Simple Circuits (OCR GCSE Physics A (Gateway)): Flashcards

Potential difference is the amount of energy transferred per unit of charge passing through the terminals of a component. It is measured in volts (V).

Current and resistance are inversely related. The greater the resistance, the lower the current for a given potential difference, and vice versa.

Potential difference is sometimes known as voltage and is measured using a voltmeter connected in parallel.

True.

The volt is defined as 1 J / C — one joule of energy transferred per coulomb of charge passing through the component.

V = IR, where V is the potential difference (V), I is the current (A) and R is the resistance (Ω).

Using V = IR:

V = 0.3 × 10

V = 3 V

Resistance is defined as the opposition to current flow and is measured in ohms (Ω).

False.

A good conductor has a low resistance, which allows current to flow easily. It is an insulator that has high resistance.

Resistance is calculated by dividing potential difference by current: R = V / I. The unit of resistance is the ohm (Ω), equivalent to one volt per ampere.

A thermistor is a temperature-dependent resistor whose resistance decreases as its temperature increases, and increases as temperature decreases.

False.

A fixed resistor has a constant resistance that does not change with current. Only components such as thermistors and LDRs have resistance that varies.

As temperature rises, more charge carriers are released in the thermistor material. This makes it easier for current to flow, so resistance decreases.

A variable resistor changes its resistance by changing the length of wire in the circuit. A longer wire has more resistance.

An LDR (light-dependent resistor) is a component whose resistance decreases as light intensity increases, and increases in the dark.

False.

In bright light, an LDR has a small resistance (tens of ohms). It is in the dark that its resistance is very large (millions of ohms).

A thermistor is used in digital thermometers, ovens and fire alarms. An LDR is used in automatic street lights and garden lights that switch on when it gets dark.

A fixed resistor has a constant resistance that cannot change. A variable resistor can be adjusted by changing the length of wire in the circuit, altering its resistance.

An ohmic conductor is a component whose I-V graph is a straight line through the origin, meaning current is directly proportional to potential difference and resistance is constant.

True.

A fixed resistor at constant temperature produces a straight line I-V graph through the origin, showing that current is directly proportional to potential difference.

As current increases, the temperature of the filament rises. This causes the atoms to vibrate more, increasing resistance and making current increase more slowly than potential difference.

A linear I-V graph indicates the component is an ohmic conductor. A non-linear graph means the resistance is not constant.

Forward bias is when a diode is connected so that current flows in the direction of the arrowhead symbol. In this direction, the diode allows current to pass through.

False.

A diode only allows current to flow in one direction (forward bias). In reverse bias, resistance is very high and no current flows.

In forward bias, current rises steeply once a threshold potential difference is reached. In reverse bias, the current is essentially zero because the diode blocks flow.

As current increases, the filament heats up. The higher temperature causes greater atomic vibration, increasing resistance. This is why the I-V graph curves rather than staying straight.

An I-V characteristic is a graph showing how the current through a component varies with the potential difference across it. Its shape reveals whether the component is ohmic or non-ohmic.

The independent variable is potential difference (V). It is varied by adjusting the variable resistor, and the resulting current is then measured.

True.

Changing the resistance of the variable resistor changes the potential difference across the component, allowing a range of I-V readings to be collected.

Leaving the circuit on allows components to heat up, changing their resistance. Switching off between readings keeps temperature controlled and ensures results are more accurate.

In the I-V experiment, current readings are taken 3 times per voltage setting and the average current is calculated and plotted.

The terminals of the power supply are reversed. This creates a negative potential difference, which allows the I-V graph to be plotted for both directions — important for showing how a diode behaves in reverse bias.

True.

If the ammeter or voltmeter does not start at zero, a zero error is introduced, shifting all readings by a fixed amount. Starting from zero prevents this systematic error.

1. Switch off the power supply between readings to prevent components overheating.

2. Do not touch wires directly when the circuit is on — high current can make wires dangerously hot.

The dependent variable is current (I). It is measured using an ammeter connected in series, and recorded for each value of potential difference applied.