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ELECTRIC CURRENT
Since electric charges experience forces, they can be made to move. The movement of
charges is called an electric current. More precisely, current is the amount of charge flowing
past some point in a given time, i.e.
Q
I= t
charge (C)
Current (ampere, A) = time (s)
C
Note: One coulomb per second is called one ampere (A), i.e. 1 A = 1 s .
QUESTION 1
How long does it take 20 C of charge to move past a given point if a current of 2 A is
flowing?
Solution
 The School For Excellence 2016
Summer School – Year 11 Physics – Book 1
Page 15
CIRCUITS
A circuit is a closed loop around which charges can flow. The simplest possible
arrangement for a circuit is shown in the following circuit diagram. Symbols are used to
represent the various circuit elements, i.e. The parts of the circuit.
Cell
Wire
Resistor
A cell is simply a source of energy – it gives energy to the electric charges that flow through
it. A cell does not produce charges; it just causes charges to move. The charges are
everywhere in the circuit – the various circuit elements and the wires are full of them.
As charges leave the cell, they have more electrical potential energy than they did when
they entered the cell.
Charges have difficulty moving through resistors. In resistors, the charges lose the energy
that they acquired when they passed through the cell.
Wires, on the other hand, present very little resistance to the flow of current. Therefore,
charges lose almost no energy at all as they pass through the wires.
All types of devices connected to an energy source are called a load. Note that all circuits
contain an energy source, a load and connecting wires. The energy source supplies
electrical energy and the load converts the electrical energy to other forms of energy – heat,
light, sound, etc.
 The School For Excellence 2016
Summer School – Year 11 Physics – Book 1
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DIRECTION OF CURRENT FLOW
A cell has a positive terminal and a negative terminal.
+
-
Positive charges would be repelled from the positive terminal and attracted to the negative
terminal. The only available path for them is therefore in the direction shown in the following
diagram.
I
This (hypothetical) flow of positive charges is known as conventional current.
 The School For Excellence 2016
Summer School – Year 11 Physics – Book 1
Page 17
CONSERVATION OF CHARGE
Charges cannot be created or destroyed. If, for example, 4 C of charge enters a resistor
every second, then 4 C of charge must leave that resistor every second. (Resistors do not
store charge.) Therefore, the current is the same on both sides of the resistor.
Note: 4 C per second = a current of 4 A.
4A
4A
The current flowing into the cell is also 4 A because that is the amount that flows out of the
resistor. It can therefore be stated that, in this simple circuit, the current is the same at all
points in the circuit.
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4A
4A
4A
4A
Summer School – Year 11 Physics – Book 1
Page 18
CONDITIONS FOR THE FLOW OF CURRENT
It follows that no current will flow if the circuit is not complete.
In the following circuit, no current can flow out of the resistor because there is nowhere for it
to go. Therefore, no current can flow into the resistor. This implies that no current can flow
out of the cell and, as a consequence, no current can flow into the cell.
It is a simple matter to control the flow of current: breaking the circuit stops the flow,
completing the circuit restores the flow of current. A device that does this is called a switch.
Switch
A switch can be placed anywhere at all in the simple circuit shown above, and still serve its
function. It does not matter where the circuit is broken – if there is no complete path for
charges, there is no current.
 The School For Excellence 2016
Summer School – Year 11 Physics – Book 1
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ELECTRICAL POTENTIAL
As charges flow through a resistor, they lose electrical potential energy. Therefore, in the
following circuit, the charges will have less energy at point B than they do at point A.
The electrical potential energy they lose in the resistor is converted to heat.
Heat
I
A
B
The amount of potential energy lost by each coulomb of charge is called the potential
difference, i.e.
V=
ΔE
Q
EMF = potential difference (volt, V) =
Δenergy( J )
charge(C )
J
Note: 1 V = 1 C
Please remember: A volt is the energy (in joules) lost by each coulomb of charge.
As charges flow through cells, they gain energy. However, in order to travel from point Y to
point X in the following circuit, positive charge must move, within the cell, from the negative
terminal to the positive terminal. Since positive charges are repelled from the positive
terminal, the only way they can get there is if they are supplied with energy. Chemical
reactions within the cell supply this energy.
X Y
I
A
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B
Summer School – Year 11 Physics – Book 1
Page 20
Whatever energy the cell supplies to the charges is the amount that they will lose as they
pass through any resistance in the circuit. If, for example, the cell is rated at 9 V, this means
that it supplies 9 joules to each coulomb of charge that flows through it (9 V = 9 J/C).
Therefore, 9 joules will be lost by each coulomb of charge as it flows through the resistor.
Remember: Energy supplied by the power source = Energy lost in the load.
The following graph shows the variation of potential around the above circuit. Remember,
potential is simply the energy possessed by each coulomb of charge; it is measured in J/C.
As the charges move through the wires, they lose no energy, so the section between points
X and A and the section between points B and Y are represented as horizontal lines on this
graph. Between points A and B, the charges are passing through the resistor and are
therefore losing energy. In going from point Y to point X, they are passing through the cell
and are therefore gaining energy.
Potential
X
A
X
V
Y
B
Distance
The following four column table may help greatly with symbols and units.
Property
Symbol
Unit
Symbol
Charge
Q
Coulomb
C
Current
I
Ampere
A
Voltage
V
Volt
V
Energy
E
Joule
J
Resistance
R
Ohm
Ω
Power
P
Watt
W
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Summer School – Year 11 Physics – Book 1
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ENERGY TRANSFER
E
If we transpose the equation V = Q in order to make E the subject, we obtain: E = VQ.
QUESTION 2
A certain cell does 120 J of work while supplying 6 J of energy to each coulomb of charge
that flows through it. How much charge passed through the cell?
Solution
QUESTION 3
If the potential difference across a certain resistor is 12 V, how much energy is lost by each
electron that flows through it?
Solution
 The School For Excellence 2016
Summer School – Year 11 Physics – Book 1
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ENERGY TRANSFER (AGAIN)
Q
Since I = t , we can write: Q = I t.
We can now substitute this into the equation E = VQ and obtain: E = V I t.
QUESTION 4
It takes a certain 12 V cell 2 s to do 48 J of work. How much current is flowing through
the cell?
Solution
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Summer School – Year 11 Physics – Book 1
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POWER
Power is the rate at which energy is converted, i.e.
E
P= t
energy (J)
Power (W) = time (s)
J
Remember: 1 W = 1 s .
E
If, instead of E, we write V I t (see above), the equation P = t becomes:
P=
VIt
=VI
t
Check the units: V is measured in J/C and I is measured in C/s. So, multiplying V by I gives:
J . C
C
s
J
= s
= W
This all makes sense. The rate at which energy is used depends on the amount of energy
carried by each coulomb (i.e. V) and on the number of coulombs per second that are going
the load (i.e. I). Think about it.
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Summer School – Year 11 Physics – Book 1
Page 24