ExpertMedium4 marksStructured
AQA GCSE · Question 05.2 · Electricity
The vending machine identifies the value of the coin by measuring the resistance of the coin.
The power dissipated by the coin is 340 mW when the current in the coin is 0.75 A.
Calculate the resistance of the coin.
Use the Physics Equations Sheet.
The vending machine identifies the value of the coin by measuring the resistance of the coin.
The power dissipated by the coin is 340 mW when the current in the coin is 0.75 A.
Calculate the resistance of the coin.
Use the Physics Equations Sheet.
How to approach this question
1. Identify the given values: Power (P) and current (I).
2. Convert the power from milliwatts (mW) to watts (W). Remember that "milli" means divide by 1000.
3. Find the equation on your sheet that links P, I, and resistance (R). It is P = I²R.
4. Rearrange the equation to make R the subject.
5. Substitute the values for P (in watts) and I into the rearranged equation and calculate R.
Full Answer
The question asks to calculate resistance (R) given power (P) and current (I).
The formula linking these is P = I²R.
Step 1: Convert the power to standard units. The power is given as 340 mW. The standard unit for power is the Watt (W). The prefix "m" (milli) means 10⁻³ or divide by 1000.
So, P = 340 mW = 340 / 1000 W = 0.340 W.
Step 2: The current is given in Amperes (A), which is the standard unit.
I = 0.75 A.
Step 3: Rearrange the formula to make R the subject.
P = I²R => R = P / I²
Step 4: Substitute these values into the equation.
R = 0.340 W / (0.75 A)²
R = 0.340 / 0.5625
R = 0.60444... Ω
Rounding to two significant figures gives R = 0.60 Ω.
Common mistakes
✗ Forgetting to convert milliwatts to watts.
✗ Using the wrong power equation (e.g., P=VI).
✗ Forgetting to square the current (I) in the calculation.
✗ Errors in rearranging the formula.
Practice the full AQA GCSE Physics Higher Tier Paper 1
20 questions · hints · full answers · grading
More questions from this exam
Q01.1Wind turbines may generate electricity when the electricity is not needed. Two methods that can b...EasyQ01.2Which energy store increases when water is pumped uphill into a reservoir?EasyQ01.3The table shows information about the two methods of storing energy.
<br/><br/>
<table border="1"...MediumQ01.4Decreasing the amount of carbon dioxide released by different activities will help slow down clim...MediumQ02.1The process of nuclear fission is used in nuclear power stations. The diagram shows the process o...Easy