It's about electricity for secondary school

1. Electricity
Revision

2. Power = energy
time
Electricity Equations
Energy transferred = charge x potential difference
Potential difference = current x resistance
Power = current x potential difference
Read the question – You
are now given the
symbols for the
equations - it will help
you work out power
equation you need to use.
Charge = current x time
Power = current2 x resistance

3. Circuit Symbols
Arrow must
point to
negative of
cell
Arrow must point
to negative of
cell

4. variable resistor
Voltmeter goes in
parallel with the
component we want to
measure.
Measuring current, voltage and resistance
Ammeter goes in series with the
component we want to measure.
Potential difference = current x
resistance
V = I x R

5. Series and Parallel
Series
• Voltage is split at each
component – adding a bulb
makes the others dimmer
• Current stays the same at
each component
Parallel
• Voltage stays the same at
each component – all bulbs
have the same brightness
• Current is split at each
component

6. Series and Parallel Circuits
Series
• In series current is the same
everywhere.
• Components in series share the
potential difference of the supply.
• The higher the resistance of the
component the bigger share of p.d.
it takes.
• V = I x R holds for individual
components and the whole circuit
Parallel
• In parallel potential difference is
the same across each branch
• The current splits up at each
branch.
• Current will travel the circuit path
that has the lowest resistance
• Total current in at a junction will
equal the total current out.

7. Ohm’s Law and Resistance
If a resistor is kept at a constant temperature, resistance remains
unchanged.
Resistance in series
• To work out total add the
resistance of each resistor
Resistance in parallel
1
𝑅𝑇
=
1
𝑅1
+
1
𝑅2
+
1
𝑅3
• Total resistance is always
smaller than the smallest
value resistor

8. Resistors in Circuits
LDR – will switch things on and off when light
levels change
• Resistance falls as light intensity increases
• Falls rapidly for low light intensity
• Falls more slowly as light intensity increases
Thermistors – will switch things on and off
when temperature changes
• Resistance falls as temperature increases
• Falls rapidly for low temperature
• Falls more slowly as temperature increases

9. Mains Electricity
Live wire touches the metal case  high current will flow to earth
through the low resistance earth wire  fuse will melt, switching off the
appliance.
Live – Brown – AC (+230v
to –230v)
Fuse is connected to live wire.
Neutral – Blue –
completes the
circuit - 0V (close
to 0V at all times)
Earth – Green and Yellow – only has current when there is a fault.
Stops electrocution!

10. National Grid
Stepping up
potential difference
reduces current.
Wires are cooler so
less thermal energy
is lost to the
environment.
The energy is
transferred more
efficiently.