Lecture 6: Electrical Power
1. Electrical energy & power 2. Electrical activity in the heart 3. Examples and Quizzes
Electrical Energy and Power
In a circuit, as a charge moves through the battery, the electrical potential energy of the system is increased by QV
The chemical potential energy of the battery decreases by the same amount
As the charge moves through a resistor, it loses this potential energy during collisions with atoms in the resistor
The temperature of the resistor will increase
Three resistors are connected in series in a circuit. The resistors A, B, and C have resistances R, 2R, and 3R, respectively. Which resistance dissipates the most power?
1. 2. 3. 4.
5
A B C They all dissipate the same power when in series.
25% 25% 25% 25%
1
0
30
A circuit element uses 50 watts of power. The current running through it is 5 A. Which of the following could be the circuit's resistance and potential difference? 1. 2.
25% 25% 25% 25%
3.
5
4.
1
0
30
Energy Transfer in the Circuit
Consider the circuit shown Imagine a quantity of positive charge, Q, moving around the circuit from point A back to point A
Energy Transfer in the Circuit, cont
Point A is the reference point
It is grounded and its potential is taken to be zero
As the charge moves through the battery from A to B, the potential energy of the system increases by QV
The chemical energy of the battery decreases by the same amount
Energy Transfer in the Circuit, final
As the charge moves through the resistor, from C to D, it loses energy in collisions with the atoms of the resistor The energy is transferred to heat When the charge returns to A, the net result is that some chemical energy of the battery has been delivered to the resistor and caused its temperature to rise
Electrical Energy and Power, cont
The rate at which the energy is lost is the power Q = V = I V t From Ohms Law, alternate forms of power are
V = I R = R
2
Electrical Energy and Power, final
The SI unit of power is Watt (W)
I must be in Amperes, R in ohms and V in Volts
The unit of energy used by electric companies is the kilowatt-hour
This is defined in terms of the unit of power and the amount of time it is supplied 1 kWh = 3.60 x 106 J
Electrical Activity in the Heart
Every action involving the bodys muscles is initiated by electrical activity Voltage pulses cause the heart to beat These voltage pulses are large enough to be detected by equipment attached to the skin
Operation of the Heart
The sinoatrial (SA) node initiates the heartbeat The electrical impulses cause the right and left artial muscles to contract When the impulse reaches the atrioventricular (AV) node, the muscles of the atria begin to relax The ventricles relax and the cycle repeats
Electrocardiogram (EKG)
A normal EKG P occurs just before the atria begin to contract The QRS pulse occurs in the ventricles just before they contract The T pulse occurs when the cells in the ventricles begin to recover
Abnormal EKG, 1
The QRS portion is wider than normal This indicates the possibility of an enlarged heart
Abnormal EKG, 2
There is no constant relationship between P and QRS pulse This suggests a blockage in the electrical conduction path between the SA and the AV nodes This leads to inefficient heart pumping
Abnormal EKG, 3
No P pulse and an irregular spacing between the QRS pulses Symptomatic of irregular atrial contraction, called fibrillation The atrial and ventricular contraction are irregular
Implanted Cardioverter Defibrillator (ICD)
Devices that can monitor, record and logically process heart signals Then supply different corrective signals to hearts that are not beating correctly
