Series and parallel circuits. Resistance. Resistors in parallel and series. 6th year Higher Physics introduction to electricity / circuits

1. Electricity
6D: Tuesday 23rd
November

2. Series circuits: Conclusions
• The voltage of the supply is equal to the sum
of the voltages across all the components in
the circuit:
Vsupply = V1 + V2 + V3 + …
• When more bulbs are added to the series
circuit the current is reduced
• The current is the same at ALL points within
the series circuit:
Isupply = I1 = I2 = I3 = …

3. Parallel circuits: Conclusions
• The sum of all currents in the parallel
branches in the circuit is equal to the current
drawn from the supply:
Isupply = I1 + I2 + I3 + …
• The voltage across components in parallel are
the same and equal to the supply voltage
Vsupply = V1 = V2 = V3 = …

4. Uses of series circuits
• Torches, old-style Christmas lights…
• Stair lighting uses two or more 2-way switches
in series:
Power Supply
Downstairs switch
Upstairs switch
Lamp
This circuit is in the OFF position
Either switch will turn the light ON

5. Uses of parallel circuits
• New style Christmas lights
• Car lighting circuits:
MM
Ignition
switch
Side & rear
lamps
Headlights
Starter
motor
Wiper
motor

6. Resistance
Learning Objectives:
• Know what resistance is and what units
we measure it in
Starter
Write down as many meanings for the
word resistance as you can in 1 minute!

7. Experimental setup
1. Set up the circuit as shown
2. Turn on the power supply and adjust the voltage across the resistor to 12V
3. Take readings of the voltage and current and record in a table in your workbook
4. Repeat this for several different values (12V down to 0V) of the voltage and
measure both voltage and current through the resistor in your table of results
5. Replace the lamp in the circuit with a lamp nd repeat steps 2-4 using a new
table and graph
6. Plot a graph of your results (voltage (V) on y-axis; current (A) on x-axis) for both
the resistor and the lamp
Variable
D.C.
power supply
0-12V

8. Definition of Resistance
• Electrical Resistance of an object is a measure
of its opposition to the passage of an electric
current
• Resistance is measured in Ohms (Ω)
Resistance = voltage / current
R=V/I V=IR I=V/R
George Simon Ohm
German Physicist
V
I R

9. Resistors
• Resistors are simple components used to
control the current and voltage in a circuit

10. Resistive heating
• For a given component, power P = IV where I
is the current through that component and V
is the voltage across that component
• Substituting from V=IR we get
P = I2
R = V2
/R = IV
Power is measured in Watts

11. Electromotive Force (e.m.f.)
• The e.m.f. of any electrical supply is the number
of joules of electrical energy given to each
coulomb of electric charge as it passes through
the supply
• This is measured in JC-1
or volts
• E.M.F.s can be generated in a variety of ways
e.g.: chemical cells, thermocouple, piezo-electric
generators, solar cells, electromagnetic
generators

12. Resistors in series:
Conservation of energy
• Applying conservation of energy to resistors in
series for one coulomb of charge.
• Energy supplied by source = energy converted
by circuit components
e.m.f. = IR1 + IR2 + IR3
IRs = IR1 + IR2 +IR3
Rs = R1 + R2 + R3
where Rs = equivalent series resistance

13. Resistors in parallel:
Conservation of charge
• Total charge per second (current) passing through R1, R2,
R3must equal the charge per second (current) supplied
by the cell, i.e. passing through RP
• Conservation of charge gives:
I = I1 + I2 + I3 (since I=Q/t for each resistor)
E/RP = E/R1 + E/R2 + E/R3 (since I=E/R for each resistor)
1/RP = 1/R1 + 1/R2 + 1/R3
where RP = equivalent parallel resistance