Chapter 19
DC Circuits
Objective: The students will be able to:
Describe the basic operation of a galvanometer
and calculate the resistance which must be
added to convert a galvanometer into an
ammeter or a voltmeter.
19.7 Electric Hazards
Even very small currents – 10 to 100 mA can
be dangerous, disrupting the nervous system.
Larger currents may also cause burns.
Household voltage can be lethal if you are wet
and in good contact with the ground. Be
careful!
19.7 Electric Hazards
A person receiving a
shock has become part
of a complete circuit.
The current in (a) passes from the high-voltage
wire through the person to to ground through the
bare feet, and back along the ground to the ground
terminal of the source. What is shoes were on?
In (b), the person touches a faulty “hot” wire
with one hand, and the other touches a sink
faucet (connected to ground via the pipe or
water in a non-metal pipe). The current will
pass through the chest, through the heart and
lungs, which makes it dangerous.
Useful rule: If one hand is touching
something electrical, put other hand
behind back and wear thick rubber-soled shoes.
19.7 Electric Hazards
Faulty wiring and improper grounding can be
hazardous. Make sure electrical work is done by
a professional.
Case b: A wire inside the device came in contact with the case and the case
is metal so it conducted electricity.
Case c: The metal case is connected directly to the ground by a separate
wire. If a “hot” wire touches the grounded case, a short circuit to ground
immediately occurs and most of current passes through the low-resistance
ground wire.
19.7 Electric Hazards
The safest plugs are those with three prongs;
they have a separate ground line.
Here is an example of household wiring – colors
can vary, though! Be sure you know which is the
hot wire before you do anything.
19.8 Ammeters and Voltmeters
An ammeter measures current; a voltmeter
measures voltage. Both are based on
galvanometers, unless they are digital.
The current in a circuit passes through the
ammeter; the ammeter should have low
resistance so as not to affect the current.
Galvanometer
• The galvanometer is the "classic" device
to measure current
• Based on the fact that a wire carrying
current in a magnetic field feels a force
8
• The current flows through a coil in a
magnetic field
• The coil experiences a torque
proportional to current
• The movement of the coil is "opposed"
by a spring
• The deflection of the needle is
proportional to current
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measuring current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
measuring current
This is how we draw an ammeter in a
circuit.
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
Ammeter
Ammeter is a low resistance galvanometer
It is used to measure the current in a circuit in amperes
Galvanometer can be converted into an ammeter by
using a low resistance wire in parallel with the
galvanometer
 the resistance of the wire depends upon the range of the
ammeter
As shunt resistance is small the combined resistance of
the galvanometer & the shunt is very low hence the
ammeter has much lower resistance than galvanometer
An ideal ammeter has zero resistance
19.8 Ammeters and Voltmeters
A voltmeter should not affect the voltage across
the circuit element it is measuring; therefore its
resistance should be very large.
measuring voltage
The ‘electrical push’ which the cell gives to the current
is called the voltage. It is measured in volts (V) on a
voltmeter
V
Voltmeter
Voltmeter is an electrical measuring device, which is
used to measure potential difference between two points
in a circuit.
CONNECTION OF VOLTMETER IN CIRCUIT
Voltmeter is always connected in parallel to a circuit.
SYMBOL
Voltmeter
Volt meter is a high resistance galvanometer
It is used to measure the potential difference between
two points of a circuit in volts
A galvanometer can be converted into a volt meter by
using a high resistance in series with the galvanometer
The value of resistance depends upon the range of the
volt meter
For voltmeter a high resistance, R is connected in series
with the galvanometer therefore resistance of voltmeter is
very large as compared to that of galvanometer
The resistance of an ideal voltmeter is infinity
measuring voltage
This is how we draw a voltmeter in a
circuit.
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
19.8 Ammeters and Voltmeters
An ohmmeter measures
resistance; it requires a
battery to provide a
current
19.8 Ammeters and Voltmeters
If the meter has too
much or (in this case)
too little resistance, it
can affect the
measurement.
Summary of Chapter 19
• A source of emf transforms energy from
some other form to electrical energy
• A battery is a source of emf in parallel with an
internal resistance
• Resistors in series:
Summary of Chapter 19
• Resistors in parallel:
• Kirchhoff’s rules:
1. sum of currents entering a junction
equals sum of currents leaving it
2. total potential difference around closed
loop is zero
Summary of Chapter 19
• Capacitors in parallel:
• Capacitors in series:
Summary of Chapter 19
• To avoid shocks, don’t allow your body to
become part of a complete circuit
• Ammeter: measures current
• Voltmeter: measures voltage
Closure:
Kahoot: 19-7 and 19-8