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 goes- the 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