What does Area 3 Electricity cover in SQA National 5 Physics?

Electricity has five key areas. Electrical charge carriers covers current as the flow of charge, Q equals I times t, and electrons as the carriers in a metal. Potential difference covers voltage as the energy per unit charge, V equals E over Q. Ohm's law covers V equals I times R and resistors in series and parallel. Practical circuits covers components, the LDR, the thermistor and the potential divider. Electrical power covers P equals I times V, P equals I squared R, P equals V squared over R and the energy and cost of running appliances.

What is the difference between current and voltage?

Current is the rate of flow of electric charge, measured in amperes with an ammeter in series, and is linked to charge by $Q = It$. Voltage (potential difference) is the energy given to each unit of charge, measured in volts with a voltmeter in parallel, and is linked to energy and charge by $V = E/Q$. In a series circuit the current is the same everywhere while the voltage is shared; in a parallel circuit the voltage is the same across each branch while the current is shared.

How do you combine resistors in series and parallel?

Resistors in series add directly, $R_T = R_1 + R_2 + \dots$, so the total is larger than any single resistor. Resistors in parallel combine by the reciprocal rule, $1/R_T = 1/R_1 + 1/R_2 + \dots$, and you must take the reciprocal at the end; the total is smaller than the smallest resistor. A quick check is that a series total is bigger than every resistor and a parallel total is smaller than every resistor.

How does a potential divider work?

A potential divider is two resistors in series across a supply; the supply voltage is shared in proportion to the resistances, with $V_2 = \frac{R_2}{R_1 + R_2} V_s$. Replacing one resistor with a light-dependent resistor (LDR) or a thermistor makes a sensor circuit, because the LDR's resistance falls as light increases and the thermistor's resistance falls as temperature increases, so the output voltage changes with the condition and can switch a device on or off.

How do you calculate electrical power and the cost of running an appliance?

Power is the energy transferred per second, $P = E/t$, in watts. The three relationships $P = IV$, $P = I^2R$ and $P = V^2/R$ all give power; choose the one matching the data given. The energy used is $E = Pt$ with time in seconds. The cost is found from the energy in kilowatt-hours (power in kW times time in hours) multiplied by the price of one unit, so a 2 kW heater run for 3 hours uses 6 kWh.

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SQA National 5 Physics Area 3 Electricity: a complete overview of charge, voltage, Ohm's law, circuits and power

A deep-dive SQA National 5 Physics guide to Area 3 Electricity. Covers charge carriers and Q equals I times t, potential difference as energy per unit charge, Ohm's law with resistors in series and parallel, practical circuits with the LDR, thermistor and potential divider, and electrical power and the cost of running an appliance.

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Electrical charge carriers
Potential difference
Ohm's law
Practical circuits
Electrical power
How Electricity is examined
For the official course specification

Area 3 Electricity is about charge, circuits and the energy electrical components transfer. It has five key areas and is, with Dynamics, one of the two most calculation-heavy areas of National 5 Physics. This guide maps each key area and the relationships you select; every key area has its own answer page with worked examples.

Electrical charge carriers

Current is the flow of electric charge, carried by free electrons in a metal. Charge, current and time are linked by $Q = It$ (with time in seconds), where charge is in coulombs. In a series circuit the current is the same everywhere; in a parallel circuit the current splits between branches, and the branch currents add to the supply current, because charge is conserved.

Potential difference

Voltage (potential difference) is the energy given to each coulomb of charge, $V = \frac{E}{Q}$ , so one volt is one joule per coulomb. A voltmeter is connected in parallel across a component. In series the supply voltage is shared between components; in parallel every branch has the full supply voltage. This is the mirror image of the current rules, so learn the pair together.

Ohm's law

Ohm's law links voltage, current and resistance, $V = IR$ , with resistance (in ohms) being the opposition to current. For a resistor at constant temperature a $V$ -against- $I$ graph is a straight line through the origin. Resistors in series add, $R_T = R_1 + R_2 + \dots$ ; resistors in parallel combine by $\frac{1}{R_T} = \frac{1}{R_1} + \frac{1}{R_2} + \dots$ and give a total less than the smallest resistor.

Practical circuits

Real circuits use standard symbols and components. Input devices include the LDR (resistance falls as light rises) and the thermistor (resistance falls as temperature rises); output devices include the lamp, LED, motor and buzzer. A potential divider splits the supply voltage in proportion to resistance, $V_2 = \frac{R_2}{R_1 + R_2} V_s$ , and replacing one resistor with an LDR or thermistor makes a sensor circuit that switches a device automatically.

Electrical power

Power is energy per second, $P = \frac{E}{t}$ , in watts. The three relationships $P = IV$ , $P = I^2R$ and $P = \frac{V^2}{R}$ all give power; pick the one that matches the data. The energy used is $E = Pt$ , and the cost is the energy in kilowatt-hours multiplied by the price per unit.

How Electricity is examined

Electricity questions are heavily numerical: selecting the right relationship, substituting and quoting the unit. They also test the series-versus-parallel rules for current, voltage and resistance, which are best learned as paired opposites. Practise potential divider questions and power-and-cost questions, both of which appear regularly, and always convert times to seconds for $Q = It$ and $E = Pt$ .

For the official course specification

The SQA publishes the full National 5 Physics course specification, data sheet, relationships sheet and past papers at sqa.org.uk. Always revise from the current specification and SQA past papers.

Sources & how we know this

SQA National 5 Physics Course Specification — SQA (2019)