Electricity and magnetism flashcards

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How can an insulating material become charged, and what actually moves?
Rubbing two insulators together transfers electrons from one to the other. The one that gains electrons becomes negative; the one that loses them becomes positive. Protons do not move.
State the law of electrostatic force between charges.
Like charges repel and unlike charges attract. The force is greater when the charges are closer together.
What is an electric field and how is it represented?
A region where a charge feels a force. Field lines point in the direction a positive charge would move: away from positive charges and towards negative charges, closer together where the field is stronger.
Define electric current and give the equation linking charge, current and time.
Current is the rate of flow of charge: I = Q/t. Current is in amperes (A), charge in coulombs (C), time in seconds (s). A current of 1 A carries 1 C each second.
What carries current in a metal and which direction is conventional current?
Free electrons carry the current, moving from negative to positive. Conventional current is defined from positive to negative, the opposite way.
How much charge flows past a point when a 2.0 A current flows for 30 s?
Q = It = 2.0 × 30 = 60 C.
Define electromotive force (e.m.f.) and potential difference (p.d.).
e.m.f. is the electrical work done by a source in driving unit charge round a complete circuit. p.d. is the work done per unit charge between two points. Both are in volts (V).
How do you connect an ammeter and a voltmeter in a circuit?
An ammeter goes in series with the component to measure current. A voltmeter goes in parallel across the component to measure p.d.
State the equation for resistance and its unit.
R = V/I. Resistance (Ω) is the p.d. across a component divided by the current through it.
A 12 V supply drives 0.50 A through a lamp. What is its resistance?
R = V/I = 12 ÷ 0.50 = 24 Ω.
Describe the I–V graph of a metal wire at constant temperature, and what happens to a filament lamp.
A wire gives a straight line through the origin: current is proportional to p.d. (ohmic). A filament lamp's graph curves: resistance increases as the filament gets hotter.
How does the resistance of a wire depend on its length and cross-sectional area?
Resistance is proportional to length (double the length, double the resistance) and inversely proportional to cross-sectional area (thicker wire, lower resistance).
Describe how an LDR, a thermistor (NTC) and a diode behave.
LDR: resistance falls as light increases. NTC thermistor: resistance falls as temperature rises. Diode: allows current in one direction only, with very high resistance the other way.
How do current and p.d. behave in a series circuit?
The current is the same everywhere. The p.d.s of the components add up to the supply voltage. The total resistance is the sum of resistances.
How do current and p.d. behave in a parallel circuit?
The p.d. across each branch equals the supply p.d. The branch currents add up to the total current. Adding more branches lowers the total resistance.
(Extended) Give the formula for the combined resistance of two resistors in parallel, and find it for 6 Ω and 3 Ω.
1/R = 1/R₁ + 1/R₂ = 1/6 + 1/3 = 1/2, so R = 2 Ω. It is always less than the smallest resistor.
(Extended) What is a potential divider and what is it used for?
Two resistors in series across a supply share the voltage in proportion to their resistances. Using an LDR or thermistor as one resistor gives an output voltage that changes with light or temperature.
Give the equations for electrical power and energy.
P = IV and E = IVt (or E = Pt). Power in watts, voltage in volts, current in amperes, time in seconds, energy in joules.
An electric kettle at 230 V draws 5.0 A. What is its power and the energy used in 2 minutes?
P = IV = 5.0 × 230 = 1150 W. E = Pt = 1150 × 120 = 138 000 J.
What is the function of a fuse and how do you choose its rating?
A fuse melts and breaks the circuit when the current exceeds its rating, preventing overheating and fire. Choose a fuse rating just above the normal working current of the appliance.
Explain the roles of the live, neutral and earth wires in a plug.
Live carries the alternating p.d. to the appliance; neutral completes the circuit; earth connects a metal case to the ground, so a fault current is large enough to blow the fuse and prevent shocks.
What are the dangers of electricity and sensible precautions?
Damaged insulation, water near electrical devices and overloaded sockets can cause shocks or fires. Use fuses, circuit breakers, earthing and double insulation, and keep water away.
Describe the pattern of the magnetic field around a bar magnet and how to plot it.
Field lines run from north to south outside the magnet, closer together where the field is stronger. Plot using a small plotting compass, marking its needle direction as you move it round.
Describe the magnetic field around a straight wire and a solenoid carrying current.
A wire has concentric circles around it. A solenoid has a field like a bar magnet's. Reversing the current reverses the field; more current or turns, or an iron core, strengthens it.
Describe how a relay works and why it is useful.
A small current in a coil magnetises an iron core, pulling a switch closed. This lets a low-voltage circuit safely switch a high-current or high-voltage circuit.
What force acts on a current-carrying wire in a magnetic field, and how do you find its direction?
The wire feels a force at right angles to both field and current. Use Fleming's left-hand rule: first finger field, second finger current, thumb force (motion).
How does a simple d.c. motor work, and what do the split-ring commutator and brushes do?
A coil in a magnetic field experiences opposite forces on its sides, so it turns. The split-ring commutator reverses the current every half turn so the coil keeps rotating in one direction.
What is electromagnetic induction and what affects the size of the induced e.m.f.?
An e.m.f. is induced when a conductor cuts magnetic field lines (or the field through a coil changes). It is bigger with faster movement, a stronger magnet or more turns on the coil.
Describe how a simple a.c. generator works.
A coil is rotated in a magnetic field, so it cuts field lines and an e.m.f. is induced. Slip rings and brushes connect it to the circuit, and the output alternates. Faster rotation gives a higher frequency and voltage.
(Extended) State the transformer equations for an ideal transformer.
Vₚ/Vₛ = Nₚ/Nₛ, and VₚIₚ = VₛIₛ (100% efficient). A step-up transformer has more turns on the secondary; step-down has fewer. Needs a.c. and an iron core.
(Extended) A transformer has 1000 primary turns on 230 V and 50 secondary turns. What is the secondary voltage?
Vₛ = Vₚ × Nₛ/Nₚ = 230 × 50 ÷ 1000 = 11.5 V.
(Extended) Why is electricity transmitted at high voltage?
High voltage means a lower current for the same power (P = IV). Power lost as heat in cables is I²R, so a lower current greatly reduces the energy wasted.

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