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5
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The atom is neutral when the number of electrons equal the number of protons
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If an electron is added to an atom, the atom has a net (-) charge and is called a negative ion
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Electric charge is quantised, meaning that the amount of charge is always a multiple of a basic unit
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The law of conservation of charge states that charge can be neither created nor destroyed
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- Conductors are materials that have many free electrons
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Coulomb's Law
Coulomb's Law states that the electric force F between two charges

and

is given by:

Where:
is the force in Newtons
is Coulomb's constant (
)
and is the charges in Coulombs
is the distance between the two charges in metres
Coulomb's Law can also be written in the form:

Where

is the permittivity of vacuum
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Coulomb's Law can also be:

Where:
is the length of the current carrying wire
and are the size of the currents in the wire
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A space can be tested for an electric field by bringing a test charge q into the space
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If q does not experience an electric force, then there is no electric field
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Where

is the electric field strength in Newtons per Coulomb

Topic 5 Page 4

Electric Current
'Free' electrons in a conductor move randomly
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If an electric field is applied to a conductor, the electrons experience
a force that pushes them into the opposite direction of the field
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which is called an Ampere


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The current can be written in the form:
Where:
is the number of electrons per unit volume
is the cross-sectional area of the wire
is the drift speed
is the charge (elementary charge)

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Electric Potential Difference
Because electric charge experiences electric force, when one charge is moved from A to B, work
is an electric field, there is a potential difference
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Whenever there

between two points is the work done per unit charge to move a point charge from one place to another
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2 - Heating of Electric Currents

Topic 5 Page 8

Resistance & Resistivity
Electric resistance determines how much current will flow for a given potential difference
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The unit is Ohms (Ω)
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The less carbon there is, the less current that will flow through the resistor
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They are:
1
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The Length of the Wire
3
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Ohm's Law
Ohm's Law states that when the temperature of most metallic conductors is kept constant, the current is proportional to the potential difference
across the conductor
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A graph of versus gives a straight line through the origin
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Topic 5 Page 10

Power Dissipation
The power dissipated in a resistor is given by:

If an electric circuit only has resistors connected to a battery, the energy from the source gets dissipated in the form of h eat
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Electric Circuits
An electric circuit is a set of conductors (like wires) and components (like resistors, lights, etc
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Resistors connected in a series are connected one after the other
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Resistors connected in parallel have

the same for all parallel components
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Kirchkoff's Law states that the sum of the currents entering a junction equals the sum of the currents leaving the junction
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Kirchhoff's Law states that the sum of the currents entering a junction equals the sum of the currents leaving the junction
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<

Applying Kirchhoff's Law:
(writing this down guarantees one mark)

<

Values for voltage and resistance will normally be given
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Topic 5 Page 14

<

5
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A chain of electrons come out of the

end into the

A primary cell is a cell that can only be used once
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The capacity of the cell is how much charge it can deliver to an external circuit in its lifetime
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Emf
The emf (of a battery) is the work done per unit charge in moving charge from one terminal of the battery to another
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A battery is a common source of potential difference in a circuit
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Topic 5 Page 17

Internal Resistance

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5
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Magnetic Force is present when two magnetic bars are next to each other
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The Earth can be thought as one
giant magnet
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Right Hand Rule
The direction of the magnetic field around a straight wire carrying a current is given by the right-hand grip rule
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The direction in which the fingers curl is the direction of the 'flow' of the magnetic field vectors (B field)
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It is different for a solenoid:

Topic 5 Page 21

Magnetic Force
An electric charge moving in a region of magnetic field experiences a magnetic force
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Parallel currents attract
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Therefore, a current carrying wire placed in a magnetic field will experience a magnetic force
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A current carrying wire acts like a magnet and can attract and repel other magnetic objects
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Current carrying wires in the opposite direction:

They repel each other
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Topic 5 Page 23



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