Electricity

By the end of this unit students will be able to:

Static Electricity

Understand that "static" electricity results from the build-up of electric charge
Understand that static charge arises because of the removal or addition of electrons to an insulator as a result of friction (insulators rubbing together)
When electrons are rubbed onto (or added to) the object, the object has more electrons than protons, so the object becomes negatively-charged.
When electrons are rubbed off (or removed from) the object, the object has less electrons than protons, so the object becomes positively-charged.
State that an object becomes charged only due to the addition or removal of electrons and not protons because protons cannot move - only electrons are transferred during friction.
Understand that like (or same) charges repel and unlike (or opposite) charges attract each other; relate this to observations about the way electrically charged objects behave.
State that charged objects can attract (but not repel) uncharged (neutral) ones (note: Year 10 students do not need to be able to explain this)
Describe conductors as substances where charges can easily move and insulators as substances that make it very difficult for charges to move.
Classify some common substances as conductors or insulators based on general knowledge or on information supplied.

Current Electricity

Give examples of substances which make good conductors and good insulators.
Describe an electric current as the flow of electrons.
State that the units of current (symbol: I) is “amperes” (symbol A).
State that current refers to the rate at which electrons flow past a point in a complete electrical circuit.
Describe ‘voltage’ (symbol: V) in terms of the electric potential energy carried by the electrons around a circuit’
State that the units for voltage is “volts” (symbol: V – same as the symbol for voltage).
Describe resistance as anything that limits the flow of electrons around a circuit.
State that the units of resistance (symbol: R) is “ohms” (symbol: Ω)
Apply the formula V=IR (where V=voltage; I=current and R= resistance) to find one of the three values where the other two are supplied in a simple problem. (Simple rearrangement may be required)
Use the correct symbols to draw circuits using these components: bulb, cell/ battery, power source, wire, ammeter, voltmeter, fuse, resistor, variable resistor, switch (open and closed). (Note: students are expected to use rulers to draw all straight lines for circuit components and wires, and to follow circuit conventions in component placement.)
Investigate the differences in series and parallel circuits in terms of their set-up - in series circuits, current flows in one circuit and is the same at each point in the circuit, while voltage is split across components. In parallel circuits, the current is the split across each branch while voltage is the same across each branch.
Calculate resistance, current and voltage values in a simple series or parallel circuit (range: two devices in series or parallel)

Static electricity

Static electricity is caused electrical charge building up in substances which are usually insulators. In an uncharged insulator, the number of positive and negative charges are equal. Such objects are neutral.

Substances can be conductors, which allow charges to move easily, or insulators which make it very difficult for charges to move. Many substances are somewhere in-between. Insulators with a static charge can hold that charge, but conductors can only hold a charge if they are isolated (not touching where the charges can leak away).

Conductors and insulators

Matter is made of atoms, which in turn consist of the nucleus (positive) and electrons (negative). In solids, the atoms must stay in place. If the electrons cannot move around by jumping from atom to atom, the substance will be a good insulator. This applies to many non-metals.

Metals conduct because they allow electrons to move around by jumping between atoms.

Charging an insulator by friction

Some insulators can become electrically charged when they rub together. This requires two different substances, one of which attracts the electrons slightly more strongly than the other. Conductors can be charged by friction if they are electrically 'isolated

An object becomes charged when electrons are lost or gained by friction.

For example, if electrons are rubbed off perspex and onto a cloth, the ‘holes’ where the electrons were on the perspex create a positive charge on the perspex rod. The extra electrons stick to the cloth because it is an insulator and they can’t go anywhere, and the cloth becomes negatively charged.

POSITIVE CHARGES NEVER MOVE. An object becomes positive by losing electrons.

Attraction and repulsion

"Like" charges repel each other i.e. positively charged objects will repel other positively charged objects; negatively charged objects will repel other negatively charged objects.

One fairly common example of this is when a child's hair stands on end because they have become electrically charged by rubbing on the plastic of a trampoline mat. The hairs all have the same charge and repel each other. They stand on end because that is how they can get as far away from each other as possible.

Attraction between charged and uncharged objects

A charged object can attract one that is uncharged. This works best with substances that allow a little movement of the charges, but which are electrically isolated (not able to be supplied with electrons from elsewhere). You will be familiar with the ability of a rubbed pen or ruler to pick up bits of paper.

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