What is current?
The flow of electric charge (e.g. electrons) around a circuit. In order for this to happen a potential difference and a complete circuit is required. The unit is A or ampere and the symbol is I.
What is potential difference?
The driving force that pushes the charge round. It is the energy transferred per coulomb of charge. The unit is V, volt.
What is resistance?
Anything that slows the flow of charge down in a circuit. The unit is ohm.
What is the formula for charge?
Charge = current x time Q = I x t Coulombs = amperes x time
What is the formula for energy transferred in circuits?
Energy transferred = charge moved x potential difference
E = Q x V
J = C x V
What is the formula linking potential difference, resistant, and current?
Potential difference = current x resistance
V = IR
V = A x ohms
Why does resistance increase with temperature? What is the exception?
Electrons collide with ions that make up the resistor as they flow through it. This gives the ions energy, so they vibrate and heat up. The more they vibrate, the harder it is for electrons to through the resistor, as there are more collisions.
The exception is a thermistor: the resistance of a thermistor decreases as temperature increases.
What is in the standard test circuit?
An ammeter in series
A voltmeter in parallel with the component under test
A variable d.c. source
What are the types of I-V graphs? Give examples.
Linear graphs - current increases in direct proportion to voltage e.g. resistors and wires
Non-linear graphs - current increases more quickly at lower voltages and tails off e.g. filament lamps, as the temperature increases
One way graphs - applies to diodes, which only allow current to flow in one direction
What do resistance-light intensity and -temperature graphs look like for LDRs and thermistors?
They curve downwards: for LDRs, resistance decreases as light intensity increases, and for thermistors, resistance decreases as temperature increases.
What are series circuits?
Components are connected in a line, end to end, between the positive and negative of the power supply. If you remove or disconnect one component, the circuit breaks and stops working. Current is the same everywhere. The total p.d. is shared between components and depends on each component’s resistance. Resistance increases as you add resistors.
What are parallel circuits?
Each component is separately connected to the positive and negative of the supply. If you remove or disconnect one, the others are hardly affected. The potential difference is the same across all components. Current is shared between branches, so the total current is equal to the sum of all currents in branches. Resistance decreases as you add resistors in parallel.
Why does adding resistors in series increase total resistance and in parallel decrease total resistance?
In series the two resistors have to share the total p.d., so the p.d. across each resistor is lower, so the current through each resistor is lower. As current is the same everywhere, total current is reduced. Total resistance is the sum of the resistances.
In parallel there are more ways for the current to go when resistors are added, so total current increases, so resistance decreases. Total resistance is less than the resistance of the smallest of the two resistors.
What is the equation for energy transferred in a circuit?
Energy transferred = Current x voltage x time
E = I x V x t
J = A x V x s
What is the equation for power?
Power = Energy transferred / Time P = E/t W = J/s
What are the equations for electrical power?
Electrical power = Current x Potential difference
P = IV
W = A x V
Power = Current^2 x Resistance P = I^2R W = A^2 x ohms
What is electricity like in the mains supply and from a battery supply?
The UK mains supply is a.c. at around 230V and 50Hz (cycles per second)
Batteries supply d.c. from a direct voltage
What are plugs like?
There is a fuse and three wires:
Neutral wire - blue. Completes the circuit, at 0V.
Live wire - brown. Carries the voltage, alternates between about 230V and -230V.
Earth wire - green and yellow. For safety, at 0V
How do fuses work?
Fuses are rated just above the normal operating current. If a fault occurs, current will increase, which melts the fuse (a thin wire). This breaks the circuit, preventing fire risk.
How does earthing work?
If a fault causes the live wire to touch the metal case of an appliance, the case becomes live. This means if you were to touch it, you would complete the circuit and be shocked. Earth wires are attached to the case, so current instead flows along the wire. A fuse would then trip, isolating the appliance, so you cannot get a shock.
What are circuit breakers?
They are used instead of fuses. A large current ‘trips’ (turns off) the circuit breaker. This happens more quickly than a fuse melting. They can be reset without having to replace them. They are also more expensive than fuses.
What does double insulated mean?
An appliance has a plastic casing and no metal parts showing. This means it does not need an earth wire, as it cannot conduct electricity. Two core cables are used, with just a live and neutral wire.