ME0005 – Foundations of Physics

Electricity 1/4

Current and Charge
All familiar with electric circuits: lighting, computers, mobile phones etc
A current is a flow of charged particles
Scientific convention: current flows from positive (+ve) to negative (-ve)  conventional current
What is flowing?
In a metal wire there are free electrons that can move about  conduction electrons  they allow a metal to conduct an electric current
Electrons are negatively charged. When an electron breaks away from an atom in the metal, it leaves behind a positively charged ion. There are an equal number of electrons and ions in a metal  the metal is charge neutral When a battery or ‘cell’ is connected, its ‘voltage’ provides a push to make the conduction electrons flow around the circuit. The negatively charged electrons flow to the positive terminal of the cell (opposite to conventional current) Current flows at all points in the circuit once the circuit is completed. Electrons are already present in the metal and do not flow from the cell
Electrical current is measured in amperes (A). How much charge is moving when a current of 1 A flows? Depends on how long the current has been flowing: charge = current  time

ME0005 – Foundations of Physics

Electricity 1/4

Charge is measured in coulombs (C). We define the coulomb as:
One coulomb is the amount of charge which moves past a point when a current of 1 ampere flows for 1 second
We can say:

Q = I t
If a current, I, flows for an amount of time t, the charge that moves during that time is Q
An electron has a charge of -1.6  10-19 C, represented by the symbol eExample:

If a current of 10 A flows through a lamp for
1 hour, how much charge moves through the lamp in this time?
Q =
=
=
=

Example:

I t
10  (60  60)
10  3600
36000 C

A car battery labelled ‘50 A h’ can supply
50 A for 1 hour. How long can the battery supply 200 A needed to start the car?

First we need to find the total charge in the battery:
Q =
I t
=
50  (60  60)
=
180000 C
Now we can find the time the battery can supply 200 A:
t

=

Q
I

=
=

180000 / 200
900 s

ME0005 – Foundations of Physics

Electricity 1/4

Resistance
What determines ‘how much’ current flows?
 voltage, V, of the cell:
 more voltage = more current
 resistance, R, of the component:
 greater resistance = smaller current
The resistance of a component tells you how much voltage is needed to push a given current though it.
We can say:

current 

voltage resistance I

V
R

Resistance is measured in ohms ()
Example:

A car headlamp has a resistance of 36 .
What current flows when it is connected to the car’s 12 V battery?

I

V
R

=
=

12 / 36
0.33 A

Re-arranging the equation for R we have: R 

V
I

We then define resistance as:
The resistance of a component in a circuit is the ratio of the voltage across the component to the current in it
So, one ohm is one volt per ampere:

1  = 1 VA-1

ME0005 – Foundations of Physics

Electricity 1/4

Measuring resistance
To measure resistance we need to know I and V
We use an ammeter to measure current: connected in series to measure the current passing through a component We use a voltmeter to measure voltage: connected in parallel to measure the voltage across a component
Varying the voltage and measuring the current we obtain an:
I / V characteristic graph
The gradient of the graph is the resistance (R = V/I)

V

voltage

A

current

For a metallic conductor we get a straight line that passes through the origin (no voltage applied, no current flows).
We say that:


the current that flows through the conductor is proportional to the voltage across it

Components with an I / V graph like this are called ohmic conductors and are said to obey:
Ohm’s Law  For a conductor at constant temperature, the current in the