OBJECTIVES:
• To define resistance and establish the relation between V, I and R
• To describe an experiment to determine resistance using a voltmeter
and an ammeter
• To relate the resistance of a wire to its length and to its diameter
Resistance (R)
DEFINITION•
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RESISTANCE is the hindrance offered by an object to the flow of current through it.
RESISTANCE is the opposition that a substance offers to the flow of electric current.
It is represented by the letter “R”.
The standard unit of resistance is the ohm (Ω).
It is a SCALAR quantity.
All wires and components have some resistance.
https://www.youtube.com/watch?v=Scq0UYwNSvM
Let’s solve these questions.
1. Find the current of an electrical circuit that has resistance of 50 Ohms and
voltage supply of 5 Volts.
2. Find the resistance of an electrical circuit that has voltage supply of 10 Volts
and current of 5mA.
• https://www.youtube.com/watch?v=iLzfe_HxrWI
• https://www.youtube.com/watch?v=3BHrMIU5f50
• https://www.youtube.com/watch?v=Vh3XGz7hgU4
• https://www.youtube.com/watch?v=hxYn74Yhkpk
,
• Ohm's Law is a formula used to calculate the relationship
between voltage, current and resistance in an electrical circuit.
• It states that at constant temperature, current through a
conductor is directly proportional to the voltage across it and
inversely proportional to it’s resistance.
If you ever need help in remembering the different equations for Ohm's law and
solving for each variable (V, I, R) you can use the triangle below.
https://quizizz.com/admin/quiz/5ea571032fa3c9001bdc92dc/igcse-physics-electrical-quantities
RESISTORS are used to reduce current flow
1. If the resistance of an electrical circuit is increased, what will happen to the current assuming the voltage remains the
same?
2. If the voltage across a resistance is doubled, what will happen to the current?
.
Explanation: If you look at the equation V= IR, if R stays the same then if you multiple V*2 (double the voltage), you
must also double the current for the equation to remain true.
Draw and label a circuit diagram to verify
OHM’s LAW
Draw and label a circuit diagram to verify
OHM’s LAW
What is a resistor?
What is a VARIABLE RESISTOR?
A variable resistor allows more control over current flow by changing the amount
of resistance. When resistance increases in a variable resistor, the amount of current
that is allowed to flow in a circuit decreases.
IT CHANGES CURRENT IN ACIRCUIT by changing resistance.
Sketch and explain the current-voltage
characteristic of an ohmic resistor
Sketch and explain the current-voltage
characteristic of a filament lamp
• The current in a circuit is affected by the resistance.
• Resistors are often used in the circuits of electrical appliances to affect the
amount of current that is present in its various components.
• By increasing or decreasing the amount of resistance in a particular branch of the
circuit, a manufacturer can increase or decrease the amount of current in that branch.
• Kitchen appliances such as electric mixers and light dimmer switches operate by altering
the current at the load by increasing or decreasing the resistance of the circuit.
• Pushing the various buttons on an electric mixer can change the mode from mixing to
beating by reducing the resistance and allowing more current to be present in the mixer.
• Similarly, turning a dial on a dimmer switch can increase the resistance of its built-in
resistor and thus reduce the current.
https://www.youtube.com/watch?v=M3fWNAIKGaM
https://www.youtube.com/watch?v=w4xT6jMoubQ
• Length of the conductor (l) :
• 1. R is directly proportional to "l"
Rαl
2. Area of cross -section (A) : R is inversely
proportional to "A"
R α 1/A
3. Nature of the material :
Conductors have low resistance and insulators
have high resistance
4. As Temperature increases – Resistance of a
conductors also increases
(a) State the relationship between 0625/31/O/N/13
(i) the resistance R and the length L of a wire of constant cross-sectional area,
..................................................................................................................................
(ii) the resistance R and the cross-sectional area A of a wire of constant length.
..................................................................................................................................
[1]
Thank you for
everything you
do, all the nights
you stay awake to
teach us, thank
you and happy
teaches day!
Chaitanya
Fig. 8.1 shows a circuit containing a battery of electromotive force (e.m.f.) 12 V and a heater of
resistance 6.0 .
(a) State what is meant by electromotive force (e.m.f.).
...................................................................................................................................................
...............................................................................................................................................[1]
(b) (i) Calculate the current in the heater.
current = .........................................................[2]
(ii) State the name of the particles that flow through the heater.
.......................................................................................................................................[1]
(iii) On Fig. 8.1, draw an arrow next to the heater symbol to show the direction of flow of
these particles through the heater. [1]
(c) Calculate the thermal energy produced in the heater in 10 minutes.
thermal energy = .........................................................[2]
[Total: 7]
b) State the relationship between
(i) the resistance R and the length l of a wire of constant cross-sectional area,
...........................................................................................................................................
(ii) the resistance R and the cross-sectional area A of a wire of constant length.
...........................................................................................................................................
[2]
(c) The 12.0 Ω and 4.0 Ω resistors in (a) are wires of the same length and are made of the same
alloy.
ratio = ............................................... [1]
(b) A cylindrical metal wire W1, of length l and cross-sectional area A, has a
resistance of 16 Ω. 0625/41/M/J/18
A second cylindrical wire W2 having length l/2
and cross-sectional area 2 A, is made from the same metal.
Determine
(i) the resistance of W2,
resistance of W2 = ...........................................................[2]