SNC1DB
The Effects of Potential Difference and Resistance on Current
Resistors are used in many
electrical devices to control the
amount of current flowing though
circuits. The voltage available at
the wall outlet may be too high for
a device so resistors are placed in
the device to prevent it from
overheating or burning up.
Remember that resistors resist the
flow of current. A material with
high resistance will decrease the
amount of current flowing through
The
coloured bands indicate the value of resistance in ohms
a circuit.
(Ω) on each resistor.
The amount of current that flows
is also affected by the amount of
potential difference supplied. A
larger potential difference, or
voltage, pushes more current
through the circuit. A smaller
potential difference results in less
current.
Wall outlets in North America
supply 120 V to loads such as
toasters, computers, and vacuum
cleaners.
Batteries
supply potential difference to a circuit.
Together, potential difference
and resistance determine the
amount of current flowing
through a circuit.
Ohm's Law
In 1827 a German scientist, Georg Ohm, first discovered that, for some resistors, there
is a mathematical relationship between potential difference and current. His discovery is
so useful that the relationship is called Ohm's Law.
Ohm's Law:
When resistance is constant, the current is proportional to the voltage.
Mathematically, Ohm's Law may be stated as



, where
V is the potential difference, measured in volts, V
I is the current, measured in amps, A
R is the resistance, measured in ohms, Ω
Check Your Understanding
1.
Rearrange the equation for Ohm's Law to solve for both current and resistance.
Did you know?
Did you know that Georg Ohm had a doctorate in physics but
eventually ended up teaching at a Jesuit high school in Cologne
in Germany? Using the school's laboratory, Ohm discovered the
simple mathematical relationship that is the mainstay of
electricity. Ohm was virtually ignored for most of his life even
though he discovered the most fundamental law of current
electricity.
Example
Problem A: The current required to operate an electric can opener is 1.5 A. What is its
resistance if the supply voltage is 120 V?
Given:
Solution:
The resistance of the can opener is 80 Ω.
Problem B: The voltage from problem A is doubled to 240 V using the same 80 Ω load.
Find the new current that would flow in the electric can opener.
Given:
Solution:
The current flowing through the new circuit is 3.0 A.
Problem C: How does the current in problem A compare to problem B? Explain.
The current is doubled in problem B compared to problem A because the voltage
has doubled using the same resistance.
Problem D: Calculate the current of 10 Ω, 16 Ω, and 20 Ω light bulbs that each uses a
120 V power source.
Given:
Solution:
The currents flowing through the 10 Ω, 16 Ω, and 20 Ω light bulbs are 12 A, 7.5 A,
and 6.0 A respectively.
Problem E: What happens to the current as the resistance increases?
The current decreases as the resistance increases in the electric circuit. When
the resistance increases it is more difficult for the electrons to move in the circuit.
Check Your Understanding
1.
Calculate the resistance of a light bulb filament when a 3 volt battery sends a current
of 0.10 A.
Answer
2.
Calculate the current through a clothes dryer that uses a 220 V power source
and has a resistance of 12 Ω in the heater coils.
Answer
Measuring Current, Potential Difference, and Resistance
You have just learned how to use Ohm's law to calculate current, potential difference, or
resistance when you are given two of these variables and the other is missing. Now you
will learn how to measure each of these quantities in a real circuit.
Consider a circuit with a light bulb, a
battery, and a switch. How would you
measure the current, the potential
difference, and the resistance?
A
simple series circuit.
Measuring Current
Current is measured using an ammeter. Since an ammeter measures the
flow of electric current through a circuit, it must be placed within the circuit to
take the measurement. Therefore, an ammeter is connected in series within a
circuit.
An ammeter has a very low internal resistance, the resistance of the
ammeter, so that current can flow through it unobstructed.
In a series circuit, the current is the same wherever you place the ammeter.
In a parallel circuit, the current will depend on where you place the ammeter,
because the current is shared among the different branches of the circuit.
Check Your Understanding
1.
What would the reading be on the above ammeter?
Answer
Measuring Potential Difference
Potential difference measures the change in
energy of electrons as they go through a
circuit. It is connected in parallel so that it
can measure the voltage across a load, in
this case a light bulb.
A voltmeter has a very high internal
resistance so that current does not bypass
the load and flow through the voltmeter.
Check Your Understanding
1.
What would the reading be on the above voltmeter?
Answer
Measuring Resistance
Similar to potential difference, resistance is
measured across a load. Therefore, an
ohmmeter is placed in parallel with the load
that is having its resistance measured. The
ohmmeter differs from ammeters and
voltmeters in that they supply their own
current, to test the resistance of a load.
Check Your Understanding
1.
What would the reading be on the above ohmmeter?
Answer
Factors Affecting Resistance
You have learned how resistance affects current and how resistance can be measured, but
how are different sized resistors made? There are four main factors that influence the
amount of resistance in an electrical circuit; the length of the wire, the type of wire material,
the cross-sectional area of the wire, and the temperature of the circuit.
Check Your Understanding
1.
How does the length of a wire affect the resistance of a circuit?
Answer
2.
How does the type of wire affect the resistance of a circuit?
Answer
3.
How does the cross-sectional area affect the resistance of a circuit?
Answer
4.
How does the temperature of the circuit affect the resistance of a circuit?