.
Check this out by doing
Experiment 1: Measuring Current and Potential Difference.
Resistance: the concept.
The resistance of a component in a circuit
is a measure of how "difficult" it is for the electrons to move through
the component, and how much energy the electrons will transfer to the load
component as they pass through it.
Good conductors
have almost no resistance. Electrons do not have to use much energy to
actually move through the material, and so the current is not limited.
As a result a good conductor becomes quite hot because the amount of heat
produced increases as the current increases.
Good insulators
have a very high, or infinite resistance.
Electrons cannot pass through an insulator.
Load components like light bulb filaments
allow electrons to flow, although at a slower rate, but some of the electron's
energy is used to move through the material of the load component. This
electrical energy is converted to some other form, eg heat and light in
the load component. The current is usually lowered sufficiently that there
is negligible heating in the conducting wires that form part of the circuit.
Resistance:
the quantity.
The resistance of a component is calculated
by dividing the potential difference or voltage drop across it by the current
passing through it.
| R= V/I |
where
-
R is resistance in ohms (W)
-
V is potential difference or voltage in volts (v)
-
I is current in amperes (A)
|
For some materials, this value remains constant
as I is increased. These are called Ohmic resistors.
What shape will a graph of V vs I be?
For others R varies as I changes.
Resistors are often used in a circuit to decrease
the current flowing in that circuit
Experiment 2: Measuring
Resistance.
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