Download Grade 9 Academic Science – Electricity

Grade 9 Academic Science – Electricity
Ohm’s Law
Section 13.9
Ohm’s Law deals with the relationship between voltage and current. In the relationship, “the potential
difference (voltage) across a resistor is proportional to the current through the resistor” (University of
Guelph, 2014).
 In your own words, what does that statement say?
From the Physics Classroom (2014), the electric potential difference between two points on a circuit (V) is
equivalent to the product of the current between those two points (I) and the total resistance of all electrical
devices present between those two points (R)
 Any better? In your own words, what does that statement say?
A complete circuit is a closed loop. A circuit contains, at least, one source of voltage (…providing an
increase of electric potential energy) and, at least, one potential drop (i.e., a place where electric potential
energy decreases).
 NOTE: Potential Difference and Potential Energy are NOT the same thing.
As we know, an increase of potential energy in a circuit causes a charge to move from a higher to a lower
potential (i.e., voltage).
Using Ohm’s Law – Example Question
A 6.0V power source is connected to a lamp and a current of 2.0A flows. All the wires are resistance-free.
Using GRASP methodology, determine the resistance of the lamp.
Given
Required
Analyze
Solve
Present
V = 6.0V
I = 2.0A
R
I = V/R ….thus, R = V/I
R = 6.0 / 2.0 = 3.0
Resistance is 3.0Ω
Ohm’s Law Questions
1. A motor rated as a 50Ω resistance operates with a current of 10A. What is the voltage?
2.
An electric lawn mower uses 25V and has a rated resistance of 2Ω. What current is used by the
mower?
We know that there is a PROPORTIONAL RELATIONSHIP between voltage, current and resistance. The
table illustrates the Ohm’s Law relationship both qualitatively and quantitatively for several circuits with
varying battery voltages and resistances.
Row
Diagram
Voltage
(V)
Resistance
( )
Current
(A)
1
1.5
3
0.50
3.0
3Ω
1.0
4.5
3
1.5
1.5
6
0.25
3.0
6
0.5
4.5
6Ω
0.75
4.5
9Ω
0.50
2
3
4
5
6
7
Rows 1, 2 and 3 illustrate that the doubling and the tripling of the battery voltage leads to a doubling and a
tripling of the current in the circuit if resistance does not change. Comparing Rows 1 and 4 or Rows 2 and
5, we see that doubling of the total resistance while maintaining a constant voltage serves to halve the
current in the circuit.
Questions – CHOOSE THE MOST CORRECT ANSWER(S) and EXPLAIN YOUR CHOICE(S)
1. Which of the following will cause the current through an electrical circuit to decrease? Choose all
that apply.
a. Decrease the voltage and keep resistance unchanged
b. Decrease the resistance and keep voltage unchanged
c. Increase the voltage and keep resistance unchanged
d. Increase the resistance and keep voltage unchanged
2.
A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the
following would cause the bulb to shine less brightly? Choose all that apply.
a. Increase the voltage of the battery
b. Decrease the voltage of the battery
c. Decrease the resistance of the circuit
d. Increase the resistance of the circuit
3.
If the resistance of a circuit was tripled and voltage is unchanged, the current through the circuit
would be ____.
a. Reduced by one-third
b. Three times higher
c. Unchanged
d. ... nonsense! There would be no way to make such a prediction.
4.
You have likely been warned to avoid contact with electrical appliances or even electrical outlets
when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry)
because wet hands cause ____.
a. The voltage of the circuit to be higher
b. The voltage of the circuit to be lower
c. Your resistance to be higher
d. Your resistance to be lower
e. The current through you to be lower
5.
If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.
a. One-fourth as much
b. Four times as much
c. Unchanged
d. There would be no way to make such a prediction
6.
Use Ohm’s Law and your understanding of Series Circuits to determine the missing values in the
diagrams
Question 1
Answers: A and D
The current in a circuit is directly proportional to the electric potential difference impressed across the
circuit and inversely proportional to the resistance of the circuit. Reducing the current can be done by
reducing the voltage (choice A) or by increasing the resistance (choice D).
Question 2
Answers: B and D
The bulb will shine less brightly if the current in it is reduced. Reducing the current can be done by
reducing the electric potential difference across the bulb (choice B) or by increasing the resistance of the
bulb (choice D).
Question 3
Answer: A
Current is inversely proportional to the resistance. A threefold increase in the resistance would cause a
threefold decrease in the current.
Question 4
Answer: D
Wet hands have less resistance and thus less hindrance to the flow of charge; the current would thus be
increased. Touching an outlet with wet hands increases the risk of charge flowing through you and causing
electrical shock or even electrocution.
Question 5
Answer: B
Current is directly proportional to the voltage. A fourfold increase in the voltage would cause a fourfold
increase in the current.