Electricity and Circuits

Question 1

What is current?

Answer 1

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.

Question 2

What is potential difference?

Answer 2

The driving force that pushes the charge round. It is the energy transferred per coulomb of charge. The unit is V, volt.

Question 3

What is resistance?

Answer 3

Anything that slows the flow of charge down in a circuit. The unit is ohm.

Question 4

What is the formula for charge?

Answer 4

Charge = current x time
Q = I x t
Coulombs = amperes x time

Question 5

What is the formula for energy transferred in circuits?

Answer 5

Energy transferred = charge moved x potential difference
E = Q x V
J = C x V

Question 6

What is the formula linking potential difference, resistant, and current?

Answer 6

Potential difference = current x resistance
V = IR
V = A x ohms

Question 7

Why does resistance increase with temperature? What is the exception?

Answer 7

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.

Question 8

What is in the standard test circuit?

Answer 8

An ammeter in series
A voltmeter in parallel with the component under test
A variable d.c. source

Question 9

What are the types of I-V graphs? Give examples.

Answer 9

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

Question 10

What do resistance-light intensity and -temperature graphs look like for LDRs and thermistors?

Answer 10

They curve downwards: for LDRs, resistance decreases as light intensity increases, and for thermistors, resistance decreases as temperature increases.

Question 11

What are series circuits?

Answer 11

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.

Question 12

What are parallel circuits?

Answer 12

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.

Question 13

Why does adding resistors in series increase total resistance and in parallel decrease total resistance?

Answer 13

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.

Question 14

What is the equation for energy transferred in a circuit?

Answer 14

Energy transferred = Current x voltage x time
E = I x V x t
J = A x V x s

Question 15

What is the equation for power?

Answer 15

Power = Energy transferred / Time
P = E/t
W = J/s

Question 16

What are the equations for electrical power?

Answer 16

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

Question 17

What is electricity like in the mains supply and from a battery supply?

Answer 17

The UK mains supply is a.c. at around 230V and 50Hz (cycles per second)
Batteries supply d.c. from a direct voltage

Question 18

What are plugs like?

Answer 18

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

Question 19

How do fuses work?

Answer 19

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.

Question 20

How does earthing work?

Answer 20

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.

Question 21

What are circuit breakers?

Answer 21

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.

Question 22

What does double insulated mean?

Answer 22

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.