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Transcript
Current, Voltage and resistance
Objective: to recap GCSE circuits
To know the circuit laws for current, voltage and
resistance.
Starter: table to fill out
Current
The net flow of charged particles
In a metal this is due to electrons, when the
circuit is not complete then the electrons just
drift around and there is no net flow.
Current
The net flow of charged particles
When a potential difference is applied the
electrons start to drift in the positive to neagtive
direction. This causes a net movement not a
rapid flow.
Annoyingly conventional current is positive to
negative.
Representing current direction
Current
Another way of thinking of current is the rate of flow of
charge.
Or
𝑄
𝐼=
𝑑
π‘β„Žπ‘Žπ‘Ÿπ‘”π‘’
πΆπ‘’π‘Ÿπ‘Ÿπ‘’π‘›π‘‘ =
π‘‘π‘–π‘šπ‘’
Units of current = Amps (A)
Units of charge = coulombs (C)
Unit of time = seconds (s)
How does current behave in series and parallel
circuits?
Set up some circuits and fill out the sheet
(15mins)
Current is the same everywhere in a series
circuit
Current splits up at each junction/branch in a
parallel circuit
Voltage
Voltage, also known as potential
difference, is a measure of the energy
provided to the charge carriers. It can be
defined as the amount of work done per
unit charge.
V (V) =
W (J)
Q (C)
Voltage is measured as a difference in potential between two points.
Thus a voltmeter must be connected in parallel and used to
measure the difference in potential across a device.
If a cell supplies 23 coulombs of charge with 776 J of energy, what is its
voltage?
V=
776
23
= 33.7 V
How does voltage(P.D) behave in series and
parallel circuits?
Set up the circuits and record your answers on
the sheet
Voltage (P.D) is divided amongst the
components in a series circuit
Voltage (P.D) is the same across each branch in a
parallel circuit
Resistance
Resistance is a measure of the opposition a material exerts against the flow of electrons.
The resistance of a material can be calculated from the
current and voltage passing through it.
R (Ξ©) =
V (V)
I (A)
Calculate the resistance in this simple
circuit.
9.7 V
3.2 A
R=
9.7
3.2
= 3.0 Ξ©
How does resistance behave in series and
parallel circuits?
R2
R1
Equivalent resistance
R3
R4
R5
R6
RT
R7
When designing or analysing circuits, complex combinations of resistors are common. To
perform calculations, for example to find a suitable fuse to protect the circuit, it is easier to
use a value for the total resistance of the circuit, RT.
RT can be called the equivalent resistance because it is the single resistor that is equivalent
to the complex combination.
Resistors
in
series
To work out the equivalent resistance of resistors in series, the resistor values can just be
added together:
10 Ξ©
20 Ξ©
equivalent resistance = 10 + 20 + 15
= 45 Ξ©
In general for a number of resistors, n:
RT = R1 + R2 + … + Rn
15 Ξ©
Resistors
in
parallel
To work out the equivalent resistance for resistors in parallel, a more complex equation
must be applied:
1
RT
=
1
R1
+
1
R2
+
…
+
1
Rn
For example:
1
RT
10 Ξ©
1
RT
20 Ξ©
15 Ξ©
1
=
=
10
+
1
20
+
6+3+4
60
=
RT
RT
1
13
60
=
60
13
Ξ©
= 4.62 Ξ©
1
15
Resistor combinations