GCSE Physics Double Award notes

1. Resistance
GCSE Physics

2. Resistance
• Book Reference
Resistance
- Pg 123
Ohm’s Law (the statement and
equation)
- page 126

3. Learning Intentions
• Describe the relationship between
voltage and current
• State what is meant by the resistance of
a circuit
• State Ohm’s Law
• Recognise how a device obeys Ohm’s Law
• Recall how to measure the resistance
of a device in a circuit

4. Voltage and Current
• For many electrical devices
there is a relationship between
the current flowing through it
and the voltage across it
• As the current flows through it
faster, what happens to the
voltage?
• Why?

5. Resistance
• What other words can we use to
explain resistance?
Impede
Obstruct
Slow down
Oppose Challenge

6. Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
The resistance of a component can be
calculated using the equation of Ohm’s Georg Simon Ohm
Law: 1789-1854
Voltage
Resistance =
Current
V V
Symbol R =
I
Volt, V
Unit Ohm, Ω = I R
Amp, A

7. Resistance
Give it to me straight…
If the resistance
V is constant the
R =
I voltage increases
at the same
Re-arranged V = R X I proportion as the
current
• Thecurrent in a metallic conductor is
directly proportional to the voltage
between its ends (provided that the
temperature and other physical conditions are
the same).

8. For a component in a circuit which obeys Ohm’s
law, measuring the current as the voltage
increases will give a graph with the shape shown
below…
Change in V
If we measure the
gradient, we get
the resistance.
Change in I
V
I = R
Notice that the graph
goes through the origin

9. An example question:
Ammeter
reads 2A
A
1) What is the resistance across this bulb?
V
R = V / I = 10 / 2 = 5Ω
2) Assuming all the bulbs are the same what
is the total resistance in this circuit?
Voltmeter
Total Voltage = 10 x 3 = 30 V
reads 10V
R = V / I = 30 / 2 = 15Ω

10. Questions from the book
• Pg 126 and 127
• Questions 17 - 22

11. Learning Intentions
• State how the resistance of a wire or
device can change
• Plot and interpret a graph for the
voltage and current passing through a
device

12. Ohm’s Law
The Stony Tube with water…

14. Resistance
In an electrical circuit RESISTANCE is the
measure of how the material opposes or
impedes the flow of electrons
• Glass and synthetic polymers have great
resistance because so many of their
electrons are not free to move from
atom to atom
• Silver and copper atoms offer only slight
resistance to an electric current because
many of their electrons are free to
move from atom to atom.

15. What affects resistance?
Here we have a piece of
- electrical wire with a set EMF
+ pushing the electrons (large
spheres) through it.
The small spheres are the atoms making up the
material
1. Cross sectional Area
If the thickness of the wire
is reduced then the
resistance increases.
The electrons obviously don’t have as much space to
flow down the wire.

16. What affects resistance?
2. Length of the material
Comparing the ‘journey’ of an electron down a longer piece
of wire, we can see that it has to deal with more
friction from the sides of the wire.
Therefore the longer the wire the higher the
resistance of the material.

17. What affects resistance?
Temperature
When the temperature of a material is turned up it
becomes hotter. This is because the atoms are vibrating
more vigorously and bouncing off each other and giving
off heat energy.
The quicker the atoms are moving the more
difficult it is for the electrons to get to where
they want to go.
The resistance increases.

18. Website Help
• http://micro.magnet.fsu.edu/electro
mag/electricity/resistance.html

19. Questions
• Page 128 Questions 23 and 24