Download Ohm`s Law - Glasgow Science Centre

Circuits and Ohms Law
Through Investigation, I understand the
relationship between current, voltage and
resistance. I can apply this knowledge to solve
practical problems. SCN 4-09a.
Today we are going to find
out…
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What the term ‘resistance’ means in the context of electricity.
What factors influence the resistance of a conducting material
What the relationship is between current, voltage and resistance
How resistors behave in series and parallel circuits.
And we will be able to…
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Provide a definition of resistance
State and describe the factors influencing resistance
Carry out numerical problems involving resistance
Describe how resistors work in series and parallel circuits.
Memory Review…
What do we mean
when we talk about a
‘conducting material’?
Like this copper penny…
…Or this copper wire!
All conducting
materials
oppose electrical current in some
way. This opposition to electrical current
is called resistance.
Resistance is given the symbol R and is
measured in Ohms. The symbol for ohms is Ω
(Omega)
The resistance of the conducting
material is determined by various
factors:
-
Thicker diameter wires have less resistance - Think
-
about how think electricity pylon cables are!
Length – Shorter cables have less resistance than longer cables
Material – copper has quite a low resistance
Temperature – heat causes resistance to increase
All conducting materials oppose
current in some way. This
includes circuit components such as lamps.
If you wanted to increase the resistance in
a circuit therefore, you could add in more
lamps and in turn, this would…
Try experimenting with
this now! What happens to the current
when you add in more bulbs to the circuit?
What happens when you take them away?
You can limit the current flowing
through the circuit in other
ways, too. You can do this using
circuit components called
resistors.
These are some
examples of circuit
symbols for
different resistors.
Why do you think
they look
different?
There are different types of
resistors. Each type has a
different symbol to represent what controls
the level of resistance.
This is a ‘standard’
resistor.
This is a variable
resistor.
This is a
‘thermistor’ - a
resistor with
resistance
controlled by
temperature.
This is a ‘light
dependent resistor or
LDR’ - a resistor with
resistance controlled
by light.
Can you think of an example
of when you would want to use…
Thinking this through…
there must be a relationship
between current, voltage and
resistance.
This relationship is called
Ohms Law
Ohms Law…
Voltage = Current X Resistance
V=IR
V
I R
Calculating Resistance in
Series Circuits…
R1
R2
R3
R4
In a series circuit
the total resistance
is the sum of the
individual resistors
R5
Rt = R1+R2+R3+R4+R5
Calculating Resistance
in Parallel Circuits…
R1
R2
R3
In a parallel
circuit the total
resistance is the
given by…
1/Rt = 1/R1 + 1/R2 + 1/R3
Today we have…
• Learnt what the term ‘resistance’ means
• Discovered what properties influence the resistance of a
conductor
• Learnt about various different kinds of resistor and what potential
uses these may have
• Discovered the relationship between Voltage, Current and
Resistance and know that this relationship is called Ohms Law.
• Learnt how to calculate resistance in series and parallel circuits.