1. Chapter 19
DC Circuits

2. 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.

3. 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!

4. 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.
If shoes were on there would be much more resistance in the circuit and less current through the person.
If wet, resistance is less and current is greater.
Useful rule: If one hand is touching
something electrical, put other hand
behind back and wear thick rubber-soled shoes.

5. 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.

6. 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.
[In the U.S., three wires normally enter a house: two hot wires at 120 V each
(which add together to 240 V for appliances or devices that run on 240 V) plus
the grounded neutral (carrying return current for the two hot wires). See
Fig. 19– 29. The “dedicated” ground wire (non-current carrying) is a fourth wire
that does not come from the electric company but enters the house from a nearby
heavy stake in the ground or a buried metal pipe. The two hot wires can feed
separate 120-V circuits in the house, so each 120-V circuit inside the house has
only three wires, including the dedicated ground.]

7. 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.

8. 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

9. 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

10. measuring current
Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A

11. measuring current A A This is how we draw an ammeter in a circuit.
PARALLEL CIRCUIT
SERIES CIRCUIT

12. 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

13. 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.

14. 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

15. 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

16. 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

17. measuring voltage V V This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT
PARALLEL CIRCUIT

18. 19.8 Ammeters and Voltmeters
An ohmmeter measures resistance; it requires a battery to provide a current

19. 19.8 Ammeters and Voltmeters
If the meter has too much or (in this case) too little resistance, it can affect the measurement.

20. 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:

21. Summary of Chapter 19 Resistors in parallel: Kirchhoff’s rules:
sum of currents entering a junction equals sum of currents leaving it
total potential difference around closed loop is zero

22. Summary of Chapter 19
Capacitors in parallel:
Capacitors in series:

23. 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

24. Closure:
Kahoot: and 19-8