Electric current is a flow of electric charge through a conductive medium (such as a wire). It
is usually carried by moving electrons.
It is essentially the amount of electrical charge transferred in a given amount of time. The
unit for current is the ampere (A).
Resistance refers to the forces that oppose the flow of electrons through a conductor.
Resistance is measured in ohms (Ω).
For example: let’s say that a water hose is the conductor, and the water flowing through it
represents the electrons. If there are pebbles or rocks in the hose, they will partially restrict
or resist the flow of water. The same principle applies to electrons flowing in a circuit.
In a circuit, cells (also called batteries) have two terminals, a
negative one and a positive one. “Like” charges (i.e. charges
that are the same) repel each other; therefore, electrons
(which are negative) are pushed away from the negative
terminal. By pushing the electrons, the cell is giving them potential energy.
Opposite charges attract, and so the electrons are trying to reach the
positive terminal of the cell. In effect, there are two forces acting on the
electrons; the one pushing them out of the negative terminal and the one
pulling them towards the positive one.
Voltage is a measure of the difference in potential energy between two
points in a circuit. The unit for voltage is volts (V). Voltmeters are used to
measure voltage. Voltmeters are always connected in parallel.
The electrons moving about in the circuit have a certain amount of potential
energy. They lose some energy if they pass through an object that has resistance, such
as a bulb. In the second circuit diagram above right, the voltmeter has been connected
in parallel to the bulb in order to measure the potential difference across the bulb (i.e.
the difference in potential energy of the electrons on either side of the bulb).
A resistor is an electrical component that implements electrical resistance. A variable
resistor is a resistor which can have its resistance changed or varied.
Voltage is directly proportional to resistance. As the resistance increases, so does the voltage. The
equation connecting resistance, voltage and current is:
Voltage
V
Resistance = Current
R= I
In other words:
 if the voltage increases, the resistance also increases.
 if the resistance increases and the voltage stays the same, the current decreases.
When electrons flow through a wire, they collide with the atoms that make up the wire. This causes
resistance. If the wire’s diameter is larger (in other words, if the wire is wider), the electrons have more
space to move about, and therefore wouldn’t collide as much with the atoms in the wire. Therefore,
there would be less resistance.
If the wire is longer, the resistance is also greater. This is because if the wire is longer, the current (the
electrons) will have a greater distance to travel, i.e. a greater distance during which they can collide with
the atoms in the wire.
A current is caused by moving electrons through a wire. Sometimes, the electrons can
collide with the atoms in the wire. This causes resistance, which makes it harder for
the current to flow; therefore, as resistance increases, current decreases, and viceversa.
Ammeters are used to measure the current flowing through a circuit (i.e. they
measure amperes or amps). Ammeters need to be connected in series.
The resistance of an object (such as a bulb) can be found by using an ammeter and a
voltmeter.
Set up the ammeter in series to the bulb in order to find the current. Set up the
voltmeter in parallel to the bulb to find the potential difference. Then, using the
formula R = V/I, divide the voltage by the current. For example, if the current is found
to be 15 amps, and the voltage 30 volts, dividing 30 by 15 yields an answer of 2 Ω.
ELECTRIC CHARGE
Electrostatic electricity is created by loose electrons. For example, if you rub a balloon against a rug or a
dry cloth, the balloon will rub some electrons off the cloth and onto the balloon. Therefore, the balloon
will have an electrostatic charge on its surface. If you then touch the balloon, you will feel a small electric
shock as the electrons pass from the balloon’s surface to your hand.
This effect can also be demonstrated by rubbing a balloon against a carpet and then running it under
water. Turn the tap on only slightly so that a thin stream comes out. Place the balloon just next to but not
touching the stream of water. Water molecules have both a positive and a negative charge on either side
of them. The positive charge will be attracted to the electrons, bending the water stream towards the
balloon.
This video demonstrates the experiment.
An electric field is created when an electrically charged object is placed near another charged or polar
object, creating a force of electrostatic attraction between the two. In the experiment above, the positive
charges in the water are attracted to the negative charges on the balloon. This creates an electric field.
Conductors are materials through which an electric current can flow freely. Examples include a lot of
metals, such as steel and copper.
Good conductors have low resistance.
Insulators are materials through which an electric current cannot flow easily. Examples include materials
such as wood, plastic, glass and textiles (cloth). Insulators have high resistance.
Power is a measure of how much energy is transferred or converted per second. Power is measured in
watts (W) (1 watt can also be expressed as 1 joule/second).
The formula for power:
Power = current * voltage
P = IV
Energy is a measure of how much work an object can do.
The formula for energy:
Energy = (current * voltage) * time
Energy = power * time