Download Potential difference - School

Electric circuits- helpsheet
Current is the flow of charge. The size of the current is the rate of flow of
electric charge.
I = current in amperes (amps), A
Q = charge in coulombs, C
t = time in seconds, s
I = Q
t
Current is measured using an Ammeter in series with the circuit.
In a series circuit the current is the same everywhere.
In a parallel circuit the current is split when it reaches a branch.
The circuit needs to be complete for the current to flow.
A
A
+- -+ - + +- -+ - +- -+ - +
+- +- +- -+ - + + ++ + - + + +- +- + - +
+- + - + +- + - + + +-
Metal atoms (ions) in a wire have delocalised
electrons which are free to move and carry the
electric current around the circuit.
The electrons moving around the circuit collide
with the ions. This is called resistance.
Components with high resistance (e.g. filament
bulbs) often get hot. This is because when the
electrons collide with the ions they transfer
energy as heat (and light), this then causes the
ions to vibrate more increasing the resistance by
making it harder for the electrons to pass through
without collisions.
Potential difference (voltage) is measured between two points and is the work done (energy transferred) per coulomb of charge that passes between the points.
V = W
Q
V
V = potential difference in volts, V
Q = charge in coulombs, C
W = work done in joules, J
3V
6V
3V
Potential difference is measured using a voltmeter connected in parallel across a component.
X
M
The sum of potential difference across one branch is equal to the sum of the potential difference of the cells or battery.
V
V
In a parallel circuit the potential difference across a single component in a branch is equal to the potential difference of the cell.
Potential difference, current and resistance are all
related by the equation;
V = I x R
V = potential difference in volts, V
I = current in amps, A
R = resistance in ohms, 
The total resistance in a
circuit is the sum of the
resistors in the circuit
6 ohms
10 ohms
The current through a resistor
(at a constant temperature) is
directly proportional to the
potential difference across a
resistor.
4V
An LED (light emitting diode)
emits light when a current
flows in the forward direction
The resistance of a bulb
increases as the
temperature of the
filament increases
An LDR (light-dependent resistor)
resistance decreases as light intensity
increases. (DARK = high resistance,
low current, LIGHT = low resistance,
high current)
The current through a diode
flows in one direction. It
has a very high resistance in
the opposite direction.
A thermistor’s resistance decreases
as the temperature increases.(
HIGH TEMP = low resistance, high
current, LOW TEMP = high
resistance, low current)
e.g. 6 ohms + 10 ohms = 16 ohms
A variable resistor can alter the
resistance in a circuit and is useful
for things like controlling volume or
light dimmer switches
2V
Household electricity has a potential difference of around 230V and a frequency of 50 Hz (Hertz), so
changes direction 50 times in a second.
Household electricity- helpsheet
Current in a simple circuit is direct (dc) because it only flows in
one direction, whereas current from the mains supply is
alternating current (ac) because it alters direction.
dc- one directio
The frequency of an AC supply is the number of oscillations per second. You can find it from the
period: Frequency = 1 ÷ period
(Remember to convert to seconds if needed. In this example, 20 ms = 20 ÷ 1000 = 0.020 s.)
Frequency = 1 ÷ 0.020 = 50 Hz
ac- alternating direction
The live terminal (pin)
potential difference
varies between a high
positive value and low
positive value.
The neutral terminal
(pin) has a potential
difference close to
earth which is zero.
The period of an AC supply is the time taken for one complete oscillation. You can find this by
looking at the time between one peak and the next.
e.g. In the oscilloscope trace, one horizontal division represents 5 ms (five milliseconds).
There are four divisions between two adjacent peaks, so the period is: 4 × 5 = 20 ms
In a plug and cable;
A plug is designed with the following;
Blue is neutral and goes on the left
Striped is Earth and goes to the top
Brown is live and goes to the right
•Plastic casing outside is an insulator
•Brass pins are good conductors
•Fuse melts if the current is too high (thus
breaking the circuit)
•Cables are made of copper because they are
bendy and good conductors
•Cables have plastic coating because they are
good insulators
This is the symbol for a fuse.
The circuits in your house are earthed
outside
Appliances with metal cases need to
be earthed inside the case.
This is just in case a live wire comes
loose- the current will then pass through
the earth wire and not you when you
touch it
Appliances with plastic casings sometimes only have 2 cables (no
Earth) as they are double insulated and have been designed so
that the live wire cannot touch the casing. This is because
the plastic acts as an insulator so the chance of
electric shock is minimal.
A fuse will melt when the current is too high
Circuit breakers
switch off the
current when
there is a fault
When the switch is
open it stays open
until it is reset
When an electrical charge flows through a resistor, the
resistor gets hot.
A lot of energy is wasted in filament bulbs as heat.
Less energy is wasted in power-saving lamps such as
Compact Fluorescent Lamps (CFLs).
There is also a choice when buying new appliances
in how efficiently they transfer energy.
The rate at which energy is transferred by an appliance called the power
Residual current
circuit breakers
(RCCB) work faster
because they cut
off current in a live
wire when it is
different from
current in a neutral
wire.
P = E
t
P = power in watts, W
E = energy in joules, J
t = time in seconds, s
P = I x V
P = power in watts, W
I = current in amps, A
V = potential difference in volts, V
Energy transferred, potential difference and charge are all related by..
E = V x Q
E = energy in joules, J
V = potential difference in volts, V
Q = charge in coulombs, C