Current, Potential Difference & Resistance Flashcards

All 33 cards in this deck

Where in an atom are protons and neutrons found?

In the nucleus at the centre of the atom.

What are the relative mass and relative charge of a proton?

Relative mass 1, relative charge $+1$.

What are the relative mass and relative charge of a neutron?

Relative mass 1, relative charge 0 (neutral).

What are the position, relative mass and relative charge of an electron?

Orbits the nucleus in shells; relative mass about $\frac{1}{1835}$ (almost zero); relative charge $-1$.

True or false? Almost all of the mass of an atom is in the nucleus.

True. Protons and neutrons are in the nucleus and electrons have almost no mass.

True or false? An atom has an overall negative charge because it contains electrons.

False. An atom has equal numbers of protons and electrons, so it is neutral overall.

How are the symbols for a cell and for a battery drawn in a circuit diagram?

Cell: one long thin line (positive terminal) and one short thick line (negative terminal). Battery: two or more cell symbols in a line.

What circuit symbols are used for an ammeter and for a voltmeter?

Ammeter: a circle containing the letter A. Voltmeter: a circle containing the letter V.

How is the symbol for a variable resistor drawn?

A rectangle (resistor) with a diagonal arrow drawn through it.

How is the symbol for a diode drawn, and how does the LED symbol differ from it?

Diode: a triangle pointing at a straight line (bar), in the direction of conventional current. LED: the same symbol with two arrows pointing away from it.

How are the symbols for a thermistor and an LDR drawn?

Thermistor: a resistor rectangle with a line across it bent up at one end. LDR: a resistor rectangle in a circle with two arrows pointing in towards it.

What are the steps to draw a circuit diagram to measure the current in and the potential difference across a resistor?

e.g. one cell and one resistor

1. Draw a closed loop: cell, then the resistor.
2. Put an ammeter in series in the loop.
3. Put a voltmeter in parallel, connected across the resistor.

What is potential difference (voltage)?

The energy transferred per unit charge passed.

What is 1 volt equal to in other units?

1 joule per coulomb.

How must a voltmeter be connected to measure the potential difference across a component?

In parallel with the component (connected across it).

True or false? A voltmeter is connected in series with the component whose potential difference is being measured.

False. A voltmeter is connected in parallel with (across) the component.

What quantity does a voltmeter measure, and in what unit?

Potential difference (voltage), in volts (V).

What is electric current?

The rate of flow of charge.

What is the current in a metal a flow of?

Electrons (free electrons moving through the metal).

How must an ammeter be connected to measure the current in a component, and what unit does it read?

In series with the component; it reads current in amps (A).

What is the equation linking charge, current and time, with units?

charge (C) = current (A) $\times$ time (s), $Q = I \times t$.

What are the steps to find the charge that flows when the current is given in mA and the time in minutes?

e.g. 500 mA for 2 minutes

1. Convert mA to A (divide by 1000): $500\,\mathrm{mA} = 0.5\,\mathrm{A}$
2. Convert minutes to seconds ($\times 60$): $2\,\mathrm{min} = 120\,\mathrm{s}$
3. $Q = I \times t = 0.5 \times 120 = 60\,\mathrm{C}$

True or false? Conventional current is drawn as flowing from the negative terminal to the positive terminal of a cell.

False. Conventional current goes from positive to negative; the electrons flow the opposite way.

What happens to current at a junction in a circuit?

Current is conserved: the total current into the junction equals the total current out of it.

What are the steps to find a missing current at a junction?

e.g. 5 A flows into a junction and splits into two branches, one carrying 2 A

1. Total current in = total current out: $5 = 2 + I$
2. Subtract the known branch current: $I = 5 - 2$
3. Missing current $= 3\,\mathrm{A}$

What must a circuit contain for there to be a current in it?

A complete (closed) circuit and a source of potential difference, such as a cell.

True or false? Some of the current is used up as it passes through a lamp.

False. Current is conserved; energy is transferred by the lamp, not current used up.

What is resistance, and what is its unit?

The opposition to the flow of charge (current), measured in ohms ($\Omega$).

What is the equation linking potential difference, current and resistance, with units?

potential difference (V) = current (A) $\times$ resistance ($\Omega$), $V = I \times R$.

What are $V = I \times R$ rearranged to make resistance the subject, and to make current the subject?

$R = \dfrac{V}{I}$ and $I = \dfrac{V}{R}$.

What are the steps to calculate the resistance of a component from meter readings?

e.g. voltmeter reads 6 V, ammeter reads 2 A

1. Write $V = I \times R$: $6 = 2 \times R$
2. Rearrange: $R = \dfrac{V}{I} = \dfrac{6}{2}$
3. Resistance $= 3\,\Omega$

What is a variable resistor used for in a circuit?

To change the resistance of the circuit, and so change the current in it.

True or false? For a fixed potential difference, doubling the resistance halves the current.

True. $I = \dfrac{V}{R}$, so current is inversely proportional to resistance.