Secondary 2 Topic

Electrical Systems – All About Resistance

These science revision notes aim to help your child:

  • Understand the concept of resistance
  • Apply and calculate resistance using the relevant formulas

 

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Learning outcomes:

  • What is resistance?

  • What are the different resistance in different materials?

Different materials have different resistance.

  • Copper and silver have very low resistance. Both are used for making electric wires.
  • Nichrome has high resistance so if allows little electrical current to flow through it.

 

Watch this video about the thermal conductivity experiment:

Question

The diagram below shows a circuit consisting of four resistors, R1, R2, R3 and R4.

 

a) Given that the current flowing through R3 is 0.5A, what is the potential difference across R3?

b) Hence, what is the potential difference across R4?

c) Calculate the amount of current flowing into R4.

d) Find the resistance of R4.

 

 

Let’s answer part (A)

a) Given that the current flowing through R3 is 0.5A, what is the potential difference across R3?

STEP 1: Recall

Potential difference = current x resistance

STEP 2: Piecing your answer together.

Answer for part (a)

Potential difference across R3 = current across R3 x resistance across R3  =  0.5 A x 9Ω = 4.5 V

 

Let’s answer part (B)

b) Hence, what is the potential difference across R4?

STEP 1: Recall

R1 , R2 and R3   are connected in parallel with each other. Each resistor has the same potential difference across it.

R4 and R3 are connected in series with each other. The sum of the potential difference across R4  and R3  is 12V.

STEP 2: Piecing your answer together.

Answer for part (B)

Potential difference across R4   = 12 V – 4.5V = 7.5 V

 

Let’s answer part (C)

c) Calculate the amount of current flowing into R4.

STEP 1: Recall

STEP 2: Piecing your answer together.

Answer for part (C)

Amount of current flowing into R4 = 1.5 + 0.75 + 0.5 = 2.75A

 

Let’s answer part (D)

d) Find the resistance of R4.

STEP 1: Recall

STEP 2: Piecing your answer together.

Answer for part (D)

R4 = 7.5V / 2.75 Ω  = 2.73 Ω (correct to 3 s.f)

 

 

Common mistakes

 

Wrong answers: The potential difference through R4 is 4.5V.

 Teachers’ comments: 

Remember that resistance of resistors in series: R = R1 + R2 + R3 +…

A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take. The current is the same through each resistor. The total resistance of the circuit is found by simply adding up the resistance values of the individual resistors. Hence, the overall resistance increases causing less current to flow through it.

 

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