Download current_and_resistance

Turn up the heating
• The electric current
• Heating effects of a current
• Voltage – Current graphs
Electric current (I)
What is an electric current?
An electric current is a flow of charges and it is
measured in amperes (A).
An electric current through a circuit is similar to water
running through a system of pipes. Describe the
features of an electric current that this model cannot
explain.
Hint: what happens if you cut the water pipe?
If we cut the water pipe the water still runs out, but
cutting a wire in the circuit breaks the circuit and the
electric current stops flowing.
Heating effects of a current
Choose the component that for a given
current will get hotter than the others.
Resistor
Diode
Filament lamp
Resistors
For relatively low currents the resistor stays
at constant temperature and its resistance
does not change.
I
V
For this value of the
current the
resistance is
constant and the
temperature does not
rise.
Back
Diodes
The current increases at higher and higher
rates, so the resistance gets lower.
I
V
For this value of the
current the
resistance is lower
and the diode does
not get hot.
Back
Filament lamps
The current decreases at higher and higher
rates, so the resistance gets higher.
I
V
For this value of the
current the
resistance is higher
and the filament
inside the lamp gets
hotter.
Back
Heating effects of a current
If we keep the same current flowing through a circuit and
we change the resistance, the higher the resistance,
the hotter the component.
Which of these wires will get hotter when the
same current flows through them?
Heating effects of a current
If we keep the same current flowing through a circuit and
we change the resistance, the higher the resistance,
the hotter the component.
Which of these wires will get hotter when the
same current flows through them?
Voltage – Current graphs
What does the gradient of this graph represent?
The gradient of a V – I graph is the resistance
V
The gradient of the line
is V/I, but
R = V/I
I
Gradient = Resistance
Voltage – Current graphs
So if you calculate the gradient of the graph you
have found the resistance!
V (V)
7
6
5
4
3
2
1
Calculate the gradient of
this graph.
5 volts
Gradient =
= 0.83 ohms
6 amps
1 2 3 4 5 6 7 8
I (A)