Every time you turn on your smartphone, millions of tiny invisible electrons rush through its internal pathways to bring the screen to life. To understand and calculate how these electrical circuits work, we rely on a set of fundamental equations linking current, charge, resistance, voltage, and power.
The foundation of any electrical calculation is understanding what is flowing and what is pushing it. Electric Current is the rate of charge flow through a component. It is measured in Amperes (A) using an ammeter, which must be connected in series. The actual "stuff" flowing is Electric Charge, measured in Coulombs (C).
The "push" that causes charge to flow is the Potential Difference (often called voltage). It is measured in Volts (V) using a voltmeter connected in parallel across a component. As charge flows, it experiences Resistance, which is the opposition to current, measured in Ohms (). For an ohmic conductor at a constant temperature, current and potential difference are directly proportional, a relationship known as Ohm's Law.
Below is a summary table of the core electrical equations you must be able to recall and apply in your OCR exam.
| Equation Name | Formula | Symbols | Units |
|---|---|---|---|
| Ohm's Law |
| = Potential Difference, = Current, = Resistance |
| Volts (V), Amperes (A), Ohms () |
| Charge Flow | = Charge, = Current, = Time | Coulombs (C), Amperes (A), Seconds (s) |
| Work Done (Energy) | = Energy, = Charge, = Potential Difference | Joules (J), Coulombs (C), Volts (V) |
| Power-Energy | = Energy, = Power, = Time | Joules (J), Watts (W), Seconds (s) |
| Electrical Power | = Power, = Current, = Potential Difference | Watts (W), Amperes (A), Volts (V) |
When calculating electrical quantities, time must always be converted into seconds. Failing to convert minutes to seconds is a common way to lose marks.
Calculate the charge that flows through a lamp when a current of is maintained for .
Step 1: Identify and convert values.
Step 2: State the formula and substitute.
Step 3: Calculate the final result with units.
A heating element has a resistance of . Calculate the potential difference required to drive a current of through it.
Step 1: State the formula.
Step 2: Substitute and calculate.
Step 3: Final result with units.
In electrical circuits, Electrical Work Done is exactly equivalent to energy transferred. When charge moves through a potential difference, energy is shifted from the power supply to the components.
A battery moves of charge through a potential difference of . Calculate the electrical work done.
Step 1: State the formula.
Step 2: Substitute and calculate.
Step 3: Final result with units.
An electric motor has a power rating of . How much energy is transferred if it operates for ?
Step 1: State the formula.
Step 2: Substitute and calculate.
Step 3: Final result with units.
Electrical Power is the rate at which energy is transferred, measured in Watts (W). It can be calculated using the potential difference and current ().
Alternatively, when studying the heating effect of a resistor, we use the equation . This specifically calculates the power dissipation—the rate at which electrical energy is transferred to the thermal store of the component.
Calculate the electrical power of a bulb that draws a current of from a power supply.
Step 1: State the formula.
Step 2: Substitute and calculate.
Step 3: Final result with units.
A resistor has a resistance of and a current of flowing through it. Calculate the power dissipation.
Step 1: State the formula.
Step 2: Substitute the values, ensuring the current is squared.
Step 3: Final result with units.
A Series Circuit is a circuit with only one single loop, meaning there is only one path for the charge to follow. Consequently, the current is identical at all points in the circuit.
If you place multiple resistors in a series circuit, they combine to create a Total Resistance (or Equivalent Resistance). You calculate this by simply adding the individual resistances together:
A circuit contains a battery connected in series to a resistor and a resistor. Calculate the current flowing through the circuit.
Step 1: Find the total equivalent resistance.
Step 2: State Ohm's Law formula for current.
Step 3: Substitute and calculate.
Step 4: Final result with units.
When using the equation P = I²R, students frequently forget to square the current value, or they accidentally square the entire (IR) calculation instead. Always evaluate I² first before multiplying by R.
OCR examiners frequently set time values in minutes or hours to test you. Always convert time into seconds before substituting it into equations like Q = It or E = Pt.
For calculation questions worth 3 or 4 marks, the OCR mark scheme awards marks specifically for showing the formula/substitution and for stating the correct unit. Even if your final number is wrong, writing down the correct equation and unit can secure half the marks.
When tackling multi-component series circuits, physically label all known resistances and voltages directly onto the circuit diagram. This helps prevent accidentally applying Ohm's Law (V=IR) using the total voltage with only one resistor's individual resistance.
Electric Current
The rate of flow of electric charge, measured in Amperes (A).
Electric Charge
A fundamental property of matter that experiences a force in an electromagnetic field, measured in Coulombs (C).
Potential Difference
The work done or energy transferred per unit of charge passing between two points in a circuit, measured in Volts (V).
Resistance
The opposition to the flow of electric current in a component, measured in Ohms (Ω).
Ohm's Law
The principle stating that the current through an ohmic conductor is directly proportional to the potential difference across it, provided the temperature remains constant.
Electrical Work Done
The energy transferred when a charge flows through a potential difference.
Electrical Power
The rate at which energy is transferred or work is done in an electrical circuit, measured in Watts (W).
Power dissipation
The rate at which a component (like a resistor) transfers electrical energy into thermal energy, causing a heating effect.
Series Circuit
A circuit in which components are connected end-to-end in a single continuous loop, providing only one path for current.
Equivalent Resistance
A single calculated resistance value that would have the exact same effect on a circuit as all the individual resistors combined.
charge flow
The physical movement of charge carriers, such as electrons, through a conductive path.
Amperes
The SI unit of electric current, equivalent to one coulomb of charge passing a point per second.
Coulombs
The SI unit of electric charge.
Volts
The SI unit of potential difference.
Ohms
The SI unit of electrical resistance.
Watts
The SI unit of power, equivalent to one joule per second.
heating effect
The process where electrical energy is converted into thermal energy as current flows through a component with resistance.
resistor
An electrical component designed to introduce a specific amount of resistance into a circuit.
Total Resistance
The sum of all resistances in a series circuit that determines the total current flow from the power source.
Put your knowledge into practice — try past paper questions for Physics A
Electric Current
The rate of flow of electric charge, measured in Amperes (A).
Electric Charge
A fundamental property of matter that experiences a force in an electromagnetic field, measured in Coulombs (C).
Potential Difference
The work done or energy transferred per unit of charge passing between two points in a circuit, measured in Volts (V).
Resistance
The opposition to the flow of electric current in a component, measured in Ohms (Ω).
Ohm's Law
The principle stating that the current through an ohmic conductor is directly proportional to the potential difference across it, provided the temperature remains constant.
Electrical Work Done
The energy transferred when a charge flows through a potential difference.
Electrical Power
The rate at which energy is transferred or work is done in an electrical circuit, measured in Watts (W).
Power dissipation
The rate at which a component (like a resistor) transfers electrical energy into thermal energy, causing a heating effect.
Series Circuit
A circuit in which components are connected end-to-end in a single continuous loop, providing only one path for current.
Equivalent Resistance
A single calculated resistance value that would have the exact same effect on a circuit as all the individual resistors combined.
charge flow
The physical movement of charge carriers, such as electrons, through a conductive path.
Amperes
The SI unit of electric current, equivalent to one coulomb of charge passing a point per second.
Coulombs
The SI unit of electric charge.
Volts
The SI unit of potential difference.
Ohms
The SI unit of electrical resistance.
Watts
The SI unit of power, equivalent to one joule per second.
heating effect
The process where electrical energy is converted into thermal energy as current flows through a component with resistance.
resistor
An electrical component designed to introduce a specific amount of resistance into a circuit.
Total Resistance
The sum of all resistances in a series circuit that determines the total current flow from the power source.