Download Electromotive Force

Internal Resistance in EMF
The voltage measured between the terminals of the battery is called the
terminal voltage Vt. Most batteries of some internal resistance so the
terminal voltage is lower than the voltage of the EMF when the battery is
discharging. When charging a battery, the terminal voltage is higher than
the voltage of the EMF.
𝑉𝑡 = 𝑉 − 𝐼𝑟
Discharging battery
𝑉𝑡 = 𝑉 + 𝐼𝑟
Charging battery
Circuit Analysis: Kirchoff’s Junction Rule
A junction is a point in the circuit where multiple wires join
together.
The junction rule is based on the conservation of charge.
The junction rule is: the total current entering the junction is
equal to the total current leaving the junction.
Circuit Analysis: Kirchoff’s Loop Rule
A closed loop in a circuit is starting from one point in the circuit
and going around the circuit to end where you started.
Kirchoff’s Loop Rule is based on the conservation of energy. The
energy that is supplied by the EMF is used by the devices in the
circuit.
Kirchoff’s Loop Rule: For any closed loop around the circuit the
total of the potential rises is equal to the potential drops.
The current through a resistor is in the direction of a potential
drop since the resistor dissipates energy. The + sign to on the left
of each resistor means that side is a higher potential than the
right side of the resistor which has the – sign.
Applying Kirchoff’s Laws
Reasoning Strategy
Applying Kirchhoff’s Rules
1. Draw the current in each branch of the circuit. Choose any direction.
If your choice is incorrect, the value obtained for the current will turn out
to be a negative number.
2. Mark each resistor with a + at one end and a – at the other end in a way
that is consistent with your choice for current direction in step 1. Outside a
battery, conventional current is always directed from a higher potential (the
end marked +) to a lower potential (the end marked -).
3. Apply the junction rule and the loop rule to the circuit, obtaining in the process
as many independent equations as there are unknown variables.
4. Solve these equations simultaneously for the unknown variables.
Measuring Current
A device to measure current is called an ammeter. The
ammeter must be connected in series for the current to
be measured. In a series circuit, the current is the same
in each device.
An ideal ammeter has an internal resistance of r = 0 Ω so
the ammeter does not affect the current.
Measuring Voltage
A device that measures voltage is called a
voltmeter. A voltmeter is connected in parallel to
the part of the circuit to measure the voltage.
Devices connected in parallel have the same
voltage across them.
An ideal voltmeter will have an internal resistance
r = ∞ to prevent current from running through
the meter and changing the voltage on the
device.
20.4 Electric Power
ELECTRIC POWER
When there is current in a circuit as a result of a voltage, the electric
power delivered to the circuit is:
P  IV
SI Unit of Power: watt (W)
Many electrical devices are essentially resistors:
P  I IR   I 2 R
V2
V 
P   V 
R
R
20.4 Electric Power
Example 5 The Power and Energy Used in a
Flashlight
In the flashlight, the current is 0.40A and the voltage
is 3.0 V. Find (a) the power delivered to the bulb and
(b) the energy dissipated in the bulb in 5.5 minutes
of operation.
20.4 Electric Power
(a)
(b)
P  IV  0.40 A3.0 V  1.2 W
E  Pt  1.2 W 330 s   4.0 102 J
Practice Questions on Electric Power
Refer to Textbook Page 573
Questions 24, 25, 27, and 28