UNIT 10 – D.C CIRCUITS
LEARNING OBJECTIVES
LEARNING OBJECTIVES
LEARNING OBJECTIVES
INTERNAL RESISTANCE
• All sources of e.m.f. have an internal resistance.
INTERNAL RESISTANCE
CLASS ACTIVITY #1
CLASS ACTIVITY #2
DETERMINING INTERNAL RESISTANCE
How can we determine the internal resistance of a power supply?
CLASS ACTIVITY #3
CLASS ACTIVITY #4
THE EFFECTS OF INTERNAL RESISTANCE
What is the maximum current that a power supply can provide?
•
The terminal p.d. of the battery depends on the resistance of the external resistor.
•
For an external resistor of resistance 1.0 Ω, the terminal p.d. is 1.5 V – half of the e.m.f.
•
The terminal p.d. approaches the value of the e.m.f. when the external resistance R is
very much greater than the internal resistance of the battery.
•
The more current a battery supplies, the more its terminal p.d. will decrease.
CLASS ACTIVITY #5
KIRCHHOFF’S FIRST LAW
Kirchhoff’s first law is an expression of the
conservation of charge. The idea is that the
total amount of charge entering a point must
exit the point.
CLASS ACTIVITY #6
KIRCHHOFF’S SECOND LAW
Kirchhoff’s second law is a consequence
of the principle of conservation of
CLASS ACTIVITY #7
CLASS ACTIVITY #7
SIGNS AND DIRECTIONS
Caution is necessary when applying Kirchhoff’s second law. You need
to take account of the ways in which the sources of e.m.f are connected
and the directions of the currents.
SIGNS AND DIRECTIONS
CLASS ACTIVITY #8
CLASS ACTIVITY #9
CLASS ACTIVITY #10
RESISTOR COMBINATIONS
RESISTORS IN SERIES
V = 𝑉1 + 𝑉2
𝑉 = 𝐼𝑅, 𝑉1 = 𝐼𝑅1 , 𝑉2 = 𝐼𝑅2
IR = 𝐼𝑅1 + 𝐼𝑅2
R = 𝑅1 + 𝑅2
RESISTORS IN PARALLEL
I = 𝐼1 + 𝐼2
𝑉
𝐼=
𝑅
𝑉
𝑉
𝑉
=
+
𝑅 𝑅1 𝑅2
1
1
1
=
+
𝑅 𝑅1 𝑅2
𝐼1 𝑅1 − 𝐼2 𝑅2 = 0𝑉
Because there is no source
of e.m.f in the loop
CLASS ACTIVITY #11
RECAP
ANY
QUESTIONS?
CLASS ACTIVITY #11
CLASS ACTIVITY #12
CLASS ACTIVITY #13
POTENTIAL DIVIDERS
How can we get an output of 3.0 V from a battery of e.m.f 6.0
V?
POTENTIAL DIVIDERS
CLASS ACTIVITY #14
CLASS ACTIVITY #15
POTENTIOMETER CIRCUITS
A potentiometer is a device used for comparing potential
differences.
CLASS ACTIVITY #16
LDR’S AS SENSORS
How is a light-dependent resistor
(LDR) used as a sensor?
The voltage of the supply is shared between the
two resistors in proportion to their resistance so,
as the light level changes and the LDR’s
resistance changes, so does the voltage across
each of the resistors.
The two resistors form a potential divider whose
output changes automatically with changing light
intensities.
THERMISTORS AS SENSORS
The thermistors that we refer to in this course are known
as negative temperature coefficient (NTC) thermistors.
This means that, when the temperature rises, the
resistance of the thermistor falls.
The thermistor can be used as a sensing device in the
same way as an LDR. Instead of sensing a change in
light level, it senses a change in temperature.
COMPARING E.M.F’S WITH POTENTIOMETERS
CLASS ACTIVITY #17
CLASS ACTIVITY #18