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Electric Circuits
Tuesday, May 23, 2017
 To calculate the size of a current from the charge flow and time
taken
Electricity
 Energy can never be created or destroyed, it can only ever be
converted from one form to another
 Energy is only useful when it is converted from one form to
another
 Electricity is so useful because it can be easily transferred…
Electricity
 Electricity is the flow of electrical power (charge) in the form of
electrons
 Electricity is a useful secondary energy source – most energy
sources (like coal, oil, nuclear, wind etc…) can be converted into
electricity
Circuits
 A bulb in the circuit is like a radiator – an electrical device uses electrical
energy, supplied by the circuit
 The wires are like pipes - they
carry the flow of electricity
(current) around the circuit
Cell
+
Wires
 The electrical current is pushed
by the cell (battery) – this is the
voltage
 The electrons flow from –ve to
+ve
Lamp
Electrical Circuits
 An electric current needs two things:  Something to make the electricity flow (battery or power pack)
 A complete circuit
 Without these two basic things, an electric current will not flow
Cell
+
Wires forming a
complete circuit
-
Component Diagrams
 Circuit symbols are used to show the components in an electrical
circuit (wires are represented by straight lines)
Symbols
 Complete the electric circuit symbols worksheet
Symbols
Symbols
Task
 Your task is to set up some simple circuits using the minimum
number of components necessary – you will have 3 minutes to set
up each circuit…
Cell
+
Wires
Switch
Lamp
Circuit 1
 Circuit 1 – basic series circuit
+
-
Circuit 2
 Circuit 2 – basic series circuit with ammeter (move this around to
and note the current at different points) and voltmeter
+
-
A
V
Circuit 3
 Circuit 3 – basic parallel circuit with ammeter and voltmeter
+
-
A
V
V
Circuit Experimentation
 Finally, experiment with the circuits (both series and parallel) –
use the ammeters and voltmeters at different points within the
circuits to try and establish some rules for the current and
voltage…
Series Circuits
 Components that are connected one after another on the same
loop of the circuit are connected in series
 If you remove or disconnect one component, the circuit is broken
and they all stop
+
6V

Voltages add to equal the supply
1.5V + 2V + 2.5V = 6V

Total resistance
3Ω + 4Ω + 5Ω = 12Ω

Current = voltage ÷ resistance
6 ÷ 12 = 0.5A
A
A
3Ω
5Ω
4Ω
V
V
V
1.5V
2V
2.5V
Series Circuits
 The same current flows through all parts of the circuit
 The total resistance is the sum of all the resistances
 The size of the current is determined by the total p.d of the cells
and the total resistance of the circuit (I = V/R)
 The total p.d of the supply is shared between the various
components, so the voltages around a series circuit always add up
to equal the total voltage of the supply
 The bigger the resistance of a component, the bigger its share of
the total p.d
Parallel Circuits
+
 Each component is separately connected to
the +ve and –ve of the supply
 Removing or disconnecting one component
hardly affects the others at all



Voltages all equal the supply
voltage (6V)
6V
-
A1
A2
4Ω
1.5A
V
6V
A3
2Ω
Total resistance the is less than
the smallest (i.e. less than 2Ω)
3A
V
Total current = the sum of all the
branches
1.5A + 3A + 1A = 5.5A
A4
6Ω
1A
V
6V
6V
Parallel Circuits
 All components get the full source p.d, so the voltage is the same
across all the components
 The current through each component depends on its resistance –
the lower the resistance, the greater the current which flows
through it
 The total current flowing around the circuit is equal to the total of
all the currents in the separate branches (the total current going
into a branch always equals the total current leaving the branch
(no current is lost))
 The total resistance of the circuit is always less than the branch
with the smallest resistance
Current
Current = Charge ÷ Time
I=Q÷t
 Current is measured in amperes (A)
 Charge is measured in coulombs (C)
 Time is measured in seconds (s)
Potential Difference
P.D. = Work done ÷ Charge
V=W÷Q
 Potential difference is measured in volts (V)
 Work done is measured in joules (J)
 Charge is measured in coulombs (C)