Transcript Electricity explained

Electricity explained
Science Consultant – Philip Storey
Imagine a lake on top of a mountain.
So what?


Water flows down the mountain due to the force
of gravity and charge flows around a circuit due
to electrostatic forces. Think what exerts this
force.
Current is the volume of charge
passing a point per second which is like
the volume of water passing per second.

Current is the same all the way around a circuit
due to resistive forces balancing electrostatic
forces
Now imagine a narrowing in the river.
So what?

The narrower section offers more resistance and reduces
the volume per second flowing down the mountain.

The reduced volume per second is the same in all sections
of the river.

The speed of the water through the narrow section is
greater than in the rest.

The water strikes the boulders at high speed in the narrow
section. More energy is transferred here and the boulders
rock back and forth.
So what?






The narrower section is like a narrower wire.
The current from the battery is reduced.
Charge rushes through the narrower section but
the reduced current is the same in all parts of a
series circuit.
The electrons carrying electric charge strike the
atoms in the narrower section harder.
These atoms vibrate more and the thin wire gets
hot.
More energy is transferred in the thinner wire.
And?
Energy transferred by each coulomb of
charge is called Voltage.
 The vast majority of energy transferred in
a series circuit is in high resistance
devices.

Think about what happens if a different material is
used instead of a different thickness.
What about two narrow sections in
Series?
So what?




If a second component is connected in series,
then the current in the whole circuit reduces.
The speed of the charge is greatest in the
narrowest section.
More energy is transferred in the section of
highest resistance (narrowest) by each charge.
The highest resistance bulb is the brightest when
the bulbs are connected in series.
Parallel circuits have branches.
So what?

The alternative route reduces the
resistance and so increases the current
(volume per second) from the mountain.

The speed of the water in the main river
and branch doesn’t change. Convinced?
Wait a minute.
Imagine a plastic sheet is lowered into the middle of the
river.
So what?

The divider won’t have any impact on the
speed on either side. The speed on both
sides is the same.

The volume of water per second is the
same on both sides if the divider is placed
in the middle. (i.e. the current is the same
if the two branches are of equal
resistance.)
So what?

The speed of the charges in parallel
branches of a circuit is always the same.
Therefore, the energy transferred by each
coulomb of charge is the same. This
means that the voltage across all parallel
components is the same.

This explains why identical bulbs placed in
parallel have the same brightness.
It doesn’t matter where the divider goesthe speed doesn’t change.
What if the bulbs aren’t identical?

The speed of the charges through each branch is
still the same.

A greater volume flows through A per second
than B ( cf. electric current is greater through a
low resistance component in parallel with a high
resistance one.)

The volume of charge flowing through each
branch is different and so more energy is
transferred where the volume per second is
greater. i.e.. in A.

Don’t use this model with the children.

Spend some time thinking about the
model.

Try these instead.



Children walking around the room using
energy to walk over stage box.
ICT – Furry Elephant – Electricity explained –to
be demonstrated.
Building circuits.
Summary – General Points.

A complete circuit is required for a current to
flow.

It doesn’t matter whether a switch is immediately
before or immediately after a bulb, if it is open
the bulb won’t light up.

Make sure that pupils know the correct symbols
and can draw circuit diagrams with more than
one of each component.

Adding an identical cell produces twice the force,
twice the voltage and therefore bulbs have twice
the brightness.
Summary –Series Circuits
Current is the same in all parts of a series
circuit.
 Identical bulbs in series have identical
brightnesses.
 Adding more bulbs in series reduces the
current in the whole circuit and the
brightness of all the bulbs.
 Bulbs can’t be switched on/off separately.

Summary – Parallel Circuits.

Brightness of a bulb don’t change if
another is placed in parallel.

Connecting more bulbs in parallel causes
the battery to run down quicker.

Connecting bulbs in parallel enables the
bulbs to be operated separately