OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply the equations relating potential difference, current, quantity of…
Full specification wording
Simple circuits: apply the equations relating potential difference, current, quantity of charge, resistance, power, energy, and time, and solve problems for circuits which include resistors in series, using the concept of equivalent resistance
This criterion asks you to select and apply the right equation from the set linking p.d.
Checked against the OCR J249 specification (opens in a new tab) on .
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This criterion asks you to select and apply the right equation from the set linking p.d. V, current I, charge Q, resistance R, power P, energy E and time t. The key relations are Q = I × t, V = I × R, P = V × I and E = P × t, with E = V × I × t combining them. For resistors in series the same current flows through each resistor, the p.d.s add, and the equivalent resistance is R = R₁ + R₂ + … . Two series resistors of 4 Ω and 6 Ω give 10 Ω; with a 5 V supply the current is 5 ÷ 10 = 0.5 A, and the p.d. across the 4 Ω resistor is 0.5 × 4 = 2 V. Choose the equation that matches the quantities given and the quantity required.
What earns the marks
- Selects the equation matching the quantities given and the quantity required, such as Q = I × t or V = I × R.
- Applies V = I × R to find an unknown p.d., current or resistance in a series circuit.
- Adds the resistances of resistors in series to obtain the equivalent resistance.
- Uses the equivalent resistance with the supply p.d. to find the current through the series circuit.
- Combines equations such as E = V × I × t to solve a multi-step problem.
Common mistakes and how to put them right
Adds the currents through series resistors.
Put it right: Keep the current the same through each series resistor and add the p.d.s instead.
Combines series resistances by a reciprocal rule.
Put it right: Add series resistances directly to obtain the equivalent resistance.
Uses the wrong rearrangement of an equation.
Put it right: Rearrange symbolically first, then substitute values with units.
Exam tips
- List the quantities given and the quantity required, then choose the equation that connects them.
- For series circuits, find the equivalent resistance first, then use it with the supply p.d. to find the current.
- Carry units through each step and check that the final unit matches the quantity asked for.
Try the lesson
The start of the Simple circuits: apply the equations relating potential difference, current, quantity of charge, resistance, power, energy, and time, and solve problems for circuits which include resistors in series, using the concept of equivalent resistance lesson, made from checked teaching notes for this specification point.
Have a go first
A charge of 30 C flows through a lamp in 10 s. What is the current through the lamp?
Listen
This criterion asks you to select and apply the right equation from the set linking p.d. V, current I, charge Q, resistance R, power P, energy E and time t. The key relations are Q = I × t, V = I × R, P = V × I and E = P × t, with E = V × I × t combining them. For resistors in series the same current flows through each resistor, the p.d.s add, and the equivalent resistance is R = R₁ + R₂ + … Two series resistors of 4 Ω and 6 Ω give 10 Ω; with a 5 V supply the current is 5 ÷ 10 = 0.5 A, and the p.d. across the 4 Ω resistor is 0.5 × 4 = 2 V. Choose the equation that matches the quantities given and the quantity required. A strong answer earns marks for these points. Selects the equation matching the quantities given and the quantity required, such as Q = I × t or V = I × R. Applies V = I × R to find an unknown p.d., current or resistance in a series circuit. Adds the resistances of resistors in series to obtain the equivalent resistance. Uses the equivalent resistance with the supply p.d. to find the current through the series circuit. Exam tip: List the quantities given and the quantity required, then choose the equation that connects them.
See it
Put it in order
Put these steps in the order they happen.
- 1. Carry units through each step and check the final unit
- 2. Combine equations such as E = V × I × t for multi-step problems
- 3. Series resistors: equivalent resistance R = R₁ + R₂ + …
- 4. Apply V = I × R to find an unknown p.d., current or resistance
- 5. List quantities given and quantity required
- 6. Choose the equation that connects them
- 7. Use equivalent resistance with supply p.d. to find current
Key words
Say what each word means before you turn the card.
Charge, current and time
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Resistance and Ohm's law
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Equivalent resistance in series
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Power and energy calculations
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See every definition
- Charge, current and time
- The relation Q = I × t links quantity of charge, current and time.
- Resistance and Ohm's law
- The relation V = I × R links potential difference, current and resistance.
- Equivalent resistance in series
- For resistors in series the same current flows through each resistor, the p.d.s add, and the equivalent resistance is R = R₁ + R₂ + … .
- Power and energy calculations
- The relations P = V × I and E = P × t link power, energy, potential difference, current and time.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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